Modulations of collicular visual responses by acoustic stimuli in rabbits

This study analyzes the influences of an acoustic stimulus upon neuronal light responses of superficial layers of the superior colliculus in anesthetized and paralyzed rabbits. The results have revealed that even if visually-responsive cells fail to be excited by sound, the latter is still capable of modifying light-evoked discharge. The influence may be "short-term" (the discharge rate recovers within 500 ms) or it may be "long-term" (the firing rate remains modified for several seconds). This audio-visual interaction depends upon several factors: the time of occurrence of both stimuli, the physical aspects of the visual target, the relative positions of the speaker and the visual receptive field, and finally, the sensitivity of the unit to movement direction. Data indicates that cells of the most dorsal (hence visual) layers of the superior colliculus are influenced by sound. It is concluded that the colliculus may use the sound as an additional cue to orientate the animal. Also, collicular cells could "memorize" for several seconds various features present in the environment.