To fixate or not to fixate. The problem of undetected strabismic subjects in visual evoked potential research.

Abstract

A common strategy in neuropsychological research involves the presentation of stimuli in left and in right visual fields in order to study possible processing differences by the right and left hemispheres of the brain. Examples of strictly behavioral studies are those of Dimond and Beaumont (1973). in which vigilance performance of the two hemispheres was compared, and of Pennal (1977), in which hemispheric asymmetries in color processing were examined. Visual event-related potential (ERP) researchers have used a similar strategy to investigate effects of visual field location on ERP recorded from over both hemispheres (e.g., Andreassi et al., 1975, and Ledlow et al., 1978). In the study by Andreassi et al., the results showed that stimuli presented in the left visual field (LVF) resulted in shorter visual ERP latencies in the right occipital area than in the left, while in the case of stimuli presented in the right visual field (RVF), the opposite occurred. This field effect on visual ERP latencies has also been reported by Ledlow et al. (1978) and by Andreassi et al. (1980). Whether a study is strictly a behavioral one or involves the measurement of ERPs, it is critical that the subject fixate a t the center of the visual field if the two hemispheres are to receive differential stimulation. We became concerned about the ability of subjects to fixate when, in one study (Andreassi et al., 1980), two of 12 subjects did not show the expected contralateral hemisphere latency advantage for eccentric stimuli. This occurred even though eye movement (vertical and horizontal) was monitored by electro-oculography and was stored on tape for later analysis. In our laboratory, we now screen' all potential subjects for visual acuity, strabism w b and depth acuity before their participation in visual ERP studies. Persons with eye muscle imbalance are not always detectable by casual observation and they may readily pass a binocular visual acuity test since they tend to use one eye in viewing. It was reasoned that individuals with strabismus would have difficulty maintaining their line of sight on some central fixation point. Therefore, even though electrooculography (EOG) is measured, these individuals may take up some different fixation point. Also, deviations from center would not be detected and the usual contralateral effect might not be observed. We believe that the absence of the contralateral hemisphere effect indicates that one or the other hemisphere is not being differentially stimulated. The depth acuity test was initiated as an additional check, since individuals with