Anatomical and physiological investigation of auditory input to the superior colliculus of the echoloting
megachiropteran bat Rousettus aegypticus
A. Thiele, R.Rübsamen & K.-P. Hoffmann
Experimental Brain Research, 112:223-236, 1996
- The objective of this study was to investigate whether a representation of auditory space in the superior colliculus
(SC) of the echolocating 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
acoustic stimuli (white noise, clicks, and pure tones) at different positions in space. Six units responded to
acoustic stimulation. Three of these located within the superficial layers and one located in the intermediate
layers were classified as omnidirectional units. Two units were located within the deep layers. One was classified
as a hemifield unit, and the other as a frontal unit. All units responded phasically to 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 units. Responses to visual stimulation were markedly
altered by acoustic stimulation in 5 units. The influence of the acoustic stimuli was temporally and spatially
restricted, and resulted either in a reduction or an elevation of unit responsiveness. Horseradish peroxidase was
injected into the SC of eight animals to investigate the auditory subcortical 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 structures retrograde labeling was found mainly in the external
nucleus of the inferior colliculus and in the nucleus of the brachium of the inferior 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 nucleus were labeled. Our results reveal only a sparse
auditory input into the SC of the flying fox, R. aegyptiacus. On the basis of single-unit 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 properties, nonetheless indicate that there
might be a representation of auditory space in the SC of R. aegyptiacus.
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