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1.  Spatial Stream Segregation by Auditory Cortical Neurons 
The Journal of Neuroscience  2013;33(27):10986-11001.
In a complex auditory scene, a “cocktail party” for example, listeners can disentangle multiple competing sequences of sounds. A recent psychophysical study in our laboratory demonstrated a robust spatial component of stream segregation showing ∼8° acuity. Here, we recorded single- and multiple-neuron responses from the primary auditory cortex of anesthetized cats while presenting interleaved sound sequences that human listeners would experience as segregated streams. Sequences of broadband sounds alternated between pairs of locations. Neurons synchronized preferentially to sounds from one or the other location, thereby segregating competing sound sequences. Neurons favoring one source location or the other tended to aggregate within the cortex, suggestive of modular organization. The spatial acuity of stream segregation was as narrow as ∼10°, markedly sharper than the broad spatial tuning for single sources that is well known in the literature. Spatial sensitivity was sharpest among neurons having high characteristic frequencies. Neural stream segregation was predicted well by a parameter-free model that incorporated single-source spatial sensitivity and a measured forward-suppression term. We found that the forward suppression was not due to post discharge adaptation in the cortex and, therefore, must have arisen in the subcortical pathway or at the level of thalamocortical synapses. A linear-classifier analysis of single-neuron responses to rhythmic stimuli like those used in our psychophysical study yielded thresholds overlapping those of human listeners. Overall, the results indicate that the ascending auditory system does the work of segregating auditory streams, bringing them to discrete modules in the cortex for selection by top-down processes.
doi:10.1523/JNEUROSCI.1065-13.2013
PMCID: PMC3718378  PMID: 23825404
2.  Specialization for Sound Localization in Fields A1, DZ, and PAF of Cat Auditory Cortex 
Cortical deactivation studies in cats have implicated the primary auditory cortex (A1), the dorsal zone (DZ), and the posterior auditory field (PAF) in sound localization behavior, and physiological studies in anesthetized conditions have demonstrated clear differences in spatial sensitivity among those areas. We trained cats to perform two listening tasks and then we recorded from cortical neurons in off-task and in both on-task conditions during single recording sessions. The results confirmed some of the results from anesthetized conditions and revealed unexpected differences. Neurons in each field showed a variety of firing patterns, including onset-only, complex onset and long latency, and suppression or offset. A substantial minority of units showed sharpening of spatial sensitivity, particularly that of onset responses, during task performance: 44 %, 35 %, and 31 % of units in areas A1, DZ, and PAF, respectively, showed significant spatial sharpening. Field DZ was distinguished by a larger percentage of neurons responding best to near-midline locations, whereas the spatial preferences of PAF neurons were distributed more uniformly throughout the contralateral hemifield. Those directional biases also were evident in measures of the accuracy with which neural spike patterns could signal sound locations. Field DZ provided the greatest accuracy for midline locations. The location dependence of accuracy in PAF was orthogonal to that of DZ, with the greatest accuracy for lateral locations. The results suggest a view of spatial representation in the auditory cortex in which DZ exhibits an overrepresentation of the frontal areas around the midline, whereas PAF provides a more uniform representation of contralateral space, including areas behind the head. Spatial preferences of area A1 neurons were intermediate between those of DZ and PAF, sharpening as needed for localization tasks.
doi:10.1007/s10162-012-0357-9
PMCID: PMC3540280  PMID: 23180228
auditory cortex; sound localization; spatial hearing; dorsal zone; posterior auditory field; primary auditory cortex; attention
3.  Mutation in the Kv3.3 Voltage-Gated Potassium Channel Causing Spinocerebellar Ataxia 13 Disrupts Sound-Localization Mechanisms 
PLoS ONE  2013;8(10):e76749.
