Zoologisches Institut, Universität München, D-80333 Munich, Germany
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Koch, U. and
B. Grothe.
Interdependence of Spatial and Temporal Coding in the
Auditory Midbrain.
J. Neurophysiol. 83: 2300-2314, 2000.
To date, most physiological studies that
investigated binaural auditory processing have addressed the topic
rather exclusively in the context of sound localization. However, there
is strong psychophysical evidence that binaural processing serves more
than only sound localization. This raises the question of how binaural processing of spatial cues interacts with cues important for feature detection. The temporal structure of a sound is one such feature important for sound recognition. As a first approach, we investigated the influence of binaural cues on temporal processing in the mammalian auditory system. Here, we present evidence that binaural cues, namely
interaural intensity differences (IIDs), have profound effects on
filter properties for stimulus periodicity of auditory midbrain neurons
in the echolocating big brown bat, Eptesicus fuscus. Our
data indicate that these effects are partially due to changes in
strength and timing of binaural inhibitory inputs. We measured filter
characteristics for the periodicity (modulation frequency) of
sinusoidally frequency modulated sounds (SFM) under different binaural
conditions. As criteria, we used 50% filter cutoff frequencies of
modulation transfer functions based on discharge rate as well as
synchronicity of discharge to the sound envelope. The binaural
conditions were contralateral stimulation only, equal stimulation at
both ears (IID = 0 dB), and more intense at the ipsilateral ear
(IID = 20,
30 dB). In 32% of neurons, the range of modulation
frequencies the neurons responded to changed considerably comparing
monaural and binaural (IID =0) stimulation. Moreover, in ~50% of
neurons the range of modulation frequencies was narrower when the
ipsilateral ear was favored (IID =
20) compared with equal
stimulation at both ears (IID = 0). In ~10% of the neurons synchronization differed when comparing different binaural cues. Blockade of the GABAergic or glycinergic inputs to the cells recorded from revealed that inhibitory inputs were at least partially
responsible for the observed changes in SFM filtering. In 25% of the
neurons, drug application abolished those changes. Experiments using
electronically introduced interaural time differences showed that the
strength of ipsilaterally evoked inhibition increased with increasing
modulation frequencies in one third of the cells tested. Thus
glycinergic and GABAergic inhibition is at least one source responsible
for the observed interdependence of temporal structure of a sound and
spatial cues.
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In most physiological studies, the neuronal
encoding of the temporal pattern of sounds has been investigated
independent of the spatial properties of the sound. In contrast
binaural processing has been discussed rather exclusively in the
context of sound localization. Most of the studies were performed under
the assumption that different sound features are processed by different
sets of neurons. Yet many psychophysical experiments do not support a
narrow concept of parallel processing but provide evidence that the
temporal sound pattern is not analyzed independent of binaural cues.
Temporal analysis and binaural processing rather contribute to each
other (Bernstein and Trahiotis 1985; Cherry
1953
; Kidd et al. 1995
; Kollmeier and
Koch 1994
). However, little is known about the underlying
physiological mechanisms.
The processing of the temporal structure of sounds, which is an
important cue for auditory feature detection including speech, has been
studied extensively. In the ascending auditory system, neurons
progressively increase their selectivity for the periodicity of
temporal structured sounds, that is, they respond best to lower modulation frequencies and tuning gets narrower (for review, see: Langner 1992; Schulze and Langner 1997
).
This increase in selectivity might be due to active neuronal filter
mechanisms. However, this response behavior has only been tested
independent of the spatial properties of the sound. Recent studies
indicate that neuronal processing of sound location and sound pattern
are mutually dependent, suggesting an interdependence of filter
mechanisms for periodic stimuli and binaural processing. First, spatial
receptive fields of neurons in the inferior colliculus (IC) of the big
brown bat (Eptesicus fuscus) depend on the stimulus type
presented (Grothe et al. 1996
). Second, filter
properties for sinusoidally amplitude modulated (SAM) sounds of
auditory midbrain neurons in the northern leopard frog (Rana p.
pipiens) are affected by the azimuthal position of an open-field
sound source or by interaural time differences in a closed-field
experiment (van Stokkum and Melssen 1991
; Xu et
al. 1996
). Third, the azimuthal position of a sound source influences filter characteristics for sinusoidally frequency modulated (SFM) sounds of many neurons in the IC of the big brown bat
(Koch and Grothe 1997
). Fourth, neurons in the medial
superior olive (MSO) of the free-tailed bat (Tadarida
brasiliensis) change their tuning for the periodicity of SAM
sounds depending on interaural intensity differences (IIDs)
(Grothe et al. 1997b
).
There is strong evidence that GABAergic and glycinergic inhibition
narrows tuning for SAM or SFM sounds of neurons in the MSO, the dorsal
nucleus of the lateral lemniscus (DNLL), and the IC (Grothe
1994; Koch and Grothe 1998
; Yang and
Pollak 1997
). On the other hand, inhibition has been shown to
be essential for processing IIDs at different levels of the auditory
system including the IC (Boudreau and Tsuchitani 1968
;
Klug et al. 1995
; Moore and Caspary 1983
;
Park and Pollak 1993
, 1994
). The IC, the focus of this
study, receives inhibitory and excitatory projections from a large
number of monaural and binaural nuclei (for review, see Oliver
and Huerta 1992
). This set of binaural projections provides an
opportunity to selectively change the interaural intensity or time
difference using dichotic stimulation. This way, it is possible to test
whether filters for the periodicity of sounds are dependent on the
relative weight or timing of ipsi- and contralateral excitation and inhibition.
Here, we present data from single neuron recordings in the IC of the
big brown bat, E. fuscus, an insectivorous bat that uses frequency modulated echolocation calls. Previous studies revealed sharp
tuning to SFM sounds in IC neurons under monaural conditions (Casseday et al. 1997). Moreover, GABAergic and
glycinergic inhibition increases selectivity for the modulation
frequency of SFM sounds and changes the response pattern under monaural
stimulus conditions (for more details, see Koch and Grothe
1998
). Additionally, we previously have shown a major impact of
the spatial position on these tuning characteristics under open-field
conditions (Koch and Grothe 1997
). In this study, we
tested the tuning to the modulation frequency of SFM stimuli under
various binaural conditions (closed-field) and performed the same tests
during iontophoretic application of GABA and glycine receptor
antagonists. In this way, we were able to address the following
questions: is there a significant dependency of periodicity coding on
binaural cues when pinna effects are excluded by using closed field
stimulation? How does GABAergic and glycinergic inhibition contribute
to the dependency of periodicity coding on binaural cues at the level
of the IC? And if so how does the relative timing of ipsi- and
contralateral excitation and inhibition influence temporal filter
properties of IC neurons?
