Temporal response features of cat auditory cortex neurons contributing to sensitivity to tones delivered in the presence of continuous noise

Adaptation in auditory processing

Adaptation is an essential feature of auditory neurons, which reduces their responses to unchanging and recurring sounds and allows their response properties to be matched to the constantly changing statistics of sounds that reach the ears. As a consequence, processing in the auditory system highlights novel or unpredictable sounds and produces an efficient representation of the vast range of sounds that animals can perceive by continually adjusting the sensitivity and, to a lesser extent, the tuning properties of neurons to the most commonly encountered stimulus values. Together with attentional modulation, adaptation to sound statistics also helps to generate neural representations of sound that are tolerant to background noise and therefore plays a vital role in auditory scene analysis. In this review, we consider the diverse forms of adaptation that are found in the auditory system in terms of the processing levels at which they arise, the underlying neural mechanisms, and their impact on neural coding and perception. We also ask what the dynamics of adaptation, which can occur over multiple timescales, reveal about the statistical properties of the environment. Finally, we examine how adaptation to sound statistics is influenced by learning and experience and changes as a result of aging and hearing loss.

Selective corticofugal modulation on sound processing in auditory thalamus of awake marmosets

Cortical feedback has long been considered crucial for the modulation of sensory perception and recognition. However, previous studies have shown varying modulatory effects of the primary auditory cortex (A1) on the auditory response of subcortical neurons, which complicate interpretations regarding the function of A1 in sound perception and recognition. This has been further complicated by studies conducted under different brain states. In the current study, we used cryo-inactivation in A1 to examine the role of corticothalamic feedback on medial geniculate body (MGB) neurons in awake marmosets. The primary effects of A1 inactivation were a frequency-specific decrease in the auditory response of most MGB neurons coupled with an increased spontaneous firing rate, which together resulted in a decrease in the signal-to-noise ratio. In addition, we report for the first time that A1 robustly modulated the long-lasting sustained response of MGB neurons, which changed the frequency tuning after A1 inactivation, e.g. some neurons are sharper with corticofugal feedback and some get broader. Taken together, our results demonstrate that corticothalamic modulation in awake marmosets serves to enhance sensory processing in a manner similar to center-surround models proposed in visual and somatosensory systems, a finding which supports common principles of corticothalamic processing across sensory systems.

[Effects of background noise on auditory response characteristics of primary auditory cortex neurons in awake mice]

OBJECTIVE

To study the effects of different continuous background noises on auditory response characteristics of primary auditory cortex (A1) neurons in awake mice.

METHODS

We performed in vivo cell-attached recordings in layer 4 neurons of the A1 of awake mice to investigate how continuous background noises of different levels affected the intensity tuning, frequency tuning and time characteristics of individual A1 neurons. According to the intensity tuning characteristics and types of stimulation, 44 neurons were devided into 4 groups: monotonic-intensity group (20 monotonic neurons), nonmonotonic-intensity group (6 nonmonotonic neurons), monotonic-frequency group (25 monotonic neurons) and monotonic-latency group (15 monotonic neurons).

RESULTS

The A1 neurons only had transient spike response within 10 to 40 ms after the onset of continuous wild-band noise stimulation. The noise intensity had no significant effects on the background firing rates of the A1 neurons (P>0.05). The increase of background noise resulted in a significant linear elevation of the intensity threshold of monotonic and nonmonotonic neurons for tone-evoked response (R²>0.90, P < 0.05). No significant difference was observed in the slopes of threshold changes between monotonic and nonmonotonic neurons (P>0.05). The best intensity of nonmonotonic neurons increased along with the intensity of the background noise, and the variation of the best intensity was positively correlated with the change of the threshold of the same neuron (r=0.944, P < 0.001). The frequency response bandwidth and the firing rate of the A1 neurons decreased as the noise intensity increased (P < 0.001), but the best frequency almost remained unchanged (P < 0.001). The increase of background noise intensity resulted in an increased first spike latency of the neurons to the same tone stimulus (P < 0.05) without affecting the time accuracy of the first action potential (P>0.05).

CONCLUSION

The acoustic response threshold of the A1 neurons increases linearly with the increase of background noise intensity. An increased background noise leads to compressed frequency band-width, a decreased firing rate and a prolonged spike latency, but the frequency selectivity and the time accuracy of auditory response to the same noise remain stable.

Binaural Background Noise Enhances Neuromagnetic Responses from Auditory Cortex

The presence of binaural low-level background noise has been shown to enhance the transient evoked N1 response at about 100 ms after sound onset. This increase in N1 amplitude is thought to reflect noise-mediated efferent feedback facilitation from the auditory cortex to lower auditory centers. To test this hypothesis, we recorded auditory-evoked fields using magnetoencephalography while participants were presented with binaural harmonic complex tones embedded in binaural or monaural background noise at signal-to-noise ratios of 25 dB (low noise) or 5 dB (higher noise). Half of the stimuli contained a gap in the middle of the sound. The source activities were measured in bilateral auditory cortices. The onset and gap N1 response increased with low binaural noise, but high binaural and low monaural noise did not affect the N1 amplitudes. P1 and P2 onset and gap responses were consistently attenuated by background noise, and noise level and binaural/monaural presentation showed distinct effects. Moreover, the evoked gamma synchronization was also reduced by background noise, and it showed a lateralized reduction for monaural noise. The effects of noise on the N1 amplitude follow a bell-shaped characteristic that could reflect an optimal representation of acoustic information for transient events embedded in noise.

Cortical Auditory Event-Related Potentials and Categorical Perception of Voice Onset Time in Children With an Auditory Neuropathy Spectrum Disorder

Objective: This study evaluated cortical encoding of voice onset time (VOT) in quiet and noise, and their potential associations with the behavioral categorical perception of VOT in children with auditory neuropathy spectrum disorder (ANSD). Design: Subjects were 11 children with ANSD ranging in age between 6.4 and 16.2 years. The stimulus was an /aba/-/apa/ vowel-consonant-vowel continuum comprising eight tokens with VOTs ranging from 0 ms (voiced endpoint) to 88 ms (voiceless endpoint). For speech in noise, speech tokens were mixed with the speech-shaped noise from the Hearing In Noise Test at a signal-to-noise ratio (SNR) of +5 dB. Speech-evoked auditory event-related potentials (ERPs) and behavioral categorization perception of VOT were measured in quiet in all subjects, and at an SNR of +5 dB in seven subjects. The stimuli were presented at 35 dB SL (re: pure tone average) or 115 dB SPL if this limit was less than 35 dB SL. In addition to the onset response, the auditory change complex (ACC) elicited by VOT was recorded in eight subjects. Results: Speech evoked ERPs recorded in all subjects consisted of a vertex positive peak (i.e., P1), followed by a trough occurring approximately 100 ms later (i.e., N2). For results measured in quiet, there was no significant difference in categorical boundaries estimated using ERP measures and behavioral procedures. Categorical boundaries estimated in quiet using both ERP and behavioral measures closely correlated with the most-recently measured Phonetically Balanced Kindergarten (PBK) scores. Adding a competing background noise did not affect categorical boundaries estimated using either behavioral or ERP procedures in three subjects. For the other four subjects, categorical boundaries estimated in noise using behavioral measures were prolonged. However, adding background noise only increased categorical boundaries measured using ERPs in three out of these four subjects. Conclusions: VCV continuum can be used to evaluate behavioral identification and the neural encoding of VOT in children with ANSD. In quiet, categorical boundaries of VOT estimated using behavioral measures and ERP recordings are closely associated with speech recognition performance in children with ANSD. Underlying mechanisms for excessive speech perception deficits in noise may vary for individual patients with ANSD.

