Department of Biology, Washington University, St. Louis, Missouri 63130
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ABSTRACT |
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Kanwal, Jagmeet S., Douglas C. Fitzpatrick, and Nobuo Suga. Facilitatory and Inhibitory Frequency Tuning of Combination-Sensitive Neurons in the Primary Auditory Cortex of Mustached Bats. J. Neurophysiol. 82: 2327-2345, 1999. Mustached bats, Pteronotus parnellii parnellii, emit echolocation pulses that consist of four harmonics with a fundamental consisting of a constant frequency (CF1-4) component followed by a short, frequency-modulated (FM1-4) component. During flight, the pulse fundamental frequency is systematically lowered by an amount proportional to the velocity of the bat relative to the background so that the Doppler-shifted echo CF2 is maintained within a narrowband centered at ~61 kHz. In the primary auditory cortex, there is an expanded representation of 60.6- to 63.0-kHz frequencies in the "Doppler-shifted CF processing" (DSCF) area where neurons show sharp, level-tolerant frequency tuning. More than 80% of DSCF neurons are facilitated by specific frequency combinations of ~25 kHz (BFlow) and ~61 kHz (BFhigh). To examine the role of these neurons for fine frequency discrimination during echolocation, we measured the basic response parameters for facilitation to synthesized echolocation signals varied in frequency, intensity, and in their temporal structure. Excitatory response areas were determined by presenting single CF tones, facilitative curves were obtained by presenting paired CF tones. All neurons showing facilitation exhibit at least two facilitative response areas, one of broad spectral tuning to frequencies centered at BFlow corresponding to a frequency in the lower half of the echolocation pulse FM1 sweep and another of sharp tuning to frequencies centered at BFhigh corresponding to the CF2 in the echo. Facilitative response areas for BFhigh are broadened by ~0.38 kHz at both the best amplitude and 50 dB above threshold response and show lower thresholds compared with the single-tone excitatory BFhigh response areas. An increase in the sensitivity of DSCF neurons would lead to target detection from farther away and/or for smaller targets than previously estimated on the basis of single-tone responses to BFhigh. About 15% of DSCF neurons show oblique excitatory and facilitatory response areas at BFhigh so that the center frequency of the frequency-response function at any amplitude decreases with increasing stimulus amplitudes. DSCF neurons also have inhibitory response areas that either skirt or overlap both the excitatory and facilitatory response areas for BFhigh and sometimes for BFlow. Inhibition by a broad range of frequencies contributes to the observed sharpness of frequency tuning in these neurons. Recordings from orthogonal penetrations show that the best frequencies for facilitation as well as excitation do not change within a cortical column. There does not appear to be any systematic representation of facilitation ratios across the cortical surface of the DSCF area.
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INTRODUCTION |
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To understand how acoustic stimuli are processed in the mammalian
auditory cortex, several studies have focused on the basic response
properties of cortical neurons, such as the sharpness of frequency
tuning, response thresholds, dynamic response range, amplitude-response
functions, etc., and how they are spatially and temporally organized
(deCharms and Merzenich 1996; Merzenich et al.
1975
; Schreiner 1995
; Schreiner and
Cynader 1984
; Schreiner and Mendelson 1990
;
Schreiner and Sutter 1992
; Schreiner et al. 1992
; Shamma et al. 1993
; Sutter and
Schreiner 1991
, 1995
; Takahashi 1995
). These
studies provide important clues to the functional organization of the
auditory cortex in mammals without necessarily attributing single-cell
response properties to species-specific behaviors. A few recent
studies, however, have made a special attempt to link neural responses
and mechanisms with the specific auditory functions such as
communication (Ehret and Schreiner 1997
;
Rauschecker 1998
; Rauschecker et al.
1995
; Wang et al. 1995
).
In mustached bats, the functional organization of the auditory cortex
has been explored with great success using species-specific stimuli
relevant to their echolocation behavior (Fitzpatrick et al.
1998; Suga 1965
, 1984
; Suga and Kanwal
1995
; Suga et al. 1983
, 1997
) and communication
behavior (Esser et al. 1997
; Kanwal 1999
; Ohlemiller et al. 1996
). These stimuli consist of
combinations of information-bearing elements in a bat's echolocation
pulse and echo (Fig. 1A). A
mustached bat's echolocation pulse consists primarily of four
harmonics (H1-H4) each
including a constant frequency (CF1-4) tone
followed by a downward sweeping FM (FM1-4). The
fundamental frequency of the FM1 component sweeps
downward from 30 to 24 kHz at a rate of 2 kHz/ms. Typically, studies on the functional organization of the bat's auditory cortex and of neural
specializations for echolocation have been coupled with analysis of the
excitatory, facilitatory, and inhibitory tuning properties of neurons
to stimulus parameters that are behaviorally relevant. On the basis of
these studies, the bat's auditory cortex has been divided into several
distinct functional areas that contain computational maps of
behaviorally relevant combinations of stimulus parameters (e.g., maps
of FM-FM delays for measuring target distance and CF/CF combinations
for measuring relative target velocities) (Suga and O'Neill
1979
) (see Fig. 1, B and C). This
organization of the auditory cortex is based strictly on the
specialized behavior of echolocation, which is essential to the bat's
ecological niche.
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The Doppler-shifted CF processing (DSCF) area lies within the main
tonotopic representation in the auditory cortex. The resting CF2 in the echolocation pulse of mustached bats
varies in different individuals, and this variation is reflected in the
neural tuning of DSCF neurons (Henson et al. 1980, 1987
;
Suga and O'Neill 1980
; Suga and Tsuzuki
1985
). The DSCF area receives projections from the ventral
division of the medial geniculate body (MGB) (Olsen 1986
; Suga 1982
). This area is considered to be
homologous to a part of the primary auditory cortex of other mammals.
Specifically, this area overrepresents a single iso-frequency contour
ranging from ~60 to 62 kHz (normalized to the average
CF2 resting frequency of 61.0 kHz) within a
radial frequency axis and a circular amplitopic axis (Suga
1977
; Suga and Manabe 1982
; Suga et al.
1987
) (Fig. 1C). On the basis of previous studies,
neurons in this area have been defined as having sharp, "level
tolerant" tuning curves and their response is restricted to
Doppler-shifted CF2 frequencies in the echo
(Suga and Jen 1976
; Suga and Manabe
1982
). Focal inactivation of the DSCF area in mustached bats
disrupts specifically the ability to make fine frequency
discriminations. This finding is consistent with the postulated role of
the DSCF area in target detection and characterization
(Riquimaroux et al. 1991
).
The response of DSCF neurons is facilitated when a 30-ms-long CF tone
burst of 60-62 kHz is paired with another equally long CF tone burst
ranging from 22 to 28 kHz or when a 3-ms-long pulse FM1 sweeping from 29 to 23 kHz precedes the onset
of the tone burst (Fitzpatrick et al. 1993;
Kanwal et al. 1991
). Furthermore, for 20-ms-long CF
tones, the facilitation is broadly tuned to echo delays between 11 and
26 ms where pulse FM1 overlaps temporally with
echo CF2. This range of tuning is quite different
and complementary to the range of tuning to echo delays (<10 ms) in
the FM-FM specialized, delay tuned areas [FM-FM, dorsal fringe (DF),
and ventral fringe (VF) areas] in the auditory cortex (Edamatsu
et al. 1989
; O'Neill and Suga 1982
; Suga
and Horikawa 1986
; Suga and O'Neill 1979
). Although the delay tuning of DSCF neurons has been studied in some
detail, the effect of combination-sensitive aspects of frequency and
amplitude tuning to CF tones at ~61 kHz frequencies is unclear. We
also do not know how facilitation affects the sharpness of frequency
tuning, thresholds and best amplitude-response levels or whether the
magnitude of facilitation varies with depth or position in the DSCF area.
