Monkey Auditory Cortex Research Articles

The effect of eye-gaze direction on speech intelligibility

In many daily situations we move our eyes to capture different visual information. Previous studies have shown interactions between the eye-gaze direction and auditory perception. The eye-gaze direction has been shown to modulate activity to acoustic stimuli in the auditory cortex of monkeys. In humans, decreased thresholds of binaural lateralization cues and improved accuracy of understanding series of digits in the presence of other speech has been shown. Here, we investigated the effect of the eye-gaze direction on speech perception of single-digit and 4-digit streams while directing the gaze at different directions containing either speech or non-speech stimuli. The speech interferers were also digits but a different gender than the target speech. The non-speech interferers consisted of speech-modulated speech shaped noise. The results showed improved speech intelligibility when the gaze direction and the auditory target direction were matching, in comparison to conditions where the gaze is pointed at a non-target direction. No differences were found comparing single-digit and 4-digit streams. Presenting speech or non-speech stimuli at the gaze direction did not affect the speech intelligibility results. These findings further clarify the influence of the gaze direction on speech perception.

Changes in neural readout of response magnitude during auditory streaming do not correlate with behavioral choice in the auditory cortex

A fundamental goal of the auditory system is to group stimuli from the auditory environment into a perceptual unit (i.e., "stream") or segregate the stimuli into multiple different streams. Although previous studies have clarified the psychophysical and neural mechanisms that may underlie this ability, the relationship between these mechanisms remains elusive. Here, we recorded multiunit activity (MUA) from the auditory cortex of monkeys while they participated in an auditory-streaming task consisting of interleaved low- and high-frequency tone bursts. As the streaming stimulus unfolded over time, MUA amplitude habituated; the magnitude of this habituation was correlated with the frequency difference between the tone bursts. An ideal-observer model could classify these time- and frequency-dependent changes into reports of "one stream" or "two streams" in a manner consistent with the behavioral literature. However, because classification was not modulated by the monkeys' behavioral choices, this MUA habituation may not directly reflect perceptual reports.

Individual Variability in Functional Organization of the Human and Monkey Auditory Cortex.

Accumulating evidence shows that auditory cortex (AC) of humans, and other primates, is involved in more complex cognitive processes than feature segregation only, which are shaped by experience-dependent plasticity and thus likely show substantial individual variability. However, thus far, individual variability of ACs has been considered a methodological impediment rather than a phenomenon of theoretical importance. Here, we examined the variability of ACs using intrinsic functional connectivity patterns in humans and macaques. Our results demonstrate that in humans, interindividual variability is greater near the nonprimary than primary ACs, indicating that variability dramatically increases across the processing hierarchy. ACs are also more variable than comparable visual areas and show higher variability in the left than in the right hemisphere, which may be related to the left lateralization of auditory-related functions such as language. Intriguingly, remarkably similar modality differences and lateralization of variability were also observed in macaques. These connectivity-based findings are consistent with a confirmatory task-based functional magnetic resonance imaging analysis. The quantification of variability in auditory function, and the similar findings in both humans and macaques, will have strong implications for understanding the evolution of advanced auditory functions in humans.

Reward cues readily direct monkeys\u2019 auditory performance resulting in broad auditory cortex modulation and interaction with sites along cholinergic and dopaminergic pathways

In natural settings, the prospect of reward often influences the focus of our attention, but how cognitive and motivational systems influence sensory cortex is not well understood. Also, challenges in training nonhuman animals on cognitive tasks complicate cross-species comparisons and interpreting results on the neurobiological bases of cognition. Incentivized attention tasks could expedite training and evaluate the impact of attention on sensory cortex. Here we develop an Incentivized Attention Paradigm (IAP) and use it to show that macaque monkeys readily learn to use auditory or visual reward cues, drastically influencing their performance within a simple auditory task. Next, this paradigm was used with functional neuroimaging to measure activation modulation in the monkey auditory cortex. The results show modulation of extensive auditory cortical regions throughout primary and non-primary regions, which although a hallmark of attentional modulation in human auditory cortex, has not been studied or observed as broadly in prior data from nonhuman animals. Psycho-physiological interactions were identified between the observed auditory cortex effects and regions including basal forebrain sites along acetylcholinergic and dopaminergic pathways. The findings reveal the impact and regional interactions in the primate brain during an incentivized attention engaging auditory task.

Homology and Specificity of Natural Sound-Encoding in Human and Monkey Auditory Cortex.

