ANATOMICAL AND PHYSIOLOGICAL INVESTIGATION OF AUDITORY INPUT TO THE SUPERIOR COLLICULUS OF THE ECHOLOCATING MEGACHIROPTERAN BAT ROUSETTUS-AEGYPTIACUS

The objective of this study was to investigate whether a representatio n of auditory space in the superior colliculus (SC) of the echolocatin g megachiropteran bat (Rousettus aegyptiacus) exists. Additionally the subcortical auditory connectivity of the SC was investigated. A total of 207 units were recorded in five awake animals while presenting aco ustic stimuli (white noise, clicks, and pure tones) at different posit ions in space. Six units responded to acoustic stimulation. Three of t hese located within the superficial layers and one located in the inte rmediate layers were classified as omnidirectional units. Two units we re located within the deep layers. One was classified as a hemifield u nit, and the other as a frontal unit. All units responded phasically t o acoustic stimulation with a latency of 4-150 ms. None of them could be activated by visual stimuli. We further examined the interaction of paired auditory and visual stimulation in 116 visually responsive uni ts. Responses to visual stimulation were markedly altered by acoustic stimulation in 5 units. The influence of the acoustic stimuli was temp orally and spatially restricted, and resulted either in a reduction or an elevation of unit responsiveness. Horseradish peroxidase was injec ted into the SC of eight animals to investigate the auditory subcortic al connectivity of the SC. Retrograde labeling in auditory structures was rare compared with labeling found in nonauditory structures (e.g., retina, substantia nigra, parabigeminal nucleus). In auditory structu res retrograde labeling was found mainly in the external nucleus of th e inferior colliculus and in the nucleus of the brachium of the inferi or colliculus. To a lesser extent it was found in the nucleus sagulum and in the area medial to the lemniscal nuclei. In one case the dorsal nucleus of the lateral lemniscus and the anterolateral periolivary nu cleus were labeled. Our results reveal only a sparse auditory input in to the SC of the flying fox, R. aegyptiacus. On the basis of single-un it recordings, we did not find an elaborate representation of auditory space as it is described for several other species. The existence of auditory and bimodal neurones, in combination with their response prop erties, nonetheless indicate that there might be a representation of a uditory space in the SC of R. aegyptiacus.