Normal sound localization requires precise comparisons of sound timing and pressure levels between the two ears. The primary localization cues are interaural time differences, ITD, and interaural level differences, ILD. Voltage-gated potassium channels, including Kv3.3, are highly expressed in the auditory brainstem and are thought to underlie the exquisite temporal precision and rapid spike rates that characterize brainstem binaural pathways. An autosomal dominant mutation in the gene encoding Kv3.3 has been demonstrated in a large Filipino kindred manifesting as spinocerebellar ataxia type 13 (SCA13). This kindred provides a rare opportunity to test in vivo the importance of a specific channel subunit for human hearing. Here, we demonstrate psychophysically that individuals with the mutant allele exhibit profound deficits in both ITD and ILD sensitivity, despite showing no obvious impairment in pure-tone sensitivity with either ear. Surprisingly, several individuals exhibited the auditory deficits even though they were pre-symptomatic for SCA13. We would expect that impairments of binaural processing as great as those observed in this family would result in prominent deficits in localization of sound sources and in loss of the "spatial release from masking" that aids in understanding speech in the presence of competing sounds.
doi:10.1371/journal.pone.0076749
PMCID: PMC3792041  PMID: 24116147
4.  Weighting of Spatial and Spectro-Temporal Cues for Auditory Scene Analysis by Human Listeners 
PLoS ONE  2013;8(3):e59815.
The auditory system creates a neuronal representation of the acoustic world based on spectral and temporal cues present at the listener's ears, including cues that potentially signal the locations of sounds. Discrimination of concurrent sounds from multiple sources is especially challenging. The current study is part of an effort to better understand the neuronal mechanisms governing this process, which has been termed “auditory scene analysis”. In particular, we are interested in spatial release from masking by which spatial cues can segregate signals from other competing sounds, thereby overcoming the tendency of overlapping spectra and/or common temporal envelopes to fuse signals with maskers. We studied detection of pulsed tones in free-field conditions in the presence of concurrent multi-tone non-speech maskers. In “energetic” masking conditions, in which the frequencies of maskers fell within the ±1/3-octave band containing the signal, spatial release from masking at low frequencies (∼600 Hz) was found to be about 10 dB. In contrast, negligible spatial release from energetic masking was seen at high frequencies (∼4000 Hz). We observed robust spatial release from masking in broadband “informational” masking conditions, in which listeners could confuse signal with masker even though there was no spectral overlap. Substantial spatial release was observed in conditions in which the onsets of the signal and all masker components were synchronized, and spatial release was even greater under asynchronous conditions. Spatial cues limited to high frequencies (>1500 Hz), which could have included interaural level differences and the better-ear effect, produced only limited improvement in signal detection. Substantially greater improvement was seen for low-frequency sounds, for which interaural time differences are the dominant spatial cue.
doi:10.1371/journal.pone.0059815
PMCID: PMC3602423  PMID: 23527271
5.  Microelectrode arrays fabricated using a novel hybrid microfabrication method 
Biomedical Microdevices  2012;14(1):193-205.
We present novel hybrid microfabrication methods for microelectrode arrays that combine microwire assembly, microelectromechanical systems (MEMS) manufacturing techniques and precision tool-based micromachining. This combination enables hybrid microfabrication to produce complex geometries and structures, increase material selection, and improve integration. A 32-channel shank microelectrode array was fabricated to highlight the hybrid microfabrication techniques. The electrode shank was 130 μm at its narrowest, had a 127 μm thickness and had iridium oxide electrode sites that were 25 μm in diameter with 150 μm spacing. Techniques used to fabricate this electrode include microassembly of insulated gold wires into a micromold, micromolding the microelectrode shank, post molding machining, sacrificial release of the microelectrode and electrodeposition of iridium oxide onto the microelectrode sites. Electrode site position accuracy was shown to have a standard deviation of less than 4 μm. Acute in vivo recordings with the 32-channel shank microelectrode array demonstrated comparable performance to that obtained with commercial microelectrode arrays . This new approach to microelectrode array fabrication will enable new microelectrodes, such as multi-sided arrays, drug eluding electrodes and biodegradable shanks.
