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PubMed1995

Time-variant structure of auditory cortex of the macaque monkey.

Supra-threshold responses of neurons in the primary auditory cortex of the macaque monkey are so complex that the characteristic frequency cannot be determined. By statistical analyses based on analysis of variance and principal component analysis, we demonstrated that the similarity of neurons from penetration perpendicular to the surface of the cortex decreases to such a level that they cannot be considered as a unit and that correlation among neurons from different cortical positions can be higher than that for neurons from penetration. This means that the frequency map is not evident for stimuli at supra-threshold. Moreover, we found that a single neuron utilizes different time segments to extract different physical aspects of stimuli. Our results indicate that encoding auditory information may involve other structures than the frequency map.

Animals↗
PubMedDate not supplied

Excitatory interactions in neuronal networks which include cells of the auditory cortex and the medial geniculate body.

The use of the method of cross-correlation analysis to elucidate the interactions between simultaneously recorded neurons from various loci of the auditory cortex (AC) and the medial geniculate body (MGB) has made it possible to identify the following characteristics of the functional organization of the excitatory interactions in the thalamocortical neuronal networks: the interdependant impulse action of neurons located at various loci of the AC and MGB was determined by reciprocal excitatory connections; the efficiency of the connections between neurons of the AC, 400-500 microns apart, and between tonotopically associated neurons of the AC and MGB was approximately identical (associations were identified in 12% of the cases); the "divergent" properties of the MGB (AC) neurons were manifested in the fact that one and same neuron could simultaneously excite both neighboring cells and neurons from one or several loci of the AC (MGB); the "convergent" properties of the AC and MGB neurons were manifested in the fact that cells located at various loci of the AC and MGB simultaneously excited one neuron. The results make it possible to explain the deviations observed in the investigation of RF of neurons of the AC and MGB from the principle of tonotopical organization. It is hypothesized that the character of the organization of the excitatory connections in the thalamocortical networks may promote the creation of the necessary conditions for the modification of the efficiency of synapses between all of the elements of the network during the stimulation of individual elements.

Acetylcholine↗
PubMed2006

Salicylate induced tinnitus: behavioral measures and neural activity in auditory cortex of awake rats.

Neurophysiological studies of salicylate-induced tinnitus have generally been carried out under anesthesia, a condition that abolishes the perception of tinnitus and depresses neural activity. To overcome these limitations, measurement of salicylate induced tinnitus were obtained from rats using schedule induced polydipsia avoidance conditioning (SIPAC) and gap pre-pulse inhibition of acoustic startle (GPIAS). Both behavioral measures indicated that tinnitus was present after treatment with 150 and 250 mg/kg of salicylate; measurements with GPIAS indicated that the pitch of the tinnitus was near 16 kHz. Chronically implanted microwire electrode arrays were used to monitor the local field potentials and spontaneous discharge rate from multiunit clusters in the auditory cortex of awake rats before and after treatment with 150 mg/kg of salicylate. The amplitude of the local field potential elicited with 60 dB SPL tone bursts increased significantly 2h after salicylate treatment particularly at 16-20 kHz; frequencies associated with the tinnitus pitch. Field potential amplitudes had largely recovered 1-2 days post-salicylate when behavioral results showed that tinnitus was absent. The mean spontaneous spike recorded from the same multiunit cluster pre- and post-salicylate decreased from 22 spikes/s before treatment to 14 spikes/s 2h post-salicylate and recovered 1 day post-treatment. These preliminary physiology data suggest that salicylate induced tinnitus is associated with sound evoked hyperactivity in auditory cortex and spontaneous hypoactivity.

Animals↗
PubMed2006

Dual time scales for categorical decision making in auditory cortex.

