Asymmetry of the human visual field in magnetic response to apparent motion.

Predominance of the lower visual field has been shown in various visual tasks, but whether the upper visual field is involved in a specific neural process is unknown. We used magnetoencephalography to study the effect of orientation and direction on the responses of five subjects to apparent motion from the human extrastriate cortex. The first magnetic response always was the largest, and the peak latency of about 200 ms did not change with the stimulus conditions. Amplitudes of the first responses were highest when motions were oriented at the horizontal meridian, decreasing with the degree of the angle between motion orientation and the horizontal meridian. There was no difference in amplitude between the two directions in the lower visual field, whereas the value of the response to downward motion in the upper visual field was significantly larger than that to upward motion. These amplitude changes are not due to differences in the anatomical distribution of neural activities because the estimated origins for the first responses always were in the same cortical area (around the occipito-parieto-temporal region) and the directions of the current vectors did not change with the stimulus conditions, and the estimated current strength changed with the stimulus conditions as did the response amplitude. These findings suggest that the human extrastriate cortex has a directional preference for downward versus upward motion in the upper visual field.