Scotopic sensitivity in schizophrenia.
Among participants with schizophrenia there is evidence for early-stage visual processing deficits, which may arise in the rod pathways. Input to the earliest level of this pathway, however, has not been tested in this population. It has been widely hypothesized that schizophrenia participants have magnocellular deficits that occur at the pre-cortical level. To address this hypothesis, we studied absolute scotopic (dark-adapted) sensitivity in fifteen schizophrenia and fifteen matched control participants. Scotopic thresholds were assessed using a 1.85-deg, 510-nm circular test stimulus located at 10 degrees eccentricity in the left visual field and presented in Maxwellian-view. Thresholds were obtained using a two-alternative forced-choice paradigm (an average of 200 trials per participant was obtained). Threshold estimates were derived using probit analysis. In this procedure the transformed binomial data (the inverse of the normal probability integral) is fit with a weighted linear regression. Noise was defined as the average deviation from this line. Lens optical density was also assessed by comparing absolute scotopic thresholds to the extinction spectrum of rhodopsin. Scotopic thresholds and lens density values of the two groups were evaluated using independent samples t-tests. The scotopic thresholds, and associated noise, did not differ between the schizophrenia and control participants. Lens density was also nearly identical between groups. These results suggest that magnocellular deficits in schizophrenia may not be due to problems at the level of the rods but are more likely to occur later in the visual pathway.