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PubMed2001

Functional organization of squirrel monkey primary auditory cortex: responses to pure tones.

The spatial organization of response parameters in squirrel monkey primary auditory cortex (AI) accessible on the temporal gyrus was determined with the excitatory receptive field to pure tone stimuli. Dense, microelectrode mapping of the temporal gyrus in four animals revealed that characteristic frequency (CF) had a smooth, monotonic gradient that systematically changed from lower values (0.5 kHz) in the caudoventral quadrant to higher values (5--6 kHz) in the rostrodorsal quadrant. The extent of AI on the temporal gyrus was approximately 4 mm in the rostrocaudal axis and 2--3 mm in the dorsoventral axis. The entire length of isofrequency contours below 6 kHz was accessible for study. Several independent, spatially organized functional response parameters were demonstrated for the squirrel monkey AI. Latency, the asymptotic minimum arrival time for spikes with increasing sound pressure levels at CF, was topographically organized as a monotonic gradient across AI nearly orthogonal to the CF gradient. Rostral AI had longer latencies (range = 4 ms). Threshold and bandwidth co-varied with the CF. Factoring out the contribution of the CF on threshold variance, residual threshold showed a monotonic gradient across AI that had higher values (range = 10 dB) caudally. The orientation of the threshold gradient was significantly different from the CF gradient. CF-corrected bandwidth, residual Q10, was spatially organized in local patches of coherent values whose loci were specific for each monkey. These data support the existence of multiple, overlying receptive field gradients within AI and form the basis to develop a conceptual framework to understand simple and complex sound coding in mammals.

Acoustic Stimulation↗
PubMed1987

Thalamic projections to fields A, AI, P, and VP in the cat auditory cortex.

Thalamocortical projections to four tonotopic fields (A, AI, P, and VP) of the cat auditory cortex were studied by using combined microelectrode mapping and retrograde axonal transport techniques. Horseradish peroxidase (HRP) or HRP combined with either tritiated bovine serum albumin or nuclear yellow was injected into identified best-frequency sites of one or two different fields in the same brain. Arrays of labeled neurons were related to thalamic nuclei defined on the basis of their cytoarchitecture and physiology. In some cases, patterns of labeling were directly compared with thalamic best-frequency maps obtained in the same brain. We compared only patterns of labeling resulting from injections into similar parts of the frequency representation in different fields to insure that observed differences in patterns of labeling did not simply reflect differences in the frequency representation at the injection sites. The thalamic projection to the four fields is divided among seven nuclei, three tonotopic nuclei (ventral nucleus, V; lateral part of the posterior group of thalamic nuclei, Po; and dorsal cap nucleus, d) and four nontonotopic nuclei (caudodorsal nucleus, cd; ventrolateral nucleus, vl; and small, Ms; and medium-large, Mg, cell regions of the medial division). Projections to each field differ, and each field receives inputs from tonotopic and nontonotopic nuclei. Field A receives its major inputs from Po and Mg, and a minor input from V. Field AI receives its major inputs from V, Po, and Mg, although Po and Mg have heavier projections to field A. Field P receives its major inputs from V, d, and vl; and minor inputs from cd, Ms, Mg, and Po. Field VP receives major inputs from V, vl, and cd; and minor inputs from d, Ms, and Mg. There are segregated territories in V and Po in which most neurons projects to one cortical field (major projection), and a smaller proportion projects to one or more other fields (minor projections). Field VP receives a major projection from the caudal pole of V. Field P receives a major projection from the caudal half of V, and from a thin band along the dorsal border of rostral V. Field AI receives a major projection from most of the rostral one-half of V, and smaller areas in Po and the caudal half of V exclusive of its caudal pole. Field A receives a major projection from most of Po.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2005

Volumetric vs. surface-based alignment for localization of auditory cortex activation.

The high degree of intersubject structural variability in the human brain is an obstacle in combining data across subjects in functional neuroimaging experiments. A common method for aligning individual data is normalization into standard 3D stereotaxic space. Since the inherent geometry of the cortex is that of a 2D sheet, higher precision can potentially be achieved if the intersubject alignment is based on landmarks in this 2D space. To examine the potential advantage of surface-based alignment for localization of auditory cortex activation, and to obtain high-resolution maps of areas activated by speech sounds, fMRI data were analyzed from the left hemisphere of subjects tested with phoneme and tone discrimination tasks. We compared Talairach stereotaxic normalization with two surface-based methods: Landmark Based Warping, in which landmarks in the auditory cortex were chosen manually, and Automated Spherical Warping, in which hemispheres were aligned automatically based on spherical representations of individual and average brains. Examination of group maps generated with these alignment methods revealed superiority of the surface-based alignment in providing precise localization of functional foci and in avoiding mis-registration due to intersubject anatomical variability. Human left hemisphere cortical areas engaged in complex auditory perception appear to lie on the superior temporal gyrus, the dorsal bank of the superior temporal sulcus, and the lateral third of Heschl's gyrus.

