2. 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

3. Cat Auditory Cortex and FAES

4. 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 Courtesy of Stein 2008 Somatosensory: SIV Visual: anterior ectosylvian visual (AEV)

5. 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.

6. 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.

7. 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+AEVBimodal: neurons response independently to separate visual and auditory stimulation Subthreshold multisensory neurons: response to only one modality but influenced by other modality

8. Response of FAES neurons to auditory, visual, and combined auditory–visual stimulation 16.5% of all neurons Within FAES

9. Multisensory influences on auditory processing in FAES MS: multisensory, bimodal & subthreshold

10. 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).