Bernal S D_2010.pdf - University of Plymouth

2.3. JLIVSORY AND OCCLUDED CONTOURS
A number of key findings strongly substantiate the hypothesis that feedback from high-level ex-
trastriate areas is responsible for subjective contours emerging in lower levels (see Figure 2.11).
To begin with, Huxlin el al. (2000) found thai monkeys with lesions of IT lost their ability io
detect illusory contours. The lemporal sequence of events is also consisleni with this hypoth­
esis, as LOC/IT regions are the lirsi lo signal the appearance of illusory contours, reporting
extremely fast response times, such as 90-100 ms (Murray etal. 2002) or 140 ms(Halgrenel al.
2003). These studies also show how the visual responses later spread to lower regions includ­
ing V3, V2 and VI. This is consistent with the multiple processing stages reported by various
groups (Yoshino et al. 2006), which distinguish between initial region-based segmentation and
laier boundary completion processes. In consonance with this finding, an IMRI study (Sian-
ley and Rubin 2(K)3) showed ihal a Kani/sa ligure with well-delined sharp contours and one
wiih blurred contours were represented equivalenlly in the LOC region. Psychophysical testing
demonstrated subjects were indeed able lo perceive the sharp and well localized edges in the
first case but not in the second, suggesting some oUier region must be responsible for neurally
coding this information, most likely V I and V2.
The above body of evidence is consistent with ihe high-resolution buffer hypothesis (Lee 2003).
the active blackboard concept (Bullier 2001) and the Reverse Hierarchy Theory (Hochstein
and Ahissar 2002) described in seclion 2.2.2. These approaches hypothesize that VI might be
involved in more complex computations usually atlribuied lo higher-level regions, by interacting
with these regions through feedback connections,
2.3.1.2 Occluded contours
It has been suggested amtxial completion processes are carried oul by the same cortical circuits
as illusory contour completion. In this section we will review experimental evidence that indeed
highlights the striking similarilies between them. These similarities suggest the same high-level
feedback mechanisms are being used. In Section 2.3.2 we will further subslanliate this argument
from a more theoretical poini of view.
Neural correlates of occluded coniours in early visual areas such as VI and V2 have been found
using iMRI (Weigelt ct al. 2007, Rauschcnberger et al. 2006). HBG (Johnson and Olshausen
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