Category formation allows us to group perceptual objects into meaningful classes and is fundamental to cognition. Categories can be derived from similarity relationships of object features by using prototypes or multiple exemplars, or from abstract relationships of features and rules . A variety of brain areas have been implicated in categorization processes, but mechanistic insights on the single-cell and local-network level are still rare and limited to the matching of individual objects to categories . For directional categorization of tone steps, as in melody recognition , abstract relationships between sequential events (higher or lower in frequency) have to be formed. To explore the neuronal mechanisms of this categorical identification of step direction, we trained monkeys for more than two years on a contour-discrimination task with multiple tone sequences. In the auditory cortex of these highly trained monkeys, we identified two interrelated types of neuronal firing: Increased phasic responses to tones categorically represented the reward-predicting downward frequency steps and not upward steps; subsequently, slow modulations of tonic firing predicted the behavioral decisions of the monkeys, including errors. Our results on neuronal mechanisms of categorical stimulus identification and of decision making attribute a cognitive role to auditory cortex, in addition to its role in signal processing.

Acoustic Stimulation↗
PubMed2006

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.

Acoustic Stimulation↗
PubMedDate not supplied

Subdivisions of auditory cortex and levels of processing in primates.

In a series of experiments on New World and Old World monkeys, architectonic features of auditory cortex were related to tone frequency maps and patterns of connections to generate and evaluate theories of cortical organization. The results suggest that cortical processing of auditory information involves a number of functionally distinct fields that can be broadly grouped into four or more levels of processing. At the first level, there are three primary-like areas, each with a discrete pattern of tonotopic organization, koniocortical histological features, and direct inputs from the ventral division of the medial geniculate complex. These three core areas are interconnected and project to a narrow surrounding belt of perhaps seven areas which receive thalamic input from the major divisions of the medial geniculate complex, the suprageniculate/limitans complex, and the medial pulvinar. The belt areas connect with a lateral parabelt region of two or more fields that are almost devoid of direct connections with the core and the ventral division of the medial geniculate complex. The parabelt fields connect with more distant cortex in the superior temporal gyrus, superior temporal sulcus, and prefrontal cortex. The results indicate that auditory processing involves 15 or more cortical areas, each of which is interconnected with a number of other fields, especially adjoining fields of the same level.

Animals↗
PubMed1998

Factors shaping the response latencies of neurons in the cat's auditory cortex.

This article addresses two issues. Firstly, the hypothesis that response latency might be a neural code for tone frequency was examined in single-neuron data from the primary auditory cortex of anesthetized cats. Minimal response latencies for characteristic frequency (CF) tones were independent of neural CF. Mean response latencies for a constant amplitude CF tone were also independent of CF. These data, and the fact that cortical neurons do not have an obvious independent referent for stimulus onset time, do not support the view that latency is a code for frequency. Secondly, to investigate a simple threshold model of spike initiation time, we describe the prolongations of response latency with increases in stimulus rise time and their dependence on the peak amplitude of the stimulus. These data show that in cortical neurons, it is not the peak stimulus intensity which determines first-spike latency, and second, that the response latencies are systematically not those expected on the basis of simple threshold model.

Acoustic Stimulation↗
PubMed1989

Repetition rate and signal level effects on neuronal responses to brief tone pulses in cat auditory cortex.

This study describes the effects on the spike count, spike timing, and entrainment of cat auditory cortex neurons of parametric variations in the repetition rate and amplitude of a brief, characteristic frequency tone pulse. Data were obtained from single neurons in barbiturate-anesthetized cats to which signals were presented monaurally to the ear contralateral to the recording electrode. All neurons showed low-pass sensitivity to tone repetition rate. In cells with a monotonic rate response, the effect of an increasing stimulus level was to elevate the response rate and to extend performance to higher repetition rates. In nonmonotonic cells, cutoff frequencies (for repetition rate) varied with overall spike count. Latent periods increased with increases in repetition rate. This effect developed over the first few stimulus trials at any given repetition rate. Spike entrainment to the tone pulses varied with both repetition rate and signal level. Increases in signal level improved entrainment for responses to stimuli presented at low repetition rates, but entrainment at high repetition rates always saturated at significantly imperfect levels.

Acoustic Stimulation↗
PubMed1980

Binaural response-specific bands in primary auditory cortex (AI) of the cat: topographical organization orthogonal to isofrequency contours.

