Time course of tonal frequency-response-area of primary auditory cortex neurons in alert cats

Automatic Sensory Predictions: A Review of Predictive Mechanisms in the Brain and Their Link to Conscious Processing

The human brain has the astonishing capacity of integrating streams of sensory information from the environment and forming predictions about future events in an automatic way. Despite being initially developed for visual processing, the bulk of predictive coding research has subsequently focused on auditory processing, with the famous mismatch negativity signal as possibly the most studied signature of a surprise or prediction error (PE) signal. Auditory PEs are present during various consciousness states. Intriguingly, their presence and characteristics have been linked with residual levels of consciousness and return of awareness. In this review we first give an overview of the neural substrates of predictive processes in the auditory modality and their relation to consciousness. Then, we focus on different states of consciousness - wakefulness, sleep, anesthesia, coma, meditation, and hypnosis - and on what mysteries predictive processing has been able to disclose about brain functioning in such states. We review studies investigating how the neural signatures of auditory predictions are modulated by states of reduced or lacking consciousness. As a future outlook, we propose the combination of electrophysiological and computational techniques that will allow investigation of which facets of sensory predictive processes are maintained when consciousness fades away.

Age-related changes in sound onset and offset intensity coding in auditory cortical fields A1 and CL of rhesus macaques

Inhibition plays a key role in shaping sensory processing in the central auditory system and has been implicated in sculpting receptive field properties such as sound intensity coding and also in shaping temporal patterns of neuronal firing such as onset- or offset-evoked responses. There is substantial evidence supporting a decrease in inhibition throughout the ascending auditory pathway in geriatric animals. We therefore examined intensity coding of onset (ON) and offset (OFF) responses in auditory cortex of aged and young monkeys. A large proportion of cells in the primary auditory cortex (A1) and the caudolateral field (CL) displayed nonmonotonic rate-level functions for OFF responses in addition to nonmonotonic coding of ON responses. Aging differentially affected ON and OFF responses; the magnitude of effects was generally greater for ON responses. In addition to higher firing rates, neurons in old monkeys exhibited a significant increase in the proportion of monotonic rate-level functions and had higher best intensities than those in young monkeys. OFF responses in young monkeys displayed a range of intensity coding relationships with ON responses of the same cells, ranging from highly similar to highly dissimilar. Dissimilarity in ON/OFF coding was greater in CL and was reduced with aging, which was largely explained by a preferential decrease in the percentage of cells with nonmonotonic coding of ON and OFF responses. The changes we observed are consistent with previously demonstrated alterations in inhibition in the ascending auditory pathway of primates and could be involved in age-related deficits in the temporal processing of sounds.NEW & NOTEWORTHY Aging has a major impact on intensity coding of neurons in auditory cortex of rhesus macaques. Neural responses to sound onset and offset were affected to different extents, and their rate-level functions became more mutually similar, which could be accounted for by the loss of nonmonotonic intensity coding in geriatric monkeys. These findings were consistent with weakened inhibition in the central auditory system and could contribute to auditory processing deficits in elderly subjects.

Spectral and spatial tuning of onset and offset response functions in auditory cortical fields A1 and CL of rhesus macaques

The mammalian auditory cortex is necessary for spectral and spatial processing of acoustic stimuli. Most physiological studies of single neurons in the auditory cortex have focused on the onset and sustained portions of evoked responses, but there have been far fewer studies on the relationship between onset and offset responses. In the current study, we compared spectral and spatial tuning of onset and offset responses of neurons in primary auditory cortex (A1) and the caudolateral (CL) belt area of awake macaque monkeys. Several different metrics were used to determine the relationship between onset and offset response profiles in both frequency and space domains. In the frequency domain, a substantial proportion of neurons in A1 and CL displayed highly dissimilar best stimuli for onset- and offset-evoked responses, although even for these neurons, there was usually a large overlap in the range of frequencies that elicited onset, and offset responses and distributions of tuning overlap metrics were mostly unimodal. In the spatial domain, the vast majority of neurons displayed very similar best locations for onset- and offset-evoked responses, along with unimodal distributions of all tuning overlap metrics considered. Finally, for both spectral and spatial tuning, a slightly larger fraction of neurons in A1 displayed nonoverlapping onset and offset response profiles, relative to CL, which supports hierarchical differences in the processing of sounds in the two areas. However, these differences are small compared with differences in proportions of simple cells (low overlap) and complex cells (high overlap) in primary and secondary visual areas.NEW & NOTEWORTHY In the current study, we examine the relationship between the tuning of neural responses evoked by the onset and offset of acoustic stimuli in the primary auditory cortex, as well as a higher-order auditory area-the caudolateral belt field-in awake rhesus macaques. In these areas, the relationship between onset and offset response profiles in frequency and space domains formed a continuum, ranging from highly overlapping to highly nonoverlapping.