Subthreshold Activity Underlying the Diversity and Selectivity of the Primary Auditory Cortex Studied by Intracellular Recordings in Awake Marmosets

Extracellular recording studies have revealed diverse and selective neural responses in the primary auditory cortex (A1) of awake animals. However, we have limited knowledge on subthreshold events that give rise to these responses, especially in non-human primates, as intracellular recordings in awake animals pose substantial technical challenges. We developed a novel intracellular recording technique in awake marmosets to systematically study subthreshold activity of A1 neurons that underlies their diverse and selective spiking responses. Our findings showed that in contrast to predominantly transient depolarization observed in A1 of anesthetized animals, both transient and sustained depolarization (during or beyond the stimulus period) were observed. Comparing with spiking responses, subthreshold responses were often longer lasting in duration and more broadly tuned in frequency, and showed narrower intensity tuning in non-monotonic neurons and lower response threshold in monotonic neurons. These observations demonstrated the enhancement of stimulus selectivity from subthreshold to spiking responses in individual A1 neurons. Furthermore, A1 neurons classified as regular- or fast-spiking subpopulation based on their spike shapes exhibited distinct response properties in frequency and intensity domains. These findings provide valuable insights into cortical integration and transformation of auditory information at the cellular level in auditory cortex of awake non-human primates.

Cortical Coding of Auditory Features

How the cerebral cortex encodes auditory features of biologically important sounds, including speech and music, is one of the most important questions in auditory neuroscience. The pursuit to understand related neural coding mechanisms in the mammalian auditory cortex can be traced back several decades to the early exploration of the cerebral cortex. Significant progress in this field has been made in the past two decades with new technical and conceptual advances. This article reviews the progress and challenges in this area of research.

Distinct Subthreshold Mechanisms Underlying Rate-Coding Principles in Primate Auditory Cortex

A key computational principle for encoding time-varying signals in auditory and somatosensory cortices of monkeys is the opponent model of rate coding by two distinct populations of neurons. However, the subthreshold mechanisms that give rise to this computation have not been revealed. Because the rate-coding neurons are only observed in awake conditions, it is especially challenging to probe their underlying cellular mechanisms. Using a novel intracellular recording technique that we developed in awake marmosets, we found that the two types of rate-coding neurons in auditory cortex exhibited distinct subthreshold responses. While the positive-monotonic neurons (monotonically increasing firing rate with increasing stimulus repetition frequency) displayed sustained depolarization at high repetition frequency, the negative-monotonic neurons (opposite trend) instead exhibited hyperpolarization at high repetition frequency but sustained depolarization at low repetition frequency. The combination of excitatory and inhibitory subthreshold events allows the cortex to represent time-varying signals through these two opponent neuronal populations.

Effects of Signal-to-Noise Ratio on Auditory Cortical Frequency Processing

The neural mechanisms that support the robust processing of acoustic signals in the presence of background noise in the auditory system remain largely unresolved. Psychophysical experiments have shown that signal detection is influenced by the signal-to-noise ratio (SNR) and the overall stimulus level, but this relationship has not been fully characterized. We evaluated the neural representation of frequency in rat primary auditory cortex by constructing tonal frequency response areas (FRAs) in primary auditory cortex for different SNRs, tone levels, and noise levels. We show that response strength and selectivity for frequency and sound level depend on interactions between SNRs and tone levels. At low SNRs, jointly increasing the tone and noise levels reduced firing rates and narrowed FRA bandwidths; at higher SNRs, however, increasing the tone and noise levels increased firing rates and expanded bandwidths, as is usually seen for FRAs obtained without background noise. These changes in frequency and intensity tuning decreased tone level and tone frequency discriminability at low SNRs. By contrast, neither response onset latencies nor noise-driven steady-state firing rates meaningfully interacted with SNRs or overall sound levels. Speech detection performance in humans was also shown to depend on the interaction between overall sound level and SNR. Together, these results indicate that signal processing difficulties imposed by high noise levels are quite general and suggest that the neurophysiological changes we see for simple sounds generalize to more complex stimuli.

SIGNIFICANCE STATEMENT

Effective processing of sounds in background noise is an important feature of the mammalian auditory system and a necessary feature for successful hearing in many listening conditions. Even mild hearing loss strongly affects this ability in humans, seriously degrading the ability to communicate. The mechanisms involved in achieving high performance in background noise are not well understood. We investigated the effects of SNR and overall stimulus level on the frequency tuning of neurons in rat primary auditory cortex. We found that the effects of noise on frequency selectivity are not determined solely by the SNR but depend also on the levels of the foreground tones and background noise. These observations can lead to improvement in therapeutic approaches for hearing-impaired patients.

Comparison of Auditory Middle-Latency Responses From Two Deconvolution Methods at 40 Hz

GOAL

Auditory middle-latency responses (MLRs) are reported to be particularly susceptible to stimulation rate. Deconvolution methods are necessary to unwrap the overlapping responses at a high rate under the linear superposition assumption. This study aims to investigate and compare the MLR characteristics at high and conventional stimulation rates.

METHODS

The characteristics were examined in healthy adults by using two closely related deconvolution paradigms, namely continuous-loop averaging deconvolution and multirate steady-state averaging deconvolution at a mean rate of 40 Hz, and a conventional low rate of 5 Hz.

RESULTS

The morphology and stability of the MLRs can benefit from a high-rate stimulation. It appears that stimulation sequencing strategies of deconvolution methods exerted divergent rate effects on MLR characteristics, which might be associated with different adaptation mechanisms.

CONCLUSION

MLRs obtained by two deconvolution methods and the conventional reference feature differently from one another.

SIGNIFICANCE

These findings have critical implications in our current understanding of the rate effects on MLR characteristics which may inspire further studies to explore the characteristics of evoked responses at high rates and deconvolution paradigms.

Thresholding of auditory cortical representation by background noise

It is generally thought that background noise can mask auditory information. However, how the noise specifically transforms neuronal auditory processing in a level-dependent manner remains to be carefully determined. Here, with in vivo loose-patch cell-attached recordings in layer 4 of the rat primary auditory cortex (A1), we systematically examined how continuous wideband noise of different levels affected receptive field properties of individual neurons. We found that the background noise, when above a certain critical/effective level, resulted in an elevation of intensity threshold for tone-evoked responses. This increase of threshold was linearly dependent on the noise intensity above the critical level. As such, the tonal receptive field (TRF) of individual neurons was translated upward as an entirety toward high intensities along the intensity domain. This resulted in preserved preferred characteristic frequency (CF) and the overall shape of TRF, but reduced frequency responding range and an enhanced frequency selectivity for the same stimulus intensity. Such translational effects on intensity threshold were observed in both excitatory and fast-spiking inhibitory neurons, as well as in both monotonic and nonmonotonic (intensity-tuned) A1 neurons. Our results suggest that in a noise background, fundamental auditory representations are modulated through a background level-dependent linear shifting along intensity domain, which is equivalent to reducing stimulus intensity.

Sensitivity of offset and onset cortical auditory evoked potentials to signals in noise

OBJECTIVE

The purpose of this study was to determine the effects of SNR and signal level on the offset response of the cortical auditory evoked potential (CAEP). Successful listening often depends on how well the auditory system can extract target signals from competing background noise. Both signal onsets and offsets are encoded neurally and contribute to successful listening in noise. Neural onset responses to signals in noise demonstrate a strong sensitivity to signal-to-noise ratio (SNR) rather than signal level; however, the sensitivity of neural offset responses to these cues is not known.

METHODS

We analyzed the offset response from two previously published datasets for which only the onset response was reported. For both datasets, CAEPs were recorded from young normal-hearing adults in response to a 1000-Hz tone. For the first dataset, tones were presented at seven different signal levels without background noise, while the second dataset varied both signal level and SNR.

RESULTS

Offset responses demonstrated sensitivity to absolute signal level in quiet, SNR, and to absolute signal level in noise.

CONCLUSIONS

Offset sensitivity to signal level when presented in noise contrasts with previously published onset results.