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Surgical procedure
Eight big brown bats (E. fuscus) were used in this study. Before surgery animals were injected subcutaneously with the neuroleptic thalamonal (Janssen; per 100 g body wt; 1 ml of 0.05 mg fentanyl and 2.5 mg droperidol) and additionally anesthetized with metofane (Janssen) by inhalation until no nociceptive response could be evoked. During surgery, the animal's head was secured in a head holder with a bite bar. The local anesthetic bupivacain (Curasan) was injected under the skin covering the skull. An incision was made across the midsagittal line of the skull and muscles and skin were reflected. The surface of the skull was cleared of tissue. A layer of cyanoacrylate adhesive was applied to the surface of the skull, and a rod was cemented onto the skull overlying the cortex using dental cement. Before the first recording session, a small hole was drilled in the skull above the IC. The location of the IC was identified visually using landmarks on the skull. Open wounds were treated with 1% H2O2.
Recording sessions started 2 days after surgery and were performed every day for 8-14 days, each session lasting 3-6 h. Between recording sessions, bats were housed in individual cages with free access to water and food. Before each recording session, bats were injected subcutaneously with 0.15-0.2 ml of the neuroleptic thalamonal. Bats then were transferred to a heated, sound-proof recording chamber where they were restrained in a cushioned holder molded to their body. The restraining cushion was attached to a custom-made stereotaxic instrument. The rod mounted on the animal's skull was secured to the stereotaxic instrument. The ground electrode was placed between the reflected muscle and the skull. The recording electrode was advanced from outside of the experimental chamber using a remote controlled hydraulic microdrive (Wells). To ensure positioning of the electrode within the central nucleus of the IC, neurons were only recorded at depths between 150 and 1,800 µm. The tonotopic organization of the central nucleus of the IC served as an additional indicator for the position of the electrode. Recording sessions lasted until the bat showed any sign of discomfort. Animals were anesthetized with pentobarbital and perfused after the last recording session, and the brain was removed for sectioning. The lesion generated by the multibarrel electrode was used to verify that the position of the electrode was within the central nucleus of the IC.
Electrodes
"Piggy back" multibarrel electrodes (Havey and
Caspary 1980) were used for recording and drug application. A
single-barrel micropipette was pulled to a tip diameter of <1 µm. A
five-barrel micropipette (H-configuration, Science products) was pulled
and the tip broken to a total diameter of 15-20 µm. The
single-barrel pipette was positioned so that the tip of the recording
electrode protruded ~10 µm beyond the tip of the multibarrel
electrode and attached with cyanoacrylate to the multibarrel electrode.
The single-barrel electrode was filled with 2 M NaCl and used for recording the extracellular neural response. Action potentials were
measured by an electrometer (Model Electro 705; World Precision Instruments), amplified and band-pass filtered (Tektronix, 5112), discriminated by a custom-made window discriminator, and fed into a
DSP-board (Tucker-Davis-Technology). Spike time resolution was 2.6 µs.
One barrel of the multibarrel electrode was filled with 2 M Na-acetate and used for balancing currents. The other four barrels were filled with bicuculline methiodide (5 mM, pH: 3), strychnine (10 mM, pH: 3.5), GABA (0.5 M, pH: 3.5), glycine (0.5 M, pH: 3.5), or glutamate (1 M, pH: 8) (all drugs: Sigma). Adjustment of pH was achieved by titrating with 1 M HCl or 1 M NaOH.
The drug electrodes and the balancing electrode were connected via
silver chloride wires to a microiontophoresis system (Medical Systems,
Neurophore BH-2), which was used to generate and monitor ejection
(10-40 nA; for glutamate: 5 to
20 nA) and retention currents (
15
nA; for glutamate: +15 nA). The sum channel that was connected to the
balancing electrode was used to eliminate current effects.
Acoustic stimuli and data acquisition
Acoustic stimuli were digitally generated by using two DSP
boards, 16-bit D/A converters (sampling rate, 377 kHz) and attenuators from Tucker-Davis Technologies. Sounds were amplified (Toellner) and
fed into custom-made earphones (Schlegel 1977) that were
placed into the funnels of the bat's ears.
The search stimulus was a 100-ms SFM sound with 0.5-ms rise/fall time
presented every 600 ms. Center frequency, modulation depth, and
modulation frequency were frequently varied while searching. On
encountering activity of a single neuron, the optimal center frequency
(19.1-69.1 kHz), modulation depth (between ±2 and ±25 kHz) and
threshold of the SFM stimulus to drive the neuron were determined
audiovisually. For all subsequent recordings stimuli 20 dB above
threshold at the contralateral ear were used. SFM signals first were
presented monaurally to the contralateral ear at best modulation depth
using a range of modulation frequencies (10-600 Hz). The same range of
stimuli then was presented binaurally by changing the ipsilateral sound
level to create interaural intensity differences (IIDs) of 0 and 20
dB (ipsilateral ear louder). To demonstrate the existence of
GABAA or glycine receptors, GABA or glycine was
applied until the response to a stimulus was inhibited completely. To
test the effects of GABAergic and glycinergic inhibition on SFM coding,
the receptor antagonists bicuculline or strychnine were applied. After
initializing drug application the neuron's response to a test tone
modulated with 50 or 100 Hz was monitored continuously. Drug
application usually caused an immediate increase in discharge rate.
Recordings did not start before discharge rate to the test stimulus had
stabilized. The same tests then were presented as under predrug
conditions. After each drug application, the retention current was
turned on, and the neuron was allowed to recover until its discharge
rate to the test stimulus dropped to levels of predrug conditions.
