Primary Auditory Areas Research Articles

Competition and convergence between auditory and cross-modal visual inputs to primary auditory cortical areas

Sensory neocortex is capable of considerable plasticity after sensory deprivation or damage to input pathways, especially early in development. Although plasticity can often be restorative, sometimes novel, ectopic inputs invade the affected cortical area. Invading inputs from other sensory modalities may compromise the original function or even take over, imposing a new function and preventing recovery. Using ferrets whose retinal axons were rerouted into auditory thalamus at birth, we were able to examine the effect of varying the degree of ectopic, cross-modal input on reorganization of developing auditory cortex. In particular, we assayed whether the invading visual inputs and the existing auditory inputs competed for or shared postsynaptic targets and whether the convergence of input modalities would induce multisensory processing. We demonstrate that although the cross-modal inputs create new visual neurons in auditory cortex, some auditory processing remains. The degree of damage to auditory input to the medial geniculate nucleus was directly related to the proportion of visual neurons in auditory cortex, suggesting that the visual and residual auditory inputs compete for cortical territory. Visual neurons were not segregated from auditory neurons but shared target space even on individual target cells, substantially increasing the proportion of multisensory neurons. Thus spatial convergence of visual and auditory input modalities may be sufficient to expand multisensory representations. Together these findings argue that early, patterned visual activity does not drive segregation of visual and auditory afferents and suggest that auditory function might be compromised by converging visual inputs. These results indicate possible ways in which multisensory cortical areas may form during development and evolution. They also suggest that rehabilitative strategies designed to promote recovery of function after sensory deprivation or damage need to take into account that sensory cortex may become substantially more multisensory after alteration of its input during development.

Dysregulation of Limbic and Auditory Networks in Tinnitus

Tinnitus is a common disorder characterized by ringing in the ear in the absence of sound. Converging evidence suggests that tinnitus pathophysiology involves damage to peripheral and/or central auditory pathways. However, whether auditory system dysfunction is sufficient to explain chronic tinnitus is unclear, especially in light of evidence implicating other networks, including the limbic system. Using functional magnetic resonance imaging and voxel-based morphometry, we assessed tinnitus-related functional and anatomical anomalies in auditory and limbic networks. Moderate hyperactivity was present in the primary and posterior auditory cortices of tinnitus patients. However, the nucleus accumbens exhibited the greatest degree of hyperactivity, specifically to sounds frequency-matched to patients' tinnitus. Complementary structural differences were identified in ventromedial prefrontal cortex, another limbic structure heavily connected to the nucleus accumbens. Furthermore, tinnitus-related anomalies were intercorrelated in the two limbic regions and between limbic and primary auditory areas, indicating the importance of auditory-limbic interactions in tinnitus.

Long-term recovery from acquired childhood aphasia and changes of cerebral blood flow

In this study, we investigated the mechanism of functional reorganization underlying long-term functional recovery from acquired childhood aphasia. We followed a 9-year-old boy with aphasia from 3 months to 10 years and 5 months after his stroke. The patient’s language ability was assessed five times by the Standard Language Test of Aphasia (SLTA) and regional cerebral blood flow (rCBF) was measured four times with 99mTc-ECD and a fully automated quantitative image analysis system. The patient showed continuous improvement of language throughout this period. Overall rCBF peaked at the time of the 2nd measurement (at the age of 12 years and 2 months) and then gradually decreased to the 4th measurement (at the age of 20 years and 2 months). However, there were several patterns of significant relative dominance of rCBF in one cerebral hemisphere compared with the other hemisphere. The Broca and supplementary motor areas showed consistent right hemisphere dominance throughout the study period, but Wernicke’s area showed left hemisphere dominance in the early stage (comparison between the 1st and 2nd measurements or the 2nd and 3rd measurements), followed by right hemisphere dominance in the late stage (comparison between the 3rd and 4th measurements). In the primary auditory area, right hemisphere dominance was only seen in the late stage (comparison between the 3rd and 4th measurements). These findings suggest that both hemispheres are involved in the long-term recovery of children from aphasia, but the location of functional reorganization varies depending on the ROIs studied and the stage after the onset.

