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PubMed2002

Heschl's gyrus is more sensitive to tone level than non-primary auditory cortex.

Previous neuroimaging studies generally demonstrate a growth in the cortical response with an increase in sound level. However, the details of the shape and topographic location of such growth remain largely unknown. One limiting methodological factor has been the relatively sparse sampling of sound intensities. Additionally, most studies have either analysed the entire auditory cortex without differentiating primary and non-primary regions or have limited their analyses to Heschl's gyrus (HG). Here, we characterise the pattern of responses to a 300-Hz tone presented in 6-dB steps from 42 to 96 dB sound pressure level as a function of its sound level, within three anatomically defined auditory areas; the primary area, on HG, and two non-primary areas, consisting of a small area lateral to the axis of HG (the anterior lateral area, ALA) and the posterior part of auditory cortex (the planum temporale, PT). Extent and magnitude of auditory activation increased non-linearly with sound level. In HG, the extent and magnitude were more sensitive to increasing level than in ALA and PT. Thus, HG appears to have a larger involvement in sound-level processing than does ALA or PT.

Acoustic Stimulation↗
PubMed1988

Layer V in rat auditory cortex: projections to the inferior colliculus and contralateral cortex.

This study compares the form and distribution within layer V of cells projecting to the inferior colliculus with that of commissural cells of origin in adult rat auditory cortex after horseradish peroxidase injections in the ipsilateral inferior colliculus or auditory cortex. The goal of this work was to determine whether every part of layer V participates equally in both projections, and if the cortical neurons in each pathway were similar. The types of neurons were defined in Golgi-Cox preparations and matched with the profiles of retrogradely labeled cells from architectonically defined cortical area 41. Inferior colliculus and commissural neurons form two populations that differ in their distribution in layer V, in somatic area, and in the form of their apical dendritic arbors. Corticocollicular neurons include the largest pyramidal cells, whose robustly filled apical dendrites ascend into layer II or farther. Commissural cells are smaller and have a more heterogeneous form. Their apical dendrites do not usually extend above layer IV, and a few of these cells may be non-pyramidal. Small pyramidal cells and inverted pyramidal cells project to the opposite cortex, but not to the inferior colliculus. Medium-sized pyramidal cells project in both systems. In addition, certain callosal cells of origin in layers V and III were morphologically similar. More than one-third of the commissural cells originate in the superficial part of layer V, where only 7% of the inferior colliculus projection neurons arise. Most corticocollicular cells lie deeper in layer V, where there are fewer commissural neurons. These findings suggest that the efferent systems projecting to telencephalic and mesencephalic targets are morphologically distinct and spatially segregated in layer V. However, the commissural projection includes similar cells in different cortical layers. The types of these efferent neurons may be more closely related to their target than to their laminar origin.

Animals↗
PubMed2003

Neuronal responses in cat primary auditory cortex to electrical cochlear stimulation: IV. Activation pattern for sinusoidal stimulation.

Patterns of threshold distributions for single-cycle sinusoidal electrical stimulation and single pulse electrical stimulation were compared in primary auditory cortex of the adult cat. Furthermore, the effects of auditory deprivation on these distributions were evaluated and compared across three groups of adult cats. Threshold distributions for single and multiple unit responses from the middle cortical layers were obtained on the ectosylvian gyrus in an acutely implanted animal; 2 wk after deafening and implantation (short-term group); and neonatally deafened animals implanted following 2-5 yr of deafness (long-term group). For all three cases, we observed similar patterns of circumscribed regions of low response thresholds in the region of primary auditory cortex (AI). A dorsal and a ventral region of low response thresholds were found separated by a narrow, anterior-posterior strip of elevated thresholds. The ventral low-threshold regions in the short-term group were cochleotopically arranged. By contrast, the dorsal region in the short-term animals and both low-threshold regions in long-term deafened animals maintained only weak cochleotopicity. Analysis of the spatial extent of the low-threshold regions revealed that the activated area for sinusoidal stimulation was smaller and more circumscribed than for pulsatile stimulation for both dorsal and ventral AI. The width of the high-threshold ridge that separated the dorsal and ventral low-threshold regions was greater for sinusoidal stimulation. Sinusoidal and pulsatile threshold behavior differed significantly for electrode configurations with low and high minimum thresholds. Differences in threshold behavior and cortical response distributions between the sinusoidal and pulsatile stimulation suggest that stimulus shape plays a significant role in the activation of cortical activity. Differences in the activation pattern for short-term and long-term deafness reflect deafness-induced reorganizational changes based on factors such as differences in excitatory and inhibitory balance that are affected by the stimulation parameters.