Recovery usually took 5-15 min. At least 10 repetitions of each
stimulus were presented for recordings. Ten stimulus repetitions are on
the lower side for quantifying responses. However, the length of the
pharmacological experiments (including recovery times) and the state of
the animal, which was only slightly sedated, required to shorten the
experimental protocol. To determine the time course of ipsilaterally
evoked inhibition, two different experiments were designed. In one set of experiments, glutamate was applied iontophoretically to the neuron
to create a stimulus independent discharge activity. The stimulus then
was presented to the ipsilateral, mostly inhibitory ear with a range of
modulation frequencies and sound intensities. This sound presentation
usually resulted in a gap in the glutamate-evoked activity, which was
created by the inhibitory action of the ipsilateral stimulus.
In another set of experiments, a sound modulated with various
modulation frequencies (25-200 Hz) was presented to the contralateral excitatory ear. For monaural stimulation, this stimulus evoked a
phase-locked response in the neuron. When the stimulus was presented binaurally with a slightly different modulation frequency at the two
ears (difference = 5 Hz = beat frequency), an ongoing
interaural time difference (ITD) of the FM phase was created, a
so-called "binaural beat." The time course of ipsilateral
inhibition could be observed by determining the ITDs where the
contralaterally evoked discharge had been suppressed. The duration of
the binaural beat stimuli was 1 s. To determine whether the time
course of inhibition was also dependent on the sound level, various
IIDs (IID = 0 to IID = 30) were tested.
Data analysis
The definition of a neuron's filter characteristics was based
on the total number of spikes (0-120 ms after stimulus onset) evoked
by each stimulus for each binaural condition. Discharge rate was
normalized to its maximum for each binaural condition. By plotting
normalized spike count versus modulation frequency we determined the
modulation transfer function (MTF) based on spike count. The upper and
lower cutoff frequencies were defined as the modulation frequencies
where the MTF dropped to 50% of its maximum. Differences in SFM tuning
for different stimulus conditions were calculated according to the
following equation: change(bin mon) = [cutoff(bin)
cutoff(mon)]/[cutoff(mon)/100]%. To illustrate changes in the
response pattern, we generated cycle histograms that showed the number
of spikes as a function of modulation phase.
Additionally, we calculated the synchronization coefficient (SC)
(Goldberg and Brown 1969) for each neuron's response to
different SFM stimuli. The SC is a measure of how precisely the
neuronal discharge is locked to a certain phase of each modulation
cycle. It ranges from 0 (no phase-locking) to 1 (perfect
phase-locking). For neurons that responded twice to each modulation
cycle of the SFM stimulus, the SC was computed separately for the
response to the upward and downward component. The on-response was
excluded from the calculation of the SC. We only calculated the SC for those modulation frequencies to which a neuron responded with
20
action potentials for the
10 repetitions of the 100-ms long stimulus.
To test whether phase-locking was significant (P < 0.01), we used the Rayleigh test as described by Batschelet
(1991)
. We also determined the SC based MTF by plotting the SC
versus the modulation frequency. The SC cutoff frequency was defined as
the modulation frequency where the SC dropped below 0.3.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Only IC neurons that under optimal stimulus conditions responded throughout the entire stimulus duration of SFM sounds were included in this study (n = 85). Each neuron's response behavior to SFM sounds was analyzed in the following way. First, the MTF based on discharge rate was calculated and the modulation frequency determined where the response had dropped to 50% of the maximal response (upper and lower cutoff frequency). Second, for a subset of neurons, the SC was calculated to determine the precision of phase-locking of the response to each stimulus cycle (see METHODS).
These results were compared between monaural stimulation and two
different binaural stimulations with an IID = 0 dB and an IID = 20 dB favoring the ipsilateral ear. To analyze the contribution of
GABAergic and glycinergic inhibition on binaural response properties to
SFM sounds, the same tests were performed while iontophoretically blocking the GABAA or glycine receptors with
bicuculline or strychnine, respectively.
Response properties of neurons to monaural stimulation with SFM sounds
For monaural stimulation, SFM tuning differed largely between
neurons. SFM tuning was defined as the range of modulation frequencies neurons responded to robustly (with 50% of their maximal discharge rate). Most neurons displayed band-pass (63%) or low-pass (33%) filter characteristics for modulation frequency. Upper cutoff frequencies ranged between 40 and >500 Hz. All lower cutoff
frequencies were <100 Hz. Furthermore, the temporal response pattern
to SFM sounds differed between neurons. First, calculating cycle
histograms showed that neurons responded either once (unidirectional)
or twice (bidirectional) to each modulation cycle, presumably to the
upward and downward part of each modulation cycle. In most neurons,
this response pattern was dependent on the modulation frequency (Fig.
1). Second, over the population of
neurons tested, phase-locking of the response varied profoundly. For
example, at a modulation frequency of 50 Hz, the SC of the neurons
varied between 0.14 (almost no synchronization) and 0.99 (nearly
perfect synchronization). About 80% of the neurons exhibited
significant phase-locking (P
0.01). For monaural
stimulation most neurons (86%) showed SC-based low-pass filter
characteristics or still exhibited significant phase-locking when the
spike number dropped below a level that allowed a statistical
significant calculation of the SC (all-pass filter characteristics). If
a neuron's response was bidirectional, we calculated the SC separately
for the response to the upward and downward part of the modulation
cycle and took the SC of the larger response for comparison. The
response behavior of IC neurons to monaural stimulation with SFM sounds
and the influence of GABAergic and glycinergic inhibition are described in more detail in Koch and Grothe (1998)
.
|
SFM tuning of neurons depends on the binaural stimulus conditions
In this study, we first compared the response to SFM stimuli for
different binaural conditions. The results show that presenting a SFM
stimulus binaurally (IID = 0) compared with monaurally changed SFM
tuning in many IC neurons. Figure 1 shows an example of a neuron that
was tested for its SFM tuning properties under both conditions
[monaural and binaural (IID = 0)]. In this case, the neuron's
response was equally strong under both conditions when tested for
modulation frequencies 75 Hz. However, for monaural stimulation, a
strong phase-locked response could be evoked for modulation frequencies
125 Hz. In contrast, for binaural sound presentation with SFMs >75
Hz, the neuron only responded to the first cycles of the stimulus. Thus
the upper cutoff of this neuron was significantly lower for binaural
compared with monaural stimulation. At low modulation frequencies (
50
Hz), the neuron responded bidirectionally (i.e., to the upward and the
downward part of the modulation) to both binaural and monaural
stimulation (see cycle histograms as insets in Fig. 1).
