Results suggest anatomical specialization of biological motion processing

The authors investigated the impact of object recognition on the perception of biological motion in a patient with severe visual agnosia. The patient showed normal perception of nonbiological motion and biological motion. However, when tested with objects constructed of coherently moving dots made up of simple geometrical figures, recognition was severely impaired.

The patient's limited perception of point-light biological motion might thus be the result of a dysfunction of cortical areas involved in the processing of biological motion. The authors conclude that their findings support models of an anatomical specialization for the processing of biological motion and emphasize the role of form analysis as a requirement for the perception of biological motion.

The patient had a normal cranial MRI but an18-Fluorodeoxyglucose positron emission tomography that revealed diffusely reduced metabolism in the temporo-occipital cortex bilaterally in comparison with healthy subjects. His results in the current study were compared with four healthy controls.

The study consisted of three experiments. The first one tested motion identification/discrimination. Subjects were presented with patterns of moving dots or static images of these dots. The patient demonstrated preserved perception of low-level motion.

The second experiment involved the ability to identify biological motion. The authors found that the patient's ability to identify full human body movements was preserved. However, he demonstrated difficulty recognizing point-light biological motion derived from recordings with a body tracker.

The third experiment tested "shape from motion," the ability to recognize shapes defined by coherently moving dots. The patient performed poorly at this activity.

The visual system can be thought of as having ventral and dorsal processing streams arising from the lateral geniculate ganglion projecting to the primary and subsequently the extrastriate visual cortex. This dual stream hypothesis consists of a ventral pathway that plays a critical role in object recognition and a dorsal visual information processing pathway that is involved in action-related visual processing and in motion processing.

Patients with brain damage of the ventral pathway typically present with signs of visual agnosia but show normal motion perception. Perception of biological motion might be impaired when nonbiological motion perception (judging the direction of the coherently moving dots) is intact or vice versa. However, the impact of object recognition on the perception of biological motion remains unclear.