doi:10.1007/s10544-011-9597-4
PMCID: PMC3289734  PMID: 21979567
Hybrid microfabrication; Microelectrode arrays; Mechanical micromachining; Neural recording; Microassembly; MEMS
6.  Selective Electrical Stimulation of the Auditory Nerve Activates a Pathway Specialized for High Temporal Acuity 
Deaf people who use cochlear implants show surprisingly poor sensitivity to the temporal fine structure of sounds. One possible reason is that conventional cochlear implants cannot activate selectively the auditory-nerve fibers having low characteristic frequencies (CFs), which, in normal hearing, phase lock to stimulus fine structure. Recently, we tested in animals an alternative mode of auditory prosthesis employing penetrating auditory-nerve electrodes that permit frequency-specific excitation in all frequency regions. We present here measures of temporal transmission through the auditory brainstem – from pulse trains presented with various auditory-nerve electrodes to phase-locked activity of neurons in the central nucleus of the inferior colliculus (ICC). On average, intraneural stimulation resulted in significant ICC phase locking at higher pulse rates (i.e., higher “limiting rates”) than did cochlear-implant stimulation. That could be attributed, however, to the larger percentage of low-CF neurons activated selectively by intraneural stimulation. Most ICC neurons with limiting rates >500 pulses per second had CFs <1.5 kHz, whereas neurons with lower limiting rates tended to have higher CFs. High limiting rates also correlated strongly with short first-spike latencies. It follows that short latencies correlated significantly with low CFs, opposite to the correlation observed with acoustical stimulation. These electrical-stimulation results reveal a high-temporal-acuity brainstem pathway characterized by low CFs, short latencies, and high-fidelity transmission of periodic stimulation. Frequency-specific stimulation of that pathway by intraneural stimulation might improve temporal acuity in human users of a future auditory prosthesis, which in turn might improve musical pitch perception and speech reception in noise.
doi:10.1523/JNEUROSCI.4949-09.2010
PMCID: PMC2828779  PMID: 20130202
Inferior colliculus; phase locking; cochlear implant; temporal acuity; latency; auditory nerve
7.  Spatial sensitivity of neurons in the anterior, posterior, and primary fields of cat auditory cortex 
Hearing research  2008;240(1-2):22-41.
We assessed the spatial-tuning properties of units in the cat’s anterior auditory field (AAF) and compared them with those observed previously in the primary (A1) and posterior auditory fields (PAF). Multi-channel, silicon-substrate probes were used to record single- and multi-unit activity from the right hemispheres of α-chloralose-anesthetized cats. Spatial tuning was assessed using broadband noise bursts that varied in azimuth or elevation. Response latencies were slightly, though significantly shorter in AAF than A1, and considerably shorter in both of those fields than in PAF. Compared to PAF, spike counts and latencies were more poorly modulated by changes in stimulus location in AAF and A1, particularly at higher sound pressure levels. Moreover, units in AAF and A1 demonstrated poorer level tolerance than units in PAF with spike rates modulated as much by changes in stimulus intensity as changes in stimulus location. Finally, spike-pattern-recognition analyses indicated that units in AAF transmitted less spatial information, on average, than did units in PAF—an observation consistent with recent evidence that PAF is necessary for sound-localization behavior, whereas AAF is not.
doi:10.1016/j.heares.2008.02.004
PMCID: PMC2515616  PMID: 18359176
8.  Partial tripolar cochlear implant stimulation: Spread of excitation and forward masking in the inferior colliculus 
Hearing research  2010;270(1-2):134-142.
This study examines patterns of neural activity in response to single biphasic electrical pulses, presented alone or following a forward masking pulse train, delivered by a cochlear implant. Recordings were made along the tonotopic axis of the central nucleus of the inferior colliculus (ICC) in ketamine/xylazine anesthetized guinea pigs. The partial tripolar electrode configuration was used, which provided a systematic way to vary the tonotopic extent of ICC activation between monopolar (broad) and tripolar (narrow) extremes while maintaining the same peak of activation. The forward masking paradigm consisted of a 200-ms masker pulse train (1017 pulses per second) followed 10 ms later by a single-pulse probe stimulus; the current fraction of the probe was set to 0 (monopolar), 1 (tripolar), or 0.5 (hybrid), and the fraction of the masker was fixed at 0.5. Forward masking tuning profiles were derived from the amount of masking current required to just suppress the activity produced by a fixed-level probe. These profiles were sharper for more focused probe configurations, approximating the pattern of neural activity elicited by single (non-masked) pulses. The result helps to bridge the gap between previous findings in animals and recent psychophysical data.
