|dc.description.abstract||Understanding how human and animal visual systems work is an important and still
largely unsolved problem. The neural mechanisms for low-level visual processing have
been studied in detail, focusing on early visual areas. Much less is known about the
neural basis of high-level perception, particularly in humans. An important issue is
whether and how lessons learned from low-level studies, such as how neurons in the
primary visual cortex respond to oriented edges, can be applied to understanding highlevel
perception, such as human processing of faces. Visual aftereffects are a useful
tool for investigating how stimuli are represented, because they reveal aspects of the
underlying neural organisation. This thesis focuses on identifying neural mechanisms
involved in high-level visual processing, by studying the relationship between low- and
high-level visual aftereffects.
Previous psychophysical studies have shown that humans exhibit reliable orientation
(tilt) aftereffects, wherein prolonged exposure to an oriented visual pattern systematically
biases perception of other orientations. Humans also show face identity
aftereffects, wherein prolonged exposure to one face systematically biases perception
of other faces. Despite these apparent similarities, previous studies have argued that the
two effects reflect different mechanisms, in part because tilt aftereffects show a characteristic
S-shaped curve, with the effect magnitude increasing and then decreasing with
orientation difference, while face aftereffects appeared to increase monotonically (in
various units of face morphing strengths) with difference from a norm (average) face.
Using computational models of orientation and face processing in the visual cortex, I
show that the same computational mechanisms derived from early cortical processing,
applied to either orientation-selective or face-selective neurons, are sufficient to replicate
both types of effects. However, the models predict that face aftereffects would
also be S-shaped, if tested on a sufficiently wide range of face stimuli.
Based on the modelling work, I designed psychophysical experiments to test this
theory. An identical experimental paradigm was used to test both face gender and tilt aftereffects, with strikingly similar S-shape curves obtained for both conditions.
Combined with the modelling results, this result provides evidence that low- and high level
visual adaptation reflect similar neural mechanisms.
Other psychophysical experiments have recently shown interactions between low and
high-level aftereffects, whereby orientation and line curvature processing (in early
visual area) can influence judgements of facial emotion (by high-level face-selective
neurons). An extended multi-level version of the face processing model replicates
this interaction across levels, but again predicts that the cross-level effects will show
similar S-shaped aftereffect curves. Future psychophysical experiments can test these
Together, these results help us to understand how stimuli are represented and processed
at each level of the visual cortex. They suggest that similar adaptation mechanisms
may underlie both high-level and low-level visual processing, which would
allow us to apply much of what we know from low-level studies to help understand