February 14, 1999
Abstract #88, Date 2/14/99, Session E3, Poster (J56)
Role of apparent signal movement in reducing masking
in a sound field
*P.S. Deliwala, G. Kidd, Jr. (Boston University,
Boston, MA)
This study examined the effect of apparent
movement on signal detection. Signal thresholds were
measured in quiet and in the presence of an energetic
and an informational masker in a quiet but
non-anechoic room. The sounds were presented via 7
speakers separated by 30 degrees, arranged in a
semicircle around the subject. Thresholds were
measured for 500-Hz and 3000-Hz signals using a 2AFC
task.
A multiple-burst paradigm was used in which the
signal and the masker were gated on/off synchronously
in a sequence of 7 bursts. The informational masker
was comprised of 7 tones, the frequencies, phases and
levels were chosen randomly on every interval. Masker
tones were chosen from a frequency range 3-octaves
wide centered around the signal frequency. In order to
minimize "energy-based" masking masker tones were not
selected from a region near the signal frequency. The
7-tone masker was presented by sending one tone to
each of the 7 speakers. Each speaker played the same
tone on every burst throughout the sequence. The
energetic masker was a Gaussian noise 3-octaves wide
and centered around the signal frequency. A different
sample of the noise was played from each of the 7
speakers. Each speaker played the same sample on every
burst throughout the sequence. The signal was
presented in 2 spatial patterns: stationary and
moving. In the stationary condition the signal was
presented from one of the 7 speakers chosen randomly
on every presentation. In the moving condition the
signal location was varied - starting from the right
and moving to the left or vice versa - during the
burst sequence. These two signal conditions were
presented as 2 interleaved adaptive tracks. The task
of the listener was to identify the interval that had
the signal.
The results for 4 subjects show that signal
movement did not significantly improve detection in
quiet. However, signal movement improved detection in
the presence of the energetic masker by 4 dB and in
the presence of the informational masker by 12 dB, on
average. Results suggest that apparent movement is
more effective in reducing informational masking than
energetic masking.
Supported by a Dudley Allen Sargent grant from Boston
University and the ONR/MURI Z883402 and NIH/NIDCD
grants DC03281 and DC00100
Abstract #103, Date 2/14/99, Session E4, Poster (B71)
Localization of near-field sources in a reverberant
room
S.G. Santarelli, N. Kopco, *B.G. Shinn-Cunningham
(Boston University Department of Cognitive and Neural
Systems)
Subjects were asked to point to the position of
sound sources within one meter of the head while
seated in a medium-sized, echoic classroom. These
localization results are compared to results from a
previous study that used an identical procedure (but a
different set of subjects) in anechoic space. Overall,
localization in the reverberant room is worse than
observed in the anechoic conditions (a surprising
result, given that, for sources near the head, the
reverberant energy is weak relative to the direct
sound reaching the ears).
In order to understand these results more fully,
gross interaural time difference (ITD) and interaural
level difference (ILD) cues were estimated for
near-field sources in anechoic space using a simple
model. This analysis is especially interesting since
iso-ITD and iso-ILD surfaces in the near field differ
from the "cone-of-confusion" (iso-binaural cue)
surface that occurs for sources relatively far from
the head. To analyze response errors for each subject,
the estimated ITD and ILD values are found for both
source and response positions. From this analysis,
localization errors are estimated in units of binaural
cues (ITD and ILD) and non-binaural cues. This
analysis implies that reverberation does not affect
interaural time and interaural intensity localization
errors in the same way. It also appears that there are
consistent subject differences in both binaural
abilities and in non-binaural localization abilities,
and that these differences capture much of the
intersubject variability observed.