Understanding homologies and differences in auditory cortical processing in human and nonhuman primates is an essential step in elucidating the neurobiology of speech and language. Using fMRI responses to natural sounds, we investigated the representation of multiple acoustic features in auditory cortex of awake macaques and humans. Comparative analyses revealed homologous large-scale topographies not only for frequency but also for temporal and spectral modulations. In both species, posterior regions preferably encoded relatively fast temporal and coarse spectral information, whereas anterior regions encoded slow temporal and fine spectral modulations. Conversely, we observed a striking interspecies difference in cortical sensitivity to temporal modulations: While decoding from macaque auditory cortex was most accurate at fast rates (> 30 Hz), humans had highest sensitivity to ~3 Hz, a relevant rate for speech analysis. These findings suggest that characteristic tuning of human auditory cortex to slow temporal modulations is unique and may have emerged as a critical step in the evolution of speech and language.

Neuronal Correlates of Auditory Streaming in Monkey Auditory Cortex for Tone Sequences without Spectral Differences

This study finds a neuronal correlate of auditory perceptual streaming in the primary auditory cortex for sequences of tone complexes that have the same amplitude spectrum but a different phase spectrum. Our finding is based on microelectrode recordings of multiunit activity from 270 cortical sites in three awake macaque monkeys. The monkeys were presented with repeated sequences of a tone triplet that consisted of an A tone, a B tone, another A tone and then a pause. The A and B tones were composed of unresolved harmonics formed by adding the harmonics in cosine phase, in alternating phase, or in random phase. A previous psychophysical study on humans revealed that when the A and B tones are similar, humans integrate them into a single auditory stream; when the A and B tones are dissimilar, humans segregate them into separate auditory streams. We found that the similarity of neuronal rate responses to the triplets was highest when all A and B tones had cosine phase. Similarity was intermediate when the A tones had cosine phase and the B tones had alternating phase. Similarity was lowest when the A tones had cosine phase and the B tones had random phase. The present study corroborates and extends previous reports, showing similar correspondences between neuronal activity in the primary auditory cortex and auditory streaming of sound sequences. It also is consistent with Fishman’s population separation model of auditory streaming.

The Map of Auditory Function

Brodmann areas 41 and 42 are located in the superior temporal gyrus and regarded as auditory cortices. The fundamental function in audition is frequency analysis; however, the findings on tonotopy maps of the human auditory cortex were not unified until recently when they were compared to the findings on inputs and outputs of the monkey auditory cortex. The auditory cortex shows plasticity after conditioned learning and surgery of cochlear implant. It is also involved in speech perception, music appreciation, and auditory hallucination in schizophrenia through interactions with other brain areas, such as the thalamus, frontal cortex, and limbic systems.