Knowledge of these response properties and the associated facilitative mechanisms is critical for understanding the behavioral role of the specializations described for neurons in this cortical area. In this study, we describe the excitatory, facilitatory, and inhibitory frequency tuning properties of DSCF neurons and also address the question of how the various stimulus parameters for facilitation are represented within the DSCF area in the primary auditory cortex of the mustached bat.
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METHODS |
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The materials and methods of the present experiments are
generally similar to those described previously (Suga
and Tsuzuki 1985). Therefore the
common procedures are described briefly, whereas those that have been
modified or are unique to this experiment are described in detail.
Surgery and recording of neural activity
Fourteen Jamaican mustached bats, Pteronotus parnellii
parnellii, weighing between 11 and 13 g were used in this
study. Before surgery, animals were placed in a Styrofoam body mold and
injected intramuscularly with 0.08 mg/kg body wt fentanyl and 4 mg/kg
droperidol mixture (Innovar), and 2 mg/kg body wt prednisolone (methyl
derivative). Innovar causes neuroleptic analgesia, while Prednisolone,
a corticosteroid hormone, reduces the level of metabolic stress. After
surgical exposure of the skull, a metal post (length = 1.5 cm) was
mounted vertically on the midline of the skull with cyanoacrylate glue (Eastman 910). The two flaps of skin were sutured loosely around the
metal post on the skull, and the bat was allowed to recover for 3 days
before the first recording session. Neural activity was recorded
biweekly from each bat for 12 wk per bat.
All recordings were made from unanesthetized, restrained animals. All experiments were carried out using protocols approved by the Animal Care and Use Committee at Washington University. The bat was placed in a Styrofoam body mold to restrain body movements. The mold was designed to provide airspace around the bat's body and move with any body movements of the bat. The mold was suspended by elastic bands in a heated (31°C), sound-proofed and echo-attenuated chamber (IAC 400A). The walls of the room were covered with echo-attenuating foam (Sonex). The head of the bat was immobilized by clamping the affixed metal post within a hollow rod clamped vertically above the recording table. Lidocaine was applied topically before manipulation of the surgical wounds. The animal was observed carefully for signs of discomfort. Drinking water was offered two to three times during the recording session.
Acoustic stimulation
Sound stimuli were presented from two condenser loudspeakers mounted on a vertical hoop and positioned 95 cm directly in front of the bat. The two loudspeakers were positioned adjacent to each other in the same azimuth in front of the bat to avoid binaural effects. The stimulus generation and delivery system consisted of three channels such that two sounds were presented from one speaker and the third was presented from a different speaker. However, when two sounds came from the same speaker, they were separated in time. The three sounds could be controlled independently in frequency, amplitude, and duration and could be delivered simultaneously or successively. These sounds could be triggered either manually or via a computer. Sounds consisted of CF tones and/or FM sweeps. The condenser loudspeakers were calibrated by placing a B&K microphone at the position of the ear and were reasonably flat between 20 and 100 kHz with a significant roll off at 120 kHz. The maximum amplitude level that could be delivered for speaker A was 98 dB SPL (re 20 µPa, RMS) at 90 kHz and that for speaker B was 95 dB SPL at 85 kHz.
A single stimulus (tone burst) was used for studying neural excitation and paired stimuli were used for studying facilitation. When time permitted, three types of response areas were obtained for each neuron. 1) Excitatory response areas were generated from neural responses to single tones varied in frequency and amplitude. The responsiveness of a neuron was scanned for frequencies from 10 to 100 kHz, and response areas were typically measured at ~25 and 61 kHz. 2) Facilitatory response areas were generated by keeping the frequency and amplitude of one tone constant and varying both of these parameters for the other tone. The amplitude of the fixed tone was generally adjusted 10 dB above its excitatory response threshold so that when presented separately, it produced a minimal response. The excitatory response was not subtracted when estimating the facilitatory response areas centered at either 25 or 61 kHz frequencies. The 30-ms-long CF tone bursts were paired so that they had simultaneous onsets. 3) Inhibition was studied with pairs or triplets of tone bursts for excitatory and facilitatory responses, respectively. The inhibitory test tone of 4-ms duration was delivered just before, without overlap, with a second excitatory tone or facilitatory tone pair. For obtaining inhibitory response areas, the excitatory and facilitatory tones were presented at the best frequency and at ~10 dB above threshold level and the inhibitory tone was varied systematically between 5 and 100 kHz. This minimized the effects of two-tone suppression. The amplitude of the first tone presented at different frequencies that caused inhibition of the response to the second was recorded. The boundaries of the inhibitory response areas were detected by audiovisually monitoring suppression of the facilitated neural response to its threshold level. The time course of the inhibition was measured by increasing the time delay between the onset of the test tone and the fixed excitatory/facilitatory tone/s.
The minimum threshold for facilitation was determined by presenting the fixed tone at its best frequency and at its minimum threshold level while simultaneously presenting the test tone at just above threshold level and attenuating it further until the response was nearly extinguished. Similarly, the upper threshold level for the facilitatory tone was determined by increasing the amplitude of the test tone until the response to the tone pair was once again minimal. The best low frequency for excitation (BFlow) was determined after establishing the best high frequency (BFhigh). Once BFlow was determined, BFhigh was checked once again and adjusted if necessary. This iterative process was repeated a few times until a stable value for both BFlow and BFhigh was obtained. To minimize experimenter bias, the frequency counter was momentarily turned off when evaluating the best response audiovisually.
To test the effect of harmonics in the pulse on the facilitative response, five neurons were tested with different combinations of harmonics in the pulse with an echo stimulus consisting of multiple-harmonics (ECF2-4) based on the best ECF2 frequency. These harmonics were generated using a custom-built harmonic generator. The fundamental frequency in the bat's own pulse is relatively weak so that the frequencies corresponding to the fundamental frequency in the echo are unlikely to contribute to a facilitative response. The first harmonic (H1) of the pulse was simulated by generating a 6-kHz FM sweep with the neuron's best frequency at its center and preceding it with a 27-ms-long CF1 at the appropriate frequency.
Data acquisition and analysis
The resting frequency of the CF2 component
of orientation sounds ("CF2 resting
frequency") emitted by each animal was measured at the beginning of
each experiment. This averaged [60.43 ± 0.83 (mean ± SD) kHz;
n = 14, males = 8, females = 6]. The
CF2 resting frequency differs among individual bats,
ranging from ~60 to ~63 kHz (Suga et al. 1987).
Neurons in the DSCF area are extremely sharply tuned to frequencies
within this range. This sharp tuning also varies according to the
bat's own CF2 resting frequency, so the best frequencies
measured for neurons in the DSCF area of different animals were
normalized to 61.0 kHz (the population average) according to the method
of Suga and Tsuzuki (1985)
. This allows comparison and
pooling of tuning curve data across different animals.