doi:10.1016/j.heares.2010.08.006
PMCID: PMC2997905  PMID: 20727397
cochlear implants; guinea pig; electrophysiology; partial-tripolar electrode configuration; forward masking; inferior colliculus
9.  Unanesthetized Auditory Cortex Exhibits Multiple Codes for Gaps in Cochlear Implant Pulse Trains 
Cochlear implant listeners receive auditory stimulation through amplitude-modulated electric pulse trains. Auditory nerve studies in animals demonstrate qualitatively different patterns of firing elicited by low versus high pulse rates, suggesting that stimulus pulse rate might influence the transmission of temporal information through the auditory pathway. We tested in awake guinea pigs the temporal acuity of auditory cortical neurons for gaps in cochlear implant pulse trains. Consistent with results using anesthetized conditions, temporal acuity improved with increasing pulse rates. Unlike the anesthetized condition, however, cortical neurons responded in the awake state to multiple distinct features of the gap-containing pulse trains, with the dominant features varying with stimulus pulse rate. Responses to the onset of the trailing pulse train (Trail-ON) provided the most sensitive gap detection at 1,017 and 4,069 pulse-per-second (pps) rates, particularly for short (25 ms) leading pulse trains. In contrast, under conditions of 254 pps rate and long (200 ms) leading pulse trains, a sizeable fraction of units demonstrated greater temporal acuity in the form of robust responses to the offsets of the leading pulse train (Lead-OFF). Finally, TONIC responses exhibited decrements in firing rate during gaps, but were rarely the most sensitive feature. Unlike results from anesthetized conditions, temporal acuity of the most sensitive units was nearly as sharp for brief as for long leading bursts. The differences in stimulus coding across pulse rates likely originate from pulse rate-dependent variations in adaptation in the auditory nerve. Two marked differences from responses to acoustic stimulation were: first, Trail-ON responses to 4,069 pps trains encoded substantially shorter gaps than have been observed with acoustic stimuli; and second, the Lead-OFF gap coding seen for <15 ms gaps in 254 pps stimuli is not seen in responses to sounds. The current results may help to explain why moderate pulse rates around 1,000 pps are favored by many cochlear implant listeners.
doi:10.1007/s10162-011-0293-0
PMCID: PMC3254721  PMID: 21969022
auditory prosthesis; pulse rate; guinea pig; temporal acuity; forward masking
10.  Unanesthetized Auditory Cortex Exhibits Multiple Codes for Gaps in Cochlear Implant Pulse Trains 
Cochlear implant listeners receive auditory stimulation through amplitude-modulated electric pulse trains. Auditory nerve studies in animals demonstrate qualitatively different patterns of firing elicited by low versus high pulse rates, suggesting that stimulus pulse rate might influence the transmission of temporal information through the auditory pathway. We tested in awake guinea pigs the temporal acuity of auditory cortical neurons for gaps in cochlear implant pulse trains. Consistent with results using anesthetized conditions, temporal acuity improved with increasing pulse rates. Unlike the anesthetized condition, however, cortical neurons responded in the awake state to multiple distinct features of the gap-containing pulse trains, with the dominant features varying with stimulus pulse rate. Responses to the onset of the trailing pulse train (Trail-ON) provided the most sensitive gap detection at 1,017 and 4,069 pulse-per-second (pps) rates, particularly for short (25 ms) leading pulse trains. In contrast, under conditions of 254 pps rate and long (200 ms) leading pulse trains, a sizeable fraction of units demonstrated greater temporal acuity in the form of robust responses to the offsets of the leading pulse train (Lead-OFF). Finally, TONIC responses exhibited decrements in firing rate during gaps, but were rarely the most sensitive feature. Unlike results from anesthetized conditions, temporal acuity of the most sensitive units was nearly as sharp for brief as for long leading bursts. The differences in stimulus coding across pulse rates likely originate from pulse rate-dependent variations in adaptation in the auditory nerve. Two marked differences from responses to acoustic stimulation were: first, Trail-ON responses to 4,069 pps trains encoded substantially shorter gaps than have been observed with acoustic stimuli; and second, the Lead-OFF gap coding seen for <15 ms gaps in 254 pps stimuli is not seen in responses to sounds. The current results may help to explain why moderate pulse rates around 1,000 pps are favored by many cochlear implant listeners.
