Ideophones in Japanese modulate the P2 and late positive complex responses

The aim of this study was (1) to provide behavioral evidence for multimodal feature integration in an object recognition task in humans and (2) to characterize the processing stages and the neural structures where multisensory interactions take place. Event-related potentials (ERPs) were recorded from 30 scalp electrodes while subjects performed a forced-choice reaction-time categorization task: At each trial, the subjects had to indicate which of two objects was presented by pressing one of two keys. The two objects were defined by auditory features alone, visual features alone, or the combination of auditory and visual features. Subjects were more accurate and rapid at identifying multimodal than unimodal objects. Spatiotemporal analysis of ERPs and scalp current densities revealed several auditory-visual interaction components temporally, spatially, and functionally distinct before 200 msec poststimulus. The effects observed were (1) in visual areas, new neural activities (as early as 40 msec poststimulus) and modulation (amplitude decrease) of the N185 wave to unimodal visual stimulus, (2) in the auditory cortex, modulation (amplitude increase) of subcomponents of the unimodal auditory N1 wave around 90 to 110 msec, and (3) new neural activity over the right fronto-temporal area (140 to 165 msec). Furthermore, when the subjects were separated into two groups according to their dominant modality to perform the task in unimodal conditions (shortest reaction time criteria), the integration effects were found to be similar for the two groups over the nonspecific fronto-temporal areas, but they clearly differed in the sensory-specific cortices, affecting predominantly the sensory areas of the nondominant modality. Taken together, the results indicate that multisensory integration is mediated by flexible, highly adaptive physiological processes that can take place very early in the sensory processing chain and operate in both sensory-specific and nonspecific cortical structures in different ways.

Synchronous presentation of stimuli to the auditory and visual systems can modify the formation of a percept in either modality. For example, perception of auditory speech is improved when the speaker's facial articulatory movements are visible. Neural convergence onto multisensory sites exhibiting supra-additivity has been proposed as the principal mechanism for integration. Recent findings, however, have suggested that putative sensory-specific cortices are responsive to inputs presented through a different modality. Consequently, when and where audiovisual representations emerge remain unsettled. In combined psychophysical and electroencephalography experiments we show that visual speech speeds up the cortical processing of auditory signals early (within 100 ms of signal onset). The auditory-visual interaction is reflected as an articulator-specific temporal facilitation (as well as a nonspecific amplitude reduction). The latency facilitation systematically depends on the degree to which the visual signal predicts possible auditory targets. The observed auditory-visual data support the view that there exist abstract internal representations that constrain the analysis of subsequent speech inputs. This is evidence for the existence of an "analysis-by-synthesis" mechanism in auditory-visual speech perception.

This article reviews the event-related potential (ERP) literature in relation to the P2 waveform of the human auditory evoked potential. Within the auditory evoked potential, a positive deflection at approximately 150-250 ms is a ubiquitous feature. Unlike other cognitive components such as N1 or the P300, remarkably little has been done to investigate the underlying neurological correlates or significance of this waveform. Indeed until recently, many researchers considered it to be an intrinsic part of the 'vertex potential' complex, involving it and the earlier N1. This review seeks to describe the evidence supportive of P2 being the result of independent processes and highlights several features, such as its persistence from wakefulness into sleep, the general consensus that unlike most other EEG phenomena it increases with age, and the fact that it can be generated using respiratory stimuli.