Bernal S D_2010.pdf - University of Plymouth

2.2. HIGH-LEVEL FEEDBACK
detailed information from low-level areas. The initial botlom-up feedforward processing is
considered to be only implicit and nol dircclly available to conscious perception. This type of
representation, denoted as vision at a glance, is obtained by generalizing low-level information
over space, size and viewpoint, leading to high-level neurons which indicate the presence of
basic categories or objects but not their precise parameters. Later, during vision with scruiiny,
feedback connections gradually incoqioratc the lower-level details into the explicit perceptual
representation. This includes features such as the precise location, retinal size, color, orcompo-
nenl motion, which are only available in the lower cortical areas with smaller receptive fields,
and were lost in Ihe neurons with larger RFs. This concept is consonant with the high-resolution
buffer and integrated model of visual processing previously described.
Vision at a glance has been associated with a lypc of search called feature search, characleri7,ed
by the amazing ability in humans to rapidly capture object categories. Although low-level
areas were thought to be responsible for feature search, several argumenis suggest ihis ability,
which has been related to fast pop-out mechanisms (approx. 100 ms from stimulus onset),
actually reflects high-level cortical activity. Feature search works for a vast range of spatial
scales, sizes and inter-element distances, including values which are greater than the small low-
level receptive field siz£s. These parameters are consistent with Ihe high-level large receptive
(ields which reflect spread attention and lead to position and si/e invanani feature detection.
Furthermore, the fast pop-out effect observed is usually related to high-level features, such as
depth from shading. 3D shapes or facial expressions. An example is shown in Figure 2.9a
where the non-face object immediately pops-out from the rest of the similar line drawings.
Another example is depicted in Figure 2.9b where an incomplete square rapidly pops out while
an identical shape is interpreted as an nccludcd sc|uarc due to aniodal completion, a feature of
implicit high-level processing.
On Ihe other hand, vision with scrutiny is associated with serial or conjunction search. This is
illustrated by initial blindness to Ihe details in a scene, which disappears after longer and re­
pealed exposure. By focusing high-level mediated altenlion to different areas or objects, details
from the low-level cortical representation are serially introduced. The extra-classical recep-
36