doi:10.1007/s10162-011-0293-0
PMCID: PMC3254721  PMID: 21969022
auditory prosthesis; pulse rate; guinea pig; temporal acuity; forward masking
11.  Auditory Cortex Spatial Sensitivity Sharpens During Task Performance 
Nature neuroscience  2010;14(1):108-114.
Activity in the primary auditory cortex (A1) is known to be essential for normal sound localization behavior, yet previous studies of the spatial sensitivity of neurons in A1 have found surprisingly broad spatial tuning. We tested the hypothesis that spatial tuning sharpens when an animal engages in an auditory task. Cats performed a task that required evaluation of the locations of sounds and another that required active listening but in which sound location was irrelevant. Some 26–44% of the units recorded in A1 showed significantly sharpened spatial tuning during the behavioral tasks compared to idle conditions, with greatest sharpening during the location-relevant task. Spatial sharpening occurred on a scale of tens of seconds and could be replicated multiple times within ~1.5-hr test sessions. Sharpening resulted primarily from increased suppression of responses to sounds at least-preferred locations. That and an observed increase in latencies suggest an important role of inhibitory mechanisms.
doi:10.1038/nn.2713
PMCID: PMC3076022  PMID: 21151120
12.  COCHLEAR IMPLANT ELECTRODE CONFIGURATION EFFECTS ON ACTIVATION THRESHOLD AND TONOTOPIC SELECTIVITY 
Hearing research  2007;235(1-2):23-38.
The multichannel design of contemporary cochlear implants (CIs) is predicated on the assumption that each channel activates a relatively restricted and independent sector of the deaf auditory nerve array, just as a sound within a restricted frequency band activates a restricted region of the normal cochlea The independence of CI channels, however, is limited; and the factors that determine their independence, the relative overlap of the activity patterns that they evoke, are poorly understood. In this study, we evaluate the spread of activity evoked by cochlear implant channels by monitoring activity at 16 sites along the tonotopic axis of the guinea pig inferior colliculus (IC). “Spatial tuning curves” (STCs) measured in this way serve as an estimate of activation spread within the cochlea and the ascending auditory pathways. We contrast natural stimulation using acoustic tones with two kinds of electrical stimulation either (1) a loose fitting banded array consisting of a cylindrical silicone elastomer carrier with a linear series of ring contacts; or (2) a space-filling array consisting of a tapered silicone elastomer carrier that is designed to fit snugly into the guinea pig scala tympani with a linear series of ball contacts positioned along it Spatial tuning curves evoked by individual acoustic tones, and by activation of each contact of each array as a monopole, bipole or tripole were recorded. Several channel configurations and a wide range of electrode separations were tested for each array, and their thresholds and selectivity were estimated.
The results indicate that the tapered space-filling arrays evoked more restricted activity patterns at lower thresholds than did the banded arrays. Monopolar stimulation (one intracochlear contact activated with an extracochlear return) using either array evoked broad activation patterns that involved the entire recording array at current levels < 6dB SL, but at relatively low thresholds. Bi- and tripolar configurations of both array types evoked more restricted activity patterns, but their thresholds were higher than those of monopolar configurations. Bipolar and tripolar configurations with closely spaced contacts evoked activity patterns that were comparable to those evoked by pure tones. As the spacing of bipolar electrodes was increased (separations > 1 mm), the activity patterns became broader and evoked patterns with two distinct threshold minima, one associated with each contact.