Acoustic Stimulation↗
PubMed1984

[Neuronal interactions in the auditory cortex of the alert cat].

Background activity of neighbouring neurons was recorded during chronic experiments in auditory cortex of unanesthetized unimmobilized cats. Functional connection between simultaneously observed neurons was estimated by cross-correlation of the spike trains. 20 pairs of neurons were studied. The most frequent type of interaction (50%) was a "shared excitatory input" from specific auditory afferents. Interaction of the "shared inhibitory input" type (5%) and complex patterns of interactions were found. No interaction between cells was observed in 10% of cases. Direct inhibitory influence of a cell on a neighbouring one was not found at all. Probable reasons of absence of direct inhibitory interactions between auditory cortical neurons are discussed. Low level of background activity in inhibitory neurons is regarded to be possible.

Action Potentials↗
PubMed1989

Cholinergic modulation of frequency receptive fields in auditory cortex: I. Frequency-specific effects of muscarinic agonists.

Previously we reported that acetylcholine (ACh) and acetyl-beta-methacholine (MCh) modify responses of neurons in auditory cortex to individual frequencies. The purpose of this study was to determine whether muscarinic agonists produce frequency-specific alterations or general changes in cellular responses. Frequency-specific modifications would be evident in alterations of frequency receptive fields (FRF) that differed across frequencies while general effects would be seen as changes that were more or less the same over frequencies. Responses of single neurons to designated sets of tones were recorded in the auditory cortex of chronically prepared awake cats before, during, and following ejection of ACh or MCh by iontophoresis or micropressure using multibarrel micropipettes. Frequency receptive fields were determined by presenting isointensity tones across a range of frequencies including the cell's best frequency (BF) to tone onset. FRF for "off" and "sustained (through)" responses were also determined quantitatively. The effects of ACh and MCh were predominantly frequency-specific (77%, 39/51 cells); general changes (19%, 10/51) and no effects (4%, 2/51) were less likely. Frequency-specific effects involved both facilitation and reduction of the same response component to different frequencies within the same neuron. For responses to tone onset (but not "through" and "off" responses), agonists were more likely to produce a decrease at the BF while simultaneously increasing responses to other frequencies. Agonists could increase or decrease frequency selectivity. Effects of agonists could be blocked by atropine, suggesting involvement of muscarinic receptors.

Acetylcholine↗
PubMed1980

Electrophysiologic studies of the auditory cortex in the awake monkey.

An overview of some recent developments in the study of central auditory processes is presented. We describe findings from single cell physiologic studies that contribute to our understanding of central auditory function, the development of behavioral techniques permitting precise evaluation of hearing in animals, and the power and potential of integrative neurophysiologic-behavioiral investigations in defining and analyzing central neural mechanisms that underlie normal perception and imperception. The influence of changes in intermodality attention on evoked activity of 25 cells of the auditory cortex is described and compared to effects of other attentional changes. A small but consistent increase in excitatory cell evoked discharge rate and a reduction in initial latency of response are shown to be correlated with a shift from a visual to an auditory task. In general, behavioral states ranging from sleep-waking to attention to a specific acoustic cue are shown to influence responsiveness of the auditory system and variability of response. Procedures are described for effective control of attentional factors and reduction of electrophysiologic variability. The contributions of these data and procedures to studies of the encoding of complex auditory signals and studies of neural mechanisms underlying such disorders as noise induced hearing loss are discussed.

Animals↗
PubMed2003

Characterisation of multiple physiological fields within the anatomical core of rat auditory cortex.

