Medical Research Council Institute of Hearing Research, University of Nottingham, Nottingham NG7 2RD, United Kingdom
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ABSTRACT |
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Palmer, Alan R.,
Dan Jiang, and
David McAlpine.
Neural Responses in the Inferior Colliculus to Binaural Masking
Level Differences Created by Inverting the Noise in One Ear.
J. Neurophysiol. 84: 844-852, 2000.
We have
measured the responses of inferior colliculus neurons in the
anesthetized guinea pig to signals which in human psychophysical experiments reveal a release of masking as a result of binaural processing (the binaural masking level difference: BMLD). More specifically we have used diotic tones at 500 Hz (So) masked by noise
that is either identical at the two ears (No) or inverted in one ear
(N). This combination of signals and noise maskers produces a
prominent masking release in humans such that the So signal is about
6-12 dB more detectable in the presence of the N
noise than the No
noise. Low-frequency inferior colliculus neurons are sensitive to the
interaural delay of the masking noise and generally respond most to the
components nearest their best frequency. Since most inferior colliculus
neurons have peaks in their delay functions close to zero interaural
time delay this means that while No noise is effective in driving the
unit, N
noise is much less effective. As the level of an So tone was
progressively increased in the presence of No and N
noises, the
first response could be either an increase or a decrease in the
activity due to the noise. However, because N
generated little or no
activity itself, the predominant response to the So tone was an
increase in discharge in this condition. Masked thresholds were defined as the point at which the standard separation D (related to
the d' of signal detection theory) = 1 in either
direction. BMLDs were measured in single neurons and in the majority of
units were in a direction consistent with the psychophysical
observations irrespective of the direction of the discharge rate change
that occurred at threshold. The lowest masked thresholds always
occurred at or near the signal frequency of 500 Hz. An average value of the single unit BMLD around 500 Hz was 3.6 dB (NoSo vs. N
So) compared with 6.6 dB for the NoSo versus NoS
BMLD we had previously reported. This lower magnitude is consistent with the hierarchy of
human psychophysical BMLDs.
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INTRODUCTION |
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The binaural detectability of a
tonal signal in a wideband masking noise is not only determined by the
spectral and temporal characteristics of the signal and noise but also
by any differences between the signals or maskers in one ear and those
in the other. Thus an identical tonal signal of (say) 500 Hz at both
ears (signal condition, So), which is just masked by identical noises
in the two ears (noise condition, No), can be made considerably more detectable (by 12-15 dB) simply by inverting the 500-Hz signal in one
ear (creating the signal condition S). This difference in threshold,
discovered by Licklider (1948)
, is termed the binaural masking level difference (BMLD). Soon after its discovery,
Hirsch (1948a
,b
) investigated many of the signal
dependencies of the BMLD and provided a hierarchy of masked thresholds
which depended on the binaural configuration increasing in magnitude as
follows: Nm-Sm (both noise and signal presented monaurally), N
-S
(both signal and noise inverted in one ear), No-So (both signal and noise identical), N
-Sm (noise inverted at one ear and signal presented monaurally), No-Sm (noise in phase at the ears and the signal
presented monaurally), N
-So (noise inverted in one ear and the
signal identical at both ears), and No-S
(signal inverted in one ear
and the noise identical).
The largest BMLD is obtained by subtracting the masked thresholds for
NoS from that for NoSo. Another large, entirely binaural, BMLD is
obtained by subtracting the masked thresholds for N
So from that for NoSo.
In several previous studies, we have described the neural mechanisms
underlying the NoSo versus NoS BMLD (Jiang et al.
