This is an application to renew an ADAMHA Research Scientist Development Award (Level II). The goal of the proposed research is to understand basic processes for formation of perceptual images. The specific example to be studied is the fusion of different stimulus dimensions into the sonar images perceived by the echolocating bat, Eptesicus fuscus. Research conducted during the first term of the RSDA has discovered an unusually accessible instance of image fusion in the way this bat processes echoes of its frequency-modulated (FM) sonar signals. The bat perceives the distance to targets, or target range, from the delay of echoes, encoding echo delay by the timing of neural discharges evoked by echoes. This discharge timing then is transformed into a topographic """"""""target range"""""""" map in the auditory cortex. The bat perceives the shape of targets in terms of the distribution of parts of the target along the target-range axis. The echoes reflected by different parts of the target overlap when they arrive at the bat's ears, creating a single complex echo whose spectrum represents target shape. The bat encodes the echo spectrum in terms of the distribution of neural activity across the frequency axis of tonotopic maps in its auditory system. The target range map has an echo delay axis, and the bat actually perceives images in terms of delay. The bat transposes information about target shape into its equivalent in terms of the delay separation of echo components to perceive the target's structure along a psychological range scale. The convergence of delay and frequency information onto a computed range scale is a concrete example of image fusion. The proposed research consists of behavioral, physiological, and acoustic experiment to determine how the bat accomplishes fusion of range and shape into whole images. The development of a theoretical understanding that can link behavioral and physiological observations is a crucial aspect of the proposal. Large steps have been made during the first RSDA term, and the present proposal is to enable the candidate to strengthen our conceptual understanding of perception through the example of echolocation.
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