SIGNIFICANCE

This sensitivity suggests a potential clinical measure of cortical encoding of signal level in noise.

Pre-attentive, context-specific representation of fear memory in the auditory cortex of rat

Neural representation in the auditory cortex is rapidly modulated by both top-down attention and bottom-up stimulus properties, in order to improve perception in a given context. Learning-induced, pre-attentive, map plasticity has been also studied in the anesthetized cortex; however, little attention has been paid to rapid, context-dependent modulation. We hypothesize that context-specific learning leads to pre-attentively modulated, multiplex representation in the auditory cortex. Here, we investigate map plasticity in the auditory cortices of anesthetized rats conditioned in a context-dependent manner, such that a conditioned stimulus (CS) of a 20-kHz tone and an unconditioned stimulus (US) of a mild electrical shock were associated only under a noisy auditory context, but not in silence. After the conditioning, although no distinct plasticity was found in the tonotopic map, tone-evoked responses were more noise-resistive than pre-conditioning. Yet, the conditioned group showed a reduced spread of activation to each tone with noise, but not with silence, associated with a sharpening of frequency tuning. The encoding accuracy index of neurons showed that conditioning deteriorated the accuracy of tone-frequency representations in noisy condition at off-CS regions, but not at CS regions, suggesting that arbitrary tones around the frequency of the CS were more likely perceived as the CS in a specific context, where CS was associated with US. These results together demonstrate that learning-induced plasticity in the auditory cortex occurs in a context-dependent manner.

Auditory cortex of the marmoset monkey - complex responses to tones and vocalizations under opiate anaesthesia in core and belt areas

Many anaesthetics commonly used in auditory research severely depress cortical responses, particularly in the supragranular layers of the primary auditory cortex and in non-primary areas. This is particularly true when stimuli other than simple tones are presented. Although awake preparations allow better preservation of the neuronal responses, there is an inherent limitation to this approach whenever the physiological data need to be combined with histological reconstruction or anatomical tracing. Here we tested the efficacy of an opiate-based anaesthetic regime to study physiological responses in the primary auditory cortex and middle lateral belt area. Adult marmosets were anaesthetized using a combination of sufentanil (8 μg/kg/h, i.v.) and N2 O (70%). Unit activity was recorded throughout the cortical layers, in response to auditory stimuli presented binaurally. Stimuli consisted of a battery of tones presented at different intensities, as well as two marmoset calls ('Tsik' and 'Twitter'). In addition to robust monotonic and non-monotonic responses to tones, we found that the neuronal activity reflected various aspects of the calls, including 'on' and 'off' components, and temporal fluctuations. Both phasic and tonic activities, as well as excitatory and inhibitory components, were observed. Furthermore, a late component (100-250 ms post-offset) was apparent. Our results indicate that the sufentanil/N2 O combination allows better preservation of response patterns in both the core and belt auditory cortex, in comparison with anaesthetics usually employed in auditory physiology. This anaesthetic regime holds promise in enabling the physiological study of complex auditory responses in acute preparations, combined with detailed anatomical and histological investigation.

Human pitch detectors are tuned on a fine scale, but are perceptually accessed on a coarse scale

Single neurons in auditory cortex display highly selective spectrotemporal properties: their receptive fields modulate over small fractions of an octave and integrate across temporal windows of 100-200 ms. We investigated how these characteristics impact auditory behavior. Human observers were asked to detect a specific sound frequency masked by broadband noise; we adopted an experimental design which required the engagement of frequency-selective mechanisms to perform above chance. We then applied psychophysical reverse correlation to derive spectrotemporal perceptual filters for the assigned task. We were able to expose signatures of neuronal-like spectrotemporal tuning on a scale of 1/10 octave and 50-100 ms, but detailed modeling of our results showed that observers were not able to rely on the explicit output of these channels. Instead, human observers pooled from a large bank of highly selective channels via a weighting envelope poorly tuned for frequency (on a scale of 1.5 octave) with sluggish temporal dynamics, followed by a highly nonlinear max-like operation. We conclude that human detection of specific frequencies embedded within complex sounds suffers from a high degree of intrinsic spectrotemporal uncertainty, resulting in low efficiency values (<1 %) for this perceptual ability. Signatures of the underlying neural circuitry can be exposed, but there does not appear to be a direct line for accessing individual neural channels on a fine scale.

Hearing aid processing changes tone burst onset: effect on cortical auditory evoked potentials in individuals with normal audiometric thresholds

PURPOSE

The validity of using the cortical auditory evoked potential (CAEP) as an objective measure of hearing aid outcome has been questioned in the literature due to stimulus modifications caused by hearing aid processing. This study aimed to investigate the effects of hearing aid processing on the CAEP elicited with tone bursts that may have altered onsets.

METHOD

CAEPs to unprocessed and hearing aid-processed tone bursts were obtained from 16 individuals with normal audiometric thresholds when the onset time, level, and signal-to-noise ratio (SNR) were matched between the 2 conditions. Tone bursts processed by the hearing aid were recorded in an anechoic box and were presented through insert receivers. Unprocessed tone bursts were superimposed with hearing aid noise floor to match the SNR of the hearing aid-processed tone bursts.

RESULTS

Shortening of rise time and overshoot at the onset of the tone burst were evident in the hearing aid-processed stimuli. Statistical analysis of data showed no significant effects of hearing aid processing on the latency or amplitude of CAEP peaks (p > .05).

CONCLUSION

The changes in rise time occurring in the tone bursts due to hearing aid processing may not confound CAEP measures that are used to validate hearing aid fitting.

Intensity-invariant coding in the auditory system

The auditory system faithfully represents sufficient details from sound sources such that downstream cognitive processes are capable of acting upon this information effectively even in the face of signal uncertainty, degradation or interference. This robust sound source representation leads to an invariance in perception vital for animals to interact effectively with their environment. Due to unique nonlinearities in the cochlea, sound representations early in the auditory system exhibit a large amount of variability as a function of stimulus intensity. In other words, changes in stimulus intensity, such as for sound sources at differing distances, create a unique challenge for the auditory system to encode sounds invariantly across the intensity dimension. This challenge and some strategies available to sensory systems to eliminate intensity as an encoding variable are discussed, with a special emphasis upon sound encoding.

Cortical encoding of signals in noise: effects of stimulus type and recording paradigm

OBJECTIVES

Perception-in-noise deficits have been demonstrated across many populations and listening conditions. Many factors contribute to successful perception of auditory stimuli in noise, including neural encoding in the central auditory system. Physiological measures such as cortical auditory-evoked potentials (CAEPs) can provide a view of neural encoding at the level of the cortex that may inform our understanding of listeners' abilities to perceive signals in the presence of background noise. To understand signal-in-noise neural encoding better, we set out to determine the effect of signal type, noise type, and evoking paradigm on the P1-N1-P2 complex.

DESIGN

Tones and speech stimuli were presented to nine individuals in quiet and in three background noise types: continuous speech spectrum noise, interrupted speech spectrum noise, and four-talker babble at a signal-to-noise ratio of -3 dB. In separate sessions, CAEPs were evoked by a passive homogenous paradigm (single repeating stimulus) and an active oddball paradigm.

RESULTS

The results for the N1 component indicated significant effects of signal type, noise type, and evoking paradigm. Although components P1 and P2 also had significant main effects of these variables, only P2 demonstrated significant interactions among these variables.

CONCLUSIONS

Signal type, noise type, and evoking paradigm all must be carefully considered when interpreting signal-in-noise evoked potentials. Furthermore, these data confirm the possible usefulness of CAEPs as an aid to understand perception-in-noise deficits.

