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Blindsight refers to residual visual abilities of patients with primary visual cortex lesions. Most of this research uses single case studies, most famously patient GY. We examined a patient (DC) after surgical resection of V1 who demonstrated robust but reversed blind field target localisation, mislocalising midline blind field targets to the periphery and vice versa. This pattern was reliable across multiple sessions and was not because of extraocular light scatter. We then used functional magnetic resonance imaging to examine neural responses to blind field motion stimuli with no evidence of motion-selective activation in DC's extrastriate cortex in the damaged hemisphere, in stark contrast to GY who showed robust bilateral activation in response to blind field stimuli. This suggests that DC's blind field performance may not represent true blindsight. Follow-up testing with the target-background contrast reversed (i.e., black targets/white background), eliminated DC's reversed localisation, strongly suggesting that she was employing an unusual decision criterion based on intraocular light scatter. DC's failure to demonstrate true blindsight may be related to the age at which she acquired her lesion-much later in life than GY.
Keywords: blindsight, light scatter, functional MRI
Patients with visual field defects arising from lesions of primary visual cortex (area Vl) are sometimes able to respond at better than chance levels to stimuli appearing in their blind field (Pöppel, Held, & Frost, 1973; Weiskrantz, Warrington, Sanders, & Marshall, 1974). Weiskrantz and colleagues (1974) coined the term "blindsight" to refer to these residual visual abilities. Their initial work, and the work of others, demonstrated that some patients could be surprisingly accurate when asked to reach to, make a saccade to or guess the location of targets briefly flashed in their nominally "blind" visual field (Weiskrantz et al, 1974; Zihl & Werth, 1984a, b). Since these early findings, a myriad of residual abilities have been demonstrated in blindsight patients including motion discrimination, colour, and form interference effects from blind field distracters, wavelength discrimination, and even semantic priming (Danckert, Maruff, Kinsella, de Graaff, & Currie, 1998; Magnussen & Mathiesen, 1989; Marcel, 1998; Morland, Jones, Finlay, Deyzac, Le, & Kemp, 1999; Stoerig & Cowey, 1989).
The many residual abilities demonstrated by some blindsight patients suggests that secondary visual pathways continue to function in the absence of input from...