The spatial distribution of neurons with different binaural response properties has been studied within the three dimensions of the primary auditory cortex (AI) in the cat. Using dichotic stimulation, 92% of neurons encountered could be classified into either the excitatory/excitatory (EE) or excitatory/inhibitory (EI) interaction class. In nearly all of almost 800 penetrations introduced along radial axes, all neurons encountered along a given penetration were of the same binaural response class. Neurons of different binaural interaction classes were spatially segregated within the plane of the cortex. Electrode penetrations made parallel to isofrequency contours traversed the mediolateral extent of AI through the middle layers of the cortex. A sharp segregation of units by binaural response class was observed in these penetrations, i.e. sequences of neurons that were all of the EE class alternated with sequences of EI neurons. The regions of uniform response to binaural stimulation formed radially organized topographical subunits that were elongated along the rostrocaudal dimension of AI. These binaural interaction bands intersect the lines of re-representation of the cochlear sensory epithelium ('isofrequency contours') and, thus, create subdivisions of AI that each contain a representation of the entire audible frequency domain. The implications of these results for the concept of AI as a unitary element in auditory processing are discussed.

Acoustic Stimulation↗
PubMed1972

Single cell activity in the auditory cortex of Rhesus monkeys: behavioral dependency.

The response to repetitive stimulation of single cells in the auditory cortex of the monkey is dependent upon behavioral performance and training of the subject in a simple auditory discrimination task. In the trained, performing animal, single cells are more responsive than in the animal that is trained but not performing in the task. In the naive monkey, evoked responses are labile and are maintained only with nonrepetitive auditory stimuli.

Acoustic Stimulation↗
PubMed2005

The effects of nembutal anesthesia on the auditory steady-state response (ASSR) from the inferior colliculus and auditory cortex of the chinchilla.

We examined the effects of nembutal anesthesia on the amplitude of the auditory steady-state response (ASSR) in the inferior colliculus (IC) and auditory cortex (AC) of the chinchilla. Tungsten electrodes were chronically implanted following anesthesia with ketamine/acepromazine. After a recovery period, the chinchillas were placed in a passive restraining device and put in a sound-attenuating booth. Recordings were made from the right IC and AC simultaneously, while a two-tone stimulus was presented to the left ear. The stimuli consisted of two equal-level tones (F1 and F2) that were mixed acoustically; F1 remained constant at 2000 Hz, while F2 varied between 2029 and 2249 Hz, in steps of approximately 20 Hz. The stimuli decreased in 10 dB steps from 80 to 30 dB pSPL. Animals were evaluated when unanesthetized, as well as when anesthetized with nembutal (on separate days). In the IC, the administration of nembutal resulted in either no change in ASSR amplitude or an amplitude increase for difference tone (DT) frequencies below 90 Hz, while an amplitude decrease was typically seen for DT frequencies at or above 90 Hz. In the AC, a decrease in amplitude was seen across DT frequencies and stimulus levels after the administration of nembutal anesthesia. Our results suggest that both the AC and IC may contribute to the scalp-recorded ASSR in the awake state. However, in the nembutal-anesthetized state, it seems unlikely that the AC contributes substantially to the surface-recorded ASSR, as the AC response was greatly attenuated under nembutal anesthesia. In contrast, the IC ASSR responses remained robust, which makes it a likely contributor to the surface-recorded responses under nembutal anesthesia.

Acoustic Stimulation↗
PubMed2006

Functional local connections with differential activity-dependence and critical periods surrounding the primary auditory cortex in rat cerebral slices.