Population coding of tone stimuli in auditory cortex: dynamic rate vector analysis

Neural representations of even temporally unstructured stimuli can show complex temporal dynamics. In many systems, neuronal population codes show 'progressive differentiation', whereby population responses to different stimuli grow further apart during a stimulus presentation. Here we analysed the response of auditory cortical populations in rats to extended tones. At onset (up to 300 ms), tone responses involved strong excitation of a large number of neurons; during sustained responses (after 500 ms) overall firing rate decreased, but most cells still showed statistically significant rate modulation. Population vector trajectories evoked by different tone frequencies expanded rapidly along an initially similar trajectory in the first tens of milliseconds after tone onset, later diverging to smaller amplitude fixed points corresponding to sustained responses. The angular difference between onset and sustained responses to the same tone was greater than between different tones in the same stimulus epoch. No clear orthogonalization of responses was found with time, and predictability of the stimulus from population activity also decreased during this period compared with onset. The question of whether population activity grew more or less sparse with time depended on the precise mathematical sense given to this term. We conclude that auditory cortical population responses to tones differ from those reported in many other systems, with progressive differentiation not seen for sustained stimuli. Sustained acoustic stimuli are typically not behaviorally salient: we hypothesize that the dynamics we observe may instead allow an animal to maintain a representation of such sounds, at low energetic cost.

Long-lasting context dependence constrains neural encoding models in rodent auditory cortex

Acoustic processing requires integration over time. We have used in vivo intracellular recording to measure neuronal integration times in anesthetized rats. Using natural sounds and other stimuli, we found that synaptic inputs to auditory cortical neurons showed a rather long context dependence, up to > or =4 s (tau approximately 1 s), even though sound-evoked excitatory and inhibitory conductances per se rarely lasted greater, similar 100 ms. Thalamic neurons showed only a much faster form of adaptation with a decay constant tau <100 ms, indicating that the long-lasting form originated from presynaptic mechanisms in the cortex, such as synaptic depression. Restricting knowledge of the stimulus history to only a few hundred milliseconds reduced the predictable response component to about half that of the optimal infinite-history model. Our results demonstrate the importance of long-range temporal effects in auditory cortex and suggest a potential neural substrate for auditory processing that requires integration over timescales of seconds or longer, such as stream segregation.

Human evoked cortical activity to signal-to-noise ratio and absolute signal level

The purpose of this study was to determine the effect of signal level and signal-to-noise ratio (SNR) on the latency and amplitude of evoked cortical activity to further our understanding of how the human central auditory system encodes signals in noise. Cortical auditory evoked potentials (CAEPs) were recorded from 15 young normal-hearing adults in response to a 1000 Hz tone presented at two tone levels in quiet and while continuous background noise levels were varied in five equivalent SNR steps. These 12 conditions were used to determine the effects of signal level and SNR level on CAEP components P1, N1, P2, and N2. Based on prior signal-in-noise experiments conducted in animals, we hypothesized that SNR, would be a key contributor to human CAEP characteristics. As hypothesized, amplitude increased and latency decreased with increasing SNR; in addition, there was no main effect of tone level across the two signal levels tested (60 and 75 dB SPL). Morphology of the P1-N1-P2 complex was driven primarily by SNR, highlighting the importance of noise when recording CAEPs. Results are discussed in terms of the current interest in recording CAEPs in hearing aid users.

Sensory responses during sleep in primate primary and secondary auditory cortex

Most sensory stimuli do not reach conscious perception during sleep. It has been thought that the thalamus prevents the relay of sensory information to cortex during sleep, but the consequences for cortical responses to sensory signals in this physiological state remain unclear. We recorded from two auditory cortical areas downstream of the thalamus in naturally sleeping marmoset monkeys. Single neurons in primary auditory cortex either increased or decreased their responses during sleep compared with wakefulness. In lateral belt, a secondary auditory cortical area, the response modulation was also bidirectional and showed no clear systematic depressive effect of sleep. When averaged across neurons, sound-evoked activity in these two auditory cortical areas was surprisingly well preserved during sleep. Neural responses to acoustic stimulation were present during both slow-wave and rapid-eye movement sleep, were repeatedly observed over multiple sleep cycles, and demonstrated similar discharge patterns to the responses recorded during wakefulness in the same neuron. Our results suggest that the thalamus is not as effective a gate for the flow of sensory information as previously thought. At the cortical stage, a novel pattern of activation/deactivation appears across neurons. Because the neural signal reaches as far as secondary auditory cortex, this leaves open the possibility of altered sensory processing of auditory information during sleep.

Heterogeneous Neuronal Responses to Frequency-Modulated Tones in the Primary Auditory Cortex of Awake Cats

Previous studies in anesthetized animals reported that the primary auditory cortex (A1) showed homogenous phasic responses to FM tones, namely a transient response to a particular instantaneous frequency when FM sweeps traversed a neuron's tone-evoked receptive field (TRF). Here, in awake cats, we report that A1 cells exhibit heterogeneous FM responses, consisting of three patterns. The first is continuous firing when a slow FM sweep traverses the receptive field of a cell with a sustained tonal response. The duration and amplitude of FM response decrease with increasing sweep speed. The second pattern is transient firing corresponding to the cell's phasic tonal response. This response could be evoked only by a fast FM sweep through the cell's TRF, suggesting a preference for fast FM. The third pattern was associated with the off response to pure tones and was composed of several discrete response peaks during slow FM stimulus. These peaks were not predictable from the cell's tonal response but reliably reflected the time when FM swept across specific frequencies. Our A1 samples often exhibited a complex response pattern, combining two or three of the basic patterns above, resulting in a heterogeneous response population. The diversity of FM responses suggests that A1 use multiple mechanisms to fully represent the whole range of FM parameters, including frequency extent, sweep speed, and direction.

Representations of Cat Meows and Human Vowels in the Primary Auditory Cortex of Awake Cats

Previous investigation of neural responses to cat meows in the primary auditory cortex (A1) of the anesthetized cat revealed a preponderance of phasic responses aligned to stimulus onset, offset, or envelope peaks. Sustained responses during stationary components of the stimulus were rarely seen. This observation motivates further investigation into how stationary components of naturalistic auditory stimuli are encoded by A1 neurons. We therefore explored neuronal response patterns in A1 of the awake cat using natural meows, time-reversed meows, and human vowels as stimuli. We found heterogeneous response types: ∼2/3 of units classified as “phasic cells” responding only to amplitude envelope variations and the remaining 1/3 were “phasic-tonic cells” with continuous responses during the stationary components. The classification was upheld across all stimuli tested for a given cell. The differences of phasic responses were correlated with amplitude-envelope differences in the early stimulus portion (<100 ms), whereas the differences between tonic responses were correlated with ongoing spectral differences in the later stimulus portion. Phasic-tonic cells usually had a characteristic frequency (CF) <5 kHz, which corresponded to the dominant spectral range of vocalizations, suggesting that the cells encode spectral information. Phasic cells had CFs across the tested frequency range (<16 kHz). Instantaneous firing rates for natural and time-reversed meows were different, but mean rates for different categories of stimuli were similar. Evidence for cat's A1 preferring conspecific meows was not found. These functionally heterogeneous responses may serve to encode ongoing changes in sound spectra or amplitude envelope occurring throughout the entirety of the sound stimulus.

First spike latency and spike count as functions of tone amplitude and frequency in the inferior colliculus of mice

Spike counts (SC) or, spike rate and first spike latency (FSL), are both used to evaluate the responses of neurons to amplitudes and frequencies of acoustic stimuli. However, it is unclear which one is more suitable as a parameter for evaluating the responses of neurons to acoustic amplitudes and frequencies, since systematic comparisons between SC and FSL tuned to different amplitudes and frequencies, are scarce. This study systematically compared the precision and stability (i.e., the resolution and the coefficient variation, CV) of SC- and FSL-function as frequencies and amplitudes in the inferior colliculus of mice. The results showed that: (1) the SC-amplitude functions were of diverse shape (monotonic, nonmonotonic and saturated) whereas the FSL-amplitude functions were in close registration, in which FSL decreased with the increase of amplitude and no paradoxical (an increase in FSL with increasing amplitude) or constant (an independence of FSL on amplitude) neuron was observed; (2) the discriminability (resolution) of differences in amplitude and frequency based on FSL are higher than those based on SC; (3) the CVs of FSL for low amplitude stimuli were smaller than those of SC; (4) the fraction of neurons for which BF=CF (within +/-500Hz) obtained from FSL was higher than that from SC at any amplitude of sound. Therefore, SC and FSL may vary, independent from each other and represent different parameters of an acoustic stimulus, but FSL with its precision and stability appears to be a better parameter than SC in evaluation of the response of a neuron to frequency and amplitude in mouse inferior colliculus.

