Holmes NP, Skaliora I, Doubell TP, King AJ (2002) Sub-threshold auditory-visual interactions in the rat superior colliculus: Linearity and independence of EPSP summation. MSc Thesis, Wolfson College, University of Oxford, 35pp.
The mammalian superior colliculus maintains topographic maps of visual, auditory, and somatosensory space that are in spatial alignment with each other. The alignment of receptive fields across different modalities for neurons in the different layers of the superior colliculus, and their interconnection with eachother and with premotor nuclei, along with the presence of many cells with bi-modal and tri-modal response properties is widely thought to play an essential role in organising orienting movements of the eyes, head, and ears to stimuli across multiple modalities. It is known that the visual projection from the retina is coarsely topographic from birth, and is refined through experience-dependent processes. The auditory map, however, must be computed from non-spatial sensory signals, and its topography is disturbed following visual deprivation or visual disturbances during development. It is thus believed that visual experience in early life guides the formation of the auditory topography through multi-sensory perceptual experience. Despite the importance of understanding the mechanism of visual guidance of the auditory map, the cellular and molecular substrates of this process are unknown. We have studied the sub-threshold integration of auditory and visual excitatory post-synaptic potentials (EPSPs) in a putative cellular substrate of this visual-auditory interaction, the layer IV multi-sensory cells of the rat superior colliculus. Linear summation of EPSPs was found for coincidental stimulation of auditory and visual projections onto the same deep layer cells. Furthermore, temporal separation of auditory and visual stimulation had no effect on the synaptic integration of these signals. In most cases, average EPSP amplitude was not different from that expected from the arithmetic sum of individual responses. It is concluded either that the mechanisms of auditory-visual guidance involve a supra-threshold component, such as long-term potentiation of coincidentally active synapses, or that the primary site of auditory-visual interaction is at an earlier processing stage in the inferior colliculus