Top down influence on visuo-tactile interaction modulates neural oscillatory responses

Multisensory integration involves bottom-up as well as top-down processes. We investigated the influences of top-down control on the neural responses to multisensory stimulation using EEG recording and time-frequency analyses. Participants were stimulated at the index or thumb of the left hand, using tactile vibrators mounted on a foam cube. Simultaneously they received a visual distractor from a light emitting diode adjacent to the active vibrator (spatially congruent trial) or adjacent to the inactive vibrator (spatially incongruent trial). The task was to respond to the elevation of the tactile stimulus (upper or lower), while ignoring the simultaneous visual distractor. To manipulate top-down control on this multisensory stimulation, the proportion of spatially congruent (vs. incongruent) trials was changed across blocks. Our results reveal that the behavioral cost of responding to incongruent than congruent trials (i.e., the crossmodal congruency effect) was modulated by the proportion of congruent trials. Most importantly, the EEG gamma band response and the gamma-theta coupling were also affected by this modulation of top-down control, whereas the late theta band response related to the congruency effect was not. These findings suggest that gamma band response is more than a marker of multisensory binding, being also sensitive to the correspondence between expected and actual multisensory stimulation. By contrast, theta band response was affected by congruency but appears to be largely immune to stimulation expectancy.

Peripersonal space: a multisensory interface for body-object interactions

Research in the last four decades has brought a considerable advance in our understanding of how the brain synthesizes information arising from different sensory modalities. Indeed, many cortical and subcortical areas, beyond those traditionally considered to be ‘associative,’ have been shown to be involved in multisensory interaction and integration (Ghazanfar and Schroeder 2006). Visuo-tactile interaction is of particular interest, because of the prominent role played by vision in guiding our actions and anticipating their tactile consequences in everyday life. In this chapter, we focus on the functional role that visuo-tactile processing may play in driving two types of body-object interactions: avoidance and approach. We will first review some basic features of visuo-tactile interactions, as revealed by electrophysiological studies in monkeys. These will prove to be relevant for interpreting the subsequent evidence arising from human studies. A crucial point that will be stressed is that these visuo-tactile mechanisms have not only sensory, but also motor-related activity that qualifies them as multisensory-motor interfaces. Evidence will then be presented for the existence of functionally homologous processing in the human brain, both from neuropsychological research in brain-damaged patients and in healthy participants. The final part of the chapter will focus on some recent studies in humans showing that the human motor system is provided with a multisensory interface that allows for continuous monitoring of the space near the body (i.e., peripersonal space). We further demonstrate that multisensory processing can be modulated on-line as a consequence of interacting with objects. This indicates that, far from being passive, the monitoring of peripersonal space is an active process subserving actions between our body and objects located in the space around us.