The activity of single neurons was recorded with sharpened,
vinyl-coated tungsten-wire electrodes with tip diameters of ~10 µm
and impedance of ~2 M. An indifferent tungsten-wire electrode was
placed in the nonauditory frontal cortex. Before insertion of the
electrode, holes of ~50 µm in diameter were made in the skull with
a sharpened needle. The recording electrode was inserted orthogonally
in 1.0- to 2.5-µm steps using a Kopf hydraulic microdrive controlled
by a stepping motor. Orthogonality of electrode penetrations was based
primarily on visual inspection with a dissection microscope. In
addition to visual inspection, the accuracy of an orthogonal penetration was confirmed by the fact that in these penetrations the
BFhigh did not change with depth as suggested by the
excitatory tuning data in a previous study (Suga and Manabe
1982
). Further confirmation was based on imaging a frontal view
of the bat's skull on a video monitor and measuring the angle of the
electrode against the tangent of the skull curvature at the penetration site on the skull. The actual depth of the recording site was estimated
by noting the reading for the cortical surface during the initial
insertion and final withdrawal of the electrode and taking the average
value. This adjusted for any effects of indentation or drying of the
cortical surface. Recordings were made from well-resolved single units
isolated from the background activity with a BAK window discriminator
(BAK DIS-1) including an analog delay (BAK AD-3). Signal-to-noise
ratios generally ranged from 3:1 to 5:1 and were occasionally higher.
Action potentials the waveform of which was restricted to the preset
time-amplitude window generated an acceptance pulse that triggered an
oscilloscope display of the shape of the delayed action potential. A
stored waveform was used as a template-match to monitor any changes in
the shape of the ongoing spike waveform and therefore ensure that the
activity of the same neuron was being recorded.
Acquisition of neural activity was controlled by Modular Instruments (MI2) software such that trigger pulse generation was synchronized with data acquisition. An electronic module was used to control stimulus amplitude by varying the level of attenuation at the output of an electronic switch. CF tones were 30 ms long and were delivered at a rate of 4/s, and neural activity was acquired for 200 ms from stimulus onset. Response was quantified as the number of impulses per 200 stimulus presentations in a 60-ms time window, although for phasic responses a 40-ms window covered the whole response (these spike count windows included a 10-ms prestimulus duration). Computer-controlled frequency scans consisted of at least five repetitions at a rate of 5/s of 22 blocks of recorded neural activity with the last block acting as a control to record spontaneous activity. The frequency of the 13th block was designated as the center-frequency and decreased/increased by specified frequency steps of 100-250 Hz in the blocks preceding/succeeding the center-frequency block. This procedure using the customized MI2 software generated a dot raster and either a cumulative or a running histogram display of the frequency-tuning of the neuron. For off-line analyses, spike times were stored on the hard drive of the computer and responses were plotted using custom software.
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RESULTS |
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Facilitation of responses by two CF tones
The response of many DSCF neurons is facilitated by the presentation of CF sounds that are contained in at least two discrete, nonoverlapping frequency ranges. A facilitative response is described as a response to two or more stimuli delivered together that is greater than the sum of the response to each stimulus delivered alone. A neuron displaying such a response is considered to be combination sensitive. The best high frequency (BFhigh) that produces a facilitative response in DSCF neurons typically ranges from 61 to 63 kHz and lies within the range of the constant frequency of the second harmonic of the Doppler-shifted echo (ECF2; see Fig. 1A). The best low frequency for facilitation (BFlow) ranges from 22 to 28 kHz and overlaps with the range of the downward frequency sweep in an echolocation pulse (PFM1 in Fig. 1). Occasionally, a third, middle frequency (BFmid) in the neighborhood of 50 kHz also triggers a facilitative response when paired with BFlow.
The majority of DSCF neurons are excited poorly or not at all when
BFlow is delivered alone and strongly when it is
paired with a BFhigh such that they exhibit
facilitation. The simultaneous delivery of two tones may lead to a
relative increase in sound pressure level (SPL) of the sound, although
at each individual frequency there will be no increase. In the DSCF
area, an increase in SPL beyond a particular value reduces the
magnitude of facilitation because of the upper thresholds in the
response areas of these neurons, particularly to
BFhigh frequencies when delivered alone or when
paired with a BFlow. Upper thresholds of DSCF
neurons when stimulated with a single tone burst at ~61 kHz are
documented in a previous study (Suga and Tsuzuki 1985).
As reported earlier (Fitzpatrick et al. 1993),
facilitation also is observed in DSCF neurons when a CF/FM signal
corresponding to the pulse H1 (1st harmonic in
the pulse) is paired at a best delay with a CF corresponding to the
echo CF2 in an echolocation signal (Fig.
2, A-C). However, this
response is generally smaller than or equal to the same neuron's
response to two CF tones delivered simultaneously. Notice that the
response to two tones at 0 ms delay is larger than the response at 31 ms "pulse H1-echo CF2" delay (Fig. 2D). These data justify our use of paired tone
bursts with a simultaneous onset for studying the tuning properties of DSCF neurons. Use of paired tone bursts is also desirable because frequency tuning for facilitation is obtained more easily with tones
than by FM sweeps.
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Classification of DSCF neurons based on excitatory and facilitatory responses
For each DSCF neuron, we calculated the mean response from three to six measurements of the response to BFlow and to BFhigh frequencies alone and to combinations of these frequencies. From these data, a facilitation ratio (FR) was calculated for each neuron by dividing the facilitated response by the sum of the responses to each component alone. FRs typically were calculated from the peak response magnitude of a neuron to tones delivered at a SPL corresponding to 10 dB above its minimum excitatory threshold. Each tone in a pair was delivered at the same SPL as when it was delivered alone. The spontaneous level of activity was subtracted from all responses. A neuron was said to be facilitated if the mean value of its FR was >1.1. The majority (66%) of neurons show FRs from 1.1 to 2.0 (Fig. 3A). The mean value of FR for DSCF neurons was 2.16 (n = 226). About 2% of the neurons had a FR >5. The mean response to BFlow frequencies alone is about one-sixth of that to the BFhigh frequencies alone at their respective best amplitudes (BAs).
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The line plotted in Fig. 3A shows the distribution of DSCF neurons based on different FRs, and the histograms show the average relative magnitude of responses to BFlow and BFhigh when presented alone for different values of facilitation. Except for some neurons with values of FR ranging from four to five, neurons respond relatively little to BFlow frequencies. When the response to each component is normalized to FR and expressed as a percentage, the majority of neurons tend to respond either equally or better to BFhigh than to BFlow.
In an attempt to identify functionally different subpopulations of DSCF
neurons, we used a nonhierarchical splitting algorithm for a three-way
classification of DSCF neurons based on their facilitative responses to
two tones (Hartigan 1975). The resultant clusters were
based on the FR versus the relative contribution of single tone
BFlow and BFhigh components
to the facilitated response and accordingly designated as
"Fhigh," "Flow,"
and "Fhigh/low" (Fig. 3B).
Although the response properties exhibit an apparent continuum of
variation, we generated a three-way classification scheme that
represented a discrete orthogonal split into two groups and a third
intermediate group. The distribution of neurons between the three
groups is shown as a scatterplot. The ellipses indicate the boundaries
of the three distributions for a 50% confidence level
(Wilkinson et al. 1998
). This information cannot be
illustrated in the bar graph, which only shows the mean response ratios
for each frequency relative to the magnitude of facilitation for
different values of FR. Fhigh class consisted of
37%, Flow class consisted of 18%, and the
Fhigh/low class consisted of the remaining 45% of the total number of neurons. The Flow group of
neurons was missed completely in previous studies of the DSCF area
because BFlow frequencies were not tested.