Diverse cortical codes for scene segmentation in primate auditory cortex

The temporal coherence of amplitude fluctuations is a critical cue for segmentation of complex auditory scenes. The auditory system must accurately demarcate the onsets and offsets of acoustic signals. We explored how and how well the timing of onsets and offsets of gated tones are encoded by auditory cortical neurons in awake rhesus macaques. Temporal features of this representation were isolated by presenting otherwise identical pure tones of differing durations. Cortical response patterns were diverse, including selective encoding of onset and offset transients, tonic firing, and sustained suppression. Spike train classification methods revealed that many neurons robustly encoded tone duration despite substantial diversity in the encoding process. Excellent discrimination performance was achieved by neurons whose responses were primarily phasic at tone offset and by those that responded robustly while the tone persisted. Although diverse cortical response patterns converged on effective duration discrimination, this diversity significantly constrained the utility of decoding models referenced to a spiking pattern averaged across all responses or averaged within the same response category. Using maximum likelihood-based decoding models, we demonstrated that the spike train recorded in a single trial could support direct estimation of stimulus onset and offset. Comparisons between different decoding models established the substantial contribution of bursts of activity at sound onset and offset to demarcating the temporal boundaries of gated tones. Our results indicate that relatively few neurons suffice to provide temporally precise estimates of such auditory "edges," particularly for models that assume and exploit the heterogeneity of neural responses in awake cortex.

Response characteristics of primary auditory cortex neurons underlying perceptual asymmetry of ramped and damped sounds

Sound envelope plays a crucial role in perception: ramped sounds (slow attack and quick decay) are louder in strength and longer in subjective duration than damped sounds (quick attack and slow decay) even if they are equal in intensity and physical duration. To explain the asymmetrical perception, the perceptual constancy hypothesis supposes that the listener eliminates the slow decay of damped sounds from the judgment of perception, while the persistence of perception hypothesis supposes asymmetrical neural responses after the source has stopped. To understand neural mechanisms underlying the perceptual asymmetry, we explored response properties of the primary auditory cortex (A1) neurons during ramped and damped stimuli in awake cats. We found two distinct types of cells tuned to specific features of the sound envelope: edge cells sensitive to the temporal edge, such as quick attack and decay, while slope cells sensitive to slow attack and decay. The former needs a short (<2.5 ms) period of stimulus duration for evoking maximal peak responses, while the latter needs a long (20 ms) period, suggesting that the timescale of processing underlies differential sensitivity between the cell types. The findings suggest that perceptual constancy is not yet be executed at A1 because the specific cells distinguishing the direction of amplitude change (attack or decay) are lacking in A1. On the other hand, there is evidence of persistence of perception: overall response duration during ramped sound reached 1.4 times longer than that during damped sound, originating mainly from the response asymmetry of the edge cell (sensitive to the quick decay of ramped sounds but not to the slow decay of damped sounds), and neuronal persistence of excitation after the termination of ramped sounds was substantially longer than that of damped sounds, corresponding to the psychological evidence of persistence of perception.

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.

