Crossmodal projections from somatosensory area SIV to the auditory field of the anterior ectosylvian sulcus (FAES) in Cat: further evidence for subthreshold forms of multisensory processing.

To date, evaluation of the neuronal basis for multisensory processing has focused on the convergence pattern that provides excitation from more than one sensory modality. However, a recent study (Dehner et al. in Cereb Cortex 14:387-401, 2004) has demonstrated excitatory-inhibitory multisensory effects that do not follow this conventional pattern and the present investigation documented a similar example of subthreshold cross-modal effects. Neuroanatomical tracers revealed that pyramidal neurons of the somatosensory area SIV project to the auditory field of the anterior ectosylvian sulcus (FAES), but subsequent electrophysiological tests showed that stimulation of SIV failed to elicit the expected orthodromic responses in FAES. Instead, combined auditory-SIV stimulation significantly suppressed FAES responses to auditory cues in approximately 25% of the neurons tested, and facilitated responses in another 5%. These modulatory responses in auditory FAES were similar in kind to those observed in somatosensory SIV and, as such, comprise further evidence for subthreshold forms of multisensory processing in cortex. Consequently, it seems likely that subthreshold cross-modal effects may impact other apparently 'unimodal' areas of the brain.