Bernal S D_2010.pdf - University of Plymouth

2.1. OBJECT RECOGNITION
of input employed (Frcgnac 2010). Results showed the same neuron could exhibit simple or
complex properties depending on whether the images presented dense or sparse noise.
It can ea.sily be concluded that many crucial elements are still missing from current models of
VI response. Estimates suggest only 35% of the variance in natural images can be accounted
for (Olshausen and Field 2005). A general point of agreement indicates the necessity to move
Ireyond botlom-up filtering models to incorporate top-down feedback modulation as one of the
basic components in any model of visual perception (Lee 2003, Olshausen and Field 2005,
Carandini el al. 2005). This is not an easy task, a.s the response of neurons in higher visual
processing areas is still very ptiorly understood.
The response properties of neurons in V2, which receive projections from area VI. are not
nearly as well documented, and il is therefore uncerUiin what type of stimuli cause V2 neu­
rons to respond optimally. Nonetheless, Hegde and Van Kssen (2(X)7) studied the responses of
a population of V2 neurons to complex contour and grating stimuli. They found .several V2
neurons responding maximally for features with angles, as well as for shapes such as intersec­
tions, tri-stars, fivepoint stars, circles, and arcs of vailing length. Additionally, the receptive
field sizesofV2 cells are approximately twice the size of those of VI. For example, at a retinal
eccentricity of 2°, V1 receptive field size is -^ 2" of visual angle, while V2 receptive held size
is "- 4" (Angelucci el al. 2(X)2). This is consistent with the hierarchical increase in the receptive
lield size and complexity proposed at the beginning of this section. Crucially, the increase of
RF size implies a decrease in spatial resolution, which is a key aspect of the modelling study in
this thesis.
Our current understanding of response selectivity in V4 neurons is also congruent with the hier­
archical increase in size and complexity (Hegde and Van Fssen 2(X)7). However, at this level il is
more difficult to characierize the exact receptive lield of neurons, as these exhibit a wider range
of preferred stimuli, and .stronger invariance to stimulus transformations. Nevertheless, lesions
of V4 in the macaque have caused impairments in pattern discrimination tasks (Van Hssen and
(iallani 1994). Further studies have shown V4 neurons can be tuned to shapes with specific
type of boundary conformation at a given position within the stimulus, e.g. concave curvature
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