Encoding of natural sounds in human and monkey auditory cortex

Event Abstract Back to Event Encoding of natural sounds in human and monkey auditory cortex Marcelo Armendariz1, Julia Erb2, 3, Federico De Martino2, 3, Elia Formisano2, 3 and Wim Vanduffel1, 4, 5* 1 KU Leuven, Laboratory of Neuro- and Psychophysiology, Belgium 2 Maastricht University, Department of Cognitive Neuroscience, Netherlands 3 Maastricht University, Maastricht Brain Imaging Center, Netherlands 4 Massachusetts General Hospital, Athinoula A. Martinos Center for Biomedical Imaging, United States 5 Harvard Medical School, Department of Radiology, United States Cortical processing of natural sounds in primates entails the representation of different complex acoustic features. Whereas the topographical representation of frequency has been reasonably well-studied, the functional organization underlying temporal and spectral modulation of sounds remains less understood. A recent human imaging study (Santoro et al. 2014) revealed the existence of regional preferences for spectral modulation, temporal modulation and frequency, and suggested a combined representation of these three features in the auditory cortex. A comparable study in monkeys (Baumann et al. 2015) corroborated the idea of a topographical representation of amplitude modulation rate. To compare the topographical representations of natural sounds in humans and monkeys, we applied high-resolution (0.75 mm isotropic voxels) contrast agent-enhanced fMRI (Vanduffel et al., 2001) using implanted phased-array coils (Janssens et al., 2012) in awake macaque monkeys while listening to identical stimuli as in Santoro et al. (2014). Our fMRI results showed representations of spectral modulation, temporal modulation and frequency in a similar manner in monkeys and humans. A model combining spectro-temporal modulations and frequency most accurately predicted the measured fMRI response. In line with previous monkey studies (Petkov et al. 2006, Joly et al. 2014), the tonotopy maps showed alternating high-low frequency patterns extending caudo-rostrally along the superior temporal gyrus and lateral sulcus. We used these features along with myelin density maps to define the borders of at least 3 core regions A1, R and RT. Surprisingly, we found that macaques showed preference for faster temporal modulation rates, with a peak at approximately 60 Hz, remarkably different from humans' modulation rate preference between 3-4 Hz. Previous studies have suggested that this latter modulation rate could be related to the processing of syllables in speech (Luo and Poeppel, 2007). In conclusion, our data revealed a highly similar lay-out of early auditory cortex in humans and monkeys with respect to spectro-temporal modulation and frequency. Differences in the modulation rate suggest that tuning of the human auditory cortex to the syllabic rate is unique and might be result of evolution of speech and language. References Santoro R, Moerel M, De Martino F, Goebel R, Ugurbil K, Yacoub E, Formisano E (2014) Encoding of Natural Sounds at Multiple Spectral and Temporal Resolutions in the Human Auditory Cortex. PLoS Comput Biol 10(1): e1003412. doi: 10.1371/journal.pcbi.1003412 Moerel M, De Martino F, Santoro R, Ugurbil K, Goebel R, Yacoub E, Formisano E (2013) Processing of natural sounds: characterization of multipeak spectral tuning in human auditory cortex. J Neurosci 33:11888–11898 Keywords: fMRI, encoding, Auditory Cortex, natural sounds, monkey brain, human brain, topographic maps, modulation Conference: Second Belgian Neuroinformatics Congress, Leuven, Belgium, 4 Dec - 4 Dec, 2015. Presentation Type: Poster Presentation Topic: Methods and Modeling Citation: Armendariz M, Erb J, De Martino F, Formisano E and Vanduffel W (2015). Encoding of natural sounds in human and monkey auditory cortex. Front. Neuroinform. Conference Abstract: Second Belgian Neuroinformatics Congress. doi: 10.3389/conf.fninf.2015.19.00019 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 13 Nov 2015; Published Online: 17 Nov 2015. * Correspondence: Dr. Wim Vanduffel, KU Leuven, Laboratory of Neuro- and Psychophysiology, Leuven, Belgium, wim@nmr.mgh.harvard.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Marcelo Armendariz Julia Erb Federico De Martino Elia Formisano Wim Vanduffel Google Marcelo Armendariz Julia Erb Federico De Martino Elia Formisano Wim Vanduffel Google Scholar Marcelo Armendariz Julia Erb Federico De Martino Elia Formisano Wim Vanduffel PubMed Marcelo Armendariz Julia Erb Federico De Martino Elia Formisano Wim Vanduffel Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Extracting information in spike time patterns with wavelets and information theory.

We present a new method to assess the information carried by temporal patterns in spike trains. The method first performs a wavelet decomposition of the spike trains, then uses Shannon information to select a subset of coefficients carrying information, and finally assesses timing information in terms of decoding performance: the ability to identify the presented stimuli from spike train patterns. We show that the method allows: 1) a robust assessment of the information carried by spike time patterns even when this is distributed across multiple time scales and time points; 2) an effective denoising of the raster plots that improves the estimate of stimulus tuning of spike trains; and 3) an assessment of the information carried by temporally coordinated spikes across neurons. Using simulated data, we demonstrate that the Wavelet-Information (WI) method performs better and is more robust to spike time-jitter, background noise, and sample size than well-established approaches, such as principal component analysis, direct estimates of information from digitized spike trains, or a metric-based method. Furthermore, when applied to real spike trains from monkey auditory cortex and from rat barrel cortex, the WI method allows extracting larger amounts of spike timing information. Importantly, the fact that the WI method incorporates multiple time scales makes it robust to the choice of partly arbitrary parameters such as temporal resolution, response window length, number of response features considered, and the number of available trials. These results highlight the potential of the proposed method for accurate and objective assessments of how spike timing encodes information.

Merging functional and structural properties of the monkey auditory cortex.

Recent neuroimaging studies in primates aim to define the functional properties of auditory cortical areas, especially areas beyond A1, in order to further our understanding of the auditory cortical organization. Precise mapping of functional magnetic resonance imaging (fMRI) results and interpretation of their localizations among all the small auditory subfields remains challenging. To facilitate this mapping, we combined here information from cortical folding, micro-anatomy, surface-based atlas and tonotopic mapping. We used for the first time, phase-encoded fMRI design for mapping the monkey tonotopic organization. From posterior to anterior, we found a high-low-high progression of frequency preference on the superior temporal plane. We show a faithful representation of the fMRI results on a locally flattened surface of the superior temporal plane. In a tentative scheme to delineate core versus belt regions which share similar tonotopic organizations we used the ratio of T1-weighted and T2-weighted MR images as a measure of cortical myelination. Our results, presented along a co-registered surface-based atlas, can be interpreted in terms of a current model of the monkey auditory cortex.

