The Presence of Viewpoint Aftereffects for Test Stimuli
of Varying Size to Adaptor Stimuli
Abstract
We experience viewpoint aftereffects when orientation-selective neurons in our primary
visual cortex adapt to a given direction and are then shown non-directional stimuli.
Investigating such visual aftereffects has important implications in our understanding of
facial recognition and general visual perception. Much research has been carried out
examining viewpoint aftereffects (He & Fang, 2005; Wilson & Daar, 2012), but little has the
effect of size of stimuli been looked into, which is what this experiment aims to investigate.
The experimenters presented participants with an orientated adaptor stimulus for 30
seconds and then a front-facing test stimulus, which was either 100% or 40% the size of the
adaptor. The participants then reported which way they thought the stimulus was facing. An
aftereffect was found for the 100% condition but not the 40% condition, leading to the
conclusion that a test stimulus of a different size to the adaptor leads to no viewpoint
aftereffect being observed. Commented [A1]: Good summary of findings and interpretation.
Introduction
A viewpoint aftereffect (first observed and reported by Addams, 1834) is a visual illusion
occurring when viewing a front-facing face or object after adaptation to a face or object
orientated in a particular direction (Fang, Ijichi, and He, 2007). We perceive the front-facing
object as facing the opposite direction to what we previously saw, as that is what our
orientation-selective neurons in our primary visual cortex (V1) had adapted to. These
neurons respond to specific directions, whilst similar nearby neurons respond to the
, opposite direction (Burr & Thompson, 2009). When neurons encoding one direction adapt,
their sensitivity decreases. The activity in surrounding neurons encoding other directions
will therefore be relatively high when presented with a neutral stimulus, meaning that the
neutral stimulus is perceived as facing the opposite direction to what was previously
adapted to. Commented [A2]: Your description here is accurate however a
little clumsy to read. Think about improving the overall flow of your
work by giving clear and concise descriptions.
He and Fang (2005) conducted an experiment aiming to determine whether there are
neurons representing specific views of objects in the human visual system. They used
several objects viewed from 15 or 30 degrees to one side to produce an aftereffect for the
participants when they viewed the same object front-facing. They found strong evidence of
a viewpoint aftereffect for every object. Commented [A3]: Give more detail, what objects were used,
what does this tell us about the mechanisms involved?
Ensure you fully expand on your descriptions of previous literature.
Wilson and Daar (2012) investigated viewpoint aftereffects in terms of facial recognition.
Participants were presented with a head outline, facial features, or the two together.
Reliable aftereffects were found in each condition. The full-face condition gave the clearest
aftereffect, suggesting that face cells in the inferior temporal cortex use both head outline
and facial features to identify faces. Commented [A4]: Explain why this is important for the current
experiment.
Morland et al. (2016) looked at viewpoint aftereffects for altered radial frequencies, some
of which resembled an approximate head outline. Various conditions gave data that
demonstrated clear aftereffects, and also provided evidence for a neural mechanism that
was tuned to the size of the stimuli. Commented [A5]: This is very relevant, more detail about this
would be useful.
Research into the topic of viewpoint aftereffects has vital implications in our understanding
of the visual system. It reveals the tuning properties of V1 neurons (Wilson & Daar, 2012),
how we perceive and recognise familiar objects (He & Fang, 2005), our mechanisms of facial
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