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Cerebral Cortex Advance Access originally published online on January 17, 2008
Cerebral Cortex 2008 18(9):2066-2076; doi:10.1093/cercor/bhm230
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© The Author 2008. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Do Cross-Modal Projections Always Result in Multisensory Integration?

Brian L. Allman1, Ruben E. Bittencourt-Navarrete2, Leslie P. Keniston1, Alexandre E. Medina1, Meng Y. Wang1 and M. Alex Meredith1

1 Department of Anatomy and Neurobiology, Virginia Commonwealth University, School of Medicine, Richmond, VA 23298, USA, 2 Instituto de Biofisica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro, Rio de Janeiro (RJ), Brazil 21941-902

Address correspondence to Brian L. Allman, PhD, Department of Anatomy and Neurobiology, Virginia Commonwealth University School of Medicine, 1101 E Marshall Street, Sanger Hall 12-067, Richmond, VA, 23298-0709, USA. Email: ballman2{at}vcu.edu

Convergence of afferents from different sensory modalities has generally been thought to produce bimodal (and trimodal) neurons (i.e., exhibit suprathreshold excitation to more than 1 sensory modality). Consequently, studies identifying cross-modal connections assume that such convergence results in bimodal (or trimodal) neurons that produce familiar forms of multisensory integration: response enhancement or depression. The present study questioned that assumption by anatomically identifying a projection from ferret auditory to visual cortex Area 21. However, electrophysiological recording within Area 21 not only failed to identify a single bimodal neuron but also familiar forms of multisensory integration were not observed either. Instead, a small proportion of neurons (9%; 27/296) showed subthreshold multisensory integration, in which visual responses were significantly modulated by auditory inputs. Such subthreshold multisensory effects were enhanced by {gamma}-aminobutyric acid antagonism, whereby a majority of neurons (87%; 20/23) now participated in a significant, multisensory population effect. Thus, multisensory convergence does not de facto result in bimodal (or trimodal) neurons or the traditional forms of multisensory integration. However, the fact that unimodal neurons exhibited a subthreshold form of multisensory integration not only affirms the relationship between convergence and integration but also expands our understanding of the functional repertoire of multisensory processing itself.

Key Words: auditory cortex • bicuculline methiodide • bimodal neuron • extra striate visual cortex • ferret • subthreshold facilitation

Received for publication June 7, 2007. Revision received November 20, 2007. Accepted for publication November 20, 2007.


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