However, it responded unidirectionally to modulation frequencies >75
Hz. Additionally, the number of cycles the neuron responded to
decreased for increasing modulation frequencies (
100 Hz) until only
the response to the first modulation cycle remained. Figure
2A shows the MTFs of the same
neuron based on total discharge rate. Compared with monaural
stimulation, the upper cutoff frequency decreased by 65 Hz (about
45%) for binaural stimulation. In this neuron, the decrease of the
upper cutoff frequency corresponded to its binaural response
characteristics in the way that it received excitation from the
contralateral and inhibition from the ipsilateral ear (EI neuron). As
in many cases, calculating the upper cutoff frequency based on the
number of spikes/cycle only displayed a small change of upper cutoff frequencies between monaural and binaural stimulation (Fig.
2B). However, because the response of the neuron as shown in
the histograms in Fig. 1 (and the MTFs in Fig. 2A)
substantially differed between monaural and binaural stimulation for
modulation frequencies >75 Hz, we chose to analyze SFM tuning
differences on the bases of total discharge rate. Furthermore for this
neuron, the precision of phase-locking was independent of whether the
sound was presented monaurally or binaurally (IID = 0; Fig.
2C). For a detailed analysis on changes in phase-locking for
different binaural conditions, see following text.
|
In total, 85 neurons were tested for SFM tuning changes based on
discharge rate comparing monaural and binaural (IID = 0) stimulation. In about one-third of the neurons (32%) the upper cutoff
frequency shifted by >25% (Fig.
3A). Seventeen neurons (20%)
responded to substantially higher modulation frequencies (25%)
during binaural compared with monaural stimulation. Ten neurons (12%)
decreased their upper cutoff frequency for binaural compared with
monaural stimulation. Additionally, minor changes of the upper cutoff
frequency (10% but <25%) could be observed in 27% of the neurons
tested. Again, as many neurons increased as decreased their upper
cutoff frequency.
|
We also observed changes of lower cutoff frequencies in about a quarter of the neurons that exhibited band-pass filter characteristic. However, because changes usually occurred at very low modulation frequencies (<50 Hz), we did not attempt to thoroughly quantify those changes because our stimulus step size was too large for those modulation frequencies.
For 52 neurons, we also determined SFM tuning properties at an IID = 20 (ipsilateral ear 20 dB louder), which is well within the
physiologically relevant range of our experimental animal. We then
compared SFM tuning properties of neurons for binaural stimulus
presentation with IID = 0 and IID =
20. Figure
4 displays the response of a neuron to an
IID = 0 (left) and an IID =
20 (right). When the stimulus was presented with an IID = 0, the neuron showed a phase-locked response up to a modulation
frequency of 200 Hz and an uncorrelated response to higher modulation
frequencies
250 Hz. At a modulation frequency of 25 Hz, the response
was almost identical for IID =
20 and IID = 0. However, at
50 Hz, the response rate was considerably lower for IID =
20
compared with IID = 0. Above a modulation frequency of 100 Hz, no
response could be elicited for IID =
20. The MTFs for discharge
rate show a decrease of the upper cutoff frequency by 95 Hz (about
66%) (Fig. 5A). This neuron
exhibited EIf properties; this means that the strongest response (at 50 Hz) was observed for IID = 0. The cycle histograms (Fig. 4,
insets) show that at a modulation frequency of 50 Hz, the
pattern of phase-locking changed for different binaural stimulus
conditions (Fig. 4). For IID = 0 the neuron responded to the
upward and downward part of each modulation cycle. However, at an
IID =
20, only one discharge to each modulation cycle could be
observed. Again, the MTFs based on SC revealed no difference for those
modulation frequencies the neuron responded to under both binaural
stimulus conditions (Fig. 5B).
|
|
In almost half of the neurons tested (22/52), the upper cutoff
frequency decreased by >25% when the intensity at the ipsilateral ear
was increased by 20 dB (Fig. 3B); i.e., these neurons
responded only to lower modulation frequencies when the sound was
favoring the ipsilateral ear. Additionally, one-quarter of the neurons showed a small decrease of the upper cutoff frequency (10%). For
about one-third of the neurons the upper cutoff frequency was highest
for sounds with an IID = 0, corresponding to a sound coming from
straight ahead.
SFM tuning is related to the neurons' EI properties
We also tested whether there is a systematic interdependence
of differences in SFM tuning between monaural and binaural (IID = 0) stimulation, and the general binaural response characteristics of
individual neurons. Discharge rates between different binaural conditions (monaural, IID = 0, and IID = 20) at the
modulation frequency that elicited the maximal response were compared.
A change in the discharge rate of >25% in the spike rate between two
binaural conditions was defined as a change evoked by additional excitation or inhibition. According to this scheme, neurons were classified as EI (contralateral excitation, ipsilateral inhibition), EE
(excitation from both ears), EO (contralateral excitation), and EIf
(excitation from contra- and ipsilateral; inhibition from ipsi- and
contralateral; strongest response for IID = 0). Almost all neurons
(9/10) for which the upper cutoff frequency decreased considerably
(less than
25%) were EI neurons (Table
1). For the majority of EE and EIf
neurons, the upper cutoff frequencies increased by >10% for binaural
compared with monaural sound presentation. Measuring changes in upper
cutoff frequency the populations of EI neurons and EE/EIf neurons were
significantly different (Mann-Whitney U test:
P < 0.05). No significant difference could be found
between the populations of EO and EI or EE/EIf neurons. Interestingly, upper cutoff frequencies also increased in a large number of EI neurons
during binaural stimulation. This suggests that a complex interaction
of inhibitory and excitatory projections causes SFM tuning changes as
well as differences in the apparent binaural properties of neurons.
|
Temporal response patterns are affected by binaural stimulus properties
For a subset of neurons (n = 42) SFM tuning changes also were analyzed based on the temporal response pattern of the neurons. The precision of phase-locking was compared between monaural and binaural stimulation as shown in Fig. 6. In this case, phase-locking was considerably better for binaural than for monaural stimulation, particularly for modulation frequencies <150 Hz (Fig. 6, A and B). However, the cycle histograms show that this change in SC also could be partially due to a change in the response from bidirectional to unidirectional (Fig. 6D). Because the responses to the upward and downward part of the modulation cycle could not be separated clearly, we calculated the SC for the entire response. Analyzing the response to SFM sounds based on spike count, this neuron responded to higher modulation frequencies for binaural than for monaural stimulation (Fig. 6C).
|
In Fig. 7A, changes in
phase-locking between monaural and binaural stimulation are quantified.
