This study evaluated subjects ability to track a constant altitude as a function of the structure in the optical flow field. Optic flow was manipulated by using four different types of ground texture (splay angle, depression angle, random dot, and block textures) crossed with two global optical flow (GOF) rates (0 and 3 eyeheights/s). The subjects were asked to maintain a constant altitude while wind disturbances randomly perturbed them on vertical, lateral, and fore-aft axes. The critical independent variables were texture type and GOF rate. Texture type was a within-subjects variable while GOF rate was a between-subjects variable. The main dependent variables included RMS height error and the correlation between subjects stick activity and the three wind disturbances. For both dependent variables, an interaction was found in that the depression angle texture provided superior performance in a hover or 0 GOF rate condition. The splay angle texture provided a constant level of performance for both GOF rates, being superior to depression angle in the higher GOF rate. These results are consistent with Flach et al.'s (1992) hypothesis that the ability to pick-up altitude information from the optic flow field depends upon the amount of optical activity that is specific to changes in altitude (signal) rather than specific to changes in lateral or fore-aft position (noise). This hypothesis provides a higher order explanation for previous results on the control of altitude which had been thought to be inconsistent.
Available at: http://works.bepress.com/john_flach/50/