Both the MEMs and OHCs are subject to descending control by signals from the central nervous system (reviewed in refs. In short, the actions of the MEMs and OHCs affect not only the response to incoming sound but also transmit vibrations backward to the eardrum. Second, within the cochlea, the outer hair cells (OHCs) are mechanically active and modify the motion of both the basilar membrane and, through mechanical coupling via the ossicles, the eardrum. Contraction of these muscles tugs on the ossicular chain, modulating middle ear sound transmission and moving the eardrum. First, the middle ear muscles (MEMs), the stapedius and tensor tympani, attach to the ossicles that connect the eardrum to the oval window of the cochlea. The auditory periphery possesses at least two means of tailoring its processing in response to descending neural control ( Fig. In this study, we tested whether eye movements affect processing in the auditory periphery. Such findings raise the question of where in the auditory pathway eye movements first impact auditory processing. In the primate brain, all of the regions previously evaluated have shown some evidence that eye movements modulate auditory processing. Accordingly, considerable effort has been devoted to determining where and how the brain incorporates information about eye movements into the visual and auditory processing streams ( 1). In species with mobile eyes (e.g., humans, monkeys), visual and auditory spatial cues bear no fixed relationship to one another but change dramatically and frequently as the eyes move, about three times per second over an 80° range of space. To derive such benefits, the brain must first link visual and auditory signals that arise from common locations in space.
Visual information can aid hearing, such as when lip reading cues facilitate speech comprehension. We discuss the possibility that the mechanisms underlying EMREOs create eye movement-related binaural cues that may aid the brain in evaluating the relationship between visual and auditory stimulus locations as the eyes move. They lasted throughout the saccade and well into subsequent periods of steady fixation. The amplitude and phase of the EMREOs depended on the direction and horizontal amplitude of the saccade. These eardrum movements, which we dub eye movement-related eardrum oscillations (EMREOs), occurred in the absence of a sound stimulus. The eardrum motion was oscillatory and began as early as 10 ms before saccade onset in humans or with saccade onset in monkeys.
Ear canal microphone measurements in humans ( n = 19 ears in 16 subjects) and monkeys ( n = 5 ears in three subjects) performing a saccadic eye movement task to visual targets indicated that the eardrum moves in conjunction with the eye movement. Here, we show a multimodal interaction evident at the eardrum. Interactions between sensory pathways such as the visual and auditory systems are known to occur in the brain, but where they first occur is uncertain.
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