Animals↗
PubMed2000

Frequency and intensity response properties of single neurons in the auditory cortex of the behaving macaque monkey.

Response properties of auditory cortical neurons measured in anesthetized preparations have provided important information on the physiological differences between neurons in different auditory cortical areas. Studies in the awake animal, however, have been much less common, and the physiological differences noted may reflect differences in the influence of anesthetics on neurons in different cortical areas. Because the behaving monkey is gaining popularity as an animal model in studies exploring auditory cortical function, it has become critical to physiologically define the response properties of auditory cortical neurons in this preparation. This study documents the response properties of single cortical neurons in the primary and surrounding auditory cortical fields in monkeys performing an auditory discrimination task. We found that neurons with the shortest latencies were located in the primary auditory cortex (AI). Neurons in the rostral field had the longest latencies and the narrowest intensity and frequency tuning, neurons in the caudomedial field had the broadest frequency tuning, and neurons in the lateral field had the most monotonic rate/level functions of the four cortical areas studied. These trends were revealed by comparing response properties across the population of studied neurons, but there was considerable variability between neurons for each response parameter other than characteristic frequency (CF) in each cortical area. Although the neuronal CFs showed a systematic spatial organization across AI, no such systematic organization was apparent for any other response property in AI or the adjacent cortical areas. The results of this study indicate that there are physiological differences between auditory cortical fields in the behaving monkey consistent with previous studies in the anesthetized animal and provide insights into the functional role of these cortical areas in processing acoustic information.

Action Potentials↗
PubMed1980

Discharge patterns of the primary auditory cortex in cats.

Single unit responses of the primary auditory cortex of unanesthetized cats were studied using three standard sets of sound stimuli, i.e., clicks, white noise bursts, and pure tone bursts. The units studied were classified into eight categories according to whether they responded to all three, two, one or none of the standard sets of stimuli; there were no units responding only to clicks. This suggests that excitation of some cortical neurons requires a special sound spectrum as well as temporal factors. Frequency-modulated (FM) sounds were usually more effective eliciting responses than the standard sets of stimuli. Responses to FM in burst seemed to be classified into three groups, i.e., sharp monopeak post-stimulus time (PST) histograms, broad monopeak, and sharp multipeaks. The existence of the last group suggests a firm convergence from the lower auditory neurons. In some units, complex stimuli were more effective in evoking responses than simple FM sounds. These units responded only to sounds with complex temporal patterns. For example, a neuron did not respond to any FM sound with a constant modulation magnitude, but showed a transient response whenever the magnitude of FM varied from a continuous pure tone to a certain range of FM. These complex units often showed a response with hysteresis.

Acoustic Stimulation↗
PubMed2006

Rapid-rate transcranial magnetic stimulation of animal auditory cortex impairs short-term but not long-term memory formation.

Bilateral rapid-rate transcranial magnetic stimulation (rTMS) of gerbil auditory cortex with a miniature coil device was used to study short-term and long-term effects on discrimination learning of frequency-modulated tones. We found previously that directional discrimination of frequency modulation (rising vs. falling) relies on auditory cortex processing and that formation of its memory depends on local protein synthesis. Here we show that, during training over 5 days, certain rTMS regimes contingent on training had differential effects on the time course of learning. When rTMS was applied several times per day, i.e. four blocks of 5 min rTMS each followed 5 min later by a 3-min training block and 15-min intervals between these blocks (experiment A), animals reached a high discrimination performance more slowly over 5 days than did controls. When rTMS preceded only the first two of four training blocks (experiment B), or when prolonged rTMS (20 min) preceded only the first block, or when blocks of experiment A had longer intervals (experiments C and D), no significant day-to-day effects were found. However, in experiment A, and to some extent in experiment B, rTMS reduced the within-session discrimination performance. Nevertheless the animals learned, as demonstrated by a higher performance the next day. Thus, our results indicate that rTMS treatments accumulate over a day but not strongly over successive days. We suggest that rTMS of sensory cortex, as used in our study, affects short-term memory but not long-term memory formation.

Analysis of Variance↗
PubMed1999

Layer VI in cat primary auditory cortex: Golgi study and sublaminar origins of projection neurons.

