Do cross-modal projections always result in multisensory integration?
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.
Allman
Brian L
BL
Department of Anatomy and Neurobiology, Virginia Commonwealth University, School of Medicine, Richmond, VA 23298, USA. ballman2@vcu.edu
Bittencourt-Navarrete
Ruben E
RE
Keniston
Leslie P
LP
Medina
Alexandre E
AE
Wang
Meng Y
MY
Meredith
M Alex
MA
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*
D018756:GABA Antagonists; C017069:bicuculline methiodide; D001640:Bicuculline