Vibration-induced auditory-cortex activation in a congenitally deaf adult.

Considerable changes take place in the number of cerebral neurons, synapses and axons during development, mainly as a result of competition between different neural activities [1-4]. Studies using animals suggest that when input from one sensory modality is deprived early in development, the affected neural structures have the potential to mediate functions for the remaining modalities [5-8]. We now show that similar potential exists in the human auditory system: vibrotactile stimuli, applied on the palm and fingers of a congenitally deaf adult, activated his auditory cortices. The recorded magnetoencephalographic (MEG) signals also indicated that the auditory cortices were able to discriminate between the applied 180 Hz and 250 Hz vibration frequencies. Our findings suggest that human cortical areas, normally subserving hearing, may process vibrotactile information in the congenitally deaf.

Persisting versus sustained neural activity: effects on transient N100m response.

Previous evidence from animal and human studies suggests that neural activity, both during a continuous tone and persisting after the offset of a first tone, can prolong the latency and enhance the amplitude of a transient response to a second tone. Our results showed that the latency of N100m to a second tone presented to the opposite ear was prolonged equally in both conditions. Unexpectedly, the effects on response strength strongly depended on stimulus laterality: the ipsilateral but not the contralateral N100m to the second tone was enhanced by the simultaneous presence of the first when compared with the effect of the preceding tone. This suggests that sustained neural activity during a continuous tone can release inhibition normally induced by ipsilateral stimulation.

Feeling vibrations: enhanced tactile sensitivity in congenitally deaf humans.

The human nervous system displays remarkable functional plasticity following long-term sensory deprivation. For example, the auditory cortex of congenitally deaf humans may start to process tactile information. To further explore this type of cross-modal plasticity, we examined the tactile accuracy of congenitally deaf and normal hearing subjects in frequency discrimination and in detection of random suprathreshold frequency changes within a monotonous sequence of vibratory stimuli. We found that congenital deafness can enhance the accuracy of suprathreshold tactile change detection while tactile frequency discrimination is not significantly changed, although there is a trend toward reduced thresholds. The enhanced tactile sensitivity in the deaf probably reflects both neural plasticity and increased attention directed to the stimuli. Whatever the underlying neural mechanisms might be, functional compensation following early sensory loss apparently leads the remaining sensory modalities to develop capacities exceeding those of the normal functional systems.

Neuromagnetic studies of human auditory cortex function and reorganization.

Much of our present understanding of sensory processing by the human brain is obtained from studies of patients with sensory impairments. Past auditory studies have focused strongly on the peripheral mechanisms of hearing disorders, and have led to an overuse of stimuli that are good for testing the transfer functions of simple acoustic features but are not suitable for understanding the perception of complex auditory stimuli or stimulus sequences. Magnetoencephalography (MEG) is a non-invasive method for studying how the human brain processes and stores auditory information. The long-term groundwork in building up the basic understanding of cortical dynamics during various "simple" stimulations now allows the use of more complex, real-life-like stimuli, and clinical applications.

Disrupting human auditory change detection: Chopin is superior to white noise.

A deviant sound in a sequence of standard sounds elicits a neuromagnetic mismatch field (MMF) reflecting change detection based on the auditory sensory memory trace. To illuminate the nature of this trace, we investigated the effects of white noise and music maskers on the MMF. The stimuli were delivered to the participant's right ear, and the maskers were delivered to the same or contralateral ear. Only maskers containing transients (music) presented to either ear abolished the MMF. In parallel, the ability to discriminate the deviants decreased dramatically, probably because of integration of transient features of the music to the neural representations of standards and deviants. As a result, the similarity of these representations prevents change detection. White noise affected the MMF amplitude only when presented to the same ear to which the stimuli were presented. All maskers decreased the M100 but not the M50 amplitude, suggesting that the neural generators behind these responses are functionally separate.

Temporal characteristics of auditory sensory memory: neuromagnetic evidence.

We investigated the temporal dependencies of N100 m, the most prominent deflection of the auditory evoked response, using whole-head neuromagnetic recordings. Stimuli were presented singly or in pairs (tones in the pair were separated by 210 ms) at interstimulus intervals (ISIs) of 0.6-8.1 s. N100 m to single stimuli and to the first tone of the pair had similar temporal recovery functions, plateauing at ISIs of 6 s. N100 m to the second tone in the pair, which was smaller than that to the first except with short ISIs, plateaued with ISIs of about 4 s. Source analysis revealed that the N100 m could be decomposed into two sources separated by about 1 cm on the supratemporal plane. The recovery function of the posterior source was not affected by stimulus presentation, whereas that of the anterior source was. Activity in the anterior area appears to reflect the effects of temporal integration. We relate these results to auditory sensory memory.