Each line represents the difference of the SC between monaural and
binaural stimulation for each neuron. Positive values indicate better
phase-locking for binaural than for monaural stimulation. Over the
population of neurons, a distinct change of the SC (0.25) was only
observed in eight neurons. Interestingly, in all except one neuron,
phase-locking was better for binaural than for monaural stimulation.
Additionally, phase-locking differed substantially for modulation
frequencies only <150 Hz.
|
The modulation frequency eliciting best synchronization and the synchronization cutoff frequency, defined as the modulation frequency where the SC dropped <0.3, were determined for monaural and binaural stimulation if spike count was high enough to calculate the SC. For many neurons (10/18), the modulation frequency with best synchronization was substantially higher (>25%) for binaural than for monaural stimulation. In contrast, for 9 of 12 neurons, the synchronization cutoff frequency was roughly equivalent for monaural and binaural stimulation (Fig. 7C).
A small number of neurons changed their response pattern from uni- to bidirectional for different binaural stimulus conditions. For example, at a modulation frequency of 50 Hz, 5 of 61 neurons, that phase-locked at this modulation frequency, responded unidirectionally during monaural and bidirectionally during binaural (IID = 0) stimulation. In three neurons that responded bidirectionally during monaural stimulation, the second response to each modulation cycle was suppressed during binaural stimulation.
Ipsilateral inhibition changes the response pattern of neurons
To test whether synaptic inhibition changes the temporal response pattern of neurons, two different experiments were performed. First, the GABAA receptor antagonist bicuculline was applied iontophoretically to the neurons that changed its response pattern (uni- or bidirectional) depending on the binaural conditions. Blocking GABAergic inhibition prevented this change in response pattern in five neurons. During drug application, these neurons responded bidirectionally independent of the binaural conditions.
In a second experiment, glutamate was applied iontophoretically,
causing the neurons to increase their discharge rate independent of the
acoustic stimulus. During glutamate application, the same range of
modulation frequencies was presented as before but only to the
ipsilateral, thus inhibitory ear. In all cases, ipsilateral sound
presentation resulted in an inhibition of glutamate-evoked activity. In
five of nine neurons, this inhibition was phase-locked when low
modulation frequencies (50 Hz) were used. Figure
8 shows the result of an experiment where
the phase-locked inhibition coincided with the suppression of the
second response during binaural sound presentation. The top
histogram shows the response of the neuron when the stimulus was
presented only to the ipsilateral ear (Fig. 8A). The
activity of the neuron was increased artificially by glutamate
application. Therefore inhibition evoked by stimulating the ipsilateral
ear can be directly viewed as a gap in the increased activity. For this
neuron, the inhibition was phase-locked to the modulation frequency (30 Hz). The middle histogram shows the response of the same
neuron to contralateral, thus monaural stimulation (Fig.
8B). The neuron responded to both the upward and downward part of the modulation cycle. The bottom histogram shows
that during binaural stimulation (IID = 0 dB) the neuron responded only once per modulation cycle (Fig. 8C). The comparison of
the three histograms indirectly suggests that the inhibition evoked by
the ipsilateral ear (Fig. 8A) coincided with the second
discharge to each modulation cycle for monaural stimulation (Fig.
8B), thereby suppressing the second, contralaterally evoked
discharge.
|
GABAergic and glycinergic inhibition is involved in binaural changes of SFM tuning
Several neuronal mechanisms might be responsible for changes in SFM tuning induced by ipsilateral stimulation. IC neurons receive multiple inhibitory projections from lower auditory nuclei. These inhibitory inputs might be responsible for the observed interdependence of SFM tuning and IIDs. To test this idea, we blocked GABAergic or glycinergic inhibition by applying the antagonists bicuculline (blocks GABAA receptors) or strychnine (blocks glycine receptors) under different binaural stimulation.
Several patterns of inhibitory influences on SFM tuning could be observed. For example, the neuron depicted in Fig. 9 showed a low-pass filter for monaural stimulation and a band-reject filter for binaural stimulation; i.e., a highly reduced response to a small range of modulation frequencies (~90 Hz) within the usual response range. The upper cutoff frequency increased by 275 Hz for binaural compared with monaural stimulation. This is consistent with the observation that this neuron received excitatory input from both the ipsi- and contralateral ear (EE neuron). Bicuculline application caused this neuron to discharge with very high rates for both conditions up to high modulation frequencies (>200 Hz). No difference between monaural and binaural (IID = 0) stimulation could be observed anymore. Normalizing the response rate as depicted in Fig. 9B shows that bicuculline completely abolished differences in SFM tuning comparing monaural and binaural stimulation. The gap in the response characteristic for binaural stimulation disappeared.
|
For the neuron displayed in Fig. 10,
SFM tuning sharpened under binaural (IID = 0) compared with
monaural stimulation, which is consistent with the EI-response
properties of this neuron. For monaural stimulation, the neuron
responded with a sustained response up to a modulation frequency of 125 Hz. For binaural stimulation, the neuron did not respond to modulation
frequencies <50 Hz. Strychnine application rendered this neuron
responsive for modulation frequencies 125 Hz and above for monaural
and binaural stimulation. The MTFs in Fig. 10B show that
differences in SFM tuning between monaural and binaural stimulation
completely disappeared during strychnine application. This neuron did
not lose its direction selectivity during strychnine application in contrast to the previous neuron that responded unidirectionally under
predrug conditions and bidirectionally during bicuculline application
(Figs. 9C and 10C). However, over the population
of neurons tested, no systematic effect on direction selectivity between GABA and glycine could be observed. For a more detailed analysis, compare Koch and Grothe (1998)
.
