Bernal S D_2010.pdf - University of Plymouth

2.3. ILLUSORY AND OCCLUDED CONTOURS
2.3.1 Experimental evidence
2.3.1.1 Illusory contours
The retinotopic aclivaiion of VI and V2 neurons in response to illusory contours, such as
Kanizsa figures, has been reponed in fMRI (Macrlens et al, 2008), EEG (Seghier and Vuilleu-
mier 2006). MliG (Halgrcn el al. 2(M)3) and single-cell recording (Lee 2003. l.ee and Nguyen
2001) studies. The illusory contour response is weaker, significantly delayed, and only arises in
a fraction of VI A'2 cells, in relation lo thai of real contours. Previous controversy as to whether
VI rcpresenis illusory conlours seems lo have been clarified by ihe results reported in the previ­
ously cited articles. Nonetheless, VI tends to show a weaker respon.se than V2 and sometimes
requires task-related altenlion lo emerge (Lee 2003), Ramsden ci al. (2001) also reported orien­
tation reversal between V2 and VI, such that the illusory contour orientation is de-emphasized
in VI, while the orthogonal orientation is enhanced. This was suggested to constitute a cortical
balancing process which could play an important role in illusory contour signaling, and was
later supptwed by psychophysical data (Dillenburger 200S).
The fact that the illusory contour response does not arise from ordinary feedforward pathways,
i.e. retina and LGN. and that it is delayed relative to real contours, suggests the involvement
of lateral and feedback connections. Interestingly, the response in VI emerges later than in V2
(Lee and Nguyen 2001, Halgren ei al. 2003, Ramsden el al. 2001, Maenens el al. 2008, Dillen­
burger 200.'i) suggesting contour completion in VI might arise as u consequence of feedback
connections from V2, The question arises as lo why is it necessary to feed back infonnalion
10 VI if the illusory contour is already represented in V2. The mosi likely reason is thai V!
neurons' smaller receptive field size provide higher spatial resolution to accurately represent
the illusorj' contour. Bigger receptive field sizes in V2 allow the system to integrate global
contextual information which is then fed back to VI circuits. When required by environmenial
demands, these circuits can then construct a more precise representalion, which explains why
illusory contours in V1 sometimes emerge as a consequence of task-related attention (Lee and
Nguyen 2001).
Furthermore, a large number of studies have reported neural correlates of illusory contours in
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