Timing of human cortical functions during cognition: role of MEG.

Understanding of sensory and cognitive brain processes requires information about activation timing within and between different brain sites. Such data can be obtained by magnetoencephalography (MEG) that tracks cortical activation sequences with a millisecond temporal accuracy. MEG is gaining a well-established role in human neuroscience, complementing with its excellent temporal resolution the spatially more focused brain imaging methods. As examples of MEG's role in cognitive neuroscience, we discuss time windows related to cortical processing of sensory and multisensory stimuli, effects of the subject's own voice on the activity of their auditory cortex, timing of brain activation in reading, and cortical dynamics of the human mirror-neuron system activated when the subject views another person's movements.

Responses of the human auditory cortex to changes in one versus two stimulus features.

Neuromagnetic responses were recorded with a 24-SQUID magnetometer in two "oddball" experiments to determine whether mismatch responses to changes in single stimulus features are additive. In experiment 1, the one-feature deviants differed from standards in interstimulus interval (ISI) or frequency, and the two-feature deviants in both ISI and frequency. In experiment 2, deviants differed in duration, frequency, or both. All deviants evoked a mismatch field (MMF) with sources close to each other in the supratemporal auditory cortex. Except for the ISI deviants, the MMF sources were about 1 cm anterior to the source of the 100-ms response, N100m, to the standards. In the two experiments, MMFs obtained in response to the two-feature deviants resembled closely the sum of MMFs in response to one-feature deviants. The results suggest that the standards leave a multiple neuronal representation in the human auditory cortex. The particular neuronal traces of the representation react independently to changes in different features of sound stimuli.

Cortical representation of sign language: comparison of deaf signers and hearing non-signers.

Numerous studies have demonstrated activation of the classical left-hemisphere language areas when native signers process sign language. More recently, specific sign language-related processing has been suggested to occur in homologous areas of the right hemisphere as well. We now show that these cortical areas are also activated in hearing non-signers during passive viewing of signs that for them are linguistically meaningless. Neuromagnetic activity was stronger in deaf signers than in hearing non-signers in the region of the right superior temporal sulcus and the left dorsal premotor cortex, probably reflecting familiarity and linguistic meaningfulness of the observed movement sequences. In contrast, the right superior parietal lobule, the mesial parieto-occipital region, and the mesial paracentral lobule were more strongly activated in hearing non-signers, apparently reflecting active visuomotor encoding of complex unfamiliar movement sequences.

Deviant auditory stimuli activate human left and right auditory cortex differently.

Infrequent "deviant' auditory stimuli embedded in a homogeneous sequence of "standard' sounds evoke a neuromagnetic mismatch field (MMF), which is assumed to reflect automatic change detection in the brain. We investigated whether MMFs would reveal hemispheric differences in cortical auditory processing. Seven healthy adults were studied with a whole-scalp neuromagnetometer. The sound sequence, delivered to one ear at time, contained three infrequent deviants (differing from standards in duration, frequency, or interstimulus interval) intermixed with standard tones. MMFs peaked 9-34 msec earlier in the right than in the left hemisphere, irrespective of the stimulated ear. Whereas deviants activated only one MMF source in the left hemisphere, two temporally overlapping but spatially separate sources, one in the temporal lobe and another in the inferior parietal cortex, were necessary to explain the right-hemisphere MMFs. We suggest that the bilateral MMF components originating in the supratemporal cortex are feature specific whereas the right-hemisphere parietal component reflects more global auditory change detection. The results imply hemispheric differences in sound processing and suggest stronger involvement of the right than the left hemisphere in change detection.

Temporal integration in auditory sensory memory: neuromagnetic evidence.

The cortical mechanisms of auditory sensory memory were investigated by analysis of neuromagnetic evoked responses. The major deflection of the auditory evoked field (N100m) appears to comprise an early posterior component (N100mP) and a late anterior component (N100mA) which is sensitive to temporal factors. When pairs of identical sounds are presented at intervals less that about 250 msec, the second sound evokes N100mA with enhanced amplitude at a latency of about 150 msec. We suggest that N100mA may index the activity of two distinct processes in auditory sensory memory. Its recovery cycle may reflect the activity of a memory trace which, according to previous studies, can retain processed information about an auditory sequence for about 10 sec. The enhancement effect may reflect the activity of a temporal integration process, whose time constant is such that sensation persists for 200-300 msec after stimulus offset, and so serves as a short memory store. Sound sequences falling within this window of integration seem to be coded holistically as unitary events.