Differences between the occipital distribution of upper and lower field pattern-evoked responses in man

Abstract

(1) The consistent reversal in the polarity of the occipital response to a reversing checkerboard stimulus in the upper and lower visual field, previously reported by the authors, has been investigated further in 9 healthy subjects, using a variety of reference electrodes, to decide between two possible explanations put forward in the earlier paper, viz., that the upper and lower field generators were situated on the lower and upper surfaces of the occipital lobe respectively, or alternatively that both responses, although of opposite surface polarity, came from the same general area of convergent information on the upper convexity. (2) Changing from a two-ear to a midfrontal reference made little difference to the form of the lower field responses, but upper field responses were significantly altered, the characteristics negativity, still present in more anterior channels, being replaced by a positivity at electrodes below the inion. (3) Comparing upper field responses to the central 2° with those to an annular stimulus from 4 to 8° out from the fixation point, the posterior positivity was particularly characteristic of the foveal stimulus, whereas the nagativity, recorded more anteriorly, was seen with the more peripheral stimulus. The difference between the midfrontal and two-ear reference recordings of the full 8° upper field stimulus was contributed almost entirely by the altered response to the central stimulus. (4) The results suggest that the upper field responses arise from the inverted neurones on the under-surface of the occipital lobe, which are sufficiently near to be picked up by the ear reference, whereas the lower field responses arise from the neurones on the upper convexity.