Perceiving Time-To-Contact under Locally Impoverished Optical Flow

As an object approaches an observer, the visual angle subtended by any two texture elements on the object surface expands, as does the solid angle corresponding to the object's contour. The inverse of the relative rate of each of these types of expansion specifies the time-to-contact (TTC) between the object and the observer. The former is referred to as local taul (LT1), and the latter as local tau2 (LT2). For LT1 and LT2 to be available, the shape of the object must remain constant during its approach. The present study examined human observers' perceptual capacity to estimate TTC of an approaching object under severely perturbed or impoverished optic flow. In Experiment 1, non-spherical, textureless objects approached the observer while rotating about either one or two axes. In Experiment 2, textured objects were depicted by random dots that varied in density. When the object rotated, the surface dots were displaced or even disappeared. These manipulations severely compromised the optical patterns specifying LT1 and LT2. However, human observers were able to estimate TTC, although less reliably, under these severely compromised conditions.