title: Do cross-modal projections always result in multisensory integration?
abstract: 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.
authors.affiliation: Department of Anatomy and Neurobiology, Virginia Commonwealth University, School of Medicine, Richmond, VA 23298, USA. ballman2@vcu.edu
authors.forename: Brian L
authors.initials: BL
authors.lastname: Allman
authors.forename: Ruben E
authors.initials: RE
authors.lastname: Bittencourt-Navarrete
authors.forename: Leslie P
authors.initials: LP
authors.lastname: Keniston
authors.forename: Alexandre E
authors.initials: AE
authors.lastname: Medina
authors.forename: Meng Y
authors.initials: MY
authors.lastname: Wang
authors.forename: M Alex
authors.initials: MA
authors.lastname: Meredith
mesh_terms: D000818:Animals; D001303:Auditory Cortex / Q000166:cytology / Q000502:physiology*; D001306:Auditory Pathways / Q000166:cytology / Q000502:physiology*; D001640:Bicuculline / Q000031:analogs & derivatives / Q000494:pharmacology; D004594:Electrophysiology; D005289:Ferrets; D018756:GABA Antagonists / Q000494:pharmacology; D007478:Iontophoresis; D009433:Neural Inhibition / Q000502:physiology; D009474:Neurons / Q000502:physiology; D010296:Parietal Lobe / Q000166:cytology / Q000502:physiology*; D017729:Presynaptic Terminals / Q000502:physiology; D012684:Sensory Thresholds / Q000187:drug effects / Q000502:physiology*; D014793:Visual Cortex / Q000166:cytology / Q000502:physiology*; D014795:Visual Pathways / Q000166:cytology / Q000502:physiology*
chemical_list: D018756:GABA Antagonists; C017069:bicuculline methiodide; D001640:Bicuculline