Supported by AFOSR grant F49620-98-1-0108
Abstract #105, Date 2/14/99, Session E4, Poster (B73)
Investigation of the relationship between three common
measures of precedence: Fusion, localization
dominance, and discrimination suppression
*R.Y. Litovsky, B.G. Shinn-Cunningham, A.D. Stein,
D.A. Singer (Boston University)
In reverberant environments listeners have a
remarkable ability to localize and identify sound
sources. The precedence effect (PE) refers to a group
of auditory phenomena that are thought to be related
to this ability. Traditionally, three measures have
been used to quantify the PE: (1) Fusion; at short
delays (1-5 ms for clicks) the lead and lag are
perceptually fused into one auditory event. (2)
Localization dominance; the leading source dominates
the perceived location of the fused image. (3)
Discrimination Suppression: at short delays, changes
in the location or interaural parameters of the lag
are difficult to discriminate compared with change in
the lead. Little is known about the relation between
these aspects of the PE, since they are rarely studied
in the same listeners. In the present study, extensive
measurements of these phenomena were made in six
normal-hearing listeners. Results from all three
measures are consistent with a simple model in which
lead and lag information interacts perceptually and
the strength of the interaction decreases with
spatiotemporal separation of the lead and lag. At
short delays, lead and lag both contribute to spatial
perception, but the lead dominates (to the extent that
only one position is even heard). At intermediate
delays, two spatial locations are and reverse traveling waves have similar
delays for frequencies below CF.
Supported by NSF and NIDCD
Abstract #106, Date 2/14/99, Session E4, Poster (B74)
The effect of expected and unexpected auditory signal location on
identification performance in a multi-source environment
*T.L. Arbogast, G. Kidd, Jr. (Boston University, Boston, MA)
Auditory spatial attention is one mechanism which may contribute to a
listener's ability to identify a particular sound source in a multi-source
environment. The role auditory spatial attention plays in such an
environment was investigated using a variation of the probe-signal method
(Greenberg and Larkin, 1968, J. Acoust. Soc. Am. 44:1513). The experiment
took place in a quiet room with 7 speakers at 30 degree intervals,
arranged in a semi-circle in front of the listener. The signal was
comprised of 8 contiguous, 60-ms pure-tone bursts arranged in either a
rising or falling frequency pattern. The maskers were also comprised of 8
contiguous pure-tone bursts but with durations which varied randomly from
20 to 100 ms. The six maskers were played simultaneously with the signal
and were constructed to produce informational, rather than energetic
masking. For each masker, frequencies were chosen randomly on each burst
from a narrow frequency band, independent from the signal frequency band.
The task was 1I-2AFC fixed-level identification. The listener was
instructed to focus attention on a speaker specified by the experimenter
(expected location) and to identify the signal as quickly as possible, but
without sacrificing accuracy. Percent correct identification and response
time were measured in two conditions: 1) the signal presented at the
expected location in 100% of trials; and 2) the signal presented at the
expected location in 75% of trials (Signal trials), and at one of the
remaining speakers, chosen randomly, in 25% of trials (Probe trials).
Accuracy and response time were compared for expected and unexpected
signal location at each speaker. Mean data indicate significantly better
identification performance and faster response times for expected signal
location, than for unexpected signal location. This suggests that
listeners can tune auditory spatial attention to aid in signal perception
in a multi-source environment.
Supported by the Dudley Allen Sargent Research Fund from Boston
University, ONR/MURI grant Z883402, and NIH/NIDCD grants DC03281 and
DC00100
Abstract #110, Date 2/14/99, Session E4, Poster (B78)
Discriminating harmonicity
*G. Kidd, Jr., C.Y.P. Chiu, C.R. Mason (Boston University, Boston, MA)
Simultaneous tones that are harmonically related tend to be grouped
perceptually to form a unitary auditory image. A single partial that is
mistuned stands out from the other tones, and two harmonic complexes with
different fundamental frequencies can readily be perceived as separate
auditory objects. These phenomena are often cited as evidence for the
strong role of 'harmonicity' in perceptual grouping and segregation of
sounds. This study was an attempt to measure the discriminability of
harmonicity. Here, the degree of harmonicity refers to how near components
fall to expected harmonic intervals. In a 2I2AFC paradigm, the listener's
task was to choose the most harmonic sound. Signals and foils were
comprised of eleven tones with the lowest component's expected frequency,
Fl, chosen randomly on every trial from 200 to 480 Hz.
