VISUAL PSYCHOPHYSICSIn the general paradigm of visual psychophysics, a human subject is presented with accurately controlled stimuli, and in certain prescribed ways, is asked what he sees. From the results of these experiments inferences can be made about the nature of visual processes.
Our most common lab setup is based on a computer-controlled apparatus comprising a calibrated color CRT screen, an SRI Eyetracker, and a stimulus deflector system. The stimulus is displayed on the color CRT with spatial, temporal and color variables controlled by a computer. The subject looks at the screen while biting on a bite board attached to the apparatus in order to keep his head still. For some experiments, the bite board plus careful fixation of the subject's sight to a fixation point on the screen are sufficient. However, in many cases, involuntary eye movements introduce unacceptable artifacts. In these cases, the subject looks at the stimulus through the combined eyetracker and optical deflector system. The eyetracker registers the subject's eye movements and transmits them to the deflector system which deflects the optical image of the color display compensating for the eye's movements. The subject has a control box which encodes and transmits a variety of responses to the computer which analyzes them and affects the parameters of the stimuli to follow, according to specific programs.
A good summary of many psychophysical studies conducted here was recently captured by one of the most prominent scientists of the visual sciences community, a cornerstone member of SRI's Visual Sciences Program who worked here for more than 25 years, Don Kelly, in his book: "Visual Science and Engineering", published by Dekker in 1994.
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