Sensory information is processed in neural networks connecting the primary sensory cortices with surrounding higher areas. Here, we investigated the properties of local connections between the primary auditory cortex (area 41) and surrounding areas (areas 20, 36, 18a and 39) in rat cerebral slices. Neural activities elicited by repetitive electrical stimulation were visualized using the activity-dependent changes in endogenous fluorescence derived from mitochondrial flavoproteins, which mostly reflect activities produced by polysynaptic glutamatergic transmission. Polysynaptic feedforward propagation was dominant compared with the corresponding polysynaptic feedback propagation between the primary (area 41) and secondary (areas 20 and 36) auditory cortices, while such a tendency was less clear in other pathways. Long inter-areal (>1 mm) propagation with the same dominancy was observed after layer V stimulation between areas 41 and 20, and was not affected by cutting the underlying white matter. Activity-dependent changes in neural activities induced by low-frequency stimulation in the presence of 1 microM bicuculline were investigated using Ca2+ imaging. Significant potentiation of the polysynaptic Ca2+ activities was only observed in polysynaptic feedforward pathways from the primary to secondary auditory cortices. Experience-dependence of the connections between areas 41 and 20 was investigated using flavoprotein fluorescence imaging. The activities from areas 41 to 20 were reduced by cochlear lesions produced at P12 but not at P28, while the activities from areas 20 to 41 were reduced by the lesions at P28, suggesting the critical period for the polysynaptic feedforward connection was before P28, while for the polysynaptic feedback connection was after P28.

Animals↗
PubMed2004

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.

Animals↗
PubMed2006

Detection of interaural correlation by neurons in the superior olivary complex, inferior colliculus and auditory cortex of the unanesthetized rabbit.

A critical binaural cue important for sound localization and detection of signals in noise is the interaural time difference (ITD), or difference in the time of arrival of sounds at each ear. The ITD can be determined by cross-correlating the sounds at the two ears and finding the ITD where the correlation is maximal. The amount of interaural correlation is affected by properties of spaces and can therefore be used to assess spatial attributes. To examine the neural basis for sensitivity to the overall level of the interaural correlation, we identified subcollicular neurons and neurons in the inferior colliculus (IC) and auditory cortex of unanesthetized rabbits that were sensitive to ITDs and examined their responses as the interaural correlation was varied. Neurons at each brain level could show linear or non-linear responses to changes in interaural correlation. The direction of the non-linearities in most neurons was to increase the slope of the response change for correlations near 1.0. The proportion of neurons with non-linear responses was similar in subcollicular and IC neurons but increased in the auditory cortex. Non-linear response functions to interaural correlation were not related to the type of response as determined by the tuning to ITDs across frequencies. The responses to interaural correlation were also not related to the frequency tuning of the neuron, unlike the responses to ITD, which broadens for neurons tuned to lower frequencies. The neural discriminibility of the ITD using frozen noise in the best neurons was similar to the behavioral acuity in humans at a reference correlation of 1.0. However, for other reference ITDs the neural discriminibility was more linear and generally better than the human discriminibility of the interaural correlation, suggesting that stimulus rather than neural variability is the basis for the decline in human performance at lower levels of interaural correlation.

Animals↗
PubMed1994

Functional organization of sound direction and sound pressure level in primary auditory cortex of the cat.

1. The functional organization of neuronal tuning to the azimuthal location and sound pressure level (SPL) of noise bursts was studied in high-frequency primary auditory cortex (AI) of barbiturate-anesthetized cats. Three data collection strategies were used to map neural responses: 1) electrode penetrations oriented normal to the cortical surface provided information on the radial organization of neurons' responses; 2) neurons' responses were examined at a few points in the middle cortical layers in multiple normal penetrations across AI to produce fine-grain maps of azimuth and level selectivity; and 3) electrode penetrations oriented tangential to the cortical surface provided information on neurons' responses along the isofrequency dimension. 2. An azimuth-level data set was obtained for each single- or multiple- (multi-) unit recording; this consisted of responses to noise bursts at five SPLs (0-80 dB in 20-dB steps) from seven azimuthal locations in the frontal hemifield (-90 to +90 degrees in 30 degrees steps; 0 degree elevation). An azimuth function was derived from these data by averaging response magnitude over all SPLs at each azimuth tested. A preferred azimuth range (PAR; range of azimuths over which the response was > or = 75% of maximum) was calculated from the azimuth function and provided a level-independent measure of azimuth selectivity. Each PAR was assigned to one of four azimuth preference categories (contralateral-, midline-, ipsilateral-preferring, or broad/multipeaked) according to its location and extent. A level function obtained from the data set (responsiveness averaged over all azimuths) was classified as monotonic if it showed a decrease of < or = 25% (relative to maximum) at the highest SPL tested (usually 80 dB), and nonmonotonic if it showed a decrease of > 25%. The percentage reduction in responsiveness, relative to maximum, at the highest level tested (termed nonmonotonic strength) and the preferred level range (PLR; range of SPLs over which responsiveness was > or = 75% of maximum) of each response was also determined. 3. Normal penetrations typically showed a predominance of one azimuth preference category and/or level function type. The majority of penetrations (26/36: 72.2%) showed statistically significant azimuth preference homogeneity, and approximately one-half (17/36: 47.2%) showed significant level function type homogeneity. Over one-third (13/36) showed significant homogeneity for both azimuth preference and level function type. 4. Mapping experiments (n = 4) sampled the azimuth and level response functions at two or more depths in closely spaced normal penetrations that covered several square millimeters of AI.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Auditory cortex: comparative aspects of maps and plasticity.

