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

Courtesy of Stein 2008

Somatosensory: SIV Visual: anterior ectosylvian visual (AEV)

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 independentlyto 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).