|
For the population of cells tested with bicuculline (n = 40) or strychnine (n = 19), several groups of neurons can be described. The diagrams in Fig. 11 depict differences in SFM tuning based on firing rate between monaural and binaural stimulation under predrug and drug conditions. Positive values indicate increased upper cutoff frequencies during binaural stimulation. Blocking GABAergic inhibition four different effects could be seen. In ~25% of the neurons, bicuculline abolished or greatly diminished binaural differences in SFM tuning (Fig. 11A, top). Under predrug conditions, most of these neurons broadened SFM tuning during binaural stimulation. In contrast, for another group of neurons (25%) SFM tuning was broader for binaural condition during bicuculline application (Fig. 11B, top). For about one-quarter of the neurons, bicuculline application had virtually no effect on their binaural SFM tuning (Fig. 11C, top). The remaining neurons (~25%) showed a reversed relationship of SFM tuning and binaural conditions under bicuculline application compared with predrug conditions (Fig. 11D), which means that differences in upper cutoff frequencies comparing binaural and monaural stimulation changed from positive to negative or vice versa. Four neurons exhibited changes in upper cutoff frequencies that were >80% comparing monaural and binaural stimulation and are not included in the graphs. For three of these neurons, tuning differences were diminished greatly during bicuculline application. During strychnine application three different effects could be observed. About 25% of the neurons did not show binaural SFM tuning differences (Fig. 11A, bottom) during strychnine application anymore. However, in contrast to bicuculline application, strychnine was only effective in neurons that sharpened SFM tuning for binaural compared with monaural stimulation (negative values). In four neurons, binaural SFM tuning differences increased during drug application (Fig. 11B, bottom). In the remaining neurons, strychnine application did not cause any changes in binaural SFM tuning differences (Fig. 11C, bottom). No reversed response behavior could be observed during strychnine application.
|
Ipsilateral inhibition gets stronger for higher modulation frequencies
Because some neurons changed their temporal response pattern in the way that at some modulation frequencies the response was unidirectional for binaural stimulation and bidirectional for monaural stimulation, we argued that the timing of ipsilateral inhibition might be an important factor for the tuning properties.
To precisely measure the temporal occurrence and duration of ipsilateral inhibition, we presented SFM sounds as "binaural beats." Several modulation frequencies were tested (25/30-200/205 Hz) with a 5-Hz modulation frequency difference between the two ears. This created an interaural phase difference (IPD) that changed continuously as the stimulus progressed and allowed us to determine at which IPDs the ipsilateral induced inhibition suppressed contralateral evoked excitation and to measure the timing and strength of the ipsilaterally evoked inhibition.
There are two pieces of evidence that ipsilateral inhibition was involved in the reduction of the response at certain IPDs: first, all neurons tested were increasingly inhibited when the intensity at the ipsilateral ear was increased gradually. Second, the maximal response during a binaural beat stimulus was equal to the response evoked by monaural stimulation. This excludes mechanisms solely based on the coincidence of two excitatory inputs.
In most neurons (14/16), inhibition was phase-locked to each modulation
cycle. For higher sound intensities at the ipsilateral ear, the
inhibition lengthened and got stronger (Fig.
12A). Figure 12B
shows the response of a neuron to a "binaural beat" SFM stimulus that was modulated for 50/55, 100/105, and 200/205 Hz at the
contralateral/ipsilateral ear respectively. For this neuron, inhibition
evoked by the ipsilateral side was weak at 50 Hz. It gradually
strengthened for higher modulation frequencies, and at 200 Hz, a strong
biphasic inhibition could be observed. At higher modulation
frequencies, the duration of inhibition remained the same although the
length of each stimulus phase decreased, leading to a higher duty cycle
of inhibitory input. To determine of what nature the observed ITD
sensitivity of this neurons was, the mean phase angles were calculated
(50/55 Hz: 0.31; 100/105 Hz: 0.36; 200/205 Hz: 026) according to
Yin and Kuwada (1983). Determining the intercept of the
regression line with the ordinate yields to a characteristic phase (CP)
of 0.36. This indicates that this neuron's ITD sensitivity is neither based on a pure EE (CP = 0) nor a pure EI (CP = 0.5)
interaction (Batra and Fitzpatrick 1997
). For this
neuron, excitation and inhibition are most likely present from ipsi-
and contralateral. This neuron decreased its upper cutoff frequency
when the sound was presented with an IID =
20 compared with
monaural stimulation (Fig. 12C). Similar properties were
observed in another four neurons. For all these neurons, the upper
cutoff frequency decreased when the intensity at the ipsilateral ear
was increased.
|
For the remaining eleven neurons, neither the strength of inhibition increased at higher modulation frequencies nor SFM tuning sharpened for IIDs favoring the ipsilateral ear.
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The present study presents evidence that binaural cues, namely
interaural intensity differences, have profound effects on filter
properties for stimulus periodicity of auditory midbrain neurons.
According to our analysis, changing the binaural stimulus conditions
does not merely scale SFM tuning linearly due to the binaural response
characteristic of the neuron but modifies it nonlinearly, partially
dependent on the binaural response type of the neuron. Our data also
indicate that these effects are partially due to changes in strength
and timing of binaural inhibitory inputs. This conclusion is based on
four main findings. First, in about one-third of the IC neurons tested
for monaural versus binaural stimulation, and in about half of the
neurons tested for IID = 0 versus IID = 20, SFM tuning
depends on the binaural stimulus conditions, in this case the
interaural intensity differences (IIDs). Second, in ~10% of the
neurons, the synchronization of the response as well as the response
pattern (once or twice per modulation cycle) is influenced by the
binaural stimulus conditions. Third, GABAergic and glycinergic
inhibition is involved in causing differences in SFM tuning for
different binaural conditions. Fourth, our data suggest that in some
neurons the strength of ipsilaterally evoked inhibition increases for
higher modulation frequencies and the duration of inhibition remains
independent of the length of the modulation phase. This in turn might
sharpen SFM tuning when ipsilateral intensity is increased.
