Abstract
Purpose: The albino visual cortex receives an unduly large input from the ipsilateral visual field (Guillery, 1986; Morland et al. 2002). To investigate how the visual cortex of humans albinos organizes this abnormal input we applied retinotopic mapping fMRI procedures. Methods: Two subjects with albinism and only small nystagmus and two control subjects underwent T2* MRI scanning of the occipital lobe during visual stimulation. In separate experiments we stimulated monocularly the nasal and temporal retina with a ‘wedge’ or ‘annulus’ of a phase reversing (6Hz) chequerboard pattern that stepped through polar angles or eccentricities respectively (Engel et al., 1997). Seven 36 s cycles of the stimulus were presented. Eight 128 × 128 voxel images (voxel size: 1.8 × 1.8 × 4 mm) perpendicular or parallel to the calcarine sulcus were acquired every 3 s. fMRI signals were projected to the flattened representation of T1 weighted images, Fourier analysed and correlated with the stimulus fundamental frequency. Results: Retinotopic mapping yielded phase maps that allowed the identification of V1 and other early visual areas in both controls and albino subjects. In the controls V1 comprised a representation of the contralateral visual field, while it comprised a representation of both the contralateral and the ipsilateral visual field in the albino subjects. Here the normal contralateral and the abnormal ipsilateral representations are, at the resolution of fMRI, arranged as overlaid maps. We obtained evidence for a similar arrangement in other early visual areas (V2 dorsal/ventral, VP). Conclusions: These results provide evidence for the presence of the “true” albino-pattern in human subjects with albinism. This pattern has previously been described in albino cats and one albino monkey.
GuilleryRW(1986) TINS 9:364–367;
EngelAS(1997) Cerebral Cortex 7:181–192;
Morland(2002) JNNP (in press)