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We examined effects of the task of categorizing linear frequency-modulated (FM) sweeps into rising and falling on auditory evoked magnetic fields (AEFs) from the human auditory cortex, recorded by means of whole-head magnetoencephalography. AEFs in this task condition were compared with those in a passive condition where subjects had been asked to just passively listen to the same stimulus material. We found that the M100-peak latency was significantly shorter for the task condition than for the passive condition in the left but not in the right hemisphere. Furthermore, the M100-peak latency was significantly shorter in the right than in the left hemisphere for the passive and the task conditions. In contrast, the M100-peak amplitude did not differ significantly between conditions, nor between hemispheres. We also analyzed the activation strength derived from the integral of the absolute magnetic field over constant time windows between stimulus onset and 260 ms. We isolated an early, narrow time range between about 60 ms and 80 ms that showed larger values in the task condition, most prominently in the right hemisphere. These results add to other imaging and lesion studies which suggest a specific role of the right auditory cortex in identifying FM sweep direction and thus in categorizing FM sweeps into rising and falling.

Neural activities elicited in the auditory system are systematically organized according to the frequency characteristics of corresponding sound inputs. This systematic frequency alignment, called 'tonotopy,' plays an important role in auditory perception. By means of magnetoencephalography (MEG) we investigated here interactions between neural groups activated by two simultaneously presented narrow-band noises (NBNs) within the human cortical tonotopic map. Auditory evoked fields indicated that the neural interactions activated by these NBNs depended on the frequency difference between them: the amplitude of the N1m-response systematically increased with increasing frequency difference between the NBNs until the critical bandwidth was reached. In contrast, the N1m decreased with frequency difference exceeding the critical bandwidth. The different N1m-response patterns within and beyond the critical band seem to result from the combination of inhibitory and excitatory neural processes in the auditory pathway and may contribute to the perception of complex sound patterns like speech and music.

Deprivation from normal sensory input has been shown to alter tonotopic organization of the human auditory cortex. In this context, cochlear implant subjects provide an interesting model in that profound deafness is made partially reversible by the cochlear implant. In restoring afferent activity, cochlear implantation may also reverse some of the central changes related to deafness. The purpose of the present study was to address whether the auditory cortex of cochlear implant subjects is tonotopically organized. The subjects were thirteen adults with at least 3 months of cochlear implant experience. Auditory event-related potentials were recorded in response to electrical stimulation delivered at different intracochlear electrodes. Topographic analysis of the auditory N1 component (approximately 85 ms latency) showed that the locations on the scalp and the relative amplitudes of the positive/negative extrema differ according to the stimulated electrode, suggesting that distinct sets of neural sources are activated. Dipole modeling confirmed electrode-dependent orientations of these sources in temporal areas, which can be explained by nearby, but distinct sites of activation in the auditory cortex. Although the cortical organization in cochlear implant users is similar to the tonotopy found in normal-hearing subjects, some differences exist. Nevertheless, a correlation was found between the N1 peak amplitude indexing cortical tonotopy and the values given by the subjects for a pitch scaling task. Hence, the pattern of N1 variation likely reflects how frequencies are coded in the brain.

A mismatch between auditory sensation and expectant imagery of syllables elicited a possible equivalent of mismatch negativity in a previous study. The purpose of this study was to verify whether auditory imagery from musical notation could also mediate such imagery-based mismatch negativity. Neuromagnetic recording was obtained from eight musicians, who were instructed to identify unpredictably occurring pitch mismatches between a random tone sequence and a visually presented musical score. The difference between incongruent and congruent responses showed a magnetic distribution consistent with two frontal-negative current dipoles bilaterally located in the vicinity of Heschl's gyrus, peaking at approximately 150 ms in latency. This imagery-based mismatch negativity may represent an early neural process of deviance detection between the sensory input and expectant imagery.

The mammalian auditory cortex undergoes continuous plasticity following auditory experience. This study demonstrates the instructive roles of sound frequency and amplitude in representational plasticity in the primary auditory cortex of the mouse. Electrical stimulation of the basal forebrain paired with a tone led to a pronounced shift in the receptive field of the cortical neurons in both frequency and amplitude domains, the shift being towards the frequency and amplitude of the tone. Importantly, the plasticity in the frequency tuning of cortical neurons appeared to be largely dependent upon frequency-specific decreases in the response threshold. The minimum threshold of cortical neurons could be reduced only if the amplitude of the presented tone was lower than the minimum threshold. This finding suggests that training with low-intensity sound can increase the sensitivity of cortical neurons. Furthermore, all of these effects evoked by basal forebrain activation could be eliminated by cortical application of atropine, the acetylcholine muscarinic receptor antagonist. The data suggest that cortical plasticity is guided by both sound frequency and amplitude. The basal forebrain promotes sound-guided cortical plasticity by facilitating neural mechanisms intrinsic to the auditory system.

We epipially mapped tone-burst-evoked potentials over the rat auditory cortex, and investigated the representation of intensity information. The experiments were designed to elucidate how the auditory cortex represents a steady-state plateau sound pressure level (SPL) and dynamic onset temporal structure, i.e., the rate of pressure change (in Pa/s), and how the representations differ across the auditory fields. The anterior and ventral fields have spatial axes of the rate of pressure change. Characteristic frequency (CF) locations mainly handle the dynamic state, while off-CF locations have the potential to code the steady state. Each field represents the intensity information differently, particularly at the off-CF locations. These results suggest that intensity information is distributed in various aspects in the multiple auditory fields.