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