Task‐related preparatory modulations multiply with acoustic processing in monkey auditory cortex

We characterised task-related top-down signals in monkey auditory cortex cells by comparing single-unit activity during passive sound exposure with neuronal activity during a predictable and unpredictable reaction-time task for a variety of spectral-temporally modulated broadband sounds. Although animals were not trained to attend to particular spectral or temporal sound modulations, their reaction times demonstrated clear acoustic spectral-temporal sensitivity for unpredictable modulation onsets. Interestingly, this sensitivity was absent for predictable trials with fast manual responses, but re-emerged for the slower reactions in these trials. Our analysis of neural activity patterns revealed a task-related dynamic modulation of auditory cortex neurons that was locked to the animal's reaction time, but invariant to the spectral and temporal acoustic modulations. This finding suggests dissociation between acoustic and behavioral signals at the single-unit level. We further demonstrated that single-unit activity during task execution can be described by a multiplicative gain modulation of acoustic-evoked activity and a task-related top-down signal, rather than by linear summation of these signals.

The Mechanisms and Meaning of the Mismatch Negativity

The mismatch negativity (MMN) is a pre-attentive auditory event-related potential (ERP) component that is elicited by a change in a repetitive acoustic pattern. It is obtained by subtracting responses evoked by frequent 'standard' sounds from responses evoked by infrequent 'deviant' sounds that differ from the standards along some acoustic dimension, e.g., frequency, intensity, or duration, or abstract feature. The MMN has been attributed to neural generators within the temporal and frontal lobes. The mechanisms and meaning of the MMN continue to be debated. Two dominant explanations for the MMN have been proposed. According to the "neural adaptation" hypothesis, repeated presentation of the standards results in adapted (i.e., attenuated) responses of feature-selective neurons in auditory cortex. Rare deviant sounds activate neurons that are less adapted than those stimulated by the frequent standard sounds, and thus elicit a larger 'obligatory' response, which yields the MMN following the subtraction procedure. In contrast, according to the "sensory memory" hypothesis, the MMN is a 'novel' (non-obligatory) ERP component that reflects a deviation between properties of an incoming sound and those of a neural 'memory trace' established by the preceding standard sounds. Here, we provide a selective review of studies which are relevant to the controversy between proponents of these two interpretations of the MMN. We also present preliminary neurophysiological data from monkey auditory cortex with potential implications for the debate. We conclude that the mechanisms and meaning of the MMN are still unresolved and offer remarks on how to make progress on these important issues.

Different Synchronization Rules in Primary and Nonprimary Auditory Cortex of Monkeys

Synchronized neuronal firing in cortex has been implicated in feature binding, attentional selection, and other cognitive processes. This study addressed the question whether different cortical fields are distinct by rules according to which neurons engage in synchronous firing. To this end, we simultaneously recorded the multiunit firing at several sites within the primary and the caudomedial auditory cortical field of anesthetized macaque monkeys, determined their responses to pure tones, and calculated the cross-correlation function of the spontaneous firing of pairs of units. In the primary field, the likelihood of synchronous firing of pairs of units increased with the similarity of their frequency tuning and their response latencies. In the caudomedial field, by contrast, the likelihood of synchronization was highest when pairs of units had an octave and other harmonic relationships and when units had different response latencies. The differences in synchrony of the two fields were not paralleled by differences in distributions of best frequency, bandwidth of tuning curves, and response latency. Our findings suggest that neuronal synchrony in different cortical fields may underlie the establishment of specific relationships between the sound features that are represented by the firing of the neurons and which follow the Gestalt laws of similarity in the primary field and good continuation in the caudomedial field.