The organization of layer VI in cat primary auditory cortex (AI) was studied in mature specimens. Golgi-impregnated neurons were classified on the basis of their dendritic and somatic form. Ipsilateral and contralateral projection neurons and the corticogeniculate cells of origin were labeled with retrograde tracers and their profiles were compared with the results from Golgi studies. Layer VI was divided into a superficial half (layer VIa) with many pyramidal neurons and a deeper part (layer VIb) that is dominated by horizontal cells. Nine types of neuron were identified; four classes had subvarieties. Classical pyramidal cells and star, fusiform, tangential, and inverted pyramidal cells occur. Nonpyramidal neurons were Martinotti, multipolar stellate, bipolar, and horizontal cells. This variety of neurons distinguished layer VI from other AI layers. Pyramidal neuron dendrites contributed to the vertical, modular organization in AI, although their apical processes did not project beyond layer IV. Their axons had vertical, intrinsic processes as well as corticofugal branches. Horizontal cell dendrites extended laterally up to 700 microm and could integrate thalamic input across wide expanses of the tonotopic domain. Connectional experiments confirmed the sublaminar arrangement seen in Nissl material. Commissural cells were concentrated in layer VIa, whereas corticocortical neurons were more numerous in layer VIb. Corticothalamic cells were distributed more equally. The cytological complexity and diverse connections of layer VI may relate to a possible role in cortical development. Layer VI contained most of the neuronal types found in other layers in AI, and these cells form many of the same intrinsic and corticofugal connections that neurons in other layers will assume in adulthood. Layer VI, thus, may play a fundamental ontogenetic role in the construction and early function of the cortex.

Animals↗
PubMed1997

Measurement of hemodynamics of auditory cortex using magnetoencephalography and near infrared spectroscopy.

Near infrared (NIR) spectroscopy, a non-invasive optical method for monitoring hemodynamics, was applied to record evoked auditory response of the human brain. Our newly developed system detected the amount of oxygenated and deoxygenated hemoglobin in the area of the local brain located beneath the probes. In the study, we used magnetoencephalography (MEG) and NIR spectroscopy to observe the blood component change near the auditory cortex. The sound stimulus of 1 kHz sine wave at 70 dB SPL was used to elicit evoked responses in both experiments. The active region of the auditory cortex was determined by MEG, and the same area was examined by the NIR system. The results from the NIR experiments showed significant increases of total hemoglobin and deoxygenated hemoglobin, indicating that both blood flow and oxygen consumption at the cortical area increased in response to the sound stimuli. The NIR recordings at the two different depths revealed that the increases of local blood flow and oxygen consumption were observed at depths ranging between 20 and 30 mm from the scalp.

Adult↗
PubMed2005

Theoretical analysis of a TIME-FREQUENCY-PCNN auditory cortex model.

A particular pulsed neural model of the auditory cortex, the Time-Frequency Pulse Coupled Neural Network (TF-PCNN), has shown to decompose its stimulus from the cochlea into characteristic pulse-coded time-frequency (TF) stop and pass regions. A derived sum of spectrograms representation with these zero- or one-valued TF weightings has already been applied for the denoising of speech in a previous work. This decomposition is now related to the concept of TF projection filters, which allows to reinterpret the model equations. The functionality imposed by the model equations can so be accessed and justified from a TF signal processing perspective, in addition to its biological motivation from experimentally observed neurophysiological behavior and simulations.

Acoustic Stimulation↗
PubMed1992

Synaptic potentials and effects of amino acid antagonists in the auditory cortex.

Neurons of in vitro guinea pig and rat auditory cortex receive a complex synaptic pattern of afferent information. As many as four synaptic responses to a single-stimulus pulse to the gray or white matter can occur; an early-EPSP followed, sequentially, by an early-IPSP, late-EPSP, and late-IPSP. Paired pulse stimulation and pharmacological studies show that the early-IPSP can modify information transmission that occurs by way of the early-EPSP. Each of these four synaptic responses differed in estimated reversal potential, and each was differentially sensitive to antagonism by pharmacological agents. DNQX (6,7-dinitroquinoxaline-2,3-dione), a quisqualate/kainate receptor antagonist, blocked the early-EPSP, and the late-EPSP was blocked by the NMDA receptor antagonist APV (D-2-amino-5-phosphonovalerate). The early-IPSP was blocked by the GABA-a receptor antagonist bicuculline, and the late-IPSP by the GABA-b receptor antagonists 2-OH saclofen or phaclofen. Presentation of stimulus trains, even at relatively low intensities, could produce a long-lasting APV-sensitive membrane depolarization. Also discussed is the possible role of these synaptic potentials in auditory cortical function and plasticity.

Amino Acids↗
PubMed2001

Long-term frequency tuning of local field potentials in the auditory cortex of the waking guinea pig.

