Slow control with eccentric targets: evidence against a position-corrective model.

Does slow control work by keeping the image at a particular retinal location (position-correction), or by keeping the image relatively stable on whatever position it occupies on the retina (velocity-correction)? The best prior evidence that slow control is not position-corrective was that a stable line of sight can be maintained anywhere on or within a small figure. This result, however, does not preclude position-correction with respect to an imagined reference position selected relative to the figure's contour. To test the importance of an imagined reference position, we compared slow control with targets for which selection of a stable reference position is easy (two points arranged symmetrically about the line of sight) and difficult (one or two points located on one side of the line of sight). We found that the stability, velocity and direction of slow control were the same with both kinds of targets. Slow drifts were in idiosyncratic directions, and not toward the eccentric target. Drift speed increased with eccentricity, but drifts did not exceed 10'/sec even at the largest eccentricity tested (4.1 degrees). The independence of slow control from the spatial configuration of the target shows that slow control does not work by bringing either a visible or an imagined reference position to the optimal fixation locus on the retina. The gradual loss of stability with increasing eccentricity is consistent with prior reports of a reduction in the number of motion detectors tuned to low velocities as eccentricity increases. We conclude that motion signals, not position signals, provide the sole sensory input to slow control.