Hierarchical Auditory Processing Directed Rostrally along the Monkey's Supratemporal Plane

Connectional anatomical evidence suggests that the auditory core, containing the tonotopic areas A1, R, and RT, constitutes the first stage of auditory cortical processing, with feedforward projections from core outward, first to the surrounding auditory belt and then to the parabelt. Connectional evidence also raises the possibility that the core itself is serially organized, with feedforward projections from A1 to R and with additional projections, although of unknown feed direction, from R to RT. We hypothesized that area RT together with more rostral parts of the supratemporal plane (rSTP) form the anterior extension of a rostrally directed stimulus quality processing stream originating in the auditory core area A1. Here, we analyzed auditory responses of single neurons in three different sectors distributed caudorostrally along the supratemporal plane (STP): sector I, mainly area A1; sector II, mainly area RT; and sector III, principally RTp (the rostrotemporal polar area), including cortex located 3 mm from the temporal tip. Mean onset latency of excitation responses and stimulus selectivity to monkey calls and other sounds, both simple and complex, increased progressively from sector I to III. Also, whereas cells in sector I responded with significantly higher firing rates to the "other" sounds than to monkey calls, those in sectors II and III responded at the same rate to both stimulus types. The pattern of results supports the proposal that the STP contains a rostrally directed, hierarchically organized auditory processing stream, with gradually increasing stimulus selectivity, and that this stream extends from the primary auditory area to the temporal pole.

A bird brain’s view of auditory processing and perception

By studying the primary forebrain auditory area of songbirds, field L, using a song-inspired synthetic stimulus and reverse correlation techniques, we found a surprisingly systematic organization of this area, with nearly all neurons narrowly tuned along the spectral dimension, the temporal dimension, or both; there were virtually no strongly orientation-sensitive cells, and in the areas that we recorded, cells broadly tuned in both time and frequency were rare. In addition, cells responsive to fast temporal frequencies predominated only in the field L input layer, suggesting that neurons with fast and slow responses are concentrated in different regions. Together with other songbird data and work from chicks and mammals, these findings suggest that sampling a range of temporal and spectral modulations, rather than orientation in time-frequency space, is the organizing principle of forebrain auditory sensitivity. We then examined the role of these acoustic parameters important to field L organization in a behavioral task. Birds’ categorization of songs fell off rapidly when songs were altered in frequency, but, despite the temporal sensitivity of field L neurons, the same birds generalized well to songs that were significantly changed in timing. These behavioral data point out that we cannot assume that animals use the information present in particular neurons without specifically testing perception.

Neuron densities vary across and within cortical areas in primates

The numbers and proportion of neurons in areas and regions of cortex were determined for a single cortical hemisphere from two prosimian galagos, one New World owl monkey, one Old World macaque monkey, and one baboon. The results suggest that there is a common plan of cortical organization across the species examined here and also differences that suggest greater specializations in the Old World monkeys. In all primates examined, primary visual cortex (V1) was the most neuron-dense cortical area and the secondary visual areas had higher-than-average densities. Primary auditory and somatosensory areas tended to have high densities in the Old World macaque and baboon. Neuronal density varies less across cortical areas in prosimian galagos than in the Old World monkeys. Thus, cortical architecture varies greatly within and across primate species, but cell density is greater in cortex devoted to the early stages of sensory processing.

Is the primary visual cortex multisensory?: Comment on “Crossmodal influences on visual perception” by Prof. Ladan Shams

In our everyday life we are confronted with a constant flow of information from several modalities (visual, auditory, proprioceptive, etc.) and multisensory integration (MSI) allows us to link the unimodal sensory analysis with the polysensory context in which they appear (crossmodal fusion). As extensively described by Shams and Kim in this issue [1], MSI results in perceptual improvements in various tasks, from simple detections to complex discriminations, memory or learning tasks. The fact that such large fields are involved in multisensory interactions implies the participation of an equally large variety of cortical networks. During the last decade, we have observed a reappraisal of the neuronal networks supporting MSI, emphasizing the original theoretical concept of a broad allocation of the cortical regions susceptible to host multisensory interactions [2]. In their review, Shams and Kim bring into focus the intriguing observation that multisensory interactions can occur at very short delays in regions historically defined as unimodal, so excluding the origin of MSI in a feedback influence from high order associative areas. Non-human primates constitute a good model to study the neuronal mechanisms of MSI as they show the same perceptual multisensory benefits as reported in humans [3]. Thus to corroborate human brain imaging studies, anatomical and electrophysiological approaches have been developed to assess if crossmodal influences can occur at early stages of sensory processing such as the primary visual cortex (V1). A recent set of anatomical studies revealed a network of heteromodal connections that link directly cortical regions processing different modalities [4]. A common feature of these heteromodal connections is a coupling between specific sensory representations across modalities (visual field or body part representations) suggesting a functional role restricted to specialized behaviors. Among these heteromodal connections, the presence of direct anatomical links between the primary visual and auditory areas suggests that this pathway can support the visuo-auditory interactions described in human and reviewed by Shams and Kim. I would like to stress that recording of single unit activity in the primary visual cortex does not reveal pure auditory responses. Instead, neurons recorded in V1 showed a modulation of their visual response as expressed as a significant reduction in visual response latencies specifically in suboptimal conditions [5] when approaching perceptual threshold. It should be noted that, in the primary auditory cortex of the monkey, other authors have reported the absence of non-specific