The organisation and response properties of the rat auditory cortex were investigated with single and multi-unit electrophysiological recording. Two tonotopically organised 'core' fields, i.e. the primary (A1) and anterior (AAF) auditory fields, as well as three non-tonotopically organised 'belt' fields, i.e. the posterodorsal (PDB), dorsal (DB) and anterodorsal (ADB) belt fields, were identified. Compared to neurones in A1, units in AAF exhibited broader frequency tuning, as well as shorter minimum, modal and mean first spike latencies. In addition, units in AAF showed significantly higher thresholds and best SPLs, as well as broader dynamic ranges. Units in PDB, DB and ADB were characterised by strong responses to white noise and showed either poor or no responses to pure tones. The differences in response properties found between the core and belt fields may reflect a functional specificity in processing different features of auditory stimuli. The present study also combined microelectrode mapping with Nissl staining to determine if the physiological differences between A1 and AAF corresponded to cytoarchitectonically defined borders. Both A1 and AAF were located within temporal cortex 1 (Te1), with AAF occupying an anteroventral subdivision of Te1, indicating that the two neighbouring, physiologically distinct fields are cytoarchitectonically homogeneous.

Acoustic Stimulation↗
PubMed2005

Perceptual organization of tone sequences in the auditory cortex of awake macaques.

Acoustic sequences such as speech and music are generally perceived as coherent auditory "streams," which can be individually attended to and followed over time. Although the psychophysical stimulus parameters governing this "auditory streaming" are well established, the brain mechanisms underlying the formation of auditory streams remain largely unknown. In particular, an essential feature of the phenomenon, which corresponds to the fact that the segregation of sounds into streams typically takes several seconds to build up, remains unexplained. Here, we show that this and other major features of auditory-stream formation measured in humans using alternating-tone sequences can be quantitatively accounted for based on single-unit responses recorded in the primary auditory cortex (A1) of awake rhesus monkeys listening to the same sound sequences.

Acoustic Stimulation↗
PubMed1999

Processing of sound sequences in macaque auditory cortex: response enhancement.

It is well established that the tone-evoked response of neurons in auditory cortex can be attenuated if another tone is presented several hundred milliseconds before. The present study explores in detail a complementary phenomenon in which the tone-evoked response is enhanced by a preceding tone. Action potentials from multiunit groups and single units were recorded from primary and caudomedial auditory cortical fields in lightly anesthetized macaque monkeys. Stimuli were two suprathreshold tones of 100-ms duration, presented in succession. The frequency of the first tone and the stimulus onset asynchrony (SOA) between the two tones were varied systematically, whereas the second tone was fixed. Compared with presenting the second tone in isolation, the response to the second tone was enhanced significantly when it was preceded by the first tone. This was observed in 87 of 130 multiunit groups and in 29 of 69 single units with no obvious difference between different auditory fields. Response enhancement occurred for a wide range of SOA (110-329 ms) and for a wide range of frequencies of the first tone. Most of the first tones that enhanced the response to the second tone evoked responses themselves. The stimulus, which on average produced maximal enhancement, was a pair with a SOA of 120 ms and with a frequency separation of about one octave. The frequency/SOA combinations that induced response enhancement were mostly different from the ones that induced response attenuation. Results suggest that response enhancement, in addition to response attenuation, provides a basic neural mechanism involved in the cortical processing of the temporal structure of sounds.

Acoustic Stimulation↗
PubMed1983

The effects of stimuli on the activity and functional connectivity of local neuronal groups in the cat auditory cortex.

Simultaneous extracellular recordings from one electrode of 'local' groups of 3-6 neurons were obtained from the auditory cortex of unanesthetized, paralyzed cats. The activity and functional connectivity of local microenvironments were examined under various auditory stimuli. Single cell response patterns were examined using peri-stimulus (PST) histograms and functional connectivity among neighboring cells by the cross renewal density (CRD) histograms. Analysis of the PST histograms suggested that a high percentage of single cells demonstrated different response patterns to different stimuli. Analysis of the CRD histograms suggested, on the one hand, that only small numbers of neighboring cells behaved as if there were direct connections from one cell to another, and that these direct connections appeared to be excitatory. On the other hand, many cell pairs shared input from shared sources which lay outside the local groups. The majority of functional connections were altered by at least one of the stimuli delivered, thus demonstrating the system's plasticity. It is suggested that long-term gates at the synaptic level are responsible for this phenomenon.

Action Potentials↗
PubMed1986

Evoked potential components in the layers of the auditory cortex of the cat.