Assessing the role of synchronization and phase coherence in neural communication comparing cortical recordings and integrate-and-fire network models

Cognitive functions likely require that the routes of neural communication can be flexibly modulated. A proposed mechanism for modulating the effective strength of the connections in the neural dynamics relies on band specific neural synchronization and phase relations [1]. Evidence of the modulation of neuronal interactions through the phase relation of rhythmic activity in the gamma band was provided in [2]. Further evidence based on the analysis of a network of integrate-and fire neurons [3] has shown that the phase relation also modulates information transfer and is not specific of the gamma band. Here we combine the study of experimental recordings of local field potentials (LFP’s) and multiple-unit activity (MUA) together with model data to better understand the origins of the phase-dependent modulation of interactions. Recordings include spontaneous activity and natural stimulus-driven activity in the monkey visual cortex V1 [4], as well as natural-stimulus driven activity in monkey auditory cortex [5]. Simulations were obtained extending the recurrent network of integrate-and-fire neurons used in [6] to model the connectivity between two different brain areas. We address some open questions regarding the generation, generality, and mechanistic nature of the phase-dependent modulation. We obtained, for any frequency band, the instantaneous power in each area (reflecting the local neural synchronization), and the instantaneous phases. We analyzed how the power correlation is modulated by the phase relation with a 1ms resolution, in contrast to the hundreds of milliseconds in [2]. We found that this modulation is accompanied by changes in the magnitude of the power of each area separately. Accordingly, we evaluated the role of the power determining the degree of phase coherence and thus the existence of a preferred phase relation. We found that the optimal phase relation associated with maximal power correlation always corresponds to the preferred phase relation for large powers. These results are not frequency band specific and were reproduced with model data, using both unidirectional and bidirectional connections, as well as both excitatory-excitatory and excitatory-inhibitory connections. Our analysis suggests that the degree of local neural synchronization that determines the power of a given rhythm in each of the interacting areas plays a role to be considered together with the one of the phase relations as part of the mechanisms that modulate dynamically the effective strength of the connections.

Analogues of simple and complex cells in rhesus monkey auditory cortex

Receptive fields (RFs) of neurons in primary visual cortex have traditionally been subdivided into two major classes: "simple" and "complex" cells. Simple cells were originally defined by the existence of segregated subregions within their RF that respond to either the on- or offset of a light bar and by spatial summation within each of these regions, whereas complex cells had ON and OFF regions that were coextensive in space [Hubel DH, et al. (1962) J Physiol 160:106-154]. Although other definitions based on the linearity of response modulation have been proposed later [Movshon JA, et al. (1978) J Physiol 283:53-77; Skottun BC, et al. (1991) Vision Res 31(7-8):1079-1086], the segregation of ON and OFF subregions has remained an important criterion for the distinction between simple and complex cells. Here we report that response profiles of neurons in primary auditory cortex of monkeys show a similar distinction: one group of cells has segregated ON and OFF subregions in frequency space; and another group shows ON and OFF responses within largely overlapping response profiles. This observation is intriguing for two reasons: (i) spectrotemporal dissociation in the auditory domain provides a basic neural mechanism for the segregation of sounds, a fundamental prerequisite for auditory figure-ground discrimination; and (ii) the existence of similar types of RF organization in visual and auditory cortex would support the existence of a common canonical processing algorithm within cortical columns.

Stable bottom‐up processing during dynamic top‐down modulations in monkey auditory cortex

It is unclear whether top-down processing in the auditory cortex (AC) interferes with its bottom-up analysis of sound. Recent studies indicated non-acoustic modulations of AC responses, and that attention changes a neuron's spectrotemporal tuning. As a result, the AC would seem ill-suited to represent a stable acoustic environment, which is deemed crucial for auditory perception. To assess whether top-down signals influence acoustic tuning in tasks without directed attention, we compared monkey single-unit AC responses to dynamic spectrotemporal sounds under different behavioral conditions. Recordings were mostly made from neurons located in primary fields (primary AC and area R of the AC) that were well tuned to pure tones, with short onset latencies. We demonstrated that responses in the AC were substantially modulated during an auditory detection task and that these modulations were systematically related to top-down processes. Importantly, despite these significant modulations, the spectrotemporal receptive fields of all neurons remained remarkably stable. Our results suggest multiplexed encoding of bottom-up acoustic and top-down task-related signals at single AC neurons. This mechanism preserves a stable representation of the acoustic environment despite strong non-acoustic modulations.

Sparse coding of harmonic vocalization in monkey auditory cortex

One of the potential foundations for the perception of harmony is the neurons in the primary auditory cortex (A1) that specifically respond to harmonically related frequencies, whose underlying organizing principles are yet unclear. We hypothesize that such harmony-related responses result from adapting to natural harmonic sounds by using sparse coding, a computational model that originally related natural image statistics to the neural properties of the primary visual cortex (V1). Our previous work has shown that a set of harmonic responses found in A1 of marmoset monkeys can emerge by sparse coding of highly harmonic sounds; however, the work was only preliminary because the specific sounds we used were unfamiliar to monkeys. In the present study, we recorded voices of marmosets, and showed that the same model applied to the conspecific vocalizations can reproduce the harmony-related responses. This result more directly supports the hypothesis that A1 adapts to natural sounds, in particular voices comprising harmonic overtones, under the principle of sparse coding.