1997a
,b
). By using a method for determining the masked
threshold, derived from signal detection theory (Sakitt
1973
), we were able to demonstrate that both single neurons and
populations of neurons in the inferior colliculus showed lower masked
thresholds to NoS
than to NoSo. We suggested that different
populations of neurons were responsible for the detection of the tone
in these two conditions and that So signals were mostly detected by an
increase in the discharge rate while S
signals were detected by a
decrease in discharge. The responses of single neurons to the binaural
unmasking signals were consistent with their sensitivity to the
interaural delay of the tones and noises. We further provided empirical
validity for prevailing computational models of the BMLD, which
suggested that the decrease in discharge rate that indicated the
presence of S
signals was caused by a desynchronization of the
activity due to the masking noise at the brain stem coincidence
detectors (Palmer et al. 1999
).
Here we extend these earlier studies to the NoSo versus NSo BMLD.
Because the noise condition against which the signal is to be detected
is no longer constant, the possibility exists for different strategies
to be employed to detect the tones at masked threshold for this BMLD
condition compared with that we have shown for NoSo versus NoS
. For
the NoSo versus NoS
condition, the No noise provided a constant
baseline activity level against which the effects of the So and S
tones could be measured. For NoSo versus N
So, the No noise generally
drove the neurons well, whereas the N
noise was often ineffective in
driving the neurons. Additionally So tones always produce more
synchronized activity at the coincidence detector. This meant that the
majority of neurons signaled So tones against both No and N
noise by
an increase in discharge. Nevertheless both individual neurons and
populations of neurons still showed masked threshold differences that
were consistent with the direction of the BMLD shown psychophysically
and with their sensitivities to interaural delay.
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METHODS |
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The detailed methods have been described previously
(Jiang et al. 1997a,b
) and will only be described
briefly here.
Anesthesia and surgical preparation
Single-unit recordings were made from the inferior colliculi of
28 pigmented guinea pigs weighing between 300 and 450 g, most of
which were also used to gather data reported in other publications (Jiang et al. 1997a,b
; McAlpine et al.
1996a
,b
; Palmer et al. 1999
). The animals were
premedicated with atropine sulfate (0.06 mg sc) and anesthetized with a
combination of urethan (1.3 g/kg in 20% solution ip) and phenoperidine
(1 mg/kg im) as described in our previous publications. All animals
were tracheotomized, and core temperature was maintained at 37°C with
a heating blanket. In some cases, the animal was artificially
ventilated with 95% oxygen and 5% CO2, and
end-tidal CO2 was monitored. The animal was
placed inside a sound-attenuating room, and sounds were presented in
closed field. Pressure equalization within the middle ear was achieved
by a narrow polythene tube (0.5 mm OD) sealed into a small hole in the
bulla on each side. The cochlear condition was assessed by monitoring
the cochlear action potential (CAP) in the left ear at intervals
throughout the experiment. The threshold of the filtered and amplified
CAP to a series of short-tone bursts was measured automatically
(Palmer et al. 1986
) at selected frequencies (0.5, 1, 2, 4, 5, 7, 10, 15, 20, and 30 kHz). The acoustic cross-talk between the
two ears of our closed-field acoustic system at frequencies from 500 to
10,000 Hz was more than 50 dB down and was better than 45 dB at all
frequencies (Palmer et al. 1990
).
Recordings were made with stereotaxically placed tungsten-in-glass
microelectrodes (Bullock et al. 1988) through the intact cortex.
Stimulus presentation
The stimuli were delivered through 12.7-mm condenser earphones (Brüel and Kjaer 4134), coupled to damped 4-mm-diam probe tubes. The output was calibrated a few millimeters from the tympanic membrane using a Brüel and Kjaer 4134 microphone fitted with a calibrated 1-mm probe tube. The sound system response on each side was flat to within ±5 dB from 100 to 10,000 Hz, and the left and right systems were matched to within ±2 dB over this range.
