COORDINATE TRANSFORMATION IN AFTEREFFECTS AND ATTENTION

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Visual scenes do not appear to shift when an eye movement causes the optical image to move across the retina, but do when the same retinal displacement occurs while the eyes are stationary. How does the visual system achieve this "position constancy" during eye movements? The goal of this project is to understand the coordinate transformations which enable position constancy by generating a body- or environment-centered description of objects from their eye-centered representation. Instead of the traditional approach to this problem, measuring mislocalization of a briefly flashed dot stimulus, we will examine various types of aftereffect/cue effect, including visual aftereffects (eg. figural, and motion), visual-motor aftereffects (eg. saccade. and arm reaching) and an attention-induced motion illusion (the "shooting" line motion effect) by employing what we call the fixation/refixation paradigm. Focusing on such effects, we could investigate the coordinate transformation process which is relevant to the overall spatial context, and thus higher-order and biologically significant. The typical experiment will consist of an adapting (cueing) phase, and a testing phase to measure negative aftereffect (or strength/direction of the illusory motion). Unlike the conventional procedure, however, there will be a refixation phase between the adapting and the testing in which the fixation point will move, and the subject will be asked to make either a saccade or smooth pursuit eye movement to it before the test presentation. This manipulation enables us to isolate the adapted location in coordinate systems or maps, including the eye-centered and the non-eyecentered, i.e. the head-, the body-, and the environment-centered. By presenting the test stimulus at various locations, we will obtain a spatial tuning curve of the effect and apply a cross-correlation. analysis to find out the degree to which the visual system maintains the eye-centered representation, and/or transfers it into a non-eye-centered description. If we find a significant non-eye-centered component, as indicated by our pilot studies, we will determine exactly which non-eye-centered system (head-, body-, or environment-centered) is relevant by rotating the observer's head and/or torso in a vestibular chair. We will further compare active and passive head/body turn conditions to identify the nature of extraretinal signals on head position which contribute to this transformation process. The results will bridge the gap between animal physiology and human psychophysics, and provide insights into-the processes underlying coordinate transformations.