Additionally, Flow neurons exhibited lower
thresholds to BFlow than to
BFhigh stimuli, whereas Fhigh and Fhigh/low neurons
exhibited lower thresholds to BFhigh than to
BFlow stimuli. Also,
Fhigh/low neurons had significantly (P < 0.001) higher values of FR (2.67 ± 1.1)
than that for Fhigh (FR = 1.82 ± 1.06)
and Flow (FR = 1.78 ± 1.38) neurons.
Examples of PST histograms obtained for the three types of DSCF neurons are shown in Fig. 4. The neuron shown in Fig. 4B is classified as a Fhigh/low neuron with an unusually high FR. The neuron shown in Fig. 4C is a typical Fhigh neuron with a much better response to BFhigh alone than to the BFlow stimulus but has an atypically high rate of spontaneous activity and a more tonic component in its response than the other two neuron types. Note that the facilitated response in Fig. 4C is preceded by a short inhibitory period. This is observed frequently for facilitated response in DSCF neurons.
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To study the time course of inhibition for a single neuron, a 4-ms-long CF prepulse at the best amplitude for inhibition precedes a two-tone, BFlow/BFhigh, combination and the delay between the prepulse and the two-tone combination is varied systematically (Fig. 5, A-G). Although only three neurons were tested systematically in this way, the data clearly show that a short, single frequency tone burst can completely suppress the facilitative response for nearly 20 ms and the response is still reduced at ~50 ms. The inhibitory frequency most effective for long-lasting inhibition is usually close to the best excitatory frequency of the neuron (e.g., see Figs. 6, 8, and 15). Thus in the example shown, the prepulse was delivered at an amplitude of 78 dB SPL had a frequency of 62.4 kHz compared with the BFhigh at 60.4 kHz. The BFlow frequency for facilitation was fixed at 24 kHz.
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Excitatory, facilitatory, and inhibitory response areas
Excitatory and/or facilitatory response areas were measured for 109 DSCF neurons and inhibitory response areas were measured for 26 neurons. Response areas obtained for three representative neurons are shown in Figs. 6, 7, and 8. The facilitative response evoked by two tones can be inhibited if a nonoverlapping third tone is presented before the pair of tones causing the facilitation. Inhibitory response areas measured for 37 DSCF neurons were commonly very broad and overlapped with or "skirted" the facilitative areas. Minimum thresholds for inhibition were usually found at both sides of the BFhigh facilitative tuning (Figs. 6-8). Typically, closed excitatory and facilitatory response areas resulted from the presence of inhibitory response areas at high stimulus levels for frequencies neighboring the BF. In Fig. 6, a broad, continuous inhibitory zone is present between 10 and 45 kHz. This and other inhibitory response areas were obtained by using a relatively high-intensity (>85 dB SPL) CF tone or "prepulse" just before and nonoverlapping with the facilitatory tone pair.
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The neuron shown in Fig. 6 is tuned to 25- and 61-kHz tone bursts. The excitatory response of this neuron to BFlow exhibits a relatively broad, closed response area. In contrast, the response area for BFhigh is very sharp and closed with no response being elicited to sound intensities >60 dB SPL. The narrow and closed BFhigh response area is replotted in the expanded frequency axis. Shaded areas indicate inhibitory response areas that typically flank the excitatory and facilitatory response areas. The shape of the facilitative response areas (see Fig. 6B) is basically the same as that of the excitatory areas shown in Fig. 6A. However, the response magnitude to both BFlow and BFhigh is enhanced. The most significant effect is on the response width of the two areas; BFlow facilitatory response area is widened in both the frequency and amplitude domains (Fig. 6B). The threshold of the BFhigh facilitatory response area is lowered by ~10 dB, and the width of the response area increases from 0.5 to 1 kHz at BA. This translates into a change in QBA (Q value at best amplitude; typically at 35 dB above minimum threshold) for BFhigh from 120.3 to 58.3.
A second type of response area is shown in Fig. 7. In this neuron, the excitatory response area to BFlow (Fig. 7A) is relatively small and the excitatory response to BFhigh is less sharply tuned than for the neuron shown in Fig. 6A. The most significant differences are a lower sensitivity (higher minimum threshold) to BFhigh and an open response area for BFlow as well as BFhigh. At 50 dB above threshold the width of the facilitative response area increases from 0.75 to 1.25 kHz. The facilitative response areas once again show both lower thresholds and an increased width compared with the excitatory response areas (Fig. 7, A and B). An unexpected result, obtained for the facilitative tuning of this neuron is the presence of an additional facilitatory response area in the range of 50 kHz (BFmid) that corresponds to the second harmonic of BFlow. As for BFlow response areas, this area was revealed by presenting a second fixed frequency at BFhigh at ~10 dB above its threshold response level. An excitatory response area at BFmid is present in only a few neurons. The magnitude of the facilitatory response to BFmid is much less than that to BFhigh but may be greater than that to BFlow. A second peak of the inhibitory response area is seen in the neighborhood of BFlow.
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Figure 8A shows excitatory response areas for BFlow and BFhigh that are closed as in Fig. 6A. The excitatory response area for BFlow is relatively small compared with the facilitatory response area at the same BF. Facilitation, in this case, results in opening of both the closed excitatory response areas so that high stimulus intensities of either BFlow or BFhigh can produce a response when presented together with the other frequency at threshold or 10 dB above threshold levels. Facilitation increases the BFlow response area by a greater percentage than the BFhigh response area, although both show an "outward" shift in the upper and lower thresholds (Fig. 8B). For this neuron, the single lobed inhibitory response area overlapped the excitatory and facilitative response area for BFlow that extends from 22 to 28 kHz. The excitatory response area at 62 kHz also is embedded within an essentially single inhibitory response area for tuning to BFhigh. These types of excitatory and facilitatory response areas are quite commonly observed, although in this case, the inhibitory response shows an unexpected additional low threshold (55 dB SPL) peak at 93 kHz. This inhibitory response area is fairly wide, ranging from 85 to 95 kHz at 30 dB above threshold.
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In the preceding three examples (Figs. 6-8), the BFhigh excitatory response areas are very sharp, whereas the BFlow response areas are fairly broad with high thresholds and show a good response only to high sound intensities. In many neurons, BFlow alone does not produce any measurable excitatory response (see Fig. 3B). Also, a neuron generally exhibits a lower threshold (sometimes by >60 dB) to BFhigh than to BFlow for both excitatory and facilitatory response areas. BFlow facilitatory response areas generally show multiple peaks of sensitivity varying by >10 dB within the range of a few kilohertz. This is a common finding for many of the neurons studied. Table 1 summarizes the tuning parameters for those neurons that showed clear facilitation (only neurons showing a FR >1.1 were used for this analysis).
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Typically, the inhibitory response areas show multiple peaks of sensitivity. The best frequency for inhibition (BFI) is defined as the frequency at which a threshold response can be obtained at the lowest SPL relative to other frequencies. BFIs are identified separately for the low- and high-frequency regions defined on the basis of excitatory and facilitatory tuning. As for the facilitatory response areas, inhibitory response areas always have lowest thresholds at frequencies neighboring the BFhigh compared to those near BFlow. BFIs are slightly higher or slightly lower than the BFs for the corresponding excitatory and facilitatory areas. For example, the BFIs for the neuron shown in Fig. 6 are 22 and 27 kHz for BFlow and 60 and 62 kHz for BFhigh. The mean difference between BFIs and BFs for each excitatory and facilitatory area, obtained for seven neurons, are listed in Table 2. For BFhigh, these neurons have BFIs that are on average 1.61 kHz lower and 0.58 kHz higher than the BFs of DSCF neurons and contribute to the sharp tuning of the facilitative response.