Stimuli
The stimuli used in this study were tones and noises presented
to the two ears. The noise used was digitally synthesized "frozen" noise output at a sampling rate of 50 kHz via a digital-analog converter (TDT QDA2) and a waveform reconstruction filter (Kemo VBF33,
cutoff slope 135 dB/octave from 5 kHz). The noise was synthesized by
the method of Klatt (1980): 16 random numbers were added
together to obtain each point resulting in a pseudo-Gaussian amplitude distribution and the resulting waveform was scaled to 16 bits. The
noise was initially synthesized with a bandwidth of 20 kHz and then
digitally filtered to give a 50-Hz to 5-kHz bandwidth. The same frozen
noise sample was used for all units. Interaural delays of the noise
were introduced during synthesis. Tonal stimuli were either from a
Hewlett Packard 3325A waveform synthesizer or digitally synthesized and
output from a digital-analog converter (TDT QDA2) and a waveform
reconstruction filter (Kemo VBF33, cutoff slope 135 dB/octave from 5 kHz). For N
noise, the digital values were multiplied by
1 before
being output to the right ear.
Data collection and analysis
Single neurons were isolated using 50-ms tone and/or noise bursts as search stimuli. The extracellularly recorded neural action potentials were amplified (Axoprobe 1A: 1,000 times), filtered (155-1,800 Hz), converted to logic pulses by an amplitude discriminator, and timed with 10-µs resolution (CED 1401 plus). The lowest binaural threshold to interaurally in-phase tones, and the frequency at which it was obtained (the best frequency, BF) were determined audiovisually. The spontaneous rate was measured over a 10-s period in the absence of any controlled acoustic stimulus.
The following analyses were carried out in the present study.
INTERAURAL PHASE DIFFERENCE (IPD) HISTOGRAMS.
IPD histograms were measured using binaural beat stimuli (Kuwada
et al. 1979). Digitally synthesized tones that differed by 1 Hz
at the two ears were used, resulting in a continuous and systematic
change in the binaural phase disparity at a rate of 1 Hz. In the
present study, the frequency of the signal delivered to the left ear
(contralateral to the recording site) was always 1 Hz greater than that
delivered to the right (ipsilateral) ear. The duration of the stimulus
was 3,000 ms, which included three complete cycles of the entire range
of possible IPDs. The stimulus was repeated 10 times with an
interstimulus interval of 6.5 s. Best delays and vector strengths
were calculated from a period histogram locked to the IPD cycles
constructed from the middle two seconds of the response (0.5-2.5 s).
NOISE DELAY FUNCTIONS (NDFS). NDFs were measured by presenting frozen noise with interaural time disparities over a range equal to three times the period of the neuron's BF, in 52 equal delay steps, starting from ipsilateral leading. The duration of the stimulus was either 50 or 333 ms, with 20 or 3 repetitions, respectively, giving a total of 1-s stimulation time at each delay.
MASKED RATE-LEVEL FUNCTIONS (MRLFS). MRLFs were obtained by measuring tone rate-level functions in the presence of a noise masker at a fixed level. Tone rate-level functions were generated by presenting tones from the HP 3325A with rolling phase (50-ms duration; rise-fall time, 1 ms) and the frozen noise (5 kHz bandwidth) simultaneously gated and varying the level of the tone pseudorandomly over a maximum range of 100 dB in 1-dB steps. For the first experimental series, the noise level was arbitrarily chosen at 10-20 dB above the No noise threshold, a level at which a reasonable No driven response and a well-tuned noise delay function was obtained. This meant that different noise levels were used in the analysis of different units. In the second experimental series, we used only a single fixed noise level (33 dB SPL/Hz1/2) that was the same for all units analyzed. Possible order effects were minimized by ensuring that each stimulus was never more than 50 dB weaker than the one preceding it. The number of spikes elicited by each tone was counted and the average MRLF was computed from 10 presentations at each level. The frequency of the tone used was 500 Hz and was always interaurally in phase (So).
DETERMINING THE MASKED THRESHOLD FROM THE MRLFS.
