Scanning, biases, and inhibition to visual stimuli in healthy and right hemisphere lesioned adults

Butler, Stephen Hugh (2007) Scanning, biases, and inhibition to visual stimuli in healthy and right hemisphere lesioned adults. PhD thesis, University of Glasgow.

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This thesis explores right hemisphere involvement in perceptual biases to
chimeric faces and posterior right hemisphere involvement in response
inhibition through an examination of the role of eye movements.
Studies of patients with focal brain lesions and neuroimaging research
indicate that face processing is predominantly based on right hemisphere
function. Additionally, experiments using chimeric faces, where the left and
the right hand side of the face are different, have shown that observers tend to
bias their responses toward the information on the left. A series of
experiments were conducted using lifelike gender based chimeric faces (Burt
and Perrett, 1997) to explore the relationship between eye movements and
perceptual biases.
A left perceptual bias was observed in experiment 1, in that subjects
based their gender decision significantly more frequently on the left side of the
chimeric faces. Additionally, analysis of the eye movement patterns indicated
a strong tendency to first fixate on the left side of the image and subsequently
a relationship between perceptual biases and eye movements.
Experiment 2 examined the issue of inversion of such facial stimuli and
provided evidence that the right hemisphere may still be more influential in
determining gender from inverted chimeric stimuli, as a significant left
perceptual bias was demonstrated to these types of stimuli. It is proposed
that the chimeric bias effects found in this experiment argue against the idea
that inversion destroys the right hemisphere superiority for faces.
Whilst experiments 1 and 2 provided evidence for right hemisphere
dominance in the processing of chimeric faces, experiments 3 and 4
investigated the influence of eye movements and exposure duration in modulating the bias. Experiment 3 and 4 demonstrate that in younger adults
but not older adults that a reliable leftward bias can be obtained when stimuli
are exposed for brief durations only. However, evidence is provided that
indicates that the perceptual bias is enhanced in the presence of eye
movements. Additionally, experiment 4 shows that the perceptual bias is
demonstrably diminished in older adults, possible mechanisms for this finding
are discussed.
Experiment 5 reviews evidence related to dysfunction in visual search in
patients with right hemisphere lesions, however what is less well understood
is how well such patients are able to inhibit a response in an otherwise simple
search task. Experiments 5 and 6 explore oculomotor capture in such
patients. Patients were asked to search for a colour target amongst distracters
and to signal target location with a saccade. On each trial an additional
distracter was presented which could be either similar or dissimilar to the
target and appear either with or without a sudden onset. Patients were
demonstrated to have higher oculomotor capture rates by the additional
distracter, and to be more susceptible to the distracting influence of sudden
Experiment 7 employed an antisaccade task and a fixation task and
demonstrated in the same group of patients further impairments in response
inhibition. In both tasks patients were demonstrated to have significant
difficulty in inhibiting an eye movement to a peripheral distracter (relative to
age matched controls). Results of experiments 5-7 indicate that patients with
right hemisphere lesions that spare the frontal lobe have demonstrable
impairments in inhibiting responses to suddenly appearing peripheral stimuli,
implicating a role for posterior brain structures in this type of inhibition.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: B Philosophy. Psychology. Religion > BF Psychology
Colleges/Schools: College of Science and Engineering > School of Psychology
Supervisor's Name: Harvey, Dr. Monika
Date of Award: 2007
Depositing User: Ms Mary Anne Meyering
Unique ID: glathesis:2007-4984
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 21 Feb 2014 11:10
Last Modified: 21 Feb 2014 11:57

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