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The difference in sensitivity for inhibition around the two best excitatory frequencies is more pronounced in the example shown in Fig. 8 than those shown in Figs. 6 and 7. Also, inhibitory response areas are generally asymmetrical around the excitatory and facilitatory response areas at BFhigh so that the inhibitory response areas on the high-frequency side of BFhigh are generally larger than those on the low-frequency side of BFhigh because of differences in width and/or sensitivity of tuning (Figs. 6-8). Thus in Fig. 6, the inhibitory response area on the low-frequency side of BFhigh is <2 kHz wide at 30 dB above threshold, and that on the high-frequency side is >10 kHz wide at the same SPL.
"Oblique "threshold-response curves
For ~15% of the neurons, the BFhigh excitatory threshold-response curves exhibited a distinctly "oblique" shape, where the BF increased with a decrease in sound pressure level of the single tone. An example of this is shown in Fig. 9A where frequency-response functions are plotted on a linear scale and stacked along an increasing amplitude scale. These types of neurons showed a similar facilitatory pattern of tuning for a combination of the BFlow and BFhigh (Fig. 9B). As explained above, the facilitatory response area examined at high resolution shows a slightly larger amplitude and width of frequency tuning than the excitatory response area at several amplitude levels (see also Table 1). In this case, this spread is toward the left (lower frequencies) for high amplitudes (>70 dB SPL) and toward the right (high frequencies) for low-stimulus amplitudes (<65 dB SPL), thus enhancing the "obliqueness" of this curve.
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To get a population estimate of the change in center frequency with
amplitude, the frequencies were plotted for both excitatory and
facilitatory tuning at 5-dB steps for four different neurons (Fig. 9,
C-F). The best frequencies for excitation and facilitation of these neurons are normalized to the values of their resting CF2 as explained above. A line of best fit for
excitation and facilitation is plotted for each neuron. On the basis of
these four plots, the mean slope for changes in the center frequency for excitatory and facilitatory tuning is 0.03 dB SPL/kHz. Within a
range of 50 dB SPL, these correspond to an average net change in the
Doppler-shifted frequency of ~1.8 and 1.3 kHz for facilitatory and
excitatory tuning, respectively.
Facilitation versus excitation: rate-level functions, minimum thresholds and best amplitudes
The overall effect of facilitation on the tuning of a neuron is best seen in a three-dimensional, frequency-amplitude-response plot (Fig. 10, A and B) and a cross-sectional profile of the frequency-response tuning at 30 dB above minimum threshold for excitation.
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The effects of facilitation include increased response magnitude, especially at amplitude levels that range from 10 dB above to 10 dB below minimum threshold for excitatory tuning, lowered threshold, and an overall greater width of tuning. All of these contribute toward a greater response volume and an abrupt plateau shape of the facilitatory tuning relative to the single tone excitatory tuning. Figure 10C shows a profile of the response magnitude for frequencies at 30 dB above minimum threshold for facilitatory and excitatory frequency tuning. The data shown in Table 1 were calculated for 30 dB above minimum threshold for excitatory tuning. Data for facilitatory tuning were calculated from response at the same dB SPL level as that for excitatory tuning. The curves based on a weighted moving average (3-point smoothing; 1:4:1) of the data are plotted on a linear frequency scale. Response widths were calculated from frequencies corresponding to a 20% level of the peak response. Responses were normalized by equating the peak response to 100 and calculating the 20% separately for curve. These data were used to calculate mean values of some of the parameters shown in Table 1.
Nearly all DSCF neurons show nonmonotonic amplitude tuning to single and paired tones, i.e., the number of impulses per tone-burst increases and then declines with higher amplitudes. The nonmonotonic tuning results from the interaction between excitation and inhibition within overlapping frequency ranges (see Figs. 6-8). For both excitation and facilitation, there is a broad spectrum of rate-level or impulse-count functions that are usually unimodal and occasionally show multipeaked amplitude tuning. The width of amplitude tuning may span the entire range (100 dB SPL) of amplitude levels tested or may be confined to <30 dB SPL. The pattern of amplitude tuning for the best excitatory and facilitatory frequencies for four of the neurons that showed distinct facilitation are presented in Fig. 11, A-D. Spontaneous levels of activity have been subtracted and total number of impulses are counted over a 100-ms window for a 30-ms tone burst. Facilitation by two tones did not result in any significant modification in the shape of the rate-level function at the best frequency for BFhigh, although the location of the peak response may shift by 20 dB (Fig. 11C).
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Facilitation usually is associated with a decrease in the minimum thresholds for excitation. The scatterplot in Fig. 12A compares the minimal amplitude levels (in dB SPL) required to obtain a threshold response for excitation and facilitation at BFhigh in 350 neurons recorded from the DSCF area. Many of the data points in this plot overlap with each other. Points lying below the diagonal indicate a lowering of minimum thresholds. In the case of neurons with oblique excitatory and facilitatory response areas, minimum and maximum thresholds shift with frequency so that the usual effect of facilitation on minimum thresholds may appear to be reversed. For the neuron shown in Fig. 11C, the facilitatory tuning at 60.15 kHz exhibits a higher minimum threshold than that for excitatory tuning. In fact, for this neuron, the minimum thresholds for excitation and facilitation were 10 and 0 dB SPL, respectively. Figure 12B is a similar comparison for seven neurons at BFlow frequencies. Minimum thresholds for BFhigh tuning may be lowered by as much as 65 dB, whereas for BFlow frequencies they may decrease by as much as 25 dB. Only seven neurons provided data for threshold comparison with BFlow tuning because single-tone BFlow stimuli alone rarely produce a measurable response spanning several dB. The best amplitude (BA) for facilitation at BFhigh also can change significantly compared with the BA for excitation for the same neuron. The data in Fig. 12C represent a subset of the neurons included in Fig. 12A and show that BAs for facilitation may be lowered by as much as 50 dB compared with the BAs for excitation at BFhigh. However, for BFlow the facilitation-induced changes in best amplitude are less dramatic and are more difficult to measure as mentioned in the preceding text.
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Response to combinations of multiple harmonics
Figure 13
shows a neuron that responds well to the combination of two tones
corresponding to BFlow and
BFhigh when these are presented together (Fig.
13A1). To examine the relevance of CF/CF tuning of DSCF
neurons to echolocation, we assumed that BFlow is
a frequency within the frequency range of pulse
FM1, and BFhigh corresponds
to the Doppler-shifted echo CF2 of a pulse-echo
pair (see Fig. 1A). To test the effects of additional
harmonics that are normally present in the echolocation signal, we
fixed the amplitude of the pulse H1 at its BA for
facilitation, which for the neuron shown in Fig. 13A
corresponded to 72 dB SPL. Additional harmonics were generated at
amplitudes corresponding to the relative attenuations of each harmonic
present in the natural pulse (Griffin 1971;
Kobler et al. 1985
). A second set of harmonics was
generated based on the neuron's BFhigh and BA at
this frequency. Because the FM-CF type of facilitative response of DSCF
neurons is delay-tuned, these stimuli were tested at the neuron's best
delay, which corresponded to 31 ms. A comparison of Fig. 13A,
2 and 3 with 1, shows that the third
harmonic in the pulse caused a greater suppression of response than the
second harmonic, and the inclusion of all three harmonics in the pulse
results in a very small response to the pulse-echo stimulus.