The goal of our study was to determine the extent of changes in frequency tuning in the auditory cortex over weeks. The subjects were awake adult male guinea pigs (n = 8) bearing electrodes chronically implanted in layers IV-VI of primary auditory cortex. Tuning was determined by presenting sequences of pure tone bursts (approximately 0.97-41.97 kHz, -20 to 80 dB, 100-ms tone duration, 5-ms rise-fall, 800-ms intertone intervals, 1.5-s intersequence interval) either in 0.5-octave steps (n = 5, 14 probes) or 0.25-octave steps (n = 3, 9 probes) delivered to the ear contralateral to recording sites. Tuning curves were determined for local field potentials (LFPs), which were tuned to frequency (negative potential, latency to peak 15-20 ms), repeatedly for up to 27 days (0.5 octave) or 12 days (0.25 octave). Characteristic frequency (CF), best frequency at 10 and 30 dB above absolute threshold (BF10, BF30), threshold (TH), and bandwidth (10 dB above threshold; BW) were measured. Absolute amplitude often decreased across weeks, necessitating normalization of amplitude. However, there were no significant trends in tuning over days for CF, BF10, or BF30 for either the half- or the quarter-octave group. Both groups exhibited random daily variations in frequency tuning, the quarter-octave group revealing larger variations averaging 0.228, 0.211, and 0.250 octave for CF, BF10, and BF30, respectively. Therefore, frequency tuning in waking animals does not exhibit directional drift over very long periods of time. However, daily tuning variations on the order of 0.20-0.25 octave indicate that the peaks of tuning curves (CF, BF) represent a preferred frequency range rather than a fixed frequency.

Animals↗
PubMed2001

Neuroarchitecture of the auditory cortex in the rufous horseshoe bat (Rhinolophus rouxi).

This study describes the location and anatomical subdivisions of the auditory cortex of the horseshoe bat, Rhinolophus rouxi. The basic cyto- and myeloarchitectural features and cytochrome oxidase reactivity patterns are evaluated in brains where auditory fields have been previously established neurophysiologically (Radtke-Schuller and Schuller 1995). Thus, the neuroanatomical findings from these brains and additional analyzed material are related to neurophysiological characteristics. The neocortex of Rhinolophus shows a typical mammalian six-layered organization. It is poorly laminated, has a low density of granular elements, a wide layer I, and a phylogenetically old pyramidal cell type in a sharply accentuated layer II. These features are generally considered 'primitive' or conservative. Frontal, parietal, temporal and occipital regions can be distinguished. In the temporal cortex, layers III and IV are found to be markedly thicker than layer V, in contrast to the parietal region, where a prominent layer V, containing a high concentration of large pyramidal cells is the most outstanding feature. The entire temporal region, most of the parietal and parts of the occipital region are responsive to auditory stimuli. The primary auditory field corresponds to most of the temporal region. The fields of the parietal region almost completely coincide with the dorsal fields of the auditory cortex. Border zones between the temporal, parietal, and occipital regions correspond to the posterior auditory field. The non-primary fields of the auditory cortex occupy a larger area of the bat's neocortex than the primary field. The accentuated neuroarchitectural features, like cortical thickness and staining intensity, are shown to coincide with the physiological representation of biologically significant parameters.

Animals↗
PubMed1983

Responses of cells in the auditory cortex of awake squirrel monkeys to normal and reversed species-specific vocalizations.

Natural vocalizations and their artificial counterparts were found to be equally effective in evoking responses in auditory cortex units of awake squirrel monkeys. Neural responsiveness was presumably based primarily on the sensitivity of the units to acoustic transients embedded in the stimuli. For the left hemisphere, a significantly higher percentage of responding units was found in the primary compared to the secondary auditory cortex. However, the difference in the percentage of responding units between the primary and secondary auditory cortices was not significant for the right hemisphere.

Animals↗
PubMed2006

Neural mechanisms underlying selectivity for the rate and direction of frequency-modulated sweeps in the auditory cortex of the pallid bat.

Frequency-modulated (FM) sweeps are common in vocalizations, including human speech. Selectivity for FM sweep rate and direction is present in the auditory cortex of many species. The present study sought to determine the mechanisms underlying FM sweep selectivity in the auditory cortex of pallid bats. In the pallid bat inferior colliculus (IC), two mechanisms underlie selectivity for FM sweep rate. The first mechanism depends on duration tuning for tones that arises as a consequence of early inhibition generated by an excitatory tone. The second mechanism depends on a narrow band of delayed high-frequency inhibition. Direction selectivity depends on a broad band of early low-frequency inhibition. Here, the contributions of these mechanisms to cortical FM sweep selectivity were determined in pentobarbital-anesthetized pallid bats. We show that the majority of cortical neurons tuned to echolocation frequencies are selective for the downward direction and rate of FM sweeps. Unlike in IC neurons tuned in the echolocation range, duration tuning is rare in cortical neurons with similar tuning. As in the IC, consistent spectrotemporal differences exist between low- and high-frequency sidebands. A narrow band of delayed high-frequency inhibition is necessary for FM rate selectivity. Low-frequency inhibition has a broad bandwidth, early arrival time, and creates direction selectivity. Cortical neurons respond better to slower FM rates and exhibit broader rate tuning than IC neurons. Relative arrival time of high-frequency inhibition is slower in the cortex than in the IC. Thus whereas similar mechanisms shape direction selectivity of neurons tuned in the echolocation range in the IC and the cortex, only one of the two mechanisms underlying rate selectivity in the IC is present in the cortex.