The intracortical distribution of evoked potentials was studied by seven-contact multielectrodes implanted into the primary auditory cortex of freely moving cats. The aim of the study was to compare the intracortical profiles of the responses evoked by click stimuli and by electrical stimulation of the medial geniculate body in different states of alertness and in Nembutal anesthesia. Only the early surface positive component showed phase reversal in the depth of the cortex. The middle latency components which were faithful indicators of vigilance appeared without phase reversal. Nembutal wiped out these components. A negative component of about 50 ms latency appeared in the attentive animal which had the highest amplitude close to the surface and decreased toward the depth indicating that it was generated in the most superficial layers of the cortex. The stimulation of the last nucleus of the specific auditory pathway elicited evoked potential patterns including also the middle latency components, which were closely similar to those induced by click stimuli.

Animals↗
PubMed1996

Response properties of units in the posterior auditory field deprived of input from the ipsilateral primary auditory cortex.

The influence of the ipsilateral primary auditory field (AI) on the response properties of neurons in the posterior auditory field (Field P) was examined in three cats anesthetized with sodium pentobarbital. Rate/level functions were obtained, by extracellular recording, from single units in Field P before (n = 38) and after (n = 50) subpial aspiration of AI. The ablations were primarily confined to the medial ectosylvian gyrus, although in one case extended into the high-frequency portion of the anterior auditory field. Comparisons between the behavior of units isolated before and after AI ablation failed to demonstrate any changes in the response properties of neurons in Field P attributable to the ablation. Nonmonotonic response profiles, first spike latency, variability in latency, threshold and maximal discharge rates of the units to acoustic stimuli were not significantly altered by the AI ablation. These results indicate that the basic response properties of neurons in Field P do not depend on input from the ipsilateral AI. This suggests that these properties are most likely determined by thalamic input or by circuitry within Field P.

Acoustic Stimulation↗
PubMed2000

Subdivisions of auditory cortex and processing streams in primates.

The auditory system of monkeys includes a large number of interconnected subcortical nuclei and cortical areas. At subcortical levels, the structural components of the auditory system of monkeys resemble those of nonprimates, but the organization at cortical levels is different. In monkeys, the ventral nucleus of the medial geniculate complex projects in parallel to a core of three primary-like auditory areas, AI, R, and RT, constituting the first stage of cortical processing. These areas interconnect and project to the homotopic and other locations in the opposite cerebral hemisphere and to a surrounding array of eight proposed belt areas as a second stage of cortical processing. The belt areas in turn project in overlapping patterns to a lateral parabelt region with at least rostral and caudal subdivisions as a third stage of cortical processing. The divisions of the parabelt distribute to adjoining auditory and multimodal regions of the temporal lobe and to four functionally distinct regions of the frontal lobe. Histochemically, chimpanzees and humans have an auditory core that closely resembles that of monkeys. The challenge for future researchers is to understand how this complex system in monkeys analyzes and utilizes auditory information.

Animals↗
PubMed2002

Context-dependent adaptive coding of interaural phase disparity in the auditory cortex of awake macaques.

In the ascending auditory pathway, the context in which a particular stimulus occurs can influence the character of the responses that encode it. Here we demonstrate that the cortical representation of a binaural cue to sound source location is profoundly context-dependent: spike rates elicited by a 0 degrees interaural phase disparity (IPD) were very different when preceded by 90 degrees versus -90 degrees IPD. The changes in firing rate associated with equivalent stimuli occurring in different contexts are comparable to changes in discharge rate that establish cortical tuning to the cue itself. Single-unit responses to trapezoidally modulated IPD stimuli were recorded in the auditory cortices of awake rhesus monkeys. Each trapezoidal stimulus consisted of linear modulations of IPD between two steady-state IPDs differing by 90 degrees. The stimulus set was constructed so that identical IPDs and sweeps through identical IPD ranges recurred as elements of disparate sequences. We routinely observed orderly context-induced shifts in IPD tuning. These shifts reflected an underlying enhancement of the contrast in the discharge rate representation of different IPDs. This process is subserved by sensitivity to stimulus events in the recent past, involving multiple adaptive mechanisms operating on timescales ranging from tens of milliseconds to seconds. These findings suggest that the cortical processing of dynamic acoustic signals is dominated by an adaptive coding strategy that prioritizes the representation of stimulus changes over actual stimulus values. We show how cortical selectivity for motion direction in real space could emerge as a consequence of this general coding principle.

Acoustic Stimulation↗
PubMed1997

Stimulus generalization of fear responses: effects of auditory cortex lesions in a computational model and in rats.