In the first experiment, the signal was varied from perfectly harmonic
(all eleven tones at the expected integer multiples of Fl) to less
harmonic by adding frequency jitter to each component (10% of Fl to 40% of
Fl). The components in the foils always had the maximum jitter (50% of Fl,
referred to as 100% jitter). Performance decreased from near perfect for
0% jitter to near chance for 80% jitter and was influenced by Fl. The
decline was not abrupt: for example, with 40% jitter, listeners were still
80% correct on average.
In the second experiment, a masker was presented simultaneously with
signals (0% jitter) and foils. The maskers were comprised of 4, 8 or 12
tones having frequencies that were randomly chosen on each presentation
from 100 to 5520 Hz. In addition to a monaural condition (SmMm), a
dichotic condition (SmMo) was tested. Adding a masker significantly
interfered with discrimination. Dichotic presentation of these audible
stimuli improved performance by 5-10 dB.
The results of these experiments suggest that the current approach is a
useful way to measure the discriminability of an acoustic variable thought
to underlie a perceptual property important in grouping and segregation.
Supported by ONR/MURI Z883402, NIH/NIDCD DC03281 and DC00100
Abstract #118, Date 2/14/99, Session E4, Poster (B86)
Failure to train away the precedence effect
*R.Y. Litovsky, M.L. Hawley, B.F. Fligor (Boston University); P.M. Zurek
(M.I.T.)
In reverberant environments listeners are able to localize and identify a
sound source, regardless of an abundance of reflections. This ability is
often studied using the precedence effect. Under headphones, stimuli
commonly consist of two pairs of clicks, each with a given ITD. Their
onset is separated by several msec to represent a direct sound and a
reflection. A common finding is that with short separations between the
two click pairs (1-3 msec) just-noticeable differences (JNDs) in the ITD
of the lagging stimulus (lag JND) are significantly higher than JNDs for
the leading stimulus (lead JND). This difference in ITD JND is thought to
reflect the fact that the auditory system attributes greater perceptual
weight to directional information contained in the leading source, and
reduced weight to information contained in the lagging source. It has been
suggested [Saberi and Perrott, J. Acoust. Soc. Amer., 87, 1732-1737, 1990]
that listeners can train away high lag JNDs by listening to the task for
12-20 hours. Saberi and Perrott reported that after training with a delay
of 2.35 msec, lag JNDs were as good as lead JNDs for all delays tested
(0.38 to 10 msec), and thus suggested that precedence may not be "hard
wired". Given the importance of this learning effect for understanding the
precedence effect, we sought to replicate it.
Eight subjects were trained using similar procedures to those of Saberi
and Perrott. One group was trained for 12-22 hours using a method of
constant stimuli to vary ITD. A second group was trained for 20 to 31hours
using an adaptive procedure to vary ITD. Results suggest that none of our
listeners showed sign of training away precedence. After 12- 31 hours of
listening, all subjects maintained high lag JNDs and low lead JNDs. Our
failure to train away the precedence effect suggests that directional
information contained in the lagging source is not always easily accessed,
and that learning to utilize that information may depend either on
particular experimental procedures that are yet to be identified, or on
subject differences. We currently have no explanation for the discrepancy
between our results and those of Saberi and Perrott.
Supported by NIH and ONR
Abstract #203, Date 2/14/99, Session E9, Poster (B171)
EarLab: A virtual hearing laboratory
*D.C. Mountain, H.M. Hoffman, V. Vajda, S.G. Deligeorges, A.E. Hubbard
(Boston University Hearing Research Center)
EarLab is a web-based resource that combines an on-line data warehouse
containing a wide variety of auditory information with a set of integrated
computational tools for the manipulation and visualization of this data.
The long range goal for EarLab is to increase understanding of the hearing
process through improved collaboration between engineers, biologists and
behavioral scientists.
A major design objective for EarLab is to create an environment that will
enable researchers from all disciplines who work on hearing to share and
compare their results electronically. The intent is not to duplicate
existing methods for publishing scientific results, but rather to augment
these methods by providing tools to compare published and unpublished data
from different laboratories, species, and techniques as well as to provide
a means to disseminate and integrate large data sets that can only be
summarized in conventional publications. Additional design objectives are
to provide online computational models that can be run over the web and to
integrate EarLab's auditory database system with other bioinformatic and
neuroinformatic resources on the world wide web. Examples of these
resources include anatomical, bibliographic, genetic, molecular, and Human
Brain Project web sites.