Fine frequency tuning in monkey auditory cortex and thalamus

The frequency resolution of neurons throughout the ascending auditory pathway is important for understanding how sounds are processed. In many animal studies, the frequency tuning widths are about 1/5th octave wide in auditory nerve fibers and much wider in auditory cortex neurons. Psychophysical studies show that humans are capable of discriminating far finer frequency differences. A recent study suggested that this is perhaps attributable to fine frequency tuning of neurons in human auditory cortex (Bitterman Y, Mukamel R, Malach R, Fried I, Nelken I. Nature 451: 197-201, 2008). We investigated whether such fine frequency tuning was restricted to human auditory cortex by examining the frequency tuning width in the awake common marmoset monkey. We show that 27% of neurons in the primary auditory cortex exhibit frequency tuning that is finer than the typical frequency tuning of the auditory nerve and substantially finer than previously reported cortical data obtained from anesthetized animals. Fine frequency tuning is also present in 76% of neurons of the auditory thalamus in awake marmosets. Frequency tuning was narrower during the sustained response compared to the onset response in auditory cortex neurons but not in thalamic neurons, suggesting that thalamocortical or intracortical dynamics shape time-dependent frequency tuning in cortex. These findings challenge the notion that the fine frequency tuning of auditory cortex is unique to human auditory cortex and that it is a de novo cortical property, suggesting that the broader tuning observed in previous animal studies may arise from the use of anesthesia during physiological recordings or from species differences.

Effect of sound intensity on tonotopic fMRI maps in the unanesthetized monkey

The monkey's auditory cortex includes a core region on the supratemporal plane (STP) made up of the tonotopically organized areas A1, R, and RT, together with a surrounding belt and a lateral parabelt region. The functional studies that yielded the tonotopic maps and corroborated the anatomical division into core, belt, and parabelt typically used low-amplitude pure tones that were often restricted to threshold-level intensities. Here we used functional magnetic resonance imaging in awake rhesus monkeys to determine whether, and if so how, the tonotopic maps and the pattern of activation in core, belt, and parabelt are affected by systematic changes in sound intensity. Blood oxygenation level-dependent (BOLD) responses to groups of low- and high-frequency pure tones 3–4 octaves apart were measured at multiple sound intensity levels. The results revealed tonotopic maps in the auditory core that reversed at the putative areal boundaries between A1 and R and between R and RT. Although these reversals of the tonotopic representations were present at all intensity levels, the lateral spread of activation depended on sound amplitude, with increasing recruitment of the adjacent belt areas as the intensities increased. Tonotopic organization along the STP was also evident in frequency-specific deactivation (i.e. “negative BOLD”), an effect that was intensity-specific as well. Regions of positive and negative BOLD were spatially interleaved, possibly reflecting lateral inhibition of high-frequency areas during activation of adjacent low-frequency areas, and vice versa. These results, which demonstrate the strong influence of tonal amplitude on activation levels, identify sound intensity as an important adjunct parameter for mapping the functional architecture of auditory cortex.

Multisensory connections of monkey auditory cerebral cortex

Functional studies have demonstrated multisensory responses in auditory cortex, even in the primary and early auditory association areas. The features of somatosensory and visual responses in auditory cortex suggest that they are involved in multiple processes including spatial, temporal and object-related perception. Tract tracing studies in monkeys have demonstrated several potential sources of somatosensory and visual inputs to auditory cortex. These include potential somatosensory inputs from the retroinsular (RI) and granular insula (Ig) cortical areas, and from the thalamic posterior (PO) nucleus. Potential sources of visual responses include peripheral field representations of areas V2 and prostriata, as well as the superior temporal polysensory area (STP) in the superior temporal sulcus, and the magnocellular medial geniculate thalamic nucleus (MGm). Besides these sources, there are several other thalamic, limbic and cortical association structures that have multisensory responses and may contribute cross-modal inputs to auditory cortex. These connections demonstrated by tract tracing provide a list of potential inputs, but in most cases their significance has not been confirmed by functional experiments. It is possible that the somatosensory and visual modulation of auditory cortex are each mediated by multiple extrinsic sources.