Processing of spectral and amplitude envelope of animal vocalizations in the human auditory cortex

In daily life, we usually identify sounds effortlessly and efficiently. Two properties are particularly salient and of importance for sound identification: the sound's overall spectral envelope and its temporal amplitude envelope. In this study, we aimed at investigating the representation of these two features in the human auditory cortex by using a functional magnetic resonance imaging adaptation paradigm. We presented pairs of sound stimuli derived from animal vocalizations that preserved the time-averaged frequency spectrum of the animal vocalizations and the amplitude envelope. We presented the pairs in four different conditions: (a) pairs with the same amplitude envelope and mean spectral envelope, (b) same amplitude envelope, but different mean spectral envelope, (c) different amplitude envelope, but same mean spectral envelope and (d) both different amplitude envelope and mean spectral envelope. We found fMRI adaptation effects for both the mean spectral envelope and the amplitude envelope of animal vocalizations in overlapping cortical areas in the bilateral superior temporal gyrus posterior to Heschl's gyrus. Areas sensitive to the amplitude envelope extended further anteriorly along the lateral superior temporal gyrus in the left hemisphere, while areas sensitive to the spectral envelope extended further anteriorly along the right lateral superior temporal gyrus. Posterior tonotopic areas within the left superior temporal lobe displayed sensitivity for the mean spectrum. Our findings suggest involvement of primary auditory areas in the representation of spectral cues and encoding of general spectro-temporal features of natural sounds in non-primary posterior and lateral superior temporal cortex.

Evaluating an acoustically quiet EPI sequence for use in fMRI studies of speech and auditory processing

Echoplanar MRI is associated with significant acoustic noise, which can interfere with the presentation of auditory stimuli, create a more challenging listening environment, and increase discomfort felt by participants. Here we investigate a scanning sequence that significantly reduces the amplitude of acoustic noise associated with echoplanar imaging (EPI). This is accomplished using a constant phase encoding gradient and a sinusoidal readout echo train to produce a narrow-band acoustic frequency spectrum, which is adapted to the scanner's frequency response function by choosing an optimum gradient switching frequency. To evaluate the effect of these nonstandard parameters we conducted a speech experiment comparing four different EPI sequences: Quiet, Sparse, Standard, and Matched Standard (using the same readout duration as Quiet). For each sequence participants listened to sentences and signal-correlated noise (SCN), which provides an unintelligible amplitude-matched control condition. We used BOLD sensitivity maps to quantify sensitivity loss caused by the longer EPI readout duration used in the Quiet and Matched Standard EPI sequences. We found that the Quiet sequence provided more robust activation for SCN in primary auditory areas and comparable activation in frontal and temporal regions for Sentences>SCN, but less sentence-related activity in inferotemporal cortex. The increased listening effort associated with the louder Standard sequence relative to the Quiet sequence resulted in increased activation in the left temporal and inferior parietal cortices. Together, these results suggest that the Quiet sequence is suitable, and perhaps preferable, for many auditory studies. However, its applicability depends on the specific brain regions of interest.