Demonstrations of EarLab in action will presented along with some examples
of research projects using EarLab.
Supported by ONR, NIDCD, and the Boston University Hearing Research Center
February 15, 1999
Abstract #252, Date 2/15/99, Session J, Symposium , 7:45p
Binaural processing of speech by hearing impaired listeners
*H.S. Colburn, M.L. Hawley (Hearing Research Center and Biomedical
Engineering, Boston University)
Binaural processing is known to improve speech intelligibility in noisy
environments and to aid in localization of sounds, at least for listeners
with normal hearing. The abilities of listeners with hearing impairments
to take advantage of binaural processing in these circumstances have been
studied much less, and the picture is not completely clear. This
presentation reviews what we know about these abilities and attempts to
relate performance in these tasks to binaural abilities in isolated tests
of binaural interaction such as interaural time discrimination. As a part
of this analysis, abilities in intelligibility and localization that are
possible with monaural listening, bilateral listening (two ears used
separately), and true binaural listening are described. Zurek [1993, chap.
in "Acoustical factors affecting hearing aid performance", Allyn and
Bacon] has studied the effect of separating a target and a single masker
on the intelligibility threshold for speech masked by noise. He showed
that the improvement with separation comes from two sources, the head
shadow effect that leads to improved signal-to-noise ratio at the more
favorably located ear and the "binaural squelch effect" that comes from
binaural interaction. With a single masker and target, the head shadow
effect for intelligibility threshold can be up to 15 dB and the binaural
squelch effect is about 2 to 4 dB. Since the intelligibility of sentence
material improves by 10-15% for each decibel of increase in
signal-to-noise ratio, small decibel improvements can lead to large
intelligibility improvements. In recent studies using multiple competing
sources, Hawley et al. [1998, JASA 103(5 Pt. 2):3051] showed that even
though overall thresholds were elevated for the listeners with hearing
impairments, the overall advantage of separating the location of the
target and competing sources is similar for listeners with normal and
impaired hearing. In localization studies, good ability requires the
comparisons of signals at each ear in order to separate stimulus
characteristics from source location characteristics. The cues that are
used for localization are different depending on the situation, with
interaural time and intensity cues dominating in the horizontal plane and
spectral cues
dominating in the vertical plane. The abilities of listeners to perform
these tasks depend on a variety of factors, including their hearing
losses, their binaural abilities, the hearing aids that they wear, and the
nature of the interference.
Supported by National Institutes of Deafness and Other Communicative
Disorders: DC00100
Abstract #331, Date 2/15/99, Session K4, Poster (B78)
Validating the measured stiffness map: A biophysical model
*R.C. Naidu, D.C. Mountain (Boston University Hearing Research Center,
Boston, MA)
Recent experimental evidence indicates that: 1) the cochlear stiffness
gradient is a 100 times smaller than that assumed by most cochlear models
(Naidu and Mountain, Hear.Res., 124, 1998); and 2) the organ of Corti (OC)
is longitudinally coupled (Naidu and Mountain, ARO 1998, 21:718). This
study investigates the effect of incorporating this lower stiffness
gradient and coupling within the OC on the results generated by a passive
biophysically based long wave model.
The model is a modified 1-D transmission line model of the gerbil cochlea
with the displacement of each section dependent on that of its neighbours
due to longitudinal coupling. Distributed representations of the stapes
and round window are included. Almost all of the parameters are based
directly on available physiological and anatomical data.
Model results were compared to experimental data obtained at high SPL. In
the frequency range over which the long wave assumption is valid, the
model correctly predicts both the magnitude and phase of basilar membrane
velocity (Xue et al, JASA, 97, 1995) and intra-cochlear pressure (Olson,
JASA, 103, 1998). Further, the good agreement between the predicted and
experimentally estimated travel times (Schmeidt and Zwislocki, JASA, 61,
1977) to several locations along the cochlea validates the range of the
experimentally determined stiffness gradient incorporated in this model.