Neural coding strategies in auditory cortex

In contrast to the visual system, the auditory system has longer subcortical pathways and more spiking synapses between the peripheral receptors and the cortex. This unique organization reflects the needs of the auditory system to extract behaviorally relevant information from a complex acoustic environment using strategies different from those used by other sensory systems. The neural representations of acoustic information in auditory cortex can be characterized by three types: (1) isomorphic (faithful) representations of acoustic structures; (2) non-isomorphic transformations of acoustic features and (3) transformations from acoustical to perceptual dimensions. The challenge facing auditory neurophysiologists is to understand the nature of the latter two transformations. In this article, I will review recent studies from our laboratory regarding temporal discharge patterns in auditory cortex of awake marmosets and cortical representations of time-varying signals. Findings from these studies show that (1) firing patterns of neurons in auditory cortex are dependent on stimulus optimality and context and (2) the auditory cortex forms internal representations of sounds that are no longer faithful replicas of their acoustic structures.

Improved cortical entrainment to infant communication calls in mothers compared with virgin mice

There is a growing interest in the use of mice as a model system for species-specific communication. In particular, ultrasonic calls emitted by mouse pups communicate distress, and elicit a search and retrieval response from mothers. Behaviorally, mothers prefer and recognize these calls in two-alternative choice tests, in contrast to pup-naïve females that do not have experience with pups. Here, we explored whether one particular acoustic feature that defines these calls-- the repetition rate of calls within a bout-- is represented differently in the auditory cortex of these two animal groups. Multiunit recordings in anesthetized CBA/CaJ mice revealed that: (i) neural entrainment to repeated stimuli extended up to the natural pup call repetition rate (5 Hz) in mothers; but (ii) neurons in naïve females followed repeated stimuli well only at slower repetition rates; and (iii) entrained responses to repeated pup calls were less sensitive to natural pup call variability in mothers than in pup-naïve females. In the broader context, our data suggest that auditory cortical responses to communication sounds are plastic, and that communicative significance is correlated with an improved cortical representation.

Interaction in the perceptual processing of interaural time and level differences

Phillips and Hall [Psychophysical evidence for adaptation of central auditory processors for interaural differences in time and level, Hear. Res., 202 (2005) 188-199.] recently described the frequency-specific, selective adaptation of perceptual channels for interaural differences in level (ILD) and time (ITD). Psychometric functions for laterality based on ITD or ILD were obtained before and after exposure to adaptor tones of two frequencies presented alternately and highly lateralized to opposite sides. Following adaptation, points of perceived centrality (PPCs) were displaced towards the sides of the adaptor tones, and in opposite directions for the two frequencies. That is, laterality judgements showed a shift away from the adapted side, particularly for test cue values near the middle of the range. These data were congruent with a two-channel, opponent-process model of sound laterality coding. The present study used the same general paradigm to explore the independence of perceptual ITD and ILD processing. Psychometric functions for laterality based on ITD or ILD were obtained for each of two frequencies concurrently, before and after exposure to adaptor tones lateralized using the complementary cue. Once again, PPCs derived from the psychometric functions were displaced towards the sides of the adaptor tones, consistent with an opponent-process account of sound laterality coding. The size of the adaptation effect was at least as great as that described in the earlier study. Thus, a quarter cycle ITD adapting stimulus effected a 3 dB shift in the mean ILD-based PPC, and a 12 dB ILD adapting stimulus effected a 100 micros shift in the mean ITD-based PPC. These data offer new evidence concerning interaction in the processing of ITDs and ILDs.

Transient Dynamics versus Fixed Points in Odor Representations by Locust Antennal Lobe Projection Neurons

Projection neurons (PNs) in the locust antennal lobe exhibit odor-specific dynamic responses. We studied a PN population, stimulated with five odorants and pulse durations between 0.3 and 10 s. Odor representations were characterized as time series of vectors of PN activity, constructed from the firing rates of all PNs in successive 50 ms time bins. Odor representations by the PN population can be described as trajectories in PN state space with three main phases: an on transient, lasting 1-2 s; a fixed point, stable for at least 8 s; and an off transient, lasting a few seconds as activity returns to baseline. Whereas all three phases are odor specific, optimal stimulus separation occurred during the transients rather than the fixed points. In addition, the PNs' own target neurons respond least when their PN-population input stabilized at a fixed point. Steady-state measures of activity thus seem inappropriate to understand the neural code in this system.

Modulation of Level Response Areas and Stimulus Selectivity of Neurons in Cat Primary Auditory Cortex

Sounds commonly occur in sequences, such as in speech. It is therefore important to understand how the occurrence of one sound affects the response to a subsequent sound. We approached this question by determining how a conditioning stimulus alters the response areas of single neurons in the primary auditory cortex (AI) of barbiturate-anesthetized cats. The response areas consisted of responses to stimuli that varied in level at the two ears and delivered at the characteristic frequency of each cell. A binaural conditioning stimulus was then presented ≥50 ms before each of the stimuli comprising the level response area. An effective preceding stimulus alters the shape and severely reduces the size and response magnitude of the level response area. This ability of the preceding stimulus depends on its proximity in the level domain to the level response area, not on its absolute level or on the size of the response it evokes. Preceding stimuli evoke a nonlinear inhibition across the level response area that results in an increased selectivity of a cortical neuron for its preferred binaural stimuli. The selectivity of AI neurons during the processing of a stream of acoustic stimuli is likely to be restricted to a portion of their level response areas apparent in the tone-alone condition. Thus rather than being static, level response areas are fluid; they can vary greatly in extent, shape and response magnitude. The dynamic modulation of the level response area and level selectivity of AI neurons might be related to several tasks confronting the central auditory system.

Long-Lasting Modulation by Stimulus Context in Primate Auditory Cortex

A sound embedded in an acoustic stream cannot be unambiguously segmented and identified without reference to its stimulus context. To understand the role of stimulus context in cortical processing, we investigated the responses of auditory cortical neurons to 2-sound sequences in awake marmosets, with a focus on stimulus properties other than carrier frequency. Both suppressive and facilitatory modulations of cortical responses were observed by using combinations of modulated tone and noise stimuli. The main findings are as follows. 1) Preceding stimuli could suppress or facilitate responses to succeeding stimuli for durations >1 s. These long-lasting effects were dependent on the duration, sound level, and modulation parameters of the preceding stimulus, in addition to the carrier frequency. They occurred regardless of whether the 2 stimuli were separated by a silent interval. 2) Suppression was often tuned such that preceding stimuli whose parameters were similar to succeeding stimuli produced the strongest suppression. However, the responses of many units could be suppressed, although often weaker, even when the 2 stimuli were dissimilar. In some cases, only a dissimilar preceding stimulus produced suppression in the responses to the succeeding stimulus. 3) In contrast to suppression, facilitation of responses to succeeding stimuli by the preceding stimulus was usually strongest when the 2 stimuli were dissimilar. 4) There was no clear correlation between the firing rate evoked by the preceding stimulus and the change in the firing rate evoked by the succeeding stimulus, indicating that the observed suppression was not simply a result of habituation or spike adaptation. These results demonstrate that persistent modulations of the responses of an auditory cortical neuron to a given stimulus can be induced by preceding stimuli. Decreases or increases of responses to the succeeding stimuli are dependent on the spectral, temporal, and intensity properties of the preceding stimulus. This indicates that cortical auditory responses to a sound are not static, but instead depend on the stimulus context in a stimulus-specific manner. The long-lasting impact of stimulus context and the prevalence of facilitation suggest that such cortical response properties are important for auditory processing beyond forward masking, such as for auditory streaming and segregation.

