Visual pursuit errors: implications for network models

Visual tracking performance is described, and from the nature of experimentally recorded errors an argument is advanced for the location of this uncertainty in the nervous system. It is shown that independent identically distributed motor command noise, n/sub 2/, which enters either A. Pellionisz´s (1988) or W. Daunicht´s (1988) eye kinematics expressions, cannot account for the observed error covariance Sigma /sub e/. Therefore, significant variability is introduced earlier, at the time of visual transduction itself, early visual processing stages, or the sensorimotor transformation. Velocity is not sensed directly, but rather requires observation of a change over time. Thus, early estimates of velocity may be poor because the denominator is so small. The authors show velocity measurements 60 ms into the eye response, an average of 180+or-18 ms after target motion initiation. The orientation of the error ellipse with major axis in the direction of motion suggests that, although speed is poorly estimated at first, direction is better resolved.<>