Changes in pattern-evoked responses in man associated with the vertical and horizontal meridians of the visual field.

1. Averaged responses have been recorded from an array of ten scalp electrodes over the occipital cortex in man to the reversal of a black-and-white checkerboard pattern, presented in different octants of the visual field.2. In all subjects a prominent wave was seen, with a peak latency of about 100 msec, which showed consistent and systematic changes with variation in the position of the stimulus in the visual field.3. With stimulation of the octants next to the vertical meridian, this component was of large amplitude, while with stimulation of the octants next to the horizontal meridian, it was small and inconspicuous.4. With upper field octants, the peak at 100 msec was surface-negative, while with lower field octants it was reversed in polarity.5. The occipital response was largest 5 or 7.5 cm above the inion, and the amplitude recorded 3 cm lateral to the mid line was larger over the hemisphere contralateral to the half field being stimulated than ipsilaterally.6. These findings are discussed in relation to the underlying anatomy of the visual cortex, and it is concluded that these responses are likely to arise mainly from extra-striate areas.