Disentangling unisensory and multisensory components in audiovisual integration using a novel multifrequency fMRI spectral analysis

Events in the world are mediated through multiple sensory inputs which are processed separately in specific unisensory areas according to the division-of-labor principle, and then need to be integrated to create a unified percept. How this is done is still not clear. For instance, recent evidence showed crossmodal activation in primary areas. We developed a novel approach to study multisensory integration using multifrequency spectral analysis to investigate the processing of audio and visual streams in a multisensory context. Auditory and visual stimuli were delivered in the same experimental condition, each in different presentation frequencies, and thus could be detected by applying Fourier spectral analysis in their different presentation frequencies. The cochleotopic and retinotopic organization of primary auditory and visual areas were found to remain intact in spite of the multisensory context. Auditory responses were also found in the Precuneus, suggesting that it might be a new auditory area responsive to pure tone stimuli, and serving as one end of a novel sensory preference gradient stretching across POG to the calcarine sulcus. Additional audiovisual areal convergence was detected both in areas in the middle of sensory preference gradients, and in primary auditory areas. Interestingly, the in/out synchronization rate of the auditory and visual streams yielded a third interaction frequency, which could be analyzed independently to reveal higher-order audiovisual interaction responses. These results were detected in one compact and natural multisensory experimental condition, which has several advantages over previous approaches. The method can be further implemented to study any type of interaction, within and across sensory modalities.

Primary and multisensory cortical activity is correlated with audiovisual percepts

Incongruent auditory and visual stimuli can elicit audiovisual illusions such as the McGurk effect where visual /ka/ and auditory /pa/ fuse into another percept such as/ta/. In the present study, human brain activity was measured with adaptation functional magnetic resonance imaging to investigate which brain areas support such audiovisual illusions. Subjects viewed trains of four movies beginning with three congruent /pa/ stimuli to induce adaptation. The fourth stimulus could be (i) another congruent /pa/, (ii) a congruent /ka/, (iii) an incongruent stimulus that evokes the McGurk effect in susceptible individuals (lips /ka/ voice /pa/), or (iv) the converse combination that does not cause the McGurk effect (lips /pa/ voice/ ka/). This paradigm was predicted to show increased release from adaptation (i.e. stronger brain activation) when the fourth movie and the related percept was increasingly different from the three previous movies. A stimulus change in either the auditory or the visual stimulus from /pa/ to /ka/ (iii, iv) produced within-modality and cross-modal responses in primary auditory and visual areas. A greater release from adaptation was observed for incongruent non-McGurk (iv) compared to incongruent McGurk (iii) trials. A network including the primary auditory and visual cortices, nonprimary auditory cortex, and several multisensory areas (superior temporal sulcus, intraparietal sulcus, insula, and pre-central cortex) showed a correlation between perceiving the McGurk effect and the fMRI signal, suggesting that these areas support the audiovisual illusion.

No need to talk, I know you: familiarity influences early multisensory integration in a songbird’s brain

It is well known that visual information can affect auditory perception, as in the famous “McGurk effect,” but little is known concerning the processes involved. To address this issue, we used the best-developed animal model to study language-related processes in the brain: songbirds. European starlings were exposed to audiovisual compared to auditory-only playback of conspecific songs, while electrophysiological recordings were made in their primary auditory area (Field L). The results show that the audiovisual condition modulated the auditory responses. Enhancement and suppression were both observed, depending on the stimulus familiarity. Seeing a familiar bird led to suppressed auditory responses while seeing an unfamiliar bird led to response enhancement, suggesting that unisensory perception may be enough if the stimulus is familiar while redundancy may be required for unfamiliar items. This is to our knowledge the first evidence that multisensory integration may occur in a low-level, putatively unisensory area of a non-mammalian vertebrate brain, and also that familiarity of the stimuli may influence modulation of auditory responses by vision.

The non-lemniscal auditory cortex in ferrets: convergence of corticotectal inputs in the superior colliculus.

Descending cortical inputs to the superior colliculus (SC) contribute to the unisensory response properties of the neurons found there and are critical for multisensory integration. However, little is known about the relative contribution of different auditory cortical areas to this projection or the distribution of their terminals in the SC. We characterized this projection in the ferret by injecting tracers in the SC and auditory cortex. Large pyramidal neurons were labeled in layer V of different parts of the ectosylvian gyrus after tracer injections in the SC. Those cells were most numerous in the anterior ectosylvian gyrus (AEG), and particularly in the anterior ventral field, which receives both auditory and visual inputs. Labeling was also found in the posterior ectosylvian gyrus (PEG), predominantly in the tonotopically organized posterior suprasylvian field. Profuse anterograde labeling was present in the SC following tracer injections at the site of acoustically responsive neurons in the AEG or PEG, with terminal fields being both more prominent and clustered for inputs originating from the AEG. Terminals from both cortical areas were located throughout the intermediate and deep layers, but were most concentrated in the posterior half of the SC, where peripheral stimulus locations are represented. No inputs were identified from primary auditory cortical areas, although some labeling was found in the surrounding sulci. Our findings suggest that higher level auditory cortical areas, including those involved in multisensory processing, may modulate SC function via their projections into its deeper layers.