Sustained firing in auditory cortex evoked by preferred stimuli

It has been well documented that neurons in the auditory cortex of anaesthetized animals generally display transient responses to acoustic stimulation, and typically respond to a brief stimulus with one or fewer action potentials. The number of action potentials evoked by each stimulus usually does not increase with increasing stimulus duration. Such observations have long puzzled researchers across disciplines and raised serious questions regarding the role of the auditory cortex in encoding ongoing acoustic signals. Contrary to these long-held views, here we show that single neurons in both primary (area A1) and lateral belt areas of the auditory cortex of awake marmoset monkeys (Callithrix jacchus) are capable of firing in a sustained manner over a prolonged period of time, especially when they are driven by their preferred stimuli. In contrast, responses become more transient or phasic when auditory cortex neurons respond to non-preferred stimuli. These findings suggest that when the auditory cortex is stimulated by a sound, a particular population of neurons fire maximally throughout the duration of the sound. Responses of other, less optimally driven neurons fade away quickly after stimulus onset. This results in a selective representation of the sound across both neuronal population and time.

Responses of neurons in cat primary auditory cortex to bird chirps: effects of temporal and spectral context

The responses of neurons to natural sounds and simplified natural sounds were recorded in the primary auditory cortex (AI) of halothane-anesthetized cats. Bird chirps were used as the base natural stimuli. They were first presented within the original acoustic context (at least 250 msec of sounds before and after each chirp). The first simplification step consisted of extracting a short segment containing just the chirp from the longer segment. For the second step, the chirp was cleaned of its accompanying background noise. Finally, each chirp was replaced by an artificial version that had approximately the same frequency trajectory but with constant amplitude. Neurons had a wide range of different response patterns to these stimuli, and many neurons had late response components in addition, or instead of, their onset responses. In general, every simplification step had a substantial influence on the responses. Neither the extracted chirp nor the clean chirp evoked a similar response to the chirp presented within its acoustic context. The extracted chirp evoked different responses than its clean version. The artificial chirps evoked stronger responses with a shorter latency than the corresponding clean chirp because of envelope differences. These results illustrate the sensitivity of neurons in AI to small perturbations of their acoustic input. In particular, they pose a challenge to models based on linear summation of energy within a spectrotemporal receptive field.

Effects of noise and cue enhancement on neural responses to speech in auditory midbrain, thalamus and cortex

Speech perception depends on the auditory system's ability to extract relevant acoustic features from competing background noise. Despite widespread acknowledgement that noise exacerbates this process, little is known about the neurophysiologic mechanisms underlying the encoding of speech in noise. Moreover, the relative contribution of different brain nuclei to these processes has not been fully established. To address these issues, aggregate neural responses were recorded from within the inferior colliculus, medial geniculate body and over primary auditory cortex of anesthetized guinea pigs to a synthetic vowel-consonant-vowel syllable /ada/ in quiet and in noise. In noise the onset response to the stop consonant /d/ was reduced or eliminated at each level, to the greatest degree in primary auditory cortex. Acoustic cue enhancements characteristic of 'clear' speech (lengthening the stop gap duration and increasing the intensity of the release burst) improved the neurophysiologic representation of the consonant at each level, especially at the cortex. Finally, the neural encoding of the vowel segment was evident at subcortical levels only, and was more resistant to noise than encoding of the dynamic portion of the consonant (release burst and formant transition). This experiment sheds light on which speech-sound elements are poorly represented in noise and demonstrates how acoustic modifications to the speech signal can improve neural responses in a normal auditory system. Implications for understanding neurophysiologic auditory signal processing in children with perceptual impairments and the design of efficient perceptual training strategies are also discussed.

Central auditory onset responses, and temporal asymmetries in auditory perception

Historically, central auditory responses have been studied for their sensitivity to various parameters of tone and noise burst stimulation, with response rate plotted as a function of the stimulus variable. The responses themselves are often quite brief, and locked in time to stimulus onset. In the stimulus amplitude domain, it has recently become clear that these responses are actually driven by properties of the stimulus' onset transient, and this has had important implications for how we interpret responses to manipulations of tone (or noise) burst plateau level. That finding was important in its own right, but a more general scrutiny of the available neurophysiological and psychophysical evidence reveals that there is a significant asymmetry in the neurophysiological and perceptual processing of stimulus onsets and offsets: sound onsets have a more elaborate neurophysiological representation, and receive a greater perceptual weighting. Hypotheses about origins of the asymmetries, derived independently from psychophysics and from neurophysiology, have in common a response threshold mechanism which adaptively tracks the ongoing level of stimulation.

The neurophysiology of auditory perception: from single units to evoked potentials

Evoked electric potential and magnetic field studies have the immense benefit that they can be conducted in awake, behaving humans and can be directly correlated with aspects of perception. As such, they are powerful objective indicators of perceptual properties. However, given a set of evoked potential and/or evoked field waveforms and their source locations, obtained for an exhaustive set of stimuli and stimulus contrasts, is it possible to determine blindly, i.e. predict, what the stimuli or stimulus contrasts were? If this can be done with some success, then a useful amount of information resides in scalp-recorded activity for, e.g., the study of auditory speech processing. In this review, we compare neural representations based on single-unit and evoked response activity for vowels and consonant-vowel phonemes with distinctions in formant glides and voice onset time. We conclude that temporal aspects of evoked responses can track some of the dominant response features present in single-unit activity. However, N1 morphology does not reliably predict phonetic identification of stimuli varying in voice onset time, and the reported appearance of a double-peak onset response in aggregate recordings from the auditory cortex does not indicate a cortical correlate of the perception of voicelessness. This suggests that temporal aspects of single-unit population activity are likely not inclusive enough for representation of categorical perception boundaries. In contrast to population activity based on single-unit recording, the ability to accurately localize the sources of scalp-evoked activity is one of the bottlenecks in obtaining an accessible neurophysiological substrate of perception. Attaining this is one of the requisites to arrive at the prospect of blind determination of stimuli on the basis of evoked responses. At the current sophistication level of recording and analysis, evoked responses remain in the realm of extremely sensitive objective indicators of stimulus change or stimulus differences. As such, they are signs of perceptual activity, but not comprehensive representations thereof.

Latency of auditory evoked field deflection N100m ruled by pitch or spectrum?

The auditory evoked field (AEF) in response to pure tones of 250 and 1000 Hz and a complex tone with a periodicity of 4 ms (composed of the frequencies 1000, 1250, 1500, 1750, and 2000 Hz), corresponding to a pitch of 250 Hz, was recorded with a 37-channel neuromagnetometer system. The intensity was 60 dB sensation level (SL). Two different stimulus durations were examined in 12 subjects: 500 ms (long tones) and 100 ms (short tones). The stimulus onset asynchrony (SOA) was uniformly distributed between 3 and 4 s for the long tones and between 0.8 and 1.2 s for the short tones. Each subject was investigated four times, to assess the intraindividual variability. The mean latency of the AEF deflection N100m turned out to be similar for the long and the short tones: about 98 and 87 ms for the pure tones of 250 Hz and 1000 Hz, respectively, and 95 ms for the complex tone with a pitch of 250 Hz. However, a great interindividual variability was observed, exhibiting no consistent relationship between the N100m latencies for the three different tones, except that the response to the pure tone of 1000 Hz generally occurred earlier. In conclusion, this study does not support the proposal that the N100m latency represents a code for pitch, although a low pitch appears to be a factor favoring a longer N100m latency.

