Population receptive field estimates in V1 lesion projection zone of patients with macular degeneration onset at different ages

Abstract

Purpose: Patients with early onset retinal dysfunction have abnormal V1 fMRI responses when passively viewing a stimulus. In contrast, patients with adult onset dysfunction do not have significant V1 responses unless they are engaged in a demanding task (Masuda et al., 2008). The development of abnormal cortical responses appears to depend on the timing of the onset of retinal dysfunction. To measure the critical period for the development of abnormal V1 responses, we used fMRI in subjects with congenital, critical period onset, and adult onset macular degeneration (MD).

Methods: We recruited MD patients with similar central retinal lesions. The retinal damage deprives a zone in the posterior of V1 of its normal input projections. We refer to this as the lesion projection zone, LPZ. In the MD and healthy controls we used moving-bar stimuli and a model-based analytical method (Dumoulin and Wandell, 2008) to measure visual field maps and population receptive field (pRF) sizes. We made the measurements twice: in one condition subjects viewed the stimuli passively and in a second condition subjects performed a stimulus judgment task. The healthy subjects were shown stimuli that matched the scotoma of the MD.

Results: There is a very large difference between the responses of the congenital and early onset (age~6) MD. There are large fMRI responses in the LPZ of these subjects during both passive and active conditions. In the adult onset MD, the LPZ responses are elicited only when the subject performed a task. There were no responses in the simulated LPZ of healthy controls under any conditions. The estimated pRF size in the LPZ of the MD far exceeds the pRF size measured in this portion of posterior calcarine cortex in control subjects.

Conclusions: In the congenital and critical period onset MD, the V1 LPZ responds to visual stimuli in both passive and task conditions. This pattern of results differs from the adult onset MD, in which the responses are only present in the task condition. The V1 LPZ responses in the congenital and critical period onset MD may be due to reorganized feed-forward circuitry. The task-dependent responses in the adult onset MD support an earlier hypothesis that the responses are elicited by the extrastriate feed-back signals. The data reveal a large effect of the critical period on cortical reorganization in MD patients.

Keywords: 755 visual cortex • 756 visual development • 696 retinal degenerations: hereditary