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In Fig. 13B, 1 and 2, the same phenomenon is shown in the form of cumulative poststimulus time histograms (PSTH) for the same neuron. From the two cumulative PSTHs shown in Fig. 13B2, it is evident that the addition of harmonics to the pulse decreases the latency of spike generation. These results are somewhat intriguing and are explained in greater detail in the discussion. Figure 13B also shows that, similar to the FM-FM area, the time of arrival of the pulse and echo CF2-4 frequencies is critical for facilitation to occur in the DSCF area.
Distribution of response latencies to BFlow and BFhigh
We measured response latencies to the BFlow and BFhigh frequencies delivered alone as well as together for 43 neurons. The latency of the response to the BFlow stimulus is usually greater than the response latency to the BFhigh stimulus (Fig. 14). The latency of the facilitated response to the BFlow/BFhigh combination is typically equal to the latency of the response to the BFhigh stimulus alone. The facilitation versus latency data illustrates three important properties of DSCF neurons. First, facilitation occurs when the latency to the BFlow frequency is greater than or equal to the latency to the BFhigh frequency. Second, highest FRs are obtained for neurons in which latency differences in the response to BFlow and BFhigh are small. Third, neurons showing differences in response latencies to BFlow versus BFhigh of >20 ms do not show any facilitation; these neurons generally are not tuned to the typical Doppler-shift-compensated frequencies in the BFlow, but rather show tuning to frequencies at ~30 kHz like most CF/CF neurons. In one of these cases, the neuron did not show an ON response, and the measured latency corresponded to an OFF response for BFlow. Fitting these data to a bivariate kernal plot (not shown) indicates that a majority of DSCF neurons have an average FR of 2, which corresponds to a mean latency difference (BFlow-BFhigh) of ~2 ms. A minority of neurons have an FR of ~4.2 for a slightly shorter latency difference. A few neurons exhibit large BFlow-BFhigh latency differences at ~21 ms and are not significantly facilitated. Interestingly, for several neurons, different latency differences also showed suppression to the combination of two tones.
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Spatial organization of response parameters
In the DSCF area, as in the CF/CF and FM-FM areas, neurons with
similar physiological properties are arranged in columns
(Taniguchi et al. 1988). There is also an anatomic basis
of this columnar organization in terms of thalamocortical projections
and corticocortical connectivity so that small, localized injections of
horseradish peroxidase in one auditory cortical area label discrete
columns in other auditory cortical areas (Fitzpatrick et al.
1998
). We were curious as to whether the excitatory and
facilitatory tuning to BFlow and the facilitatory tuning to
BFhigh also is organized columnarly. Figure
15 shows BAs and BFs marked on
inhibitory response areas obtained from an orthogonal penetration in
the DSCF area. These data provide estimates of several parameters of
the neural response within a column and clearly show that all of the
response parameters including the shape of inhibitory areas do not show dramatic changes with depth. For example, at all depths ranging from
317 to 610 µm, the inhibitory response areas show a similar peak at
near ~90 kHz, although not all neurons in the DSCF area exhibit
inhibitory tuning in this frequency range. The lowest thresholds for
inhibition in the BFhigh region may vary, however, by as
much as 20 dB. Within a column, the shape of inhibitory tuning is
significantly variable only in the BFlow region, where neurons show relatively broad excitatory and facilitatory tuning.
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The bar graph in Fig. 16A is a density plot of the distribution of FRs within the DSCF areas in a single hemisphere. The distribution is unimodal, although there is a large spread of FRs, and it is heavily skewed to the left of a normal distribution. The source of the differences in the FRs among DSCF neurons is unknown. The magnitude of FR is based on three to four measurements of the response to 200 repetitions of each stimulus. These data show that within a single hemisphere there can be a large range of FRs represented. Several types of analyses were performed to estimate the variation in FR between neurons and to identify the parameters that may show either a causal or a coincidental relationship. However, no clear relationship was observed between FR and the normalized BFhigh for excitation/facilitation of a neuron. Similarly, as shown in Fig. 16B, there is a lack of any systematic relationship between the cortical depth at which a neuron is located and the value of FR for that neuron. This is an important result considering the frequency-specific columnar organization within the auditory cortex. Thus neurons within a column that are tuned to similar frequencies may also exhibit similar FRs.
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DISCUSSION |
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Our previous study showed that neural specializations, such
as combination sensitivity and delay tuning, co-exist within an expanded representation of systematically mapped best frequencies and
amplitudes in the DSCF area (Fitzpatrick et al. 1993;
see also Suga 1977
; Suga and Manabe
1982
). We describe here several basic response parameters
associated with spectral facilitation of the neural response observed
in the DSCF area and interpret the data in light of this area's
behavioral role in echolocation. Our analysis in concert with previous
studies on the auditory cortex of the mustached bat suggests that
neurons in an area designated as an expanded representation of one
stimulus parameter, in fact, are specialized to respond to those
combinations of stimulus parameters in the frequency and time domain
that naturally occur during a relevant behavior such as echolocation
during insect pursuit. Furthermore DSCF neurons appear to be
specialized simultaneously to process communication as well as
echolocation sounds (Kanwal et al. 1992
). However, this
study focuses on the functional significance of excitatory,
facilitatory and inhibitory tuning of DSCF neurons for echolocation.
Combination sensitivity in the primary auditory cortex
The DSCF area in the mustached bat contains radial frequency and
circular amplitopic maps for frequencies in the 61- to 63-kHz range
(Suga 1977; Suga and Jen 1976
). These
maps, however, are based on frequency tuning of neurons to single-tone
stimuli (Suga 1982
). Although detailed mapping was not
the focus of this study, the data on response parameters of
facilitatory tuning to BFhigh frequencies
indicate that the BFs are unaffected by the presentation of a second
tone. Minimum thresholds, however, are lowered by
17 dB. BAs for
facilitation also may change by as much as 25 dB (see Fig. 11).
Responses obtained from neurons at different depths within a single
orthogonal penetration also confirm the columnar organization for
tuning to BFhigh in the DSCF area (Suga and Manabe 1982
). However, no clear map of FRs or correlation with BFs was observed.
In several vertebrate species, combinations of stimulus parameters have
been shown to correspond to two or more components of a behaviorally
relevant complex stimulus, e.g., unique frequency-modulated patterns or
combinations of harmonics in sounds generated by a prey or a predator
or within species-specific communication sounds (frogs: Gerhardt
and Doherty 1988; Lopez and Narins 1991
; birds: Margoliash and Fortune 1992
; bats: Esser
1994
). However, combination sensitivity within the primary
auditory cortex has not been observed in a nonbat species even though
other mammalian species have been studied extensively to map various
response parameters (Schreiner 1992
). In one study in
the cat, a few multipeaked neurons exhibited decreased response
latencies and thresholds and a relative enhancement of response by
two-tone combinations, but, as stated, these were not facilitatory by
the classic definition (Sutter and Schreiner 1991
).
Recent studies in the nonprimary auditory cortex of macaques also have
shown facilitation of the neural response to narrow frequency bands
extracted from species-specific communication calls (Rauschecker
et al. 1995
)
Spectral convergence in the primary auditory cortex
As a consequence of combination sensitivity, the DSCF area may be considered to overrepresent 22- to 28-kHz (BFlow) as well as 61- to 63-kHz (BFhigh) frequencies. Most DSCF neurons do not show good responses to a BFlow presented alone, although as shown in the response area data a good response is obtained when a BFhigh is simultaneously presented at or just above its threshold level (see Fig. 4). The intra- versus intercolumnar variation in BFlow frequency tuning is difficult to measure because of the absence of a single BF within the irregular threshold boundary of the response areas.