To determine the masked threshold for a tone from the MRLF, we used an
analysis technique derived from signal detection theory (Green
and Swets 1966). However, the classic detectability index (d') metric assumes that the responses of the neurons are
normally distributed with equal variances, an assumption that does not necessarily hold for neurons in the auditory pathway. Accordingly, we
employed a modified version of d, the Standard Separation
(D), described by Sakitt (1973)
, which allows
a simple interpretation that is independent of any assumptions about
the underlying distributions:
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(1) |
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RESULTS |
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Data were obtained from 169 units (BFs ranged from 125 to 2,090 Hz) in 28 guinea pigs. In the first series of 22 animals, we were mainly concerned with measuring the full response profile to provide explanatory leverage, and the yield was consequently modest. We maximized the number of positive BMLD responses by positioning the noise level at 10-20 dB above the No noise threshold for a particular unit. Because this meant that the noise was at different levels for different units, we have labeled these data "variable noise level" in summary histograms. In the second series of six guinea pigs, we were more concerned with summarizing the response of a population of inferior colliculus neurons to the BMLD stimulus, and we obtained higher yields with less analysis of each unit using a single noise level: these data we have labeled as "fixed noise level" in summary histograms. In general, the results from both parts of the study were compatible.
Responses to No and N noise alone
The baseline from which the tonal responses arise depends on the
level of activity evoked by the background noise stimulus. In our
previous work describing only the responses to So and S signals in
No noise, the majority of units were well driven by the No noise
because most had peak responses at or near zero interaural time delay
(ITD). For the same reason, the majority of units are poorly
driven by N
noise. In some units, the difference in response to No
and N
noise was negligible, and in a small proportion the No noise
gave a smaller response than the N
noise representing units that had
a trough at zero ITD and a peak at or near a delay of half the BF period.
Shapes of individual rate-level functions to So tones in No and
N noise
In Fig. 1 we show the different
shapes of the MRLFs in response to the So tone in the presence of the
No and N noise. Two examples are given for each of the main response
types, and the second column shows the conversion to D
values. The most common type of response [73% (38/52) of the variable
noise data and 38% (45/117) of the fixed noise data, see Fig. 3] is
shown in Fig. 1A. The nomenclature reflects the sign of the
D value at the masked threshold as we have used previously
(Jiang et al. 1997a
,b
). Thus positive-positive
(PP) units achieved masked threshold by an increase in
discharge rate above the noise-evoked baseline for both No and N
noise, generating a positive D in each case. For the two PP
examples shown, the No noise (
) drives the unit better than the N
noise (
), and the BMLD to the 500-Hz tones is positive (in the same
direction as psychophysical data) for the first and negative for the
second.
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The second most common type [10% (5/52) of the variable noise data
and 21% (25/117) of the fixed noise data] is negative-negative (NN), i.e., the masked threshold was achieved by a decrease in discharge rate for both No and N noise, resulting in a negative D. In Fig. 1B, we again show examples in which
the BMLD that resulted was negative in one case and positive in the
other and the effectiveness of the two noises were different: N
was
more effective in the first and No in the second. Differences in the
effectiveness of the noise reflects the noise delay sensitivities as we
have described in the preceding text. At higher levels of the So 500-Hz
tone the first unit in Fig. 1B shows an increasing
discharge. Previously we attributed this either to monaural effects of
the tone or simply that the So tone completely dominates the response.
The second NN example shows a decrease to reach the D threshold but
continues to decrease possibly suggestive of an inhibitory effect
similar to that described previously (Palmer et al.
1999
, Fig. 8F).
Negative-positive (NP) responses in which D was
achieved by a discharge rate decrease in No and an increase in N
made up 8% (4/52) of the variable noise data and 11% (13/117) of the
fixed noise data. The two examples of NP responses that we show in Fig. 1C illustrate a problem with the D approach when
applied to physiological data without due regard to the initial
response of the neuron to N
noise. In both units, the N
noise
appeared to be strongly suppressive, so much so that the unit was
completely silenced. This means that the SD of the discharge was zero
and no D could be calculated. Equally clear is the fact that at some
level the So tone did activate the units. We therefore simply took the
point at which the discharge rose above zero as the masked threshold.
