International audienceThe occipital lobe contains a retinotopic representation of the visual field. The representation of the central retina in early visual areas (V1-3) is found at the occipital pole. When the central retina is lesioned in both eyes by macular degeneration, this region of visual cortex at the occipital pole is accordingly deprived of input. However, even when such lesions occur in adulthood, some visually driven activity in and around the occipital pole can be observed. It has been suggested that this activity is due to remapping of this area, so that it now responds to inputs from intact, peripheral retina. We evaluated whether or not remapping of visual cortex underlies this activity. Our fMRI results provide no evidence of remapping, questioning the contemporary view that early visual areas of the adult human brain have the capacity to reorganize extensively

Two subdivisions of human V5/MT+: one located posteriorly (MT/TO-1) and the other more anteriorly (MST/TO-2) were identified in human participants using functional magnetic resonance imaging on the basis of their representations of the ipsilateral versus contralateral visual field. These subdivisions were then targeted for disruption by the application of repetitive transcranial magnetic stimulation (rTMS). The rTMS was delivered to cortical areas while participants performed direction discrimination tasks involving 3 different types of moving stimuli defined by the translational, radial, or rotational motion of dot patterns. For translational motion, performance was significantly reduced relative to baseline when rTMS was applied to both MT/TO-1 and MST/TO-2. For radial motion, there was a differential effect between MT/TO-1 and MST/TO-2, with only disruption of the latter area affecting performance. The rTMS failed to reveal a complete dissociation between MT/TO-1 and MST/TO-2 in terms of their contribution to the perception of rotational motion. On the basis of these results, MT/TO-1 and MST/TO-2 appear to be functionally distinct subdivisions of hV5/MT+. While both areas appear to be implicated in the processing of translational motion, only the anterior region (MST/TO-2) makes a causal contribution to the perception of radial motion.

We describe a compelling demonstration of large-scale developmental reorganization in the human visual pathways. The developmental reorganization was observed in rod monochromats, a rare group of congenitally colorblind individuals who virtually lack cone photoreceptor function. Normal controls had a cortical region, spanning several square centimeters, that responded to signals initiated in the all-cone foveola but was inactive under rod viewing conditions; in rod monochromats this cortical region responded powerfully to rod-initiated signals. The measurements trace a causal pathway that begins with a genetic anomaly that directly influences sensory cells and ultimately results in a substantial central reorganization.

In albinism there is an abnormal projection of part of the temporal retina to the visual cortex contralateral to the eye. This projection, together with the normally routed fibers from nasal retina, provides a cortical hemisphere with visual input from more than the normal hemifield of visual space. In many mammalian models of albinism, a possible sensory mismatch in the visual cortex is avoided either by reorganization of the thalamocortical connections to give the abnormal input an exclusive cortical representation, or by the abnormal input being substantially suppressed. In this study we examine, with fMRI, how the human visual cortex topographically maps its input in albinism. We find that the input from temporal retina is not substantially suppressed and forms a retinotopic mapping that is superimposed on the mapping of the nasal retina in striate and extrastriate areas. The abnormal routing of temporal fibers is not total, with the line of decussation shifting to between 6 and 14 degrees into temporal retina. Our results indicate that the abnormal input to visual cortex in human albinism does not undergo topographic reorganization between the thalamus and cortex. Furthermore, the abnormal input is not significantly suppressed in either striate or extrastriate areas. The topographic mapping that we report in human does not conform, therefore, to the commonly observed patterns in other mammals but takes the form of the "true albino" pattern that has been reported rarely in cat and in the only other individual primate studied.