Longitudinal coupling did not affect the passive velocity response of the
OC although in a pseudo-active situation (achieved by reducing the damping
of each resonant section), the peak of the traveling wave was broadened.
In the pseudo-active model, there was also an increased accumulation of
phase around the characteristic frequency. The former result is consistent
with the conclusions of Wickesberg and Geisler (In: Peripheral Auditory
Mechanisms, Springer-Verlag, 1985).
Supported by NIDCD
Abstract #332, Date 2/15/99, Session K4, Poster (B79)
Experimental evidence does not support feed-forward outer hair cell forces
*K.D. Karavitaki (Massachusetts Institute of Technology, Cambridge, MA and
Boston University Hearing Research Center, Boston, MA ); D.C. Mountain
(Boston University Hearing Research Center, Boston, MA)
Recent models (Steele et al., in Biophysics of Hair Cell Sensory Systems,
world Scientific, 1993, pp. 207-215, Geisler and Sang, Hearing Res., 1995,
pp. 132-146) refer to the outer hair cells (OHC) as being part of a
'feed-forward' system where forces produced at the basolateral end of the
cells cause hairbundle deflections at a more apical region. This
hypothesis is based on anatomical observations (Voldrich, in Mechanics of
Hearing, Delft University Press, 1983, pp. 163-167) showing that the OHC
are situated in an oblique angle with respect to the longitudinal axis
running from the base to the apex of the cochlea. We investigated this
hypothesis by estimating the radial and longitudinal motion of OHC in an
excised gerbil cochlea. If the 'feed-forward' hypothesis is correct we
expect that the longitudinal motion would be comparable to the radial
motion.
Data for this study were collected from the apical turn of excised gerbil
cochleas using a video stroboscopy system. AC current was delivered
between the scala vestibuli of the apical turn and scala tympani of the
second turn at frequencies of 20 to 4 kHz. Electrically-evoked motion was
estimated using a 2-dimensional cross correlation technique. Plots of the
frequency response of both the radial and the longitudinal components of
motion were generated.
The experimental results indicate that the longitudinal component of OHC
motion in the apex is about ten times smaller than the radial component.
These results are inconsistent with the prediction from the 'feed-forward'
hypothesis.
Supported by NIDCD
Abstract #333, Date 2/15/99, Session K4, Poster (B80)
Forward and reverse traveling waves in the cochlea
*H.H. Nakajima, R.C. Naidu, A.E. Hubbard, D.C. Mountain (Boston Univ.
Hearing Research Center, Boston, MA)
Classical cochlear models predict that forward and reverse traveling waves
have the same velocity at a given location. Therefore the delay for a wave
propagating from the tympanic membrane to a fixed point along the cochlear
partition should be the same as the delay for a wave originating from that
point and propagating back to the tympanic membrane.
To test this prediction, we estimated the forward traveling wave delay
from the cochlear microphonic (CM) phase and the reverse traveling wave
delay from the electrically-evoked otoacoustic emission (EEOE) phase. The
group delays of this system were obtained from the slopes of the phase
curves. A glass microelectrode with a 2 to 3 micrometer tip was
introduced into the scala media at different locations along the cochlea.
This electrode was used to record the CM and to stimulate with current to
produce EEOEs in the same cochlea. The characteristic frequency (CF) of
the electrode location was estimated by summating potential and CM
recordings. The location of the electrode was determined from digitized
images of the cochlea. Our estimate of the CF of the electrode location
was consistent with the gerbil frequency-place map obtained by Muller
(1996, Hear. Res. 94, 148-156).
We calculated group delays for frequencies below and above the CF of the
electrode location. Above CF, the EEOE group delay decreases while the CM
group delay remains approximately the same. Because the CM and EEOE are
distributed responses, they may not accurately reflect basilar membrane
motion for frequencies near or above CF. For frequencies below CF, we
found that the EEOE and CM group delays are approximately the same.
Therefore we conclude that the forward and reverse traveling waves have
similar delays for frequencies below CF.