Finally in Fig. 1D, we show positive-negative (PN)
responses [6% (3/52) of the variable noise data and 3% (3/117) of
the fixed noise data] in which No gave positive D and N
gave negative D, but both BMLDs were positive. Again, at
higher levels in the second unit the So 500-Hz tone dominated the response.
As in our previous studies, the most common response corresponds to those units that show a peak in the delay functions to noise and tones located near zero ITD (see later detailed comments).
For some units, only one configuration actually yielded a masked
threshold estimate. Examples are shown in Fig.
2. Here, despite the fact that No drove
two of the units well (Fig. 2, A and C), the
500-Hz So tone was ineffective in altering the activity due to the No
noise by sufficient amounts to reach D = 1. In these cases, the So tone achieved masked threshold in N noise by an increase (Fig. 2A) or by a decrease in discharge (Fig.
2B). Even restricting our sampling to delay-sensitive
low-frequency units we did encounter some units for which the 500-Hz
tone was ineffective in altering the discharge rate, as shown in Fig.
2C. The relative proportions of the various response types
that we encountered both in the first (variable noise at +20 dB re
threshold) and second series (fixed noise level) are shown in Fig.
3, where "undefined" is used to
designate those units where we only obtained one masked threshold.
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BMLD measured in individual neurons
The magnitude of the BMLD in individual neurons is shown in Fig.
4. As in our previous NoSo versus NoS
data the majority of neurons (64% for the variable noise data and 67%
of the fixed noise data after exclusion of single-threshold data) have
positive BMLDs with magnitudes above 3 dB. For those units for which
the tone only provided a masked threshold in one condition, the BMLD is
at least equal to the difference between this masked threshold and the
maximum output of the sound system. In the majority, it was the N
So
condition that yielded the single masked threshold resulting in large
positive BMLDs (Fig. 4B). The bin Fig. 4B, left,
indicates those units for which the 500-Hz tone was completely ineffective in changing the discharge rate due to the noise (as in Fig.
2C).
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Relationship of MRLF shape to delay sensitivities
Our main aim was to examine the magnitude of the BMLD using NoSo
versus NSo, but in many instances, we obtained other data that
allowed us to suggest the mechanisms that underlie the observed BMLD
responses. Figure 5 shows two
representative examples of the PP response type (which were in the
majority in these data). In Fig. 5, A-C, we show the most
typical result. Here both the noise delay function (Fig. 5B)
and the binaural-beat response show a peak close to zero interaural
phase difference (see lower abscissa in Fig. 5C).
The No noise therefore is more effective in driving the unit than the
N
noise (Fig. 5A), which completely suppresses the unit.
The coincident activity evoked by the So tone raises the discharge rate
in response to both No and N
noise. This effect we have described in
some detail previously (Palmer et al. 1999
).
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A second PP unit is shown in Fig. 5, D-F. At first sight
this appears to be a paradoxical result since both No noise (Fig. 5E) and the So tone (Fig. 5F) occur at a trough
in the discharge rate and one might therefore expect the discharge rate
to the noise to be reduced by adding the So tone. However, this unit is
not a simple trough unit [a unit in which the major feature of the
delay function is a trough at or near zero ITD probably generated by
inhibition in the brain stem (Batra et al. 1997)] and
may represent either a unit with a long best delay or a mixed unit in
which both trough and peak features are present (see McAlpine et
al. 1998
). This is evident when the monaural responses to noise are considered (marked as C for contralateral and I for ipsilateral on
Fig. 5E). The peaks in the delay function are a result of
facilitation above the contralateral response while the troughs are
relatively shallow not reaching down to the response to the ipsilateral
ear. If the trough is due to coincidence between excitation from one ear and inhibition from the other, the relatively shallow trough represents a lack of perfect coincidence; i.e., every excitatory spike
is not cancelled by a corresponding inhibitory input. The MRLF (Fig.
5D) indicates that No is slightly less effective in driving
the unit than N
, but that the So tone produces an increase for both
No and N
.