Dynamics of phase-independent spectro-temporal tuning in primary auditory cortex of the awake ferret

Tuning of cortical neurons is often measured as a static property, or during a steady-state regime, despite a number of studies suggesting that tuning depends on when it is measured during a neuron's response (e.g., onset vs. sustained vs. offset). We have previously shown that phase-locked tuning to feature transients evolves as a dynamic quantity from the onset of the sound. In this follow-up study, we examined the phase-independent tuning during feature transients. Based on previous results, we hypothesized phase-independent tuning should evolve on the same timescale as phase-locked tuning. We used stimuli of constant level, but alternating between flat spectro-temporal envelope and a modulated envelope with well-defined spectral density and temporal periodicity. This allowed the measure of changes in tuning to novel spectro-temporal content, as happens during running speech and other sounds with rapid transitions without a confounding change in sound level. For 95% of neurons, tuning changed significantly from the onset, over the course of the response. For a majority of these cells, the change occurred within the first 40ms following a feature onset, often even around 10-20ms. This solidifies the idea that tuning can change rapidly from onset tuning to the sustained, steady-state tuning.

Comparison of LFP-based and spike-based spectro-temporal receptive fields and cross-correlation in cat primary auditory cortex

Multi-electrode array recordings of spike and local field potential (LFP) activity were made from primary auditory cortex of 12 normal hearing, ketamine-anesthetized cats. We evaluated 259 spectro-temporal receptive fields (STRFs) and 492 frequency-tuning curves (FTCs) based on LFPs and spikes simultaneously recorded on the same electrode. We compared their characteristic frequency (CF) gradients and their cross-correlation distances. The CF gradient for spike-based FTCs was about twice that for 2–40 Hz-filtered LFP-based FTCs, indicating greatly reduced frequency selectivity for LFPs. We also present comparisons for LFPs band-pass filtered between 4–8 Hz, 8–16 Hz and 16–40 Hz, with spike-based STRFs, on the basis of their marginal frequency distributions. We find on average a significantly larger correlation between the spike based marginal frequency distributions and those based on the 16–40 Hz filtered LFP, compared to those based on the 4–8 Hz, 8–16 Hz and 2–40 Hz filtered LFP. This suggests greater frequency specificity for the 16–40 Hz LFPs compared to those of lower frequency content. For spontaneous LFP and spike activity we evaluated 1373 pair correlations for pairs with >200 spikes in 900 s per electrode. Peak correlation-coefficient space constants were similar for the 2–40 Hz filtered LFP (5.5 mm) and the 16–40 Hz LFP (7.4 mm), whereas for spike-pair correlations it was about half that, at 3.2 mm. Comparing spike-pairs with 2–40 Hz (and 16–40 Hz) LFP-pair correlations showed that about 16% (9%) of the variance in the spike-pair correlations could be explained from LFP-pair correlations recorded on the same electrodes within the same electrode array. This larger correlation distance combined with the reduced CF gradient and much broader frequency selectivity suggests that LFPs are not a substitute for spike activity in primary auditory cortex.

Brief sounds evoke prolonged responses in anesthetized ferret auditory cortex

Neurons in the auditory cortex of anesthetized animals are generally considered to generate phasic responses to simple stimuli such as tones or noise bursts. In this paper, we show that under ketamine/medetomidine anesthesia, neurons in ferret auditory cortex usually exhibit complex sustained responses. We presented 100-ms broad-band noise bursts at a range of interaural level differences (ILDs) and average binaural levels (ABLs), and used extracellular electrodes to monitor evoked activity over 700 ms poststimulus onset. We estimated the degree of randomness (noise) in the response functions of individual neurons over poststimulus time; we found that neural activity was significantly modulated by sound for up to approximately 500 ms following stimulus offset. Pooling data from all neurons, we found that spiking activity carries significant information about stimulus identity over this same time period. However, information about ILD decayed much more quickly over time compared with information about ABL. In addition, ILD and ABL are coded independently by the neural population even though this is not the case at individual neurons. Though most neurons responded more strongly to ILDs corresponding to the opposite side of space, as a population, they were equally informative about both contra- and ipsilateral stimuli.