SFM tuning based on firing rate changes for different binaural situations
Most studies that investigated response characteristics of
IC neurons to periodic stimuli used monaural sound presentations exclusively (Casseday et al. 1997; Langner and
Schreiner 1988
; Schuller 1979
). On the other
hand, most studies investigating the binaural properties of IC neurons
only used pure tones or broadband noise. Consequently, these
independent sets of data cannot be related to each other. The present
study, however, indicates that neuronal processing of periodic stimuli
and the analysis of IIDs are not accomplished by two completely
independent mechanisms but rather influence each other.
In a different set of experiments, we previously showed that the
azimuthal position of a free-field speaker influences the tuning for
the modulation frequency of SFM sounds (Koch and Grothe 1997). Using the same criterion for changes in the tuning to
the modulation frequency (
25%), ~55% of the neurons showed
position-dependent filtering. This is an equivalent number of neurons
as we found in the present study comparing differences in SFM tuning
between IID = 0 and IID =
20. This indicates that the
frequency-dependent attenuation of the pinna only plays a minor role in
determining SFM tuning of IC neurons and that neuronal processing is
the main factor. It also demonstrates that the differences in SFM
tuning in the present study are most likely not due to different total energy levels that exist for different binaural conditions in our
present, more artificial experimental design.
A similar dependency of response characteristics for periodic stimuli
on sound location has been reported before but in a different class of
vertebrates. In the northern leopard frog's torus semicircularis, the
amphibian homologue of the mammalian IC, more than half of the neurons
sharpened their tuning for the modulation frequency of SAM sounds when
a free-field loudspeaker was rotated from the contralateral hemisphere
into the ipsilateral hemisphere (Xu et al. 1996).
Similar results from the grass frog midbrain have demonstrated that
most neuron's selectivity for the temporal characteristic of a sound
and its tuning for interaural time disparities (ITD) are intricately
coupled (Melssen and van Stokkum 1988
; Melssen et
al. 1990
; van Stokkum and Melssen 1991
). However, the tympanic membrane of these animals acts as a pressure gradient receiver comparing sound pressure reaching the tympanic membrane through the external ear and the Eustachian tube from both
sides. As a consequence, frequency tuning of auditory nerve fibers in
these animals is already dependent on the spatial position of the
stimulus (Feng and Shofner 1981
; Narins et al.
1988
). Therefore changes in SAM tuning of frog IC neurons might
be at least partially a reflection of the peripheral response
properties. However, similar results also were obtained from IC neurons
in the big brown bat, the animal used in the present study. There
neurons sharpened their receptive field when pulse repetition rate of
pure tones was increased (Wu and Jen 1996
). Moreover,
receptive fields of individual neurons in the big brown bat IC differ
for different stimuli (Grothe et al. 1996
).
Interestingly, in the present study, only a small number of neurons
could be classified as true EE neurons when a SFM stimulus was used.
Most neurons responded only weakly by ipsilateral stimulation alone and
also exhibited some inhibition from the ipsilateral side. A possible
explanation could be that IC neurons integrate projections from the
presumed EI and EE neurons of the MSO and lateral superior olive (LSO).
It is possible that by using pure tone stimulation, this integration remains occluded in many cases. Another study that used binaural SAM
stimulation reported a similar small percentage of true EE neurons
(Batra et al. 1989
). Despite species-specific
differences that certainly exist, all these results indicate that
processing of sound location and sound pattern is not independent from
each other.
Synchronization and response pattern changes with different IIDs
In the present study, only 10% of the neurons changed their
precision of synchronization to the modulation cycles when the stimulus
was presented binaurally compared with the degree of phase locking in
response to monaural sound presentation. In our study, most of these
neurons synchronized better under binaural than under monaural
stimulation. In contrast, under free-field conditions, we observed a
change in synchronization in 50% of the neurons (Koch and
Grothe 1997). The discrepancy might be explained by the
different analysis used. In the free-field study, the SC was calculated
the same way regardless of whether the response was uni- or
bidirectional. A considerable difference exists to neurons of the
rabbit IC. There most neurons synchronized significantly better to SAM
sounds when they were presented as binaural beat stimuli compared with
contralateral stimulation only (Batra et al. 1989
).
However, neurons in the rabbit IC synchronized to much higher
modulation frequencies compared with the neurons in the present study.
On the other hand, in the frog IC there are no significant differences
in the degree of synchronization due to different binaural test
conditions (Xu et al. 1996
). Again, there seem to be
profound species-specific differences.
GABA and glycine affects binaural tuning in different ways
There are several potential mechanisms that could accomplish
IID-dependent changes in SFM tuning of IC neurons. Data from the MSO of
the free-tailed bat (T. brasiliensis) demonstrate that in
the majority of neurons SAM tuning is dependent on IIDs (Grothe et al. 1997b), whereas in LSO neurons, these parameters are
processed independently (Grothe et al. 1997a
). This
suggests that this interdependency might be at least partially created
at much earlier stages of binaural processing. However, in this study,
we present evidence that GABAergic and glycinergic inhibition is
involved in changing SFM tuning at the level of the IC. Therefore at
least some of these changes are likely to be created within the IC.
Our results indicate that GABAergic and glycinergic inhibition
differ in their effects on changing SFM tuning for different binaural
conditions. A schematic of the major GABAergic and glycinergic projections to the IC is shown in Fig.
13. The major source of GABAergic
inhibition to IC neurons is the dorsal nucleus of the lateral lemniscus
(DNLL). Neurons in the DNLL stain for GABA and provide strong bilateral
projections to both ICs (Adams and Mugnaini 1984;
Roberts and Ribak 1987a
,b
; Thompson et al.
1985
; Vater et al. 1992
). Many neurons in the
DNLL respond with a phase-locked discharge up to high modulation
frequencies of SAM sounds (Yang and Pollak 1997
).
