Pursuit affects precision of perceived heading for small viewing apertures

We investigated the interaction between extra-retinal rotation signals and retinal motion signals in heading perception during pursuit eye movement. F or limited viewing aperture, the variability in perceived heading strongly depends on the pattern of motion directions. Heading towards a point outsid e the aperture generates nearly parallel aperture flow. This results in low er precision of perceived heading than heading that renders the radial patt ern of flow visible. We ask if the precision is limited by the pattern of f low visible on the retina or that on the screen. During fixation, the two p atterns are identical. They are decoupled during pursuit, since pursuit cha nges radial flow within the aperture on the screen into nearly parallel flo w on the retina, and vice versa. The extra-retinal signal is known to reduc e systematic errors in the direction of pursuit, thus compensating for the rotational flow during pursuit. We now ask if the extra-retinal signal also affects the precision of heading percepts. It might if at the spatial inte gration stage the rotational flow has been subtracted out already. A compen sation beyond the integration stage, however, cannot undo the change in ret inal motion directions so that an effect of pursuit on precision cannot be avoided. We measured the variable and systematic errors in perceived headin g during fixation and pursuit for a frontal plane approach, while varying d uration, dot lifetime and aperture size. We found precision is effected by pursuit as much as predicted from the pattern of retinal flow, while compen sation is significantly greater than zero. This means that the interaction between the extra-retinal signal and visual motion signals takes place afte r spatial integration of local motion signals. Furthermore, compensation in creased significantly with longer duration (0.5-3.0 s), but not with larger aperture size (10-50 degrees). A larger aperture size did increase the ecc entricity of perceived heading. (C) 2001 Elsevier Science Ltd. All rights r eserved.