Illusory contours activate specific regions in human visual cortex: evidence from functional magnetic resonance imaging.

Abstract

The neural basis for perceptual grouping operations in the human visual system, including the processes which generate illusory contours, is fundamental to understanding human vision. We have employed functional magnetic resonance imaging to investigate these processes noninvasively. Images were acquired on a GE Signa 1.5T scanner equipped for echo planar imaging with an in-plane resolution of 1.5 x 1.5 mm and slice thicknesses of 3.0 or 5.0 mm. Visual stimuli included nonaligned inducers (pacmen) that created no perceptual contours, similar inducers at the corners of a Kanizsa square that created illusory contours, and a real square formed by continuous contours. Multiple contiguous axial slices were acquired during baseline, visual stimulation, and poststimulation periods. Activated regions were identified by a multistage statistical analysis of the activation. for each volume element sampled and were compared across conditions. Specific brain regions were activated in extrastriate cortex when the illusory contours were perceived but not during conditions when the illusory contours were absent. These unique regions were found primarily in the right hemisphere for all four subjects and demonstrate that specific brain regions are activated during the kind of perceptual grouping operations involved in illusory contour perception.Our goal is to identify areas of human cortex involved in the operation of perceptual grouping of local features into a global percept. We used functional magnetic resonance imaging (fMRI), a noninvasive neuroimaging technique which relies on local variations in blood supply and 02 concentration during neural activity (1-3), to investigate these processes. Neural activation within the cerebral cortex is believed to be associated with an increase in blood flow that is out of proportion to the 02 consumption, thus decreasing the capillary 02 extraction fraction and delivering more oxyhemoglobin to the local venous circulation (4-7). The resulting decrease in the local capillary and venous deoxyhemoglobin concentration results in an increase in the T2* weighted magnetic resonance signal due to the decreased paramagnetic effects of deoxyhemoglobin (8, 9). These local changes in blood chemistry can be observed without the use of exogenous contrast enhancing agents on clinical magnetic resonance scanners (3,(10)(11)(12), which has enabled fMRI localization of visual (10-16), motor (3,(17)(18)(19), auditory (20), speech (21), taste (22), and olfactory (23) processing in the human brain. Since fMRI is now established as a method for exploring the functional organization of the human brain (24), we applied this technique to investigate the global processes of perceptual grouping in vision.Borders between visual objects and their background are usually defined by changes in luminance or color. However, perceptual borders can be created by inducing elements distant from the perceived border, as in Kanizsa's triangle (25). The phenomena of perceived visual borders not associated with net lumi...

Cognitive and emotional processes during dreaming: A neuroimaging view

“…Similarly, brain activity related to the perception of illusory contours using Kanizsa triangle elicited significant activation in early visual areas (Ffytche & Zeki, 1996;Hirsch et al, 1995) and imagining a visual detail with high resolution causes early visual cortex activation (Kosslyn & Thompson, 2003). During dreams subjects do not often report perceiving objects with a very high degree of detail, which is congruent with the relative quiescence of early visual cortices during REM sleep (e.g.…”

Illusory contours: a window onto the neurophysiology of constructing perception

“…However, the poor temporal resolution of these methods cannot differentiate feed-forward vs. feedback mechanisms contributing to the same effect. In a first blocked-design fMRI study of 4 healthy subjects, regions in extra-striate visual cortex -most likely area V2 -responded preferentially to IC presence [62]. These results were considered as "consistent with a data-driven, bottom-up approach to illusory-contour perception" (p. 6472 of [62]; see also [63] for nearly 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 contemporaneous PET results).…”

“…In a first blocked-design fMRI study of 4 healthy subjects, regions in extra-striate visual cortex -most likely area V2 -responded preferentially to IC presence [62]. These results were considered as "consistent with a data-driven, bottom-up approach to illusory-contour perception" (p. 6472 of [62]; see also [63] for nearly 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 contemporaneous PET results). Another blocked-design fMRI study [64] and V1 when viewing ICs but not real contours (see also [17] for qualitatively similar findings in monkeys), which was taken as consistent with the role of feedback signals, potentially originating from the fusiform gyrus [65].…”

Electrophysiological indexes of illusory contours perception in humans

“…absence of illusory Kanizsa square. The first goal was to investigate whether there was right hemispheric lateralisation in brain activation, as suggested by several studies [14]. Secondly the study aimed at understanding if illusory contour formation occurred at early stages of visual processing pathways or if it involved associative higher level cortical regions.…”

“…Besides the right-sided activation of the fusiform gyrus recorded in the Larsson et al [19] PET study, there seems to be further evidence of hemispheric asymmetry. For instance Hirsch et al [14] in the fMRI study showed the predominance of brain activity related to illusory contours perception in the right extra-striate area. According to Hirsch et al brain activation could involve striate cortex (area 17), too.…”