Multisensory Enhancement in the Optic Tectum of the Barn Owl: Spike Count and Spike Timing

Department of Physiology and Biophysics, The Rappaport Faculty of Medicine and Research Institute, Technion, Haifa, Israel Submitted 9 November 2008; accepted in final form 2 March 2009 Temporal and spatial correlations between auditory and visual stimuli facilitate the perception of unitary events and improve behavioral responses. However, it is not clear how combined visual and auditory information is processed in single neurons. Here we studied responses of multisensory neurons in the barn owl's optic tectum (the avian homologue of the superior colliculus) to visual, auditory, and bimodal stimuli. We specifically focused on responses to sequences of repeated stimuli. We first report that bimodal stimulation tends to elicit more spikes than in the responses to its unimodal components (a phenomenon known as multisensory enhancement). However, this tendency was found to be history-dependent; multisensory enhancement was mostly apparent in the first stimulus of the sequence and to a much lesser extent in the subsequent stimuli. Next, a vector-strength analysis was applied to quantify the phase locking of the responses to the stimuli. We report that in a substantial number of multisensory neurons responses to sequences of bimodal stimuli elicited spike trains that were better phase locked to the stimulus than spike trains elicited by stimulating with the unimodal counterparts (visual or auditory). We conclude that multisensory enhancement can be manifested in better phase locking to the stimulus as well as in more spikes. Address for reprint requests and other correspondence: Y. Gutfreund, Dept. of Physiology and Biophysics, The Bruce Rappaport Medical School, The Technion, Haifa, 31096, Israel (E-mail: yoramg{at}tx.technion.ac.il )