For the other, less common, response types a single unifying
explanation based on their other response properties was more difficult. For the NN type units, the only common factor seemed to be
that both No and N noise fell at positions on their noise delay
functions that evoked a reasonably strong response and thus decreases
in discharge rate were possible. While in all of the NN units there was
a trough of some kind near zero ITD, it was generally not down to zero
spikes/stimulus. Assuming this trough is the result of coincidences at
the brain stem due to the No noise and therefore presumably inhibitory,
adding an So tone could further reduce the activity in the trough. The
activity at N
on this basis would represent lack of coincidence and
might even represent activity mainly due to contralateral input alone:
under such conditions adding So tones will also reduce the activity to
the N
noise since its coincident activation of the brain stem generates inhibition.
Both PN units showed a trough close to zero IPD in the noise delay function, but neither this fact nor other data using tonal stimuli gave any plausible explanation linking their delay sensitivities with the direction and magnitude of their individual BMLDs.
Finally, for one NP unit we also measured the effect on its sensitivity
to interaurally delayed noise of either desynchronizing the noise at
the two ears or adding So tones to the noise (see Palmer et al.
1999 for details of these paradigms). The effect of the So tone
was to desynchronize the activity due to the No noise, as demonstrated
by a flattening of the noise interaural delay function (similar to that
which we have previously described for 51 and 56 dB SPL So tones in
Fig. 8F of Palmer et al. 1999
). This
desynchronization has the effect of decreasing the activity to No and
increasing the activity to N
. Further increases in the level for
this NP unit resulted in strong excitation, possibly as a result of a
monaural response to the So. It is notable that the BMLD in this unit
was large and in the opposite direction to the psychophysics.
Responses of populations of inferior colliculus neurons to a single signal-to-noise ratio
In Fig. 6A, the masked
threshold for 117 units from six animals are plotted separately for the
NoSo () and N
So (
) conditions with the noise level fixed at 33 dB SPL/Hz1/2. Unsurprisingly, consistent with our
previous NoSo versus NoS
data, the lowest masked thresholds are
measured in units with BFs at the signal frequency (in this case 500 Hz). All measured masked thresholds are shown in this figure including
those when only one condition yielded a value. Also just discernible in
this figure is the fact that the NoSo thresholds (
) are generally higher than the N
So thresholds (
) reflecting the preponderance of
positive BMLDs in individual units (Fig. 4). To emphasize this point,
we have plotted in Fig. 6B the average masked threshold for
NoSo and N
So computed from those units with BFs around the signal
frequency (300-800 Hz, dotted vertical lines in Fig. 6A). The average value for NoSo is higher than that for N
So, yielding an
average BMLD of 3.6 dB. This value is smaller than that we calculated
in a previous study for NoSo versus NoS
(Jiang et al.
1997b
), a result consistent with the ordering of psychophysical magnitudes. The average BMLD value for the whole BF population shown in
Fig. 6 is 3.4 dB.
|
In Fig. 7 we show the D values
for the NoSo and NSo at the levels of the 500-Hz tone shown by the
horizontal dashed lines in Fig. 6A. At 58 dB SPL, the
presence of the tone is signaled by increases in the discharge rate of
units near 500 Hz for both No and N
noise (Fig. 7, A and
B) with larger changes occurring for N
noise. When the
level of the tone is reduced by 6 dB to near the lowest N
So
thresholds at 52 dB SPL (Fig. 7, C and D) only
sporadic units reach a D of 1 for No noise but a population of units near 500 Hz still shows highly statistically significant (D >2) increases in discharge for the N
noise.
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DISCUSSION |
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The results of the present study may be summarized as follows:
when masked by either No or N noise, 500-Hz tones identical at the
two ears are detected at the lowest signal-to-noise ratio by neurons
with best frequencies at or near the signal frequency; N
noise is
generally less effective in activating inferior colliculus neurons than
No noise because the majority of neurons have peaks in their interaural
delay functions near zero ITD (see also Yin et al. 1983
,
1986
); for the majority of neurons, the first detectable response to So 500-Hz tones masked by N
noise is an increase in
discharge rate above that due to the N
noise alone; in the majority
of neurons, the BMLD is in a direction consistent with psychophysical
observations; So tones are more detectable in N
noise than in No
noise; and the average BMLD in neurons around 500 Hz is smaller for
NoSo versus N
So than for NoSo versus NoS
. This is consistent with
psychophysical observations.