In addition to the excitatory bands, DSCF neurons also exhibit
broad and/or notched inhibitory response areas as seen in Figs. 5 and
6. This suggests that DSCF neurons receive a wide range of
high-frequency inhibitory inputs. These data indicate that a
substantial spectrum of the bats' audiogram, including frequencies not
contained in echolocation signals, is represented within a single
region of the primary auditory cortex. These results mean that many
DSCF neurons will respond in some way to most frequencies within its
audiogram and argues against a simple cochleotopic/tonotopic representation of frequencies within the primary auditory cortex. This
is an important conclusion as most previous studies have focused only
on the tonotopic representation and have largely ignored representation
of inhibitory frequencies. (Merzenich et al. 1975;
Reale and Imig 1980
; Romani et al. 1982
;
Tunturi 1952
). The convergence of the excitatory and
various inhibitory frequency components in a mustached bat's auditory
system most likely takes place at subcortical levels as in the case of
FM-FM and CF/CF neurons (Olsen 1986
; Olsen and
Suga 1991
). In that case, the cortical processing of the
frequency information may have to do with the sharpening of frequency
tuning as suggested by our observation of large inhibitory side bands.
The site of this convergence can be clarified further on the basis of
neuropharmacological studies (Zhang and Suga 1997
).
The sensitivity of auditory neurons to a wide range of frequencies has
been noted in the auditory cortex of other species as well as in other
parts of the auditory pathway. Several types of neurons in the dorsal
cochlear nucleus of mammals have narrow excitatory regions with
inhibitory surrounds (Evans and Nelson 1973;
Shofner and Young 1985
; Young and Brownell
1976
). Some of these show nonmonotonic rate-level functions
similar to those of neurons in the DSCF area. In the cat primary
auditory cortex, nonmonotonic neurons are segregated at least partially
from monotonic neurons and generally are suppressed by noise
(Phillips and Cynader 1985
; Phillips et al. 1985
,
1994
; Reale et al. 1979). In general, in the
auditory cortex, neurons that show nonmononotonic rate-level functions
do not respond to wideband noise that includes their excitatory as well
as inhibitory receptive fields. Although DSCF neurons have not been
tested with noise, their rate-level functions are typically
nonmonotonic and generally have wide inhibitory areas.
In the auditory cortex of ferrets, an organized distribution of
inhibitory response areas has been implicated in complex sound recognition (Shamma et al. 1993). The shapes of these
response areas show a columnar organization as in the DSCF area of the mustached bat. It is noteworthy that inhibitory areas in the dorsal cochlear nucleus and in auditory cortical neurons in the ferret, although generally wider than the excitatory regions, are not as broad
as in the DSCF area of the mustached bat. We have not yet determined if
inhibitory regions show systematic changes across the surface of the
DSCF area. Inhibitory response areas do vary among spatially separated
DSCF neurons in spite of the similarity in the basic patterns of
excitatory and facilitatory response areas. The exceptionally broad
inhibition may be necessary to prevent DSCF neurons from responding to
high frequency ambient noise and to echolocation signals emitted by
conspecifics; the latter have energy in restricted frequency regions
but at widely spaced intervals according to the harmonic structure of
the signal. This relationship of the inhibitory regions to bands within
the echolocation signal will be more fully described in a later section.
Neural mechanisms for facilitation and inhibition
To further appreciate the functional significance of the
combination sensitivity of DSCF neurons to BFlow
and BFhigh and the complex patterns of inhibitory
tuning, one has to consider the behavioral role/s of the DSCF area.
Inactivation of the DSCF area leads to deterioration of perception of
small differences in the frequencies of tone bursts in the
BFhigh region (Riquimaroux et al.
1991). As previously alluded to, we also know that
BFlow and BFhigh match the
frequencies within the echolocation pulse FM1 and
echo CF2, respectively (see Fig. 1A).
Therefore it is imperative to discuss the facilitation and inhibition
at least in reference to the mustached bat's echolocation behavior.
Latency differences to pulse and echo components constitute an
important underlying mechanism for determining the best delay of DSCF
neurons. In this study, CF tones were used to measure response
latencies to pulse and echo components. We examined the distribution of
response onset latencies of each component triggering the facilitation
response. These data show that to evoke facilitation in DSCF neurons
the latency difference between pulse and echo can be as long as 10 ms,
although a latency difference of 0 to 2 ms shows the maximum
facilitation (see Fig. 13). The latency difference for the response to
the minimal pulse and echo components corresponds to the duration of
the pulse CF1 in an echolocation signal. Thus
during insect pursuit, where pulse duration changes from 30 to 20 ms
during the search and early approach phases, DSCF neurons will show
facilitation to pulse-echo delays of 35 ± 5 ms (for 30 ms pulses)
or to 25 ± 5 ms (for 20 ms pulses), respectively. These data are
consistent with our previous study of delay tuning of DSCF neurons,
which showed that delay tuning changes with pulse duration and that
long discriminable delays calculated from receiver operating
characteristic (ROC) curves of neural responses to pulse-echo pairs are
~32 ms (Fitzpatrick et al. 1993). These data together
support the idea that in contrast to CF/CF and FM-FM neurons, a few
DSCF neurons can produce a facilitative response with maximal FRs for
objects that are relatively far off; most likely from objects that are
6 m away. This takes into consideration a relative velocity of
approach of 2-3 m/s. Because the echo reflected from a small (~2 cm
sphere) target at 6 m would be attenuated by >90 dB but from a
reflective background by <40 dB (Lawrence and Simmons
1982
; Moss and Schnitzler 1995
), these long
delays presumably correspond to echoes returning from background objects rather than an insect.
Pulse response latencies shorter than those for echoes by ~0.5 ms may
also show facilitation (see Fig. 14). This may represent jitter in the
system for very short pulse-echo delays, i.e., when the target is very
close and the CF duration is very small to nonexistent. The negative
(pulse-echo), latency characteristic of some DSCF neurons suggests that
the echo CF2 must produce an excitatory postsynaptic
potential (EPSP) that lasts for ~1 ms to account for this phenomenon.
In contrast, the positive (pulse-echo) latency difference of other
neurons suggests that stimulation by a pulse FM1 component
(corresponding to BFlow) results in the generation of a
long duration (10 ms) or a delayed subthreshold EPSP in either
cortical or subcortical neurons. This means that the facilitative
response can track an approaching target independent of the duration of
the emitted pulse.
Facilitatory tuning for insect pursuit
The insect-hunting behavior of mustached bats is grossly divisible
into three phases, namely search, approach, and terminal (Novick
1963; Novick and Vaisnys 1964
). The stimulus
parameters causing facilitation in the response of DSCF neurons suggest
additional adaptations, besides target detection and characterization,
of these neurons for echolocation, e.g., some DSCF neurons also can track a closing-in target during insect pursuit. The evidence for this
is twofold. Our previous data on delay tuning indicated that in some
neurons tuned to long delays, the best delay changes (decreases) with a
decrease in CF duration (Fitzpatrick et al. 1993
).
Furthermore, as an inevitable consequence of target approach, the echo
intensity gradually increases. A profound echo-intensity compensation
is observed for fixed targets/backgrounds when bats are swung on a
pendulum (Gaioni et al. 1990
; Kobler et al.
