Reflections on blindsight: neuroimaging and behavioural explorations clarify a case of reversed localisation in the blind field of a patient with hemianopia.
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.
Danckert
James
J
Department of Psychology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada. jdancker@watarts.uwaterloo.ca
Culham
Jody C
JC
D019575:Blindness, Cortical / Q000150:complications / Q000503:physiopathology*; D001931:Brain Mapping; D005260:Female; D006423:Hemianopsia / Q000150:complications / Q000503:physiopathology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008875:Middle Aged; D010775:Photic Stimulation; D011930:Reaction Time; D014793:Visual Cortex / Q000503:physiopathology / Q000601:surgery*; D014794:Visual Fields / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology