Sensitivity to biological motion drops by ~½ log-unit with inversion, and is unaffected by amblyopia

The low-level deficits associated with amblyopia have been studied extensively, but very little is known about potential impairments to higher-level visual processing such as object recognition or structure-from-motion. Studies on biological motion, a complex form of structure-from-motion depicting human actions, have demonstrated that normal observers can analyze these patterns more effectively when they are shown in their original upright configuration as opposed to inverted upside-down (feet-up head-down). We measured this inversion effect quantitatively for both the dominant and amblyopic eyes of amblyopic observers. We found a modest ( approximately 30%) loss in sensitivity in the amblyopic eye for both upright and inverted actors, which we attribute to low-level deficits. However, we found no difference in the inversion effect between the two eyes, both showing an average 1/2 log-unit drop in sensitivity between upright and inverted displays. Our data provide a quantitative estimate of the inversion effect for biological motion, and demonstrate that higher-level processing in the motion hierarchy is not affected by amblyopia.