Comparison of Human Occipital Cortical Activation to Lower and Upper Visual Field Stimuli

We compared cortical responses to lower and upper quadrant and full hemifield stimuli (90° and 180° sectors of circular checkerboards) measured from 15 healthy subjects with a Neuromag-122™ whole-scalp neuromagnetometer. The 0.2 s stimuli were presented once every second, while the subjects fixated a black dot in the centre of the screen.

The first evoked responses, peaking at 70 ms in the contralateral hemisphere, were stronger for lower than for upper field stimulation (13/15 subjects, LVF; 11/15 RVF). The sources of the evoked responses, modelled as equivalent current dipoles, clustered around the calcarine fissure, with a trend for stronger sources after lower than after upper field stimulation (on average 12% LVF; 40% RVF; ns).

Attention-related visual processing may be enhanced in the lower compared with the upper visual field (Rubin et al, 1996 Science 271 651 – 653). Although our data showed a strong tendency to larger responses for lower than for upper visual field stimuli, this difference was not significant for source strengths, mainly because of different source depths for upper and lower field stimuli. However, the marked similarity of source current directions for full hemifield and lower quadrant stimuli (15° - 35° upwards from the horizontal axis, viewed from back, compared with directions 15° - 25° downwards for upper field stimuli) suggest that visual input from the lower field is preferred already at early stages of the human visual system.