Preserved visual abilities following large bilateral lesions of the occipitotemporal cortex

Abstract

MC is a 38-year-old woman with large bilateral lesions of occipitotemporal cortex, including almost the entire ventral stream and sparing only a tag of tissue in the rostral calcarine cortex. Formal and informal testing revealed that MC has some preserved motion perception (Riddoch phenomenon) and is able to track moving targets. Although MC reports seeing colour, formal testing revealed that she cannot correctly identify (or discriminate between) different hues or visual textures. MC can identify simple geometric shapes, even large letters, although there is a strong suggestion that she does this by using ‘tracing’ strategies, either explicitly with her finger or implicitly with eye and head movements. With more complex shapes, her performance plummets; she cannot recognize line drawings (or real exemplars) of familiar objects. Despite the fact that MC cannot distinguish between rectangular objects with the same surface area but different dimensions, when she reaches out to pick up such objects, she shows in-flight grip scaling. Moreover, her grip scaling for these objects (but not her manual estimation of their size) was significantly improved when the objects were strobe-illuminated at a temporal frequency of 4Hz. When picking up objects of varying shape, MC made subtle in-flight adjustments of her finger and thumb to ensure that she acquired the object using stable grasp points, even though she could not distinguish between pairs of these same objects. MC also showed evidence that she could detect obstacles when walking over them or when reaching out to objects. She is also able to intercept a tennis ball rolled towards her. All of this suggests that the visual perception of motion and the visual control of a broad range of actions can occur without input from the ventral stream or early visual areas, including (perhaps) primary visual cortex (Goodale et al., VSS 2008).

Footnotes

 Supported by grants from Canadian Institute of Health Research to MG and JC. We are very grateful to the patient and her family for their generous participation.