The conditioning of fear responses to a simple acoustic stimulus (pure tone) paired with footshock can be mediated by the transmission of auditory information to the lateral nucleus of the amygdala from either the auditory thalamus or the auditory cortex. We examined the processing capacity of the thalamo-amygdala pathway by making lesions of the auditory cortex and testing the extent to which conditioned fear responses generalized to tones other than the one paired with footshock. Two studies were performed, one in an anatomically constrained computational model of the fear conditioning network and the other in rats. Stimulus generalization was unaffected in both. These findings support the validity of the model as an approach to studying the neural basis of conditioned fear learning, and in addition suggest that the thalamo-amygdala pathway, possibly by the use of population coding, is capable of performing at least crude stimulus discriminations.

Acoustic Stimulation↗
PubMed2002

Redefining the tonotopic core of rat auditory cortex: physiological evidence for a posterior field.

Previous physiological studies have identified a tonotopically organized primary auditory cortical field (AI) in the rat. Some of this prior research suggests that the rat, like other mammals, may have additional fields surrounding AI. We, therefore, recorded in the Sprague-Dawley rat extracellular responses of single neurons throughout AI, and continued posteriorly to verify the existence of a posterior field (P) and to compare the neuronal properties in the two regions. Acoustic stimuli, including tones, bandpass noise, broadband noise, and temporally modulated stimuli, were delivered dichotically via sealed systems. Consistent with previous findings, AI was characterized by an anterior-to-posterior tonotopic progression from high to low frequencies (ranging from >40 kHz to <1 kHz). A frequency reversal at the posterior border of AI marked entry into a second core tonotopic region, P, with progressively higher frequencies encountered further posteriorly, up to a point (approximately 8 kHz) where cells were no longer tone responsive. Nevertheless, bandpass noise was an effective stimulus in P, enabling characterization of cells up to 15 kHz. Compared with AI, the frequency tuning of response areas was relatively broader in P, the response latency was often longer and more variable, and the response magnitude was more commonly a nonmonotonic function of stimulus level. In both fields, most neurons were binaurally influenced. The presence of multiple auditory cortical fields in the rat is consistent with auditory cortical organization in other mammals. Moreover, the response properties of P relative to AI in the rat also resemble those found in other mammals. Finally, the physiological data suggest that core auditory cortex (temporal area TE1) is composed not only of AI as previously thought, but also of at least two other subdivisions, P and an anterior field (A). Furthermore, our physiological characterization of TE1 reveals that it is larger than suggested by previous anatomical characterizations.

Acoustic Stimulation↗
PubMed1984

[Inhibitory responses of neurons of an isolated strip of auditory cortex to intracortical stimulation in the cat].

Neuronal reactions to intracortical stimulation at a depth of 1.2-1.4 mm have been investigated in an acutely isolated auditory cortex slab from the cat brain (A-1). It is shown that among neuronal reactions to such stimulation primary inhibitory postsynaptical potentials (IPSPs) prevailed. Latent periods of the inhibitory reactions did not exceed 10 ms. Amplitude and duration of IPSPs did not differ from those of inhibitory responses of neurons in the intact auditory cortex. Some peculiarities in the development of inhibitory responses were found in different layers. The distribution of neurons reacting by IPSP was analyzed according to the slab depth, the duration of primary IPSPs and their latent periods, depending on the distance between the recording microelectrode and stimulating electrode. The mechanisms of inhibitory process spreading in an isolated slab and the mechanisms providing prolongation of cortical inhibitory responses are discussed.

Animals↗
PubMed1991

Activation of the human auditory cortex by speech sounds.

Magnetic evoked responses were recorded to different speech sounds in healthy humans. (i) Short words consisting of fricative consonant/vowel combinations evoked strong responses at the auditory cortex about 100 ms after the vowel onset. The response is specific to acoustic rather than phonetic aspects of the sounds. (ii) In a categorization task, words elicited a transient response followed by a sustained field (SF). When the subject counted the number of target words, SF was clearly increased. There were no consistent differences between the hemispheres and a similar increase of SF was observed when the subject classified the duration of two tones. (iii) When tone 'probes' were presented randomly to either ear and speech sounds to one ear, the 100-ms response was dampened and delayed bilaterally. The dampening was not specific to speech masking but dependent on the amount of frequency and amplitude transitions in the masker. All these experiments suggest that the auditory system performs a very similar analysis of both speech signals and other sounds. (iv) In a recent study, more closely related to speech perception, visual input from articulatory movements of the speaker was found to affect the activity of the auditory cortex. It seems that MEG studies can be useful in the study of brain mechanisms underlying speech perception in intact humans.

Acoustic Stimulation↗