Multisensory connections of monkey auditory cerebral cortex

Rats are born deaf and start hearing at the end of the second postnatal week, when the ear canals open and low-intensity sounds start to evoke responses in the auditory cortex. Here, using μECoG electrode arrays and intracortical silicon probe recordings, we found that bone-conducted (BC) sounds evoked biphasic responses in the auditory cortex starting from postnatal day (P) 8. The initial phase of these responses, generated by thalamocortical input, was followed by intracortical propagation within supragranular layers. BC-evoked responses co-localized with the responses evoked by electrical stimulation of the cochlea and the deepest layers of the inferior colliculus prior to onset of low-threshold hearing (P13), as well as with the responses evoked by high-frequency (30 kHz) low-intensity (70 dB) air-conducted sounds after that. Thus, BC signals reach high-frequency processing regions of the auditory cortex well before the onset of low-threshold hearing, reflecting early integrity of the auditory system.

Characteristics of neurons in auditory cortex of monkeys performing a simple auditory task.

Repetition of complex frequency-modulated sweeps enhances neuromagnetic responses in the human auditory cortex

Accumulating evidence demonstrates that responses in auditory cortex to auditory consequences of self-generated actions are modified relative to the responses evoked by identical sounds generated by an external source. Such modifications have been suggested to occur through a corollary discharge sent from the motor system, although the exact neuroanatomical origin is unknown. Furthermore, since tactile input has also been shown to modify responses in auditory cortex, it is not even clear whether the source of such modifications is motor output or somatosensory feedback. We recorded functional magnetic resonance imaging (fMRI) data from healthy human subjects (n = 11) while manipulating the rate at which they performed sound-producing actions with their right hand. In addition, we manipulated the amount of tactile feedback to examine the relative roles of motor and somatosensory cortices in modifying evoked activity in auditory cortex (superior temporal gyrus). We found an enhanced fMRI signal in left auditory cortex during perception of self-generated sounds relative to passive listening to identical sounds. Moreover, the signal difference between active and passive conditions in left auditory cortex covaried with the rate of sound-producing actions and was invariant to the amount of tactile feedback. Together with functional connectivity analysis, our results suggest motor output from supplementary motor area and left primary motor cortex as the source of signal modification in auditory cortex during perception of self-generated sounds. Motor signals from these regions could represent a predictive signal of the expected auditory consequences of the performed action.

Recent studies using functional imaging and electrophysiology demonstrate that processes related to sensory integration are not restricted to higher association cortices but already occur in early sensory cortices, such as primary auditory cortex. While anatomical studies suggest the superior temporal sulcus (STS) as likely source of visual input to auditory cortex, little evidence exists to support this notion at the functional level. Here we tested this hypothesis by simultaneously recording from sites in auditory cortex and STS in alert animals stimulated with dynamic naturalistic audio–visual scenes. Using Granger causality and directed transfer functions we first quantified causal interactions at the level of field potentials, and subsequently determined those frequency bands that show effective interactions, i.e. interactions that are relevant for influencing neuronal firing at the target site. We found that effective interactions from auditory cortex to STS prevail below 20 Hz, while interactions from STS to auditory cortex prevail above 20 Hz. In addition, we found that directed interactions from STS to auditory cortex make a significant contribution to multisensory influences in auditory cortex: Sites in auditory cortex showing multisensory enhancement received stronger feed-back from STS during audio–visual than during auditory stimulation, while sites with multisensory suppression received weaker feed-back. These findings suggest that beta frequencies might be important for inter-areal coupling in the temporal lobe and demonstrate that superior temporal regions indeed provide one major source of visual influences to auditory cortex.

Reliability and Representational Bandwidth in the Auditory Cortex

Sequences of repeating tones can be masked by other tones of different frequency. When these tone sequences are perceived, nevertheless, a prominent neural response in the auditory cortex is evoked by each tone of the sequence. When the targets are detected based on their isochrony, participants know that they are listening to the target once they detected it. To explore if the neural activity is more closely related to this detection task or to perceptual awareness, this magnetoencephalography (MEG) study used targets that could only be identified with cues provided after or before the masked target. In experiment 1, multiple mono-tone streams with jittered inter-stimulus interval were used, and the tone frequency of the target was indicated by a cue. Results showed no differential auditory cortex activity between hit and miss trials with post-stimulus cues. A late negative response for hit trials was only observed for pre-stimulus cues, suggesting a task-related component. Since experiment 1 provided no evidence for a link of a difference response with tone awareness, experiment 2 was planned to probe if detection of tone streams was linked to a difference response in auditory cortex. Random-tone sequences were presented in the presence of a multi-tone masker, and the sequence was repeated without masker thereafter. Results showed a prominent difference wave for hit compared to miss trials in experiment 2 evoked by targets in the presence of the masker. These results suggest that perceptual awareness of tone streams is linked to neural activity in 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.

Formation of associations in auditory cortex by slow changes of tonic firing

The present work clarifies the neural underpinning of inattentional deafness under high visual load. The findings of near-simultaneous load effects on both visual and auditory evoked responses suggest shared audiovisual processing capacity. Temporary depletion of shared capacity in perceptually demanding visual tasks leads to a momentary reduction in sensory processing of auditory stimuli, resulting in inattentional deafness. The dynamic "push-pull" pattern of load effects on visual and auditory processing furthers our understanding of both the neural mechanisms of attention and of cross-modal effects across visual and auditory processing. These results also offer an explanation for many previous failures to find cross-modal effects in experiments where the visual load effects may not have coincided directly with auditory sensory processing.

The Effects of Salicylate on Auditory Evoked Potential Amplitwde from the Auditory Cortex and Auditory Brainstem

Differential patterns of multisensory interactions in core and belt areas of human auditory cortex

Neural restoration of degraded audiovisual speech

Age-related auditory nerve deficits propagate central gain throughout the auditory system: Associations with cortical microstructure and speech recognition.

Objectives. To investigate the activation pattern of the human auditory cortex to monaural sound stimuli using functional magnetic resonance imaging (fMRI) in volunteers with normal hearing or unilateral conductive hearing loss. Method. Ten normal hearing volunteers and seven volunteers with unilateral conductive hearing loss were subjected to a random sequence of monaural sound stimuli and silence, whilst undergoing fMRI in a 3 Tesla scanner. The sound stimuli consisted of narrowband noise (250 Hz and 4000 Hz), as well as wideband noise (250–4000 Hz). A signal averaging algorithm was used to detect the response in the primary auditory cortex. A laterality index was calculated and compared between the study groups. Results. In the normal hearing group the auditory areas of both hemispheres showed activity in response to monaural stimuli, but the responses were consistently greater in the contralateral primary auditory cortex. Wideband stimuli produced the greatest degree of contralaterality and the largest response across the auditory cortex. The responses to high and low frequency stimuli were found to be conform with established tonotopic organization of the primary auditory cortex. There was no difference in the laterality index between the study groups. Conclusions. We have developed a non‐invasive way to investigate the response pattern of the primary auditory cortex to sound stimuli using fMRI. A similar activation pattern of the auditory cortex was found in both study groups suggesting a preservation of the organisation of the ascending auditory pathways and auditory cortex in unilateral conductive hearing loss.