The neural correlates of crowding-induced changes in appearance.
Object recognition in the peripheral visual field is limited by crowding: the disruptive influence of nearby clutter. Despite its severity, little is known about the cortical locus of crowding. Here, we examined the neural correlates of crowding by combining event-related fMRI adaptation with a change-detection paradigm. Crowding can change the appearance of objects, such that items become perceptually matched to surrounding objects; we used this change in appearance as a signature of crowding and measured brain activity that correlated with the crowded percept. Observers adapted to a peripheral patch of noise surrounded by four Gabor flankers. When crowded, the noise appears oriented and perceptually indistinguishable from the flankers. Consequently, substitution of the noise for a Gabor identical to the flankers ("change-same") is rarely detected, whereas substitution for an orthogonal Gabor ("change-different") is rarely missed. We predicted that brain areas representing the crowded percept would show repetition suppression in change-same trials but release from adaptation in change-different trials. This predicted pattern was observed throughout cortical visual areas V1-V4, increasing in strength from early to late visual areas. These results depict crowding as a multistage process, involving even the earliest cortical visual areas, with perceptual consequences that are increasingly influenced by later visual areas.
UCL Institute of Ophthalmology, 11-43 Bath Street, London EC1V 9EL, UK. e.anderson@ucl.ac.uk
Elaine J
EJ
Anderson
Steven C
SC
Dakin
D Samuel
DS
Schwarzkopf
Geraint
G
Rees
John A
JA
Greenwood
D000328:Adult; D006801:Humans; D008279:Magnetic Resonance Imaging; D033281:Nontherapeutic Human Experimentation; D010775:Photic Stimulation / Q000379:methods; D014793:Visual Cortex / Q000502:physiology*; D014794:Visual Fields; D014796:Visual Perception / Q000502:physiology*