Supported by NSF and NIDCD
February 16, 1999
Abstract #502, Date 2/16/99, Session R1, Poster (J2)
Intralabyrinthine infusion of BrdU potentiates
gentamicin damage in the chick auditory epithelium
*D.W. Roberson (Boston Children's Hospital and Harvard
Department of Otology and Laryngology); E.P. Messana,
J.A. Alosi (Boston Children's Hospital); D.A. Cotanche
(Boston Children's Hospital and Harvard Department of
Otology and Laryngology)
A number of laboratories have used systems to
continuously infuse the inner ear with various
substances (Roberson et al., Auditory Neuroscience,
2:195; Rubel et al., Science, 267:701). Previous work
has shown that intralabyrinthine infusion of
trititated thymidine does not itself cause hair cell
death in the chick. This study was designed to
determine whether infusion of BrdU alters the effect
of gentamicin toxicity on the chick cochlea.
Nine chicks had a pump implanted which
continuously infused the left inner ear with 0.5
microliters/hour of BrdU (20 mg/ml, in saline). Ten
chicks did not have pumps placed. Two days after pump
placement, half the chicks in each group were given a
single dose of 200 mg/kg of gentamicin; the other half
were given 300 mg/kg of gentamicin. Four days after
gentamicin injection, all chicks were sacrificed and
all ears harvested. Cochleae were stained with
phalloidin and examined with immunofluorescent
microscopy. Extent of total and partial hair cell loss
was expressed as a percent of the length of the
cochlea.
In both pump ears and non-pump ears 300 mg/kg of
gentamicin caused greater damage than 200 mg/kg.
Within each dosage group, however, damage in the right
ear was similar regardless of whether the left ear had
pump placement or not. Therefore data from all ears
which did not have pump placement (right ears of pump
animals, and both ears of non-pump animals) were
pooled.
In chicks injected with 200 mg/kg of gentamicin,
damage was greater in the pump ears (N = 5; total HC
loss = 43 7%, partial HC loss = 55 7%) than in the
non-pump ears (N = 13; total HC loss = 22 5%;
partial HC loss = 31 6%).
In chicks injected with 300 mg/kg of gentamicin,
damage was also greater in the pump ears (N = 4; total
HC loss = 63 26%, partial HC loss = 76 17%) than
in the non-pump ears (N = 13; total HC loss = 32 4%;
partial HC loss = 43 5%).
Continuous intralabyrinthine infusion of BrdU in
saline potentiates hair cell loss due to systemic
gentamicin injection. Whether this potentiation is due
to BrdU toxicity or to perilymph disruption from the
saline infusion is unclear.
NIDCD DC-01689, NIDCD DC-00152, The National
Organization for Hearing Research & The Children's
Hospital Otolaryngology Foundation Research Fund
Abstract #505, Date 2/16/99, Session R1, Poster (J5)
Hair cell loss and supporting cell rearrangement in
the chick cochlea after gentamicin treatment
*S. Goklani (WT Clarke High School, Westbury, LI, NY);
E.P. Messana (Children's Hospital); D.A. Cotanche
(Children's Hospital and Harvard Medical School)
Many studies have used aminoglycoside treatment
to induce hair cell regeneration in the avian cochlea.
While details of these studies have differed, the
basic result has generally been the same, i.e. loss of
both tall and short hair cells from the proximal (high
frequency) end of the cochlea and a gradual
replacement of these missing cells by regeneration.
SEM and LM studies of damaged cochleae have led to the
hypothesis that the dying hair cells were ejected up
into the scala media and overlying tectorial membrane.
Thus, the surviving cells in the damaged region were
thought to be almost exclusively supporting cells.