Moreover, DNLL neurons are excited by contralateral stimulation and
inhibited by ipsilateral stimulation (Covey and Casseday
1991
; Yang and Pollak 1997
).
|
As depicted in Fig. 11A (top), bicuculline was
most effective in abolishing SFM tuning differences in neurons that
broadened their tuning for binaural stimulation. This suggests that
GABAergic inhibition is stronger at high modulation frequencies for
contralateral stimulation than for binaural stimulation. This is in
agreement with findings that contralateral stimulation excites neurons
in the ipsilateral DNLL, which in turn exert GABAergic inhibition on
neurons in the ipsilateral IC. As the sound intensity at the ipsilateral ear increases (binaural stimulation), neurons in the ipsilateral DNLL are increasingly inhibited and cease to inhibit IC
neurons at higher modulation frequencies. In a number of neurons, blocking GABAergic inhibition broadened SFM tuning for binaural stimulation (Fig. 11B, top). This effect might be due to
inhibitory GABAergic inputs from the contralateral DNLL. In this DNLL
neurons do not respond to contralateral stimulation alone; however,
they do start to exert inhibition on IC neurons during binaural
stimulation. Another GABAergic projection arises from the contralateral
IC. In vitro recordings of IC neurons have shown that stimulation of
the commissure elicits short and long latency inhibitory postsynaptic potentials (IPSPs) that can be blocked by bicuculline (Moore et al. 1998; Smith 1992
).
In contrast, blocking glycinergic inhibition was only effective in
abolishing differences in SFM tuning in neurons that sharpened their
SFM tuning for more negative IIDs (Fig. 11A, bottom). This might be due to the prominent glycinergic projection from the ipsilateral LSO (Glendenning et al. 1992; Saint
Marie and Baker 1990
; Saint Marie et al. 1989
),
which is excited by stimulating the ipsilateral ear and inhibited by
stimulating the contralateral ear (Tsuchitani 1977
).
Another glycinergic projection arises from the monaural nuclei of the
lateral lemniscus on the ipsilateral side (Gonzalez-Hernandes et
al. 1996
; Vater et al. 1997
). However, neurons
in the ventral nucleus of the lateral lemniscus (VNLL) and the
intermediate nucleus of the lateral lemniscus (INLL) receive input only
from the contralateral ear. Hence their impact on SFM tuning should not
change for binaural stimulation because the intensity at the
contralateral ear was kept constant in the present study.
Ipsilaterally evoked inhibition gets stronger at higher modulation frequencies: a possible neuronal mechanism for creating low-pass filters
As already suggested by Melssen and Epping (1992),
relative timing of excitation and inhibition might be an important
factor in changing periodicity tuning for different IIDs. Well
understood is the impact of delayed inhibitory inputs on SAM coding in
the MSO (Grothe 1994
, Grothe et al.
1997b
) and the DNLL (Yang and Pollak 1997
) where
the response to one cycle is suppressed by inhibition evoked by the
preceding cycle. In combination with temporal summation of inhibition
at very high repetition rates, which has been shown at different levels
of the auditory system and for hippocampal neurons, this creates a very
effective low-pass filter mechanism for periodic sounds (Buhl et
al. 1995
; Davies and Collinridge 1996
;
Fuzessery 1997
; Grothe and Sanes
1994
). Moreover, the effect and duration of ipsilaterally
induced inhibition has been shown to increase with increasing intensity
of the ipsilateral signal (Park et al. 1996
;
Sanes 1990
; Wu and Kelly 1992
). Our data
indicate that inhibition strengthens at higher modulation frequencies
and the duration of inhibition stays the same in absolute terms and
hence increases relative to the duration of the modulation cycle.
Additionally, the absolute duration increases for higher sound
intensities. Therefore different IIDs might contribute to the filter
effect through the mechanism of time-intensity trading. Increasing the
stimulus intensity at one ear results in shorter delays of excitation
and inhibition in many IC and LSO neurons that can be compensated for
by artificially introducing interaural time differences (Irvine
et al. 1995
; Park et al. 1996
; Pollak 1988
; Yin et al. 1985
). On the contrary, one
other study suggests that in the IC of the free-tailed bat inhibition
is only little involved in low-pass filtering the neuronal response to
SAM sounds (Burger and Pollak 1999
). This indicates that
other neuronal filter mechanisms apart from inhibition, which are
presently unknown, must be effective in that case.
Unfortunately because inhibitory and excitatory inputs to each IC neuron are plentiful, it seems impossible to determine the temporal and binaural changes of each input and to dissect out their contributions to the entire response behavior of each neuron.
We also are aware of the fact that the amount of cycling of the response to a binaural beat stimulus depends on the precision of phase-locking of the excitatory and inhibitory inputs. However, phase-locking decreased for increasing modulation frequencies during contralateral (mostly excitatory) stimulation (Fig. 12A). Therefore the inhibitory input is most likely to be responsible for these changes.
Behavioral relevance
At first glance, the observed ambiguity in neuronal coding of
sound pattern depending on sound location seems to be enigmatic. Indeed, one psychoacoustic experiment shows that the extent of lateralization of a SAM sound depends on its modulation frequency (Bernstein and Trahiotis 1985).
However, spatial cross-correlation models suggest a different
interpretation for this phenomenon. In these models, processing of
sound location using binaural information is coupled to the analysis of
sound pattern such as pitch or periodicity (Loeb et al.
1983; Shamma et al. 1989
). A similar dependency
of spectral tuning of neurons on IIDs or ITDs can be seen in the
"stereausis" model, proposed by Shamma. In this model, the change
in the spectral tuning of neurons is supposed to provide additional
information for sound localization (Shamma et al. 1989
).
Moreover, these models have properties that might account for certain
complex binaural, psychophysical observations such as the cocktail
party effect.
![]() |
ACKNOWLEDGMENTS |
---|
We thank J. H. Casseday, G. Neuweiler, and the anonymous reviewers for helpful comments on the manuscript. We also thank P. Schlegel for technical support and J. H. Casseday, E. Covey, and D. Molter for providing software.
Present address of U. Koch: Physiology Dept., University College London, London WC1E 6BT, United Kingdom.
![]() |
FOOTNOTES |
---|
Present address and address for reprint requests: B. Grothe, Max-Planck-Institute of Neurobiology, Am Klopferspitz 18a, D-82152 Martinsried, Germany.
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 28 May 1999; accepted in final form 4 January 2000.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|