Frequency specificity of the human auditory brainstem and middle latency responses using notched noise masking

This study investigated the frequency specificity of the auditory brainstem and middle latency responses to 80 and 90 dB ppe SPL 500-Hz and 90 dB ppe SPL 2000-Hz tonebursts. The stimuli were brief (2-1-2 cycle) linear-gated tonebursts. ABR/MLRs were recorded using two electrode montages: (1) Cz-nape of neck and (2) Cz-ipsilateral earlobe. Cochlear contributions to ABR wave V-Na and MLR waves Na-Pa and Pa-Nb were assessed by plotting notched noise tuning curves which showed amplitudes and latencies as a function of center frequency of the noise masker [Abdala and Folsom, J. Acoust. Soc. Am. 97, 2394 (1995); ibid. 98, 921 (1995)]. Maxima in the response amplitude profiles for the ABR and MLR to 80 dB ppe SPL tonebursts occurred within one-half octave of the nominal stimulus frequency, with minimal contributions to the responses from frequencies greater than one octave away. At 90 dB ppe SPL, contributions came from a slightly broader frequency region for both stimulus frequencies. Thus, the ABR/MLR to 80 dB ppe SPL tonebursts shows good frequency specificity which decreases at 90 dB ppe SPL. No significant differences exist in frequency specificity of: (1) ABR wave V-Na versus MLR waves Na-Pa and Pa-Nb at either stimulus frequency or intensity; and (2) ABR/MLRs recorded using the two electrode montages.

Sensitivity of auditory cortical neurons to locations of signals and competing noise sources

The present study examined cortical parallels to psychophysical signal detection and sound localization in the presence of background noise. The activity of single units or of small clusters of units was recorded in cortical area A2 of chloralose-anesthetized cats. Signals were 80-ms click trains that varied in location in the horizontal plane around the animal. Maskers were continuous broadband noises. In the focal masker condition, a single masker source was tested at various azimuths. In the diffuse masker condition, uncorrelated noise was presented from two speakers at +/-90 degrees lateral to the animal. For about 2/3 of units ("type A"), the presence of the masker generally reduced neural sensitivity to signals, and the effects of the masker depended on the relative locations of signal and masker sources. For the remaining 1/3 of units ("type B"), the masker reduced spike rates at low signal levels but often augmented spike rates at higher signal levels. Increases in spike rates of type B units were most common for signal sources in front of the ear contralateral to the recording site but tended to be independent of masker source location. For type A units, masker effects could be modeled as a shift toward higher levels of spike-rate- and spike-latency-versus-level functions. For a focal masker, the shift size decreased with increasing separation of signal and masker. That result resembled psychophysical spatial unmasking, i.e., improved signal detection by spatial separation of the signal from the noise source. For the diffuse masker condition, the shift size generally was constant across signal locations. For type A units, we examined the effects of maskers on cortical signaling of sound-source location, using an artificial-neural-network (ANN) algorithm. First, an ANN was trained to estimate the signal location in the quiet condition by recognizing the spike patterns of single units. Then we tested ANN responses for spike patterns recorded under various masker conditions. Addition of a masker generally altered spike patterns and disrupted ANN identification of signal location. That disruption was smaller, however, for signal and masker configurations in which the masker did not severely reduce units' spike rates. That result compared well with the psychophysical observation that listeners maintain good localization performance as long as signals are clearly audible.

Sensitivity of unanesthetized chinchilla auditory system to noise burst onset, and the effects of carboplatin

The gross near-field responses of the auditory nerve and inferior colliculus to noise burst stimuli were recorded through intracranially implanted electrodes in six unanesthetized chinchillas. Responses were studied as a function of stimulus plateau amplitude and rise time, both before and after a systemic dose of 75 mg/kg of carboplatin. Both recording sites showed sensitivity to stimulus level and rise time. Increases in stimulus level and decreases in stimulus rise time each produced increases in the response magnitude, and decreases in response latency. When the stimuli were re-specified as rate of pressure change at sound onset (Pa/s), the amplitude and latency of responses at each site were found to be a direct function of rate of sound pressure change. These data provide the first confirmation in unanesthetized animals of previous single unit observations in barbiturate-anesthetized cats. Carboplatin treatment resulted in a 20-80% loss of inner hair cells, a modest threshold elevation, and a 50-75% reduction in peak response amplitudes. The general patterns of sensitivity to stimulus level and rise time were not markedly affected by carboplatin, nor was the fashion in which response parameters (amplitude and latency) were ruled by rate of pressure change at sound onset.

Regional variations of noise-induced changes in operating range in cat AI

Regional differences in spectral integration of neurons in cat primary auditory cortex (AI) suggest that regions differ in effects of background noise on operating characteristics of neurons. Therefore, tone-response threshold, best level (peak-rate intensity), dynamic range, and sharpness of tuning in quiet and in continuous broadband noise were mapped for single neurons along the isofrequency domain of AI. Neurons did not show an excitatory response to the noise. Noise invariably increased the tone-response threshold and best levels. Consequently, the dynamic ranges and receptive fields shifted to higher intensity levels without changes of average sharpness of tuning. These shifts were linearly related to noise level and showed little inter-neuronal variability for neurons in the central, mostly sharply tuned part of AI. In more dorsal and ventral parts of AI, neurons were more variable in tone-response threshold, dynamic range and best level, and no systematic relationship between increase in noise level, threshold increase and best-level increase was observed. We conclude that linear shifts in the operating range of neurons in central AI in the presence of continuous noise backgrounds do not affect other response properties and may relate to the unaltered analysis and representation of spectral components of sounds. In contrast, neurons in dorsal and ventral AI change response properties in a non-predictable way in the presence of noise in accordance with the more complex receptive field properties in those areas.

Spatial receptive fields of primary auditory cortical neurons in quiet and in the presence of continuous background noise

Spatial receptive fields of primary auditory (AI) neurons were studied by delivering, binaurally, synthesized virtual-space signals via earphones to cats under barbiturate anesthesia. Signals were broadband or narrowband transients presented in quiet anechoic space or in acoustic space filled with uncorrelated continuous broadband noise. In the absence of background noise, AI virtual space receptive fields (VSRFs) are typically large, representing a quadrant or more of acoustic space. Within the receptive field, onset latency and firing strength form functional gradients. We hypothesized earlier that functional gradients in the receptive field provide information about sound-source direction. Previous studies indicated that spatial gradients could remain relatively constant across changes in signal intensity. In the current experiments we tested the hypothesis that directional sensitivity to a transient signal, as reflected in the gradient structure of VSRFs of AI neurons, is also retained in the presence of a continuous background noise. When background noise was introduced three major affects on VSRFs were observed. 1) The size of the VSRF was reduced, accompanied by a reduction of firing strength and lengthening of response latency for signals at an acoustic axis and on-lines of constant azimuth and elevation passing through the acoustic axis. These effects were monotonically related to the intensity of the background noise over a noise intensity range of approximately 30 dB. 2) The noise intensity-dependent changes in VSRFs were mirrored by the changes that occurred when the signal intensity was changed in signal-alone conditions. Thus adding background noise was equivalent to a shift in the threshold of a directional signal, and this shift was seen across the spatial receptive field. 3) The spatial gradients of response strength and latency remained evident over the range of background noise intensity that reduced spike count and lengthened onset latency. Those gradients along the azimuth that spanned the frontal midline tended to remain constant in slope and position in the face of increasing intensity of background noise. These findings are consistent with our hypothesis that, under background noise conditions, information that underlies directional acuity and accuracy is retained within the spatial receptive fields of an ensemble of AI neurons.