1985
). In a natural situation, however, echo-intensity
compensation, like Doppler-shift compensation, is probably based on
echoes from the background vegetation so that echoes from approaching
targets may continue to increase in intensity, especially at close
range (Trappe and Schnitzler 1982
). Our data on
frequency tuning indicate that the BFs in some of the
Fhigh type of neurons decrease as echo intensity
increases. Furthermore because of the broad tuning to
BFlow, some DSCF neurons can track changes in
echo CF2 frequencies independent of changes in
H1 frequencies owing to Doppler-shift compensation. Although it is not clear that the duration versus delay
tracking and the frequency versus intensity tracking occur in the same
neuron, the available data indicate that both types of effects can be
observed in DSCF neurons.
In Rhinolophus, the flying speed of a bat is reduced from
6.1 m/s in the search phase to ~2.5 m/s during the terminal phase of
insect capture (Kick and Simmons 1984). Mustached bats
normally fly at a speed 4.5 to 9 m/s during the search phase but may
slow down considerably when trying to capture insects in their tail wing (Griffin 1958
). The net change in frequency tuning
of a single neuron (1.70 ± 0.64 kHz) suggests that these tracking
neurons operate within relative velocities decreasing from, e.g., 9 to ~3 meters/s as the echo intensity increases (The frequency of the
CF2 in the returning echo from the target
presumably is lowered down to the Doppler-shift compensated pulse
CF1 frequency as the bat reorients itself to
intercept a target or slows its flying speed for target capture).
Other major effects of facilitation are a lowering of the minimum
thresholds and a widening of the tuning of the neural response. Both of
these effects increase the probability of target detection by extending
the acoustic boundaries at which a target can be perceived in terms of
relative distance and motion domains. On the basis of these data and
those reported previously (Suga and Manabe 1982;
Suga et al. 1983
), it appears that DSCF neurons signal target detection and may track targets during the characterization process.
Inhibitory tuning for noise reduction during roosting and foraging
Mustached bats are gregarious, and several hundred or even
thousands of bats may roost in a single cave in the same or adjacent colonies during the nonflying and nonhunting periods of their daily
life. Roosting bats routinely emit echolocation pulses and communication sounds at >100 dB SPL so that the sound intensity for
the echolocation frequencies heard all around by any bat is easily 100 dB SPL (Kanwal et al. 1994). Echolocation pulse-echo pairs, however, carry information that is useful only to the emitter. For other bats, the high sound intensities of pulse and sometimes echo
frequencies that vary only by a few hundred hertz represent a noisy
environment that may interfere with the processing of the bat's own
pulse-echo pairs. Thus the normal spontaneous noise level of ~2 to 3 spikes/s per DSCF neuron could more than double for a bat roosting
inside a cave, leading to the possibility of energy and information
loss. Instead, the mustached bat has evolved a set of adaptive
mechanisms that regulate the noise-driven neural activity in a
behaviorally relevant signal-specific manner. These adaptations are
based on the patterns of inhibitory as well as excitatory and
facilitatory tuning of DSCF neurons.
At the auditory periphery, sharply tuned neurons may respond to various
components of echolocation and communication sounds emitted by other
bats. If these sounds coincide or precede the bat's own pulse or echo,
then a cortical neuron on which several frequency channels converge may
be in an absolute or relative refractory phase and therefore unable to
respond adequately to the bat's own pulse-echo frequencies.
Consequently several mechanisms have been enumerated that either reduce
or prevent masking and/or jamming of a neuron's response
(Henson et al. 1980; Suga et al. 1987
).
For example, small individual variations among the bats' resting
CF2 and sharp tuning to the corresponding
Doppler-shifted frequencies allow the bat's auditory cortex to be
personalized to listen only to its own signals.
The data obtained in this study suggest that the neural properties
(described in the preceding text) that prevent masking basically are
unmodified by facilitation although stimulus combinations do lead to a
slight broadening of the excitatory response area. Our data further
show that additional specializations exist for rejecting echolocation
pulses emitted by other bats. These specializations supplement the
proposed "personalization" mechanisms and are related to the
inhibition of cortical auditory neurons. They allow the auditory system
to adjust to a noisy environment by rejecting/attenuating noise without
losing information contained in their own echolocation signals. We
label these adaptations for rejecting sounds produced by conspecifics
as "privatization" mechanisms to contrast them from those
adaptations that allow a bat to "tune-in" to their personalized
echolocation signals (Suga et al. 1987).
One important privatization mechanism is related to the broadening of
inhibitory response areas at high intensities. According to the
inhibitory-tuning data, high sound pressure levels of inhibitory frequencies in the H1 and
H2 of the pulse and/or echo may lead to
inhibition of most DSCF neurons. This additional broadening of the
inhibitory tuning at higher intensities suggests that inhibition may
serve as a mechanism to protect against overstimulation of neurons by
some communication sounds and high-intensity echoes from large
background objects or by echolocation pulses emitted by other bats in a
small enclosed space as during roosting in caves. This problem of
overstimulation does not exist for high intensities of sounds in the
bat's own pulse because of the attenuation by middle ear muscles
alluded to earlier (Suga and Shlegel 1972).
A second privatization mechanism is related to the presence of a low
threshold of inhibition to frequencies near ~90 kHz in some DSCF
neurons. This explains the intriguing observation that 90-kHz
frequencies are exclusively inhibitory for some DSCF neurons when
presented from an external source (Figs. 13 and 15). Interestingly, the
minimum threshold for inhibition for these frequencies is several
decibels higher and the BFI is generally lower than that present in
ECF3. These data suggest that the 90-kHz
inhibition is to the pulse CF3 component of other
bats echolocating in the vicinity. Once again, pulse
CF3 frequencies in a bat's own pulse may be
attenuated specifically by vocalization triggered neural feedback or by
contraction of middle-ear muscles when a bat emits the echolocation
pulse (Suga and Shlegel 1972). Weaker
ECF3 components in the echoes are probably
ineffective in causing inhibition. Any pulse
CF3-triggered neural activity arriving at cortical
neurons just prior to a bat's own pulse FM1-echo
CF2 combination are ineffective in causing
inhibition in the presence of the robust facilitatory response. On the
contrary, the pulse CF3 may trigger transient suppression
(resetting) of ongoing neural activity, thus preparing a DSCF neuron
for processing of the incoming, information-bearing echo. Thus, the
different patterns of inhibitory response areas in DSCF neurons may
improve signal to noise ratios for echolocation during foraging as well
as communal roosting in spite of an acoustically cluttered environment.
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ACKNOWLEDGMENTS |
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We thank Dr. J. Butman for modifications of MI2 software, Dr. M. Suzuki for providing custom-written programs for some of the analyses, and Dr. K. K. Ohlemiller and two anonymous reviewers for critically reading the manuscript. We also thank the Department of Agriculture and the Natural Resource Conservation Authority of Jamaica for permission to export bats.
This research was supported in part by National Institutes of Health Grants NS-07057 and DC-02054 to J. Kanwal and DC-00175 to N. Suga.
Present address of D.C. Fitzpatrick: Dept. of Anatomy, University of Connecticut Health Center, Farmington, CT 06030.
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FOOTNOTES |
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Present address and address for reprint requests: J. S. Kanwal, Georgetown Institute for Cognitive and Computational Sciences, New Research Bldg., Room WP09, Georgetown University Medical Center, 3970 Reservoir Road NW, Washington, DC 20007-2197.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 26 January 1999; accepted in final form 10 June 1999.
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REFERENCES |
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