doi:10.1016/j.heares.2007.09.013
PMCID: PMC2387102  PMID: 18037252
Cochlear implant; cochlear prosthesis; deafness; auditory nervous system; multichannel recording; auditory prosthesis; inferior colliculus
13.  Auditory Prosthesis with a Penetrating Nerve Array 
Contemporary auditory prostheses (“cochlear implants”) employ arrays of stimulating electrodes implanted in the scala tympani of the cochlea. Such arrays have been implanted in some 100,000 profoundly or severely deaf people worldwide and arguably are the most successful of present-day neural prostheses. Nevertheless, most implant users show poor understanding of speech in noisy backgrounds, poor pitch recognition, and poor spatial hearing, even when using bilateral implants. Many of these limitations can be attributed to the remote location of stimulating electrodes relative to excitable cochlear neural elements. That is, a scala tympani electrode array lies within a bony compartment filled with electrically conductive fluid. Moreover, scala tympani arrays typically do not extend to the apical turn of the cochlea in which low frequencies are represented. In the present study, we have tested in an animal model an alternative to the conventional cochlear implant: a multielectrode array implanted directly into the auditory nerve. We monitored the specificity of stimulation of the auditory pathway by recording extracellular unit activity at 32 sites along the tonotopic axis of the inferior colliculus. The results demonstrate the activation of specific auditory nerve populations throughout essentially the entire frequency range that is represented by characteristic frequencies in the inferior colliculus. Compared to conventional scala tympani stimulation, thresholds for neural excitation are as much as 50-fold lower and interference between electrodes stimulated simultaneously is markedly reduced. The results suggest that if an intraneural stimulating array were incorporated into an auditory prosthesis system for humans, it could offer substantial improvement in hearing replacement compared to contemporary cochlear implants.
doi:10.1007/s10162-007-0070-2
PMCID: PMC2538356  PMID: 17265124
auditory nerve; cat; cochlear implant; cochlear nerve; intraneural electrical stimulation; inferior colliculus
14.  Topographic Spread of Inferior Colliculus Activation in Response to Acoustic and Intracochlear Electric Stimulation 
The design of contemporary multichannel cochlear implants is predicated on the presumption that they activate multiple independent sectors of the auditory nerve array. The independence of these channels, however, is limited by the spread of activation from each intracochlear electrode across the auditory nerve array. In this study, we evaluated factors that influence intracochlear spread of activation using two types of intracochlear electrodes: (1) a clinical-type device consisting of a linear series of ring contacts positioned along a silicon elastomer carrier, and (2) a pair of visually placed (VP) ball electrodes that could be positioned independently relative to particular intracochlear structures, e.g., the spiral ganglion. Activation spread was estimated by recording multineuronal evoked activity along the cochleotopic axis of the central nucleus of the inferior colliculus (ICC). This activity was recorded using silicon-based single-shank, 16-site recording probes, which were fixed within the ICC at a depth defined by responses to acoustic tones. After deafening, electric stimuli consisting of single biphasic electric pulses were presented with each electrode type in various stimulation configurations (monopolar, bipolar, tripolar) and/or various electrode orientations (radial, off-radial, longitudinal). The results indicate that monopolar (MP) stimulation with either electrode type produced widepread excitation across the ICC. Bipolar (BP) stimulation with banded pairs of electrodes oriented longitudinally produced activation that was somewhat less broad than MP stimulation, and tripolar (TP) stimulation produced activation that was more restricted than MP or BP stimulation. Bipolar stimulation with radially oriented pairs of VP ball electrodes produced the most restricted activation. The activity patterns evoked by radial VP balls were comparable to those produced by pure tones in normal-hearing animals. Variations in distance between radially oriented VP balls had little effect on activation spread, although increases in interelectrode spacing tended to reduce thresholds. Bipolar stimulation with longitudinally oriented VP electrodes produced broad activation that tended to broaden as the separation between electrodes increased.