Interaction of syntax, prosody and spatial lateralization during the processing of language

Event Abstract Back to Event Interaction of syntax, prosody and spatial lateralization during the processing of language Björn Herrmann1*, Burkhard Maess1, Anja Hahne1, Erich Schröger2 and Angela Friederici1 1 Max Planck Institute for Human Cognitive and Brain Sciences, Germany 2 University of Leipzig, Germany Syntactic processing during sentence comprehension is believed to be a higher cognitive function which recruits regions outside primary sensory cortices. The primary cortices, on the other hand, are supposed to be restricted to the processing of perceptual information. In a recent visual sentence processing study this view was challenged by showing sensitivity of the visual sensory cortex to syntactic anomalies, a finding introduced as part of a "sensory hypothesis". The present study aimed to test the sensory hypothesis in the auditory domain by localizing the neuronal generators underlying syntactic processing. We also sought to localize the processing of a perceptual feature in order to have a true perceptual reference. Using whole head magnetoencephalography (MEG) and distributed source modeling, we investigated the processing of congruent sentences, sentences that were either syntactically/prosodically incongruent, included an interaural time difference (ITD) change or both deviations. A number of early effects were observed. Very early activations (50 – 90 ms) of superior temporal gyrus (STG) were found for processing of syntactic/prosodic and spatial anomalies. These effects were interpreted to reflect a stimulus filtering mechanism "gating in" important information. Following these effects, syntactically/prosodically incongruent sentences elicited activations in the anterior STG around 110 – 160 ms, whereas infrequent ITD changes elicited activations located more posterior in the STG. Hence, our results do not support the sensory hypothesis in its simplest form as activations were found outside primary auditory sensory areas. The present findings, however, provide clear evidence for a dissociation of speech-related processes and the processing of auditory spatial information in the superior temporal cortex. Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010. Presentation Type: Poster Presentation Topic: Language Citation: Herrmann B, Maess B, Hahne A, Schröger E and Friederici A (2010). Interaction of syntax, prosody and spatial lateralization during the processing of language. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00206 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: 30 Mar 2010; Published Online: 30 Mar 2010. * Correspondence: Björn Herrmann, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, bherrmann@cbs.mpg.de 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 Björn Herrmann Burkhard Maess Anja Hahne Erich Schröger Angela Friederici Google Björn Herrmann Burkhard Maess Anja Hahne Erich Schröger Angela Friederici Google Scholar Björn Herrmann Burkhard Maess Anja Hahne Erich Schröger Angela Friederici PubMed Björn Herrmann Burkhard Maess Anja Hahne Erich Schröger Angela Friederici 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.

GABA receptor subunits in human auditory cortex in normal and stroke cases.

GABA receptors are ubiquitous in the cerebral cortex and play a major role in shaping responses of cortical neurons. GABA(A) and GABA(B) receptor subunit expression was visualized by immunohistochemistry in human auditory areas from both hemispheres in 9 normal subjects (aged 43-85 years; time between death and fixation 6-24 hours) and in 4 stroke patients (aged 59-87 years; time between death and fixation 7-24 hours) and analyzed qualitatively for GABA(A) and semiquantitatively for GABA(B) receptor subunits. In normal brains, the primary auditory area (TC) and the surrounding areas TB and TA displayed distinct GABA(A) receptor subunit labeling with differences among cortical layers and areas. In postacute and chronic stroke we found a layer-selective downregulation of the alpha-2 subunit in the anatomically intact cerebral cortex of the intact and of the lesioned hemisphere, whereas the alpha-1, alpha-3 and beta-2/3 subunits maintained normal levels of expression. The GABA(B) receptors had a distinct laminar pattern in auditory areas and minor differences among areas. Unlike in other pathologies, there is no modulation of the GABA(B) receptor expression in subacute or chronic stroke.