To allow comparison of the masked thresholds in this paper (which are
given in dB SPL) with equivalent human values requires a conversion to
signal-to-noise ratio (S/N). Because the neural bandwidth is frequency
dependent, the within-channel S/N will vary. However, the main channel
of interest is at 500 Hz (the signal frequency) because this is where
the lowest masked thresholds are obtained. The bandwidths of guinea pig
auditory filters are wider than those in humans by 2-2.5 times, thus
at 500 Hz the equivalent rectangular bandwidth is about 220 Hz
(Evans et al. 1992). Integration of the noise within
this bandwidth represents an increase of 23.4 dB over the noise
spectral density of 33 dB giving 56.4 dB of noise energy within the
500-Hz channel. The lowest masked thresholds (i.e., those most likely
responsible for detection; Fig. 6) within the band at 500 Hz excluding
one outlier (2% of the sample) in each case are about 48 dB for N
So and 52 dB for NoSo, giving within-channel S/N ratios at masked threshold of
8.4 and
4.4, respectively. These are somewhat higher than typically reported human values. To be more specific: a recent report gave S/N ratios for 19 human subjects for 500-Hz tones in
wideband maskers for NoSo and NoS
conditions (Bernstein et al., 1998
). Their S/N ratios were quoted as E/No: a
measure that includes signal duration. Calculating E/No for our data
with a duration of 0.05 s gives 48 + 10 log (0.05)
33 = 2 dB for N
So and 6 dB for NoSo. The NoSo value here is 3.8 dB less
than Bernstein's 19 subjects (mean = 9.8 dB), and the N
So
value is almost 6 dB less (mean =
3.9 dB) than the NoS
. We
would expect NoS
to be slightly lower than N
So (see
INTRODUCTION), but another likely contribution to these
discrepancies is the relatively low level of the masking noise that we
have employed in this study compared with that of Bernstein et al., who
used 50-dB spectrum level. The relatively low level was chosen to
ensure that the signals remained within the dynamic range of the 500-Hz
neurons. The BMLD increases with noise level above threshold by as much
as 10 dB (e.g., Hirsh 1948a
; McFadden
1968
) and is almost maximal at 50 dB spectrum level. We have
also observed similar increases in neural BMLDs with noise level
(McAlpine et al. 1996a
, Fig. 12). As the BMLD gets
larger, the difference between the NoSo and N
So conditions increases
as a result of larger improvements in the N
So masked thresholds than
in the NoSo masked thresholds. Testing the guinea pig at a higher noise
level should therefore reduce the discrepancy with the human data.
In our previous study (Jiang et al. 1997a), we compared
the pattern of responses to NoSo versus NoS
to those predicted from a generic cross-correlation model of binaural hearing based on that
originally developed by Colburn (1973
, 1977
, 1996
) and
found that the physiological results were in good agreement with such a
model. These models include the assumption that the auditory system
uses internal delays to compensate for delays in the waveforms reaching
the ears. In Fig. 8, we compare the
output of such a model for NoSo versus NoS
with that for NoSo versus
N
So. Figure 8A is a recomputation of the figure we showed
previously (Fig. 12 in Jiang et al. 1997a
). The ordinate
represents the level of activity within a single frequency channel
centered at the signal frequency; the abscissa represents the most
effective or best delays of a population of delay-sensitive neurons.