However, it has recently been proposed that the hair
cells eject only their sensory bundles in response to
aminoglycosides and that the remaining portion of the
hair cell rebuilds a new bundle in order to regain its
functional capacity. We have set out to test this idea
in the two-week-old chick cochlea damaged by a single
injection of gentamicin at 300 mg/kg. Whole-mount
preparations of the cochlea were made at 3, 4, and 5
days after the gentamicin injection and were labeled
with phalloidin to detect the actin cytoskeleton,
pan-cadherin antibodies to outline the cells apical
junctional complexes, and myosin VIIa antibodies
(kindly provided by Dr. Tama Hasson, Yale University)
to specifically identify hair cells. On day 3 all of
the hair cells were ejected from ts on the behavior of models with
linear somas. This is motivated by a key finding in
Agmon-Snir et al., that soma excitation is facilitated
when synaptic inputs occur coincidentally on distinct
four-compartment dendrites compared to when they occur
directly on a single dendrite. This behavior is also
observed when inputs are on simplified,
single-compartment dendrites. Our analyses are used to
evaluate the effects and importance of the number and
strength of synaptic inputs, the number of
compartments, and dendritic length. Dendritic lengths
are intriguing in NL neurons because of the
observations (Smith and Rubel, 1979) that the lengths
of dendrites are adjusted during development to
progress from long to short as the characteristic
frequencies of cells progress from low to high
frequency. The number and strength of inputs are
especially interesting in the analysis because varying
these parameters leads to an enhancement or reduction
of the advantage of binaural over monaural coincidence
detection. Results from models with linear and
nonlinear cell bodies will be compared with in vitro
brain-slice experiments from the chick (Reyes et al.,
1996). Specifically, the empirical results obtained by
applying current-clamped and conductance-clamped
inputs at the soma will be related to modeling results
where inputs are placed at the soma and/or dendrites.
Supported by NIDCD and ONR.
Abstract #571, Date 2/16/99, Session R3, Poster (B71)
Simple models of nucleus laminaris neuron
*V.K. Dasika (Hearing Research Center and Biomedical Engineering,
Boston University); J.A. White (Biomedical Engineering, Boston
University); H.S. Colburn (Hearing Research Center and Biomedical
Engineering, Boston University)
Physiologically abstracted and simplified models of nucleus laminaris
(NL) neurons are described and tested. These models are closely related
to models of MSO neurons (e.g., Han and Colburn, Hearing Research
1993: Brughera et al., Auditory Neuroscience, 1996) and to general models
of coincidence detection (Agmon-Snir et al., Nature 1998). The underlying
goal of this study is to use simple models to isolate the sources of various
effects that are observed physiologically and in complex models. We
study the effects of dendrites on the behavior of models with linear somas.
This is motivated by a key finding in Agmon-Snir et al., that soma
excitation is facilitated when synaptic inputs occur coincidentally on
distinct four-compartment dendrites compared to when they occur directly
on a single dendrite. This behavior is also observed when inputs are on
simplified, single-compartment dendrites. Our analyses are used to
evaluate the effects and importance of the number and strength of
synaptic inputs, the number of compartments, and dendritic length.
Dendritic lengths are intriguing in NL neurons because of the observations
(Smith and Rubel, 1979) that the lengths of dendrites are adjusted during
development to progress from long to short as the characteristic
frequencies of cells progress from low to high frequency. The number and
strength of inputs are especially interesting in the analysis because
varying these parameters leads to an enhancement or reduction of the
advantage of binaural over monaural coincidence detection. Results from
models with linear and nonlinear cell bodies will be compared with in vitro
brain-slice experiments from the chick (Reyes et al., 1996). Specifically,
the empirical results obtained by applying current-clamped and
conductance-clamped inputs at the soma will be related to modeling
results where inputs are placed at the soma and/or dendrites.
Supported by NIDCD and ONR.
February 17, 1999
Abstract #847, Date 2/17/99, Session Y5, Poster (B112)
Performance limits for frequency and intensity
discrimination based on a computational auditory-nerve
model
*M.G. Heinz (Speech and Hearing Sciences Program, MIT,
Cambridge, MA); L.H. Carney, H.S. Colburn (Hearing
Research Center and Department of Biomedical
Engineering,Boston University, Boston, MA)
Limits on psychophysical performance for
intensity and frequency discrimination have been
previously evaluated using optimal decision theory
(ODT) (Siebert, 1968, in: Recognizing Patterns, and
1970, IEEE); however, the use of an analytical AN
model limited the parameter space over which
predictions were valid. Heinz et al. (ARO 1998)
combined ODT with a computational AN model to evaluate
frequency discrimination over a wide parameter space.