Nonlinear spectrotemporal interactions underlying selectivity for complex sounds in auditory cortex

In the auditory cortex of awake animals, a substantial number of neurons do not respond to pure tones. These neurons have historically been classified as "unresponsive" and even been speculated as being nonauditory. We discovered, however, that many of these neurons in the primary auditory cortex (A1) of awake marmoset monkeys were in fact highly selective for complex sound features. We then investigated how such selectivity might arise from the tone-tuned inputs that these neurons likely receive. We found that these non-tone responsive neurons exhibited nonlinear combination-sensitive responses that require precise spectral and temporal combinations of two tone pips. The nonlinear spectrotemporal maps derived from these neurons were correlated with their selectivity for complex acoustic features. These non-tone responsive and nonlinear neurons were commonly encountered at superficial cortical depths in A1. Our findings demonstrate how temporally and spectrally specific nonlinear integration of putative tone-tuned inputs might underlie a diverse range of high selectivity of A1 neurons in awake animals. We propose that describing A1 neurons with complex response properties in terms of tone-tuned input channels can conceptually unify a wide variety of observed neural selectivity to complex sounds into a lower dimensional description.

Temporally dynamic frequency tuning of population responses in monkey primary auditory cortex

Frequency tuning of auditory cortical neurons is typically determined by integrating spikes over the entire duration of a tone stimulus. However, this approach may mask functionally significant variations in tuning over the time course of the response. To explore this possibility, frequency response functions (FRFs) based on population multiunit activity evoked by pure tones of 175 or 200 ms duration were examined within four time windows relative to stimulus onset corresponding to "on" (10-30 ms), "early sustained" (30-100 ms), "late sustained" (100-175 ms), and "off" (185-235 or 210-260 ms) portions of responses in primary auditory cortex (A1) of 5 awake macaques. FRFs of "on" and "early sustained" responses displayed a good concordance, with best frequencies (BFs) differing, on average, by less than 0.25 octaves. In contrast, FRFs of "on" and "late sustained" responses differed considerably, with a mean difference in BF of 0.68 octaves. At many sites, tuning of "off" responses was inversely related to that of "on" responses, with "off" FRFs displaying a trough at the BF of "on" responses. Inversely correlated "on" and "off" FRFs were more common at sites with a higher "on" BF, thus suggesting functional differences between sites with low and high "on" BF. These results indicate that frequency tuning of population responses in A1 may vary considerably over the course of the response to a tone, thus revealing a temporal dimension to the representation of sound spectrum in A1.

Neurons and objects: the case of auditory cortex

Sounds are encoded into electrical activity in the inner ear, where they are represented (roughly) as patterns of energy in narrow frequency bands. However, sounds are perceived in terms of their high-order properties. It is generally believed that this transformation is performed along the auditory hierarchy, with low-level physical cues computed at early stages of the auditory system and high-level abstract qualities at high-order cortical areas. The functional position of primary auditory cortex (A1) in this scheme is unclear – is it ‘early’, encoding physical cues, or is it ‘late’, already encoding abstract qualities? Here we argue that neurons in cat A1 show sensitivity to high-level features of sounds. In particular, these neurons may already show sensitivity to ‘auditory objects’. The evidence for this claim comes from studies in which individual sounds are presented singly and in mixtures. Many neurons in cat A1 respond to mixtures in the same way they respond to one of the individual components of the mixture, and in many cases neurons may respond to a low-level component of the mixture rather than to the acoustically dominant one, even though the same neurons respond to the acoustically-dominant component when presented alone.

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.

Ultra-fine frequency tuning revealed in single neurons of human auditory cortex

Just-noticeable differences of physical parameters are often limited by the resolution of the peripheral sensory apparatus. Thus, two-point discrimination in vision is limited by the size of individual photoreceptors. Frequency selectivity is a basic property of neurons in the mammalian auditory pathway. However, just-noticeable differences of frequency are substantially smaller than the bandwidth of the peripheral sensors. Here we report that frequency tuning in single neurons recorded from human auditory cortex in response to random-chord stimuli is far narrower than that typically described in any other mammalian species (besides bats), and substantially exceeds that attributed to the human auditory periphery. Interestingly, simple spectral filter models failed to predict the neuronal responses to natural stimuli, including speech and music. Thus, natural sounds engage additional processing mechanisms beyond the exquisite frequency tuning probed by the random-chord stimuli.