Much recent work in the field of auditory cortex analysis consists of an intensified search for complex sound representation and sound localization mechanisms using tonotopic maps as a frame of reference. Mammalian species rely on parallel processing in multiple tonotopic and non-tonotopic maps but show different degrees of unit complexity, and orderly representation of acoustic dimensions in such maps depending on the predictability of sounds in their environment. Birds appear to rely chiefly on one tonotopic map which harbours multidimensional complex representations. During development and after partial hearing loss, tonotopic organization changes in a predictable manner. Learning also modifies the spatial representation of sounds and even modifies tonotopic organization, but the spatial rules involved in this process have not yet emerged.

Animals↗
PubMed1986

Searching for neural correlates of the hearing sensation fluctuation strength in the auditory cortex of squirrel monkeys.

Sounds with slow (less than 20 Hz) fluctuations may elicit the hearing sensation fluctuation strength. For AM tones, neural correlates of fluctuation strength were searched in the auditory cortex of unanesthetized squirrel monkeys. To enable a comparison of psychophysical and physiological data, the 'modulation' of the peristimulus time histogram was fitted by a sinusoidal function. The dependence of the amplitude of this function on modulation frequency, modulation depth and sound pressure level was often comparable to the dependence of fluctuation strength on the same stimulus parameters. In particular, as a function of modulation frequency, the neural data also show a bandpass characteristic at low modulation frequencies as was found for the hearing sensation fluctuation strength.

Animals↗
PubMed1981

Ipsilateral corticocortical projections related to binaural columns in cat primary auditory cortex.

Projections from the high-frequency representations of auditory cortical fields A and P were autoradiographically labeled using anterograde transport of a mixture of tritiated proline and leucine. A single injection of isotope into either cortical field resulted in multiple regions of dense labeling within ipsilateral AI. As the result of an injection of radioactive label into field A in one experiment, densely labeled regions were found to be systematically related to the binaural map throughout the high-frequency representation of AI. Contralateral dominant suppression responses were located in regions of dense labeling while summation responses were located in regions of less dense labeling. In contrast, it was previously found that callosal axon terminations were more densely concentrated in summation columns than in contralateral dominant suppression columns (Imig and Brugge, '78). Thus, the two classes of binaural columns differ with respect to the density of innervation they receive from these ipsilateral and contralateral populations of neurons. In other experiments in which isotope was injected into field A, a systematic relationship between density of ipsilateral labeling and binaural response class was only seen in a portion of AI; in other regions no relationship was evident. A simple interpretation of these data follows. Within a contiguous territory in field A is a population of neurons whose axons provide more dense innervation to contralateral dominant suppression columns than to summation columns in ipsilateral AI. Injections of radioactivity confined to this territory result in a systematic relationship between the density of labeling and the binaural map throughout AI. Outside this territory is a population of neurons whose axon terminations are not systematically related to the binaural map in AI. Isotope injections which engage both territories may result in a systematic relation between density of labeling and the binaural map in one portion of AI, while in another, no relationship may be evident. There is some indication that projections from field P may also be related to binaural columns in AI in the same manner as are the projections from field A.

Acoustic Stimulation↗