Effects of illumination intensity and direction on object coding in macaque inferior temporal cortex.

Single unit activity in area TE was recorded from two macaques as they viewed 3D appearing rendered objects that were illuminated from different directions (without cast shadows) and intensities of illumination. The average modulation produced by changes in illumination intensity or direction was rather moderate, with the majority of the neurons responding invariantly to these lighting variables. When neural activity was affected by illumination direction, it was not manifested as a preference for a particular direction of illumination by a given neuron. Instead, the tuning appeared to be to the relative brightness of a given shaped surface at a given orientation. The modulation to changes in illumination direction was considerably smaller than that produced by changes in object shape. Most of the neurons that were unaffected by changes in illumination direction responded much less to silhouettes of these objects, indicating that these neurons were also sensitive to an object's inner features. The neuronal invariance for shading variations may provide the basis for the invariance of object recognition under changes in illumination.