The curve is the result of a computation of the interaural
cross-correlation using simulated spike train probabilities after
peripheral filtering at 500 Hz. Thus in Fig. 8A, the No
noise alone produces greatest activity in a population of neurons which
are most sensitive to ITDs at or near zero ITD. The BMLD for NoSo
versus NoS
results from an asymmetry in the modifications to the
responses to the No noise as a result of the So or S
tones. The No
noise generates a peak of activation in neurons with best delays close
to zero ITD, and the So tone at masked threshold produces only a small increase in the amplitude of this peak. In contrast, at the same S/N
ratio the S
signal produces a larger decrease in the peak amplitude.
Assuming increases and decreases in discharge are equally detectable,
to achieve the same detectability the S
noise can be reduced in
level giving a substantial BMLD. We subsequently demonstrated that the
reduction in the activity caused by the S
signal is consistent with
a desynchronizing effect of that signal on the responses to the No
noise (Palmer et al. 1999
).
|
In Fig. 8B we show a similar treatment for the NSo
condition. Of particular note here is the fact that N
noise produces a minimum or trough in activation centered on zero ITD. This
corresponds to the relatively poorer activation to N
noise of
neurons with peaks in their delay functions near zero. The addition of
So signal at zero ITD raises the discharge rate of neurons with best
delays near zero ITD (i.e., in the majority of neurons). This
represents a desynchronizing effect of the So noise. Addition of
identical tones to the filtered noise waveforms from each ear should
produces a shift of the instantaneous phase in opposite directions by
the same amount and thus reduces the negative cross-correlation due to
the noise and hence increases the number of coincident spikes delivered
to the coincidence detector. Again assuming equal changes in the curves
represent equal detectability changes, it is noteworthy that the So
signal produces a larger change to the N
noise response than it does
to the No noise response shown in Fig. 8A, but that this is
a smaller change than that due to the S
signal on the No noise. This
is consistent with a smaller BMLD for NoSo versus N
So than that for
NoSo versus NoS
. Note that detection of So signals in N
noise in
neurons with very long best delays is equivalent to detection of So in
No noise in neurons with short best delays. However, at least in our
data, neurons with characteristics of simple coincidence detectors, but
with very long best delays have not been found (McAlpine et al.
1996b
, Fig. 6).
We conclude from comparison of Fig. 8 with the results presented in this paper that there is good agreement between the empirically measured physiological data and those predicted from the cross-correlation model of binaural interaction.
Intuitively, one might expect that the lower the firing rate of a cell
to the noise masker alone, the lower the detection threshold for a tone
because a smaller amount of signal energy would be required to produce
a given change in firing rate. In which case, one would expect that the
So and S thresholds would be lowest for the N
masker. However,
while this is true for So tones, the lowest detection thresholds for
S
tones are with an No masker, which produces a larger firing rate
than N
. This is not a paradox because the argument misses two
important features of the situation. First, the masked firing rate is
not determined solely by the energy in the stimulus, it depends mostly
on the interaction between the phases of the signal and masker. Second, in determining detectability it is the magnitude of the change in
firing rate compared with the variability of the firing
rate, as embodied in the statistic D, which is important.
When the signal interaural phase is different from the noise interaural
phase (i.e., NoS
and N
So), the addition of a low-level signal
causes the noise masker to become desynchronized and hence, in the
majority of neurons, causes the No firing rate to decrease
significantly and the N
rate to increase significantly.
These firing rate changes are often larger than the firing rate change
caused by the additional energy added by the signal, which is the only
detection cue available in the NoSo and N
S
cases where the
interaural phases of signal and noise are the same.
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ACKNOWLEDGMENTS |
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We thank P. Moorjani for assistance with data analysis and Drs. Trevor Shackleton, Michael Akeroyd, and Mark Wallace for helpful comments on this manuscript. Special thanks also to M. Akeroyd for computation of the model results plotted in Fig. 8. Thanks also to S. Colburn and T. Yin for excellent refereeing.
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FOOTNOTES |
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Address for reprint requests: A. R. Palmer, MRC Institute of Hearing Research, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom (E-mail: arp{at}ihr.mrc.ac.uk).
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 14 July 1999; accepted in final form 17 April 2000.
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