Distinct representations of phonemes, syllables, and supra-syllabic sequences in the speech production network

Functional neuroimaging studies have converged on a core network of brain regions that supports speech production, but the sublexical processing stages performed by the different parts of this network remain unclear. Using an fMRI adaptation paradigm and quantitative analysis of patterns of activation rather than contrast subtractions alone, we were able to identify a set of neural substrates predominantly engaged in phonemic, syllabic, and supra-syllabic levels of processing during speech. Phoneme-level processes were found in the left SMA, pallidum, posterior superior temporal gyrus, and superior lateral cerebellum. Syllable-level processes were found in the left ventral premotor cortex, and supra-syllabic processes related to phonological chunking were found in the right superior lateral cerebellum. Active regions that were not sensitive to sublexical manipulations included primary motor and auditory cortical areas, and medial cerebellum. These results offer a quantitative technique for localizing sublexical neural processes that are difficult to dissociate using non-invasive imaging techniques and provide the beginnings of a “brain map” for language output.

Projection from Visual Areas V2 and Prostriata to Caudal Auditory Cortex in the Monkey

Studies in humans and monkeys report widespread multisensory interactions at or near primary visual and auditory areas of neocortex. The range and scale of these effects has prompted increased interest in interconnectivity between the putatively "unisensory" cortices at lower hierarchical levels. Recent anatomical tract-tracing studies have revealed direct projections from auditory cortex to primary visual area (V1) and secondary visual area (V2) that could serve as a substrate for auditory influences over low-level visual processing. To better understand the significance of these connections, we looked for reciprocal projections from visual cortex to caudal auditory cortical areas in macaque monkeys. We found direct projections from area prostriata and the peripheral visual representations of area V2. Projections were more abundant after injections of temporoparietal area and caudal parabelt than after injections of caudal medial belt and the contiguous areas near the fundus of the lateral sulcus. Only one injection was confined to primary auditory cortex (area A1) and did not demonstrate visual connections. The projections from visual areas originated mainly from infragranular layers, suggestive of a "feedback"-type projection. The selective localization of these connections to peripheral visual areas and caudal auditory cortex suggests that they are involved in spatial localization.

Integration of auditory and visual information in human face discrimination in pigeons: Behavioral and anatomical study

Social stimuli are often multi-sensory. Animals commonly recognize a conspecific through visuo-auditory and olfactory sensory cues. In this study pigeons were trained to discriminate between a human face and the face of a Japanese macaque then tested with morphing images of human and macaque faces (Experiment 1). These images were also presented in conjunction with two different auditory stimuli, namely, vocalization of either a human or a macaque. Results showed that the human vocalization facilitated pigeons’ recognition of human faces, but a comparable facilitation of recognition of macaque faces accompanied by a macaque vocalization was not observed. The subjects had previous exposure to human vocalizations (due to living in laboratory cages) but not to the macaque face or vocalization. This difference in experience can explain differences that such auditory stimuli have on visual discrimination in pigeons. In primates, the primary auditory area and the primary visual area connect to each other in the telencephalon. An anatomical study (Experiment 2) provided evidence that visual and auditory pathways were independent at the level of the primary sensory area in the telencephalon (entopallium and field L) suggesting an anatomical difference in visuo-auditory integration between primates and pigeons. Thus, the visual and auditory systems should be integrated in higher associative area, such as nidopallium caudo-lateralis (NCL) or in an earlier stage, such as the midbrain in the avian brain.

Dichotic pitch activates pitch processing centre in Heschl's gyrus

Although several neuroimaging studies have reported pitch-evoked activations at the lateral end of Heschl's gyrus, it is still under debate whether these findings truly represent activity in relation to the perception of pitch or merely stimulus-related features of pitch-evoking sounds. We investigated this issue in a functional magnetic resonance imaging (fMRI) experiment using pure tones in noise and dichotic pitch sequences, which either contained a melody or a fixed pitch. Dichotic pitch evokes a sensation of pitch only in binaural listening conditions, while the monaural signal cannot be distinguished from random noise. Our data show similar neural activations for both tones in noise and dichotic pitch, which are perceptually similar, but physically different. Pitch-related activation was found at the lateral end of Heschl's gyrus in both hemispheres, providing new evidence for a general involvement of this region in pitch processing. In line with prior studies, we found melody-related activation in Planum temporale and Planum polare, but not in primary auditory areas. These results support the view of a general representation of pitch in auditory cortex, irrespective of the physical attributes of the pitch-evoking sound.

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.