Eye movements continuously produce shifts in the visual image on the retina. Despite these dynamic inputs, we perceive stability or constancy of the visual world. Constancy is presumably based on an internal representation of space derived from both retinal and non-retinal signals. This proposal aims to understand how the brain transforms across and separately utilizes different coordinate systems (representations), what extra-retinal signals are used for the modulation or coordinate-transformation processes, and what regions of the brain are responsible for such processes. We will apply a gaze manipulation paradigm to sensory-motor aftereffects such as saccade and arm-reach adaptation, attention-modulated perceptual effects and reaction times, as well as spatial localization and memory. We extend our paradigm to head and body turns utilizing a vestibular chair, and also employ Transcranial Magnetic Stimulation (TMS) to obtain insights into the underlying neural mechanism. A typical experiment with the gaze manipulation paradigm will consist of an adapting (cueing or target presentation) phase, followed by a testing phase to measure negative aftereffect, attentional modulation, or mislocalization. Unlike the conventional procedure, however, there will be a refixation phase between the adapting/cueing and the testing in which the observer has to move gaze to a new location. This enables us to isolate the adapted/cued location in eye-centered and the non-eye-centered, i.e. the head-, the body-, and the environment-centered coordinate frames of reference. By presenting the test stimulus at various locations, we will obtain a spatial tuning curve 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 either gaze-dependent modulation or a 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 and what extra-retinal signals are critical by rotating the observer's head and/or torso in a vestibular chair. We will further apply TMS to the parietal region of the observer who is performing an attentional cueing task with a gaze shift, to see if TMS can effectively block the transformation process. The results will bridge the gap between animal physiology and human psychophysics, providing insights into coordinate transformation. ? ?
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