doi:10.1007/s10162-004-4026-5
PMCID: PMC2504547  PMID: 15492888
cochlear implant; cochlear prosthesis; deafness; auditory nervous system; multichannel recording; auditory prosthesis
15.  Cortical Responses to Cochlear Implant Stimulation: Channel Interactions  
This study examined the interactions between electrical stimuli presented through two channels of a cochlear implant. Experiments were conducted in anesthetized guinea pigs. Multiunit spike activity recorded from the auditory cortex reflected the cumulative effects of electric field interactions in the cochlea as well as any neural interactions along the ascending auditory pathway. The cochlea was stimulated electrically through a 6-electrode intracochlear array. The stimulus on each channel was a single 80-µs/phase biphasic pulse. Channel interactions were quantified as changes in the thresholds for elevation of cortical spike rates. Experimental parameters were interchannel temporal offset (0 to ±2000 µs), interelectrode cochlear spacing (1.5 or 2.25 mm), electrode configuration (monopolar, bipolar, or tripolar), and relative polarity between channels (same or inverted). In most conditions, presentation of a subthreshold pulse on one channel reduced the threshold for a pulse on a second channel. Threshold shifts were greatest for simultaneous pulses, but appreciable threshold reductions could persist for temporal offsets up to 640 µs. Channel interactions varied strongly with electrode configuration: threshold shifts increased in magnitude in the order tripolar, bipolar, monopolar. Channel interactions were greater for closer electrode spacing. The results have implications for design of speech processors for cochlear implants.
doi:10.1007/s10162-003-3057-7
PMCID: PMC2538368  PMID: 14564662
cochlear implants; auditory cortex; guinea pig; channel interaction
16.  Auditory Cortical Images of Tones and Noise Bands 
We examined the representation of stimulus center frequencies by the distribution of cortical activity. Recordings were made from the primary auditory cortex (area A1) of ketamine-anesthetized guinea pigs. Cortical images of tones and noise bands were visualized as the simultaneously recorded spike activity of neurons at 16 sites along the tonotopic gradient of cortical frequency representation. The cortical image of a pure tone showed a restricted focus of activity along the tonotopic gradient. As the stimulus frequency was increased, the location of the activation focus shifted from rostral to caudal. When cochlear activation was broadened by increasing the stimulus level or bandwidth, the cortical image broadened. An artificial neural network algorithm was used to quantify the accuracy of center-frequency representation by small populations of cortical neurons. The artificial neural network identified stimulus center frequency based on single-trial spike counts at as few as ten sites. The performance of the artificial neural network under various conditions of stimulus level and bandwidth suggests that the accuracy of representation of center frequency is largely insensitive to changes in the width of cortical images.
doi:10.1007/s101620010036
PMCID: PMC2504540  PMID: 11545145
auditory cortex; guinea pig; tonotopy; neural ensembles; functional imaging
17.  Location Coding by Opponent Neural Populations in the Auditory Cortex 
PLoS Biology  2005;3(3):e78.
Although the auditory cortex plays a necessary role in sound localization, physiological investigations in the cortex reveal inhomogeneous sampling of auditory space that is difficult to reconcile with localization behavior under the assumption of local spatial coding. Most neurons respond maximally to sounds located far to the left or right side, with few neurons tuned to the frontal midline. Paradoxically, psychophysical studies show optimal spatial acuity across the frontal midline. In this paper, we revisit the problem of inhomogeneous spatial sampling in three fields of cat auditory cortex. In each field, we confirm that neural responses tend to be greatest for lateral positions, but show the greatest modulation for near-midline source locations. Moreover, identification of source locations based on cortical responses shows sharp discrimination of left from right but relatively inaccurate discrimination of locations within each half of space. Motivated by these findings, we explore an opponent-process theory in which sound-source locations are represented by differences in the activity of two broadly tuned channels formed by contra- and ipsilaterally preferring neurons. Finally, we demonstrate a simple model, based on spike-count differences across cortical populations, that provides bias-free, level-invariant localization—and thus also a solution to the “binding problem” of associating spatial information with other nonspatial attributes of sounds.
A model relying on properties of auditory cortical neurons recorded in the cat can account for the accurate localization of sounds
doi:10.1371/journal.pbio.0030078
PMCID: PMC1044834  PMID: 15736980

Results 1-17 (17)