Comparison between offset and onset responses of primary auditory cortex ON-OFF neurons in awake cats

Primary auditory cortex (A1) neurons are believed not to carry much information about tonal offsets because A1 neurons in barbiturate-anesthetized animals are usually described as having only onset responses. We investigated tonal offset responses in comparison with onset responses in the caudal part of A1 of awake cats. Cells responding to both onsets and offsets were commonly found (59.2% of recorded cells). Offset responses usually co-occurred with phasic onset responses or phasic components of sustained responses. These ON-OFF cells had diverse combinations of offset- and onset-frequency-receptive field (FRF): offset-FRF was similar to onset-FRF, or narrower, wider, lower, or higher than onset-FRF. The distribution of FRF patterns was diffuse with no boundaries between the different FRF-pattern groups. The onset- versus offset-FRF pattern of each cell remained unchanged across multiple stimulus intensities. Mean offset response showed similar peak latency (19.5 vs. 21.5 ms), longer half-decay time (74.5 vs. 48.5 ms), and lower peak amplitude (20.4 vs. 35.9 spikes/s) compared with the mean onset response. Although offset responses were facilitated when preceded by the suppression of spike activity, they were still elicited without preceding spike suppression. It is concluded that neurons showing paired onset and offset responses are predominant in the caudal A1. Their frequency-filtering property is usually not static but dynamic, changing between sound onsets and offsets. Offset responses are similarly precise and salient as onset responses for effectively encoding sound offsets. They may be elicited as active spike responses to sound offset rather than simple rebound facilitation.

Responses of neurons in primary auditory cortex (A1) to pure tones in the halothane-anesthetized cat

The responses of primary auditory cortex (A1) neurons to pure tones in anesthetized animals are usually described as having mostly narrow, unimodal frequency tuning and phasic responses. Thus A1 neurons are believed not to carry much information about pure tones beyond sound onset. In awake cats, however, tuning may be wider and responses may have substantially longer duration. Here we analyze frequency-response areas (FRAs) and temporal-response patterns of 1,828 units in A1 of halothane-anesthetized cats. Tuning was generally wide: the total bandwidth at 40 dB above threshold was 4 octaves on average. FRA shapes were highly variable and many were diffuse, not fitting into standard classification schemes. Analyzing the temporal patterns of the largest responses of each unit revealed that only 9% of the units had pure onset responses. About 40% of the units had sustained responses throughout stimulus duration (115 ms) and 13% of the units had significant and informative responses lasting 300 ms and more after stimulus offset. We conclude that under halothane anesthesia, neural responses show many of the characteristics of awake responses. Furthermore, A1 units maintain sensory information in their activity not only throughout sound presentation but also for hundreds of milliseconds after stimulus offset, thus possibly playing a role in sensory memory.

Spectral-shape preference of primary auditory cortex neurons in awake cats

The study of the influence of spectral envelopes of complex tones on cortical neuron discharges is important with respect to understanding the formant processing of vowels. That the sharpness of formants can contribute to the vowel quality has previously been shown in psychophysical experiments. However, it is unknown how this parameter is reflected by cortical neuron discharges, especially when a formant falls into different portions of the neuronal frequency receptive field (FRF). To show this is the aim of this study focusing on the sustained discharging neurons in the low frequency portion of cat primary auditory cortex (A1). The stimuli were multi-frequency tones whose spectral envelope was sinusoidally modulated with a peak fixed at a neuron's best frequency. The modulation depth, defined as damping-amplitude (DA), varied systematically. Stimulus bandwidth also varied systematically. Large DA at off-center frequencies produces a prominent spectral peak with steep slope, whereas small DA has a less well-defined spectral peak with gentle slope. Single frequency and two tone stimuli served to delineate the excitatory and inhibitory subfields of FRF. The A1 neuron preferred large DA when the sound energy fell into FRF with the relatively large inhibitory-subfield, while the same cell preferred small DA when it fell into FRF with the relatively small inhibitory-subfield. It is concluded that the A1 neuron can estimate steepness of slope of a spectral peak, and that a preference for steep slopes stems from a shift of balance toward the spectral inhibition, whereas a preference for gentle slopes, toward the spectral excitation.

