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Multisensory convergence • Traditionally, represented by inputs from more than one sensory modality, each of which can independently activate the target neuron. (50 yr) • Recently, multisensory neuron excited to suprathreshold levels by only one sensory modality, yet inputs from a second modality can significantly modulate these responses through facilitation or suppression (25 yr) Subthreshold multisensory effects may be a general feature Cat Auditory Cortex and FAES Auditory field of the anterior ectosylvian sulcus (FAES) • Prefer complex sounds /c multiple frequencies & sensitivity to sound location • Reversible deactivation of leads to deficits in sound localization • Bimodal neurons identified largely along shared borders • Nonbimodal forms of multisensory processing in visual and somatosensory cortices. Allman 2007 Somatosensory: SIV Visual: anterior ectosylvian visual (AEV) Courtesy of Stein 2008 Hypothesis • Subthreshold-processing patterns might be ubiquitous to multisensory systems • Multisensory processing is subserved by not just bimodal neurons, but by a range of multisensory convergence patterns. Methods • Craniotomy to expose auditory cortex (deep anesth. pentobarbital) • a glass-insulated tungsten electrode was inserted vertically and advanced into the FAES (shallow anesth. with ketamine + acepromazine, still some reflex kept) • Visual cues: moving light or dark bars projected onto a translucent hemisphere • Auditory cues: clicks, claps, whistles, & hisses • Somatosensory stimuli: air puffs, brushes, taps to the body surface, as well as compression of deep tissues and joint rotation. FAES & the sensory responses Auditory neurons: 70% (135/193) Visual: 13.5% (26/193) ventrally (AEV) A+V (bimodal): 14.5% (28/193) 2% unresponsive No other sensory convergence (v+s) FAES+AEV Bimodal: neurons response independently to separate visual and auditory stimulation Subthreshold multisensory neurons: response to only one modality but influenced by other modality Response of FAES neurons to auditory, visual, and combined auditory–visual stimulation 16.5% of all neurons Within FAES Multisensory influences on auditory processing in FAES MS: multisensory, bimodal & subthreshold Conclusion • Neurons in auditory FAES exhibit both bimodal and subthreshold forms of multisensory convergence and processing. • Subthreshold multisensory neurons produce response levels intermediate to those elicited in bimodal (multisensory convergence) or in unimodal neurons (no convergence).