Dopamine and visual function in schizophrenia : a psychophysical investigation using the tilt after-effect and contrast sensitivity tests
The study used a non-invasive technique, the Tilt Aftereffect (TAE) test, to investigate the visual changes influenced by dopamine (DA) in schizophrenia. One-third of forty five schizophrenic patients could not complete the TAE test and this group was found to have impaired sustained attention and to have different demographic and clinical characteristics from the schizophrenic patients who could complete the TAE test. TAE changes were found only in response to DA changes brought about by altering neuroleptic levels, e.g. 'before' compared with 'after' a depot injection. These changes were only found when gratings of 2 c/d were used and not for 10 c/d. No illness or drug effect was found using the TAE test. A similarity in the pattern of change over time was found for the TAE test using 2 c/d gratings and the peak spatial frequency (SF) from the Contrast Sensitivity (CS) test in response to changes in levels of DA. A decrease in DA caused a shift of the CS peak to lower SFs, with a decrease in sensitivity for mid to high SFs (3 - 10 c/d) and an increase in sensitivity for low SFs (0.5, 1 c/d). The findings suggest that both the TAE and CS tests are influenced by DA and that lateral inhibition alone cannot explain the production of the TAE since the influence of the orientation of the adapting gratings on the size of the TAE indicates that an adaptation process is involved. It is suggested that the transient system, which is sensitive to low spatial and high temporal frequencies, may be selectively affected by DA changes.