Hemispheric representation of the central retina of commissurotomized subjects

It is controversial whether a stimulus projected within 1 to 3 degrees from the boundary between the right and left hemiretina is transmitted to only one cerebral hemisphere or to both cerebral hemispheres. In order to resolve this issue, letter- and word-stimuli were presented for 200 msec with a new type of tachistoscope, called the fundus tachistoscope, in and about the central retina, (i.e. fovea, 1.2 degrees in horizontal diameter) of the right eyes of two commissurotomized subjects (N.G. and A.A.). During stimulus presentation the subjects were attempting to fixate a fixation target. The fundus tachistoscope combined with image analysis of the fundus enables us to measure the position of the stimulus on the retina, relative to the foveal center, as well as whether or not the eye moved during stimulus presentation. The results indicate that the region of the right (temporal) hemiretina represented by both hemispheres in letter processing, if it exists, was estimated as less than 0.6 degrees from the foveal center. The two subjects frequently (27% in N.G. and 46% in A.A.) fixated the fixation target eccentrically, i.e. with a retinal point other than the foveal center, during fixation, namely stimulus presentation. Their eccentric fixations were small with magnitude almost all falling between 1.35 degrees right and 1.25 degrees left of the foveal center. It is therefore recommended that letter-stimuli be presented at least 2.0 degrees from the foveal center in ordinary tachistoscopic studies of cerebral hemispheric differences. Eye movements, which varied in 0.11 degrees and 1.43 degrees horizontally, occurred in about 8% of all the trials during fixation. On the average of the two subjects, the eye movements caused or worsened eccentric fixation in only about one third of the trials, and corrected eccentric fixation in about two thirds of the trials.