This analysis has been extended to include intensity
discrimination and random-amplitude frequency
discrimination.
Our AN model included linear gamma-tone filters,
saturating rate-level curves, low-threshold
high-spontaneous-rate fibers, rolloff in
phase-locking, neural adaptation, and realistic onsets
and offsets. Independent Poisson processes were used
to model 30,000 AN fibers spaced according to a human
cochlear map. Performance limits based on AN discharge
times (all-information) were compared to limits based
on discharge counts (rate-place).
Rate-place predictions for frequency
discrimination were not consistent with human
performance ({uc delta}f) versus frequency at high
frequencies. However, all-information {uc delta}f was
smaller than and matched the shape of human {uc
delta}f versus frequency up to at least 8 kHz.
All-information {uc delta}f improved much too rapidly
versus duration, and continued to improve with level
above 30 dB SPL, unlike humans. Intensity
discrimination ({uc delta}L) based on rate-place was
within a factor of two of all-information {uc delta}L.
Both schemes were roughly consistent with human {uc
delta}L, i.e., "near-miss" to Weber's Law, shallow
slope versus duration, and flat with frequency.
Random-amplitude variation did not affect optimal
frequency discrimination based on either
all-information or rate-place.
There is more than sufficient rate-place
information to explain human performance in all tasks
studied. However, our results suggest temporal
information is used for frequency discrimination at
all frequencies, but with restrictions in the temporal
extent of comparisons between AN discharges. Such a
decision process is physiologically realistic and
could likely account for both frequency and intensity
discrimination.
Supported by NIH Grant T32DC00038 and ONR Grant Agmt
#Z883402
Abstract #848, Date 2/17/99, Session Y5, Poster (B113)
Spatiotemporal coding of sound level: Quantifying the
information provided by level-dependent phase cues
*L.H. Carney (Hearing Research Center & Dept. of
Biomedical Engineering, Boston Univ., Boston, MA);
M.G. Heinz (Speech and Hearing Sciences Program, MIT,
Cambridge, MA); H.S. Colburn (Hearing Research Center
& Dept. of Biomedical Engineering, Boston Univ.,
Boston, MA)
This study represents an extension of previous
work related to understanding the significance of
auditory-nerve (AN) response properties in level
coding (Siebert, 1968, in: Recognizing Patterns, MIT
Press; Colburn, 1981, RLE Report, MIT). This study
focused on changes in the phase of phase-locked action
potentials as a function of level and how these
changes in phase may contribute to the encoding of
level. Most previous studies of level coding based on
the properties of AN discharges have focussed on
discharge rates and synchronization of discharges to
the stimulus waveform. These studies typically have
not included the nonlinear tuning of the auditory
periphery and the concomitant effects on the timing of
phaselocked AN responses (Anderson et al., 1971, JASA
49:1131-1139) associated with the compressive
nonlinearity. These nonlinear changes in phase vary
with characteristic frequency (CF) and thus introduce
level-dependent cues in the relative timing across AN
fibers. These cues can be detected by comparing
discharge times across fibers with different CFs.
A simple analytical model for AN fibers included
rate- and synchrony-level functions, a simple
description of the changes in phase with level, and
Poisson statistics. The available information from
nonlinear phase changes was quantified to allow their
potential contribution to level discrimination to be
directly compared to the contributions of rate and
synchrony measures. Changes in the phase of AN
responses provide cues for level discrimination that
are comparable or greater than those provided by
discharge rate and synchrony, and the phase cues
persist over a wider dynamic range. Heinz et al. (ARO
1998, 1999) have shown that temporal cues can
contribute significantly over a wide frequency range.
Evaluating the potential significance of the nonlinear
response properties of AN fibers associated with the
compressive nonlinearity is important for
understanding neural mechanisms involved in level
coding. The compressive nonlinearity, which is
associated with the cochlear amplifier, is a fragile
property of the ear that is believed to be affected in
common forms of hearing impairment.
Supported by: NSF Grant IBN9601215 (LHC), and NIH
Grant T32DC00038 (MGH)