Cortical visual processing in patients with congenital achromatopsia: coherent form, motion and biological motion perception

摘要

Congenital achromatopsia is characterised by an absence of functioning cone photoreceptors, resulting in poor visual acuity and no colour vision. While previous psychophysical studies have investigated basic functions including acuity and contrast sensitivity, little is known about the effects of achromatopsia on mid- and higher-level vision. We measured coherence thresholds for form, motion and biological motion in four patients and six controls under varying light levels, including scotopic conditions in which controls and patients were comparable in being reliant only on rods. The patient group showed higher coherence thresholds at all light levels for the form and motion tests, while their biological motion thresholds were comparable to controls. Additional tests confirmed that poor contrast sensitivity could not account for these results in three out of four of the patients. The second phase of the study will use vERPs to measure and localise cortical responses to these stimuli. Previous research with controls has shown distinct patterns of activation in response to coherent form and motion, namely a midline response to motion and a lateralised response to form [Wattam-Bell et al, 2010, Current Biology, 20, 411-415]. Data we have collected with controls has also demonstrated a specific right hemisphere response to biological motion. These results will be compared to those from patients in order to determine the extent to which elevated form and motion thresholds reflect atypical patterns of cortical processing. Our results show that visual impairment in congenital achromatopsia extends beyond previously examined low-level visual functions, to include coherent form and motion perception. This suggests that atypical photoreceptor function can affect the development of cortical visual processing. Potential advances in treatments for genetic visual disorders including retinal gene replacement therapy raise questions regarding neural plasticity, including the extent to which cortical visual processing can be reorganised following restoration of photoreceptor function.