Acoustic Stimulation↗
PubMed1984

Projections of the medial geniculate nucleus to layer 1 of the auditory cortex in the cat traced with horseradish peroxidase.

Thalamic afferents to layer 1 of the auditory cortex in the cat have been studied using retrograde axonal transport of horseradish peroxidase. The magnocellular part of the medial geniculate nucleus sends fibers to layer 1 of the primary and secondary auditory areas and of the dorsal division of the posterior ectosylvian area. The dorsal principal part and the ventromedial portion of the ventral principal part send only a few fibers to layer 1 of these cortical areas.

Animals↗
PubMed2005

Functional organization and hemispheric comparison of primary auditory cortex in the common marmoset (Callithrix jacchus).

Hemispheric fine-grain maps of primary auditory cortex (AI) were derived from microelectrode penetrations in the temporal gyrus of the common marmoset (Callithrix jacchus) to 1) compare the functional organization of AI in the marmoset with other mammalian species and 2) compare the right and left AI maps in individual monkeys. Frequency receptive fields (FRFs) were recorded with pure tones. Five FRF parameters were analyzed: characteristic frequency, threshold, sharpness of tuning 10 dB and 40 dB above threshold, and minimum response latency. The present study confirms that the functional organization of AI is characterized by a robust tonotopic frequency gradient overlaid with spatially clustered distributions of other FRF parameters. Globally, this functional organization of AI in the common marmoset is similar to that in other mammalian species. With respect to within-subject hemispheric comparisons of the five FRF parameters, a coherent pattern of asymmetry is not evident in marmoset AI. The overall results indicate that the basic functional organization between hemispheres is similar but not identical.

Acoustic Stimulation↗
PubMed2000

Organisation of binaural interactions in the primary and dorsocaudal fields of the guinea pig auditory cortex.

This study investigated the nature and topography of binaural interactions in the primary auditory field (AI) and dorsocaudal field (DC) of the urethane anaesthetised guinea pig auditory cortex. Single and multi-units were classified by their responses to monaural and binaural stimulation. In both AI and DC, units displayed binaural facilitation, binaural inhibition, or a level dependent mixture of facilitation and inhibition. There was a significant difference in the distribution of binaural response types between the two fields. Facilitated units predominated in DC (facilitated: 58%; inhibited: 24%; mixed: 6%; non-interacting: 12%), while inhibited units were the most common class in AI (facilitated: 15%; inhibited: 44%; mixed: 18%; non-interacting: 22%). It has previously been suggested that inhibited and facilitated units are concerned with processing different areas of space suggesting a possible separation of function between the two core fields. Topographically, the binaural response properties in AI and DC varied along isofrequency bands, with neurones displaying similar interactions aggregating in clusters. These clusters were similar in size for the two fields and often overlapped neighbouring isofrequency bands. However, their shape and position varied between different animals. This clustered organisation of binaural interactions is similar to that reported in recent studies of AI in other mammals.

Acoustic Stimulation↗
PubMed1996

Sound lateralization in patients with lesions including the auditory cortex: comparison of interaural time difference (ITD) discrimination and interaural intensity difference (IID) discrimination.

We examined sound lateralization using dichotic presentation of noises in 15 patients with left unilateral (12 patients) or bilateral (3 patients) temporal lobe lesions, that included the auditory cortex, and evaluated their abilities to discriminate interaural time and intensity difference (ITD, IID) separately. On the ITD discrimination test, discrimination thresholds in patients with left unilateral lesions were significantly higher than those in normal subjects, but all patients with left unilateral lesions could detect ITD. However, none of 3 patients with bilateral lesions could detect ITD. On the IID discrimination test, all patients with either unilateral or bilateral lesions could detect IID. IID discrimination thresholds in these patients were significantly higher than those in normal subjects. The auditory cortex plays an important role in discriminating both cues, but appears to be necessary for discriminating ITD.

Adolescent↗