Obstacle avoidance during locomotion is unaffected in a patient with visual form agnosia

A patient (D.F.) who developed visual form agnosia following carbon monoxide-induced anoxia was assessed on three tests designed to measure her sensitivity to obstacle height in a locomotor task. Although her verbal estimates of the height of the obstacles were correlated with their actual height, the slope of the line relating estimated and actual obstacle height was much shallower than for control subjects. Similarly, when asked to estimate the height of the obstacle by raising one leg while standing nearby, the slope of line relating toe elevation and obstacle height was shallower than in control subjects. In contrast, D.F. was able to negotiate the same obstacles during locomotion as well as control subjects: toe elevation increased linearly as a function of obstacle height with similar slopes and correlation for the line relating toe elevation and obstacle height. These results provide additional support for the proposal by Goodale and Milner that the cortical pathways mediating the required transformations for the visual control of skilled actions are separate from those mediating experiential perception of the visual world.