Interaction of Excitatory and Inhibitory Frequency-receptive Fields in Determining Fundamental Frequency Sensitivity of Primary Auditory Cortex Neurons in Awake Cats

Harmonic complex tones produce pitch-height perception corresponding to the fundamental frequency (F0). This study investigates how the spectral cue of F0 is processed in neurons of the primary auditory cortex (A1) with sustained-response properties. We found F0-sensitive and -insensitive cells: the former discriminated between harmonics and noise, while the latter did not. F0-sensitive cells preferred F0s corresponding to the best frequency (BF) and 0.5 x BF. The F0-sensitivity to F0=0.5 x BF was preserved for missing F0, but abolished by eliminating both F0 and the second harmonic. The inhibitory subfield of the frequency-receptive field was restricted to the spectral region between the preferred harmonics in F0-sensitive cells, while it was frequency unspecific in F0-insensitive cells. We conclude that (i) A1 is well organized for discrimination between harmonics and noise; (ii) pitch-height is represented along with the tonotopic axis; (iii) all aspects of the sustained neural responses to harmonic and noise stimuli are consequences of spectral filtering; and (iv) although the observed cell behavior explains some psychophysical pitch perception behaviors, such as pitch-chroma (helical pitch perception with frequency elevation), pitch-level tolerance and adaptive behavior, F0-encoding in A1 remains at the incomplete perceptual level (dominance of the third to fifth harmonics for pitch strength is unexplainable by the cell behavior).

Spectral-edge sensitivity of primary auditory cortex neurons in alert cats

Although psychophysical studies have revealed involvement of spectral edges in auditory perception, little is known about neural processing. This study investigates how spectral edges are processed in neurons of alert cat primary-auditory-cortex (A1) with sustained response property. Stimuli are low-pass, high-pass and band-pass tones with sharp spectral edges whose edge-frequencies were systematically shifted, constructing edge-frequency response functions. Pure- and two-tone stimuli served to delineate excitatory and inhibitory subfields of the frequency response field (FRF). Based on the response function characteristics, cells were divided into edge-sensitive and edge-insensitive cells: the edge sensitive cells had narrow tuning to the high-edge (type-II cells) or low-edge (type-III cells) frequencies, while the edge insensitive cells were driven by any static stimuli with energy on FRF (type-I) or only very narrowband stimuli with energy confined to FRF (type-IV cells). Edge-sensitive cells showed a close correlation between the best frequencies of the single-frequency (BFSF) and edge-frequency (BFEF) response functions and between their half-height bandwidths, suggesting that the edge-frequency identification is processed along the tonotopic axis in A1. BFSF shifted (mean 0.11 octaves) into the stimulus band from the BFEF (closely corresponding to pitch shift into stimulus band from the edge frequency in human psychophysical data of edge-pitch), suggesting central mechanism of edge-pitch sensation. Type-I cells had non-significant inhibitory subfields of FRF; type-II cells had the significant inhibitory subfield on the higher frequency side; type-III cells, on the lower frequency side; and type-IV cells, on both sides, suggesting that the inhibitory mechanism characterizes the cell-type specific spectral-edge sensitivity.

Suppression of auditory cortical activities in awake cats by pure tone stimuli

Based on the time courses of excitatory spike-responses to pure-tone stimuli, neurons in the primary auditory cortex (A1) of awake cats have been classified into the phasic cell (P-cell), the tonic cell (T-cell), and the phasic-tonic cell (PT-cell). In the present study, taking advantage of the presence of the spontaneous spike-activities of A1 neurons in awake animals, time courses of suppressive spike-responses to pure-tone stimuli were studied by constructing spectro-temporal spike-activity diagrams. In P-cell, the suppression and excitation temporally alternated and spectrally co-occurred, restricting excitatory spike-responses within narrow temporal limits but not setting the spectral limits. In T-cell, the suppression and excitation spectrally alternated and temporally co-occurred, restricting excitatory frequency-tuning but not setting the time limits. PT-cell has mixed response properties of P- and T-cells. The findings suggest that: (1) P-cell analyzes temporal information of the sound without active spectral limits, (2) T-cell analyzes spectral information without limits of time, and (3) PT-cell analyzes spectrally and temporally complex auditory information. Taken in the light of recent findings that the shift of the balance of the synaptic excitation and inhibition results in the spike-activity modulation of the cerebral cortical neurons, it is suggested that the temporal shift of the balance of the excitation and inhibition works in P-cell; the spectral shift, in T-cell; and the combination of the temporal and spectral shifts, in PT-cell, underlying the functional differences between cell-types.