The columnar and layer-specific response properties of neurons in the primary auditory cortex of Mongolian gerbils

The columnar and layer-specific response properties of neurons in the primary auditory cortex (AI) of Mongolian gerbils were studied using single-unit recordings of responses to tone-burst stimuli presented to the ear contralateral to the recording side. During near-radial microelectrode penetrations of the AI in 100-microm steps, the best frequency (BF), best threshold (BT), best amplitude (BA), latency, tuning curve and Q10dB were recorded. Neurons encountered during single penetrations showed similar BFs, indicating a columnar frequency organization, but their latencies and Q10dBs differed. The BAs and BTs recorded within single penetrations often showed a similar value in the middle cortical layers. The latencies and Q10dBs of these neurons exhibited a tendency toward a layer-specific distribution. The latencies of neurons located in layers I-V were longer than those located in layer VI. The Q10dBs of neurons located in layers III and IV were higher than those located in layers I and VI. These results are almost consistent with those of previous studies on frequency representation, and indicated the existence of an integrative mechanism of frequency processing in the AI. This is the first study in which a layer-specific, partially columnar organization for stimulus amplitude is described.

Auditory cortical onset responses revisited. I. First-spike timing

Sound onsets are salient and behaviorally relevant, and most auditory neurons discharge spikes locked to such transients. The acoustic parameters of sound onsets that shape such onset responses are unknown. In this paper is analyzed the timing of spikes of single neurons in the primary auditory cortex of barbiturate-anesthetized cats to the onsets of tone bursts. By parametric variation of sound pressure level, rise time, and rise function (linear or cosine-squared), the time courses of peak pressure, rate of change of peak pressure, and acceleration of peak pressure during the tones' onsets were systematically varied. For cosine-squared rise function tones of a given frequency and laterality, any neuron's mean first-spike latency was an invariant and inverse function of the maximum acceleration of peak pressure occurring at tone onset. For linear rise function tones, latency was an invariant and inverse function of the rate of change of peak pressure. Thus latency is independent of rise time or sound pressure level per se. Latency-acceleration functions, obtained with cosine-squared rise function tones under different stimulus conditions (frequency, laterality) from any given neuron and across the neuronal pool, were of strikingly similar shape. The same was true for latency-rate of change of peak pressure functions obtained with linear rise function tones. Latency-acceleration/rate of change of peak pressure functions could differ in their extent and in their position within the coordinate system. The positional differences reflect neuronal differences in minimum latency Lmin and in a sensitivity S to acceleration and rate of change of peak pressure (transient sensitivity), a hitherto unrecognized neuronal property that is distinctly different from firing threshold. Estimates of Lmin and S, which were derived by fitting a simple function to the neuronal latency-acceleration/rate of change of peak pressure functions, were independent of rise function. On average, Lmin decreased with increasing characteristic frequency (CF), but varied widely for neurons with the same CF. S varied with CF in a fashion similar to the cat's audiogram and, for a given neuron, varied with frequency. SD of first-spike latency was roughly proportional to the slope of the functions relating latency to acceleration/rate of change of peak pressure. Thus SD increased exponentially, rather than linearly, with mean latency, and did so at about twice the rate for linear than for cosine-squared rise function tones. The proportionality coefficients were quite similar across the neuronal pool and similar for both rise functions. Minimum SD increased nonlinearly with increasing Lmin. These findings suggest a peripheral origin of S and a peripheral establishment of latency-acceleration/rate of change of peak pressure functions. Because of the striking similarity in the shapes of such functions across the neuronal pool, sound onsets will produce orderly and predictable spatiotemporal patterns of first-spike timing, which could be used to instantaneously track rapid transients and to represent transient features by partly scale-invariant temporal codes.

Time course of forward masking tuning curves in cat primary auditory cortex

Nonsimultaneous two-tone interactions were studied in the primary auditory cortex of anesthetized cats. Poststimulatory effects of pure tone bursts (masker) on the evoked activity of a fixed tone burst (probe) were investigated. The temporal interval from masker onset to probe onset (stimulus onset asynchrony), masker frequency, and intensity were parametrically varied. For all of the 53 single units and 58 multiple-unit clusters, the neural activity of the probe signal was either inhibited, facilitated, and/or delayed by a limited set of masker stimuli. The stimulus range from which forward inhibition of the probe was induced typically was centered at and had approximately the size of the neuron's excitatory receptive field. This "masking tuning curve" was usually V shaped, i.e., the frequency range of inhibiting masker stimuli increased with the masker intensity. Forward inhibition was induced at the shortest stimulus onset asynchrony between masker and probe. With longer stimulus onset asynchronies, the frequency range of inhibiting maskers gradually became smaller. Recovery from forward inhibition occurred first at the lower- and higher-frequency borders of the masking tuning curve and lasted the longest for frequencies close to the neuron's characteristic frequency. The maximal duration of forward inhibition was measured as the longest period over which reduction of probe responses was observed. It was in the range of 53-430 ms, with an average of 143 +/- 71 (SD) ms. Amount, duration and type of forward inhibition were weakly but significantly correlated with "static" neural receptive field properties like characteristic frequency, bandwidth, and latency. For the majority of neurons, the minimal inhibitory masker intensity increased when the stimulus onset asynchrony became longer. In most cases the highest masker intensities induced the longest forward inhibition. A significant number of neurons, however, exhibited longest periods of inhibition after maskers of intermediate intensity. The results show that the ability of cortical cells to respond with an excitatory activity depends on the temporal stimulus context. Neurons can follow higher repetition rates of stimulus sequences when successive stimuli differ in their spectral content. The differential sensitivity to temporal sound sequences within the receptive field of cortical cells as well as across different cells could contribute to the neural processing of temporally structured stimuli like speech and animal vocalizations.

Responses of inferior colliculus neurons in C57BL/6J mice with and without sensorineural hearing loss: effects of changing the azimuthal location of a continuous noise masker on responses to contralateral tones

Extracellular recordings were obtained from inferior colliculus neurons of young adult (2-month-old) C57 mice with normal hearing and middle-aged (6-month-old) C57 mice with sensorineural hearing loss as they responded to best frequency (BF) tones (signal) in the presence of a continuous background noise (masker). Rate/level functions were obtained for the signal alone, noise bursts alone, and the signal in continuous noise as a function of masker location. For both groups of mice, thresholds for BF tones were significantly elevated in the presence of noise at all three noise locations. Separating the signal and masker sources significantly improved masked tone thresholds of 2-month-old mice but not hearing-impaired mice. The decreased ability of middle-aged mice to benefit from separation of the signal and masker sources may reflect alterations in binaural processing as a result of sensorineural hearing loss.

The influence of temporal pattern of stimulation on delay tuning of neurons in the auditory cortex of the FM bat, Myotis lucifugus

In echolocating bats, delay-sensitive neurons show facilitative responses to simulated pulse-echo pairs at particular echo delays. Three experiments examined how the temporal pattern of stimulation affected the delay tuning of neurons in the auditory cortex of the awake FM bat, Myotis lucifugus. First, delay tuning was compared using a series of pulse-echo pairs fixed in echo delay ('standard' stimuli), and a series of pulse-echo pairs in which successive sound pairs decreased by a fixed echo-delay step ('approach' stimuli). Similar best delays were measured with both stimulation patterns presented at repetition rates in which the neuron was delay-sensitive. At the higher delay-sensitive pulse repetition rates, approach stimuli evoked larger delay-dependent responses. Second, approach stimuli were fixed at different intertrial intervals. The best delay was unaffected by intertrial interval, although some neurons showed larger responses for longer intertrial intervals (0.5, 1.0 s), especially at the higher delay-sensitive pulse repetition rates. Third, approach stimuli were fixed at different echo-delay steps to simulate target velocity. The majority of neurons showed some sensitivity to echo-delay step, with clear preference for target velocity mainly between 1.8-7.0 m/s. This suggests that delay-sensitive neurons compute target velocity by rate of change of echo delay over successive echoes. Thus, response properties of cortical neurons are influenced by dynamic acoustic conditions found in target-directed flight.