Physiological studies of visual perception

Thesis

The problem of investigating the neural basis of higher cognitive function is receiving considerable attention with the increasing availability of functional neuroimaging. The aim of this thesis was to investigate how participants perceive a bistable, rotating structure from motion (SFM) cylinder, looking in particular at which areas of the brain are responsible for generating the internal switch that causes a perceptual change. Another aim was to discover how participants with autism, a neurodevelopmental condition thought to affect higher functioning, might differ in their perception of a similar stimulus. This thesis aimed to assess the interplay of bottom-up and top-down processing and the affect autism might have on the perception of multiple rotating SFM cylinders. Quantitative data were collected using Magnetoencephalography (MEG) and psychophysical testing. Participants viewed a continually rotating bistable SFM cylinder, giving responses in the form of a button press, whilst functional imaging was recorded using MEG. They were either presented; an ambiguous SFM cylinder in which participants were able to report perceptual switches, or a stable SFM 3D cylinder in which participants could report stimulus induced switches when the disparity of the stimulus was reversed. The results of this study showed a statistically significant increase in left frontal cortex activity 1180ms prior to the behavioural response to perceptual switches (p<0.05 when comparing perceptual switches to stimulus induced switches). When testing participants with autism, a behavioural psychophysics set up was used and participants indicated the direction of rotation of a continuously rotating SFM cylinder. Participants with autism showed borderline statistically significant reduced perceptual durations when viewing a constantly rotating bistable SFM cylinder (p<0.10). The increase in activity in the left frontal area was interpreted as evidence of the internal switch responsible for the perceptual change participants reported and could be the neuronal signature for the top-down control of perceptual switches. The reduced perceptual durations seen in participants with autism could be due to a weaker involvement of top-down control leading to a less stable perception, with local instability taking a greater role, however more participants would be required in order to get statistically significant results. Further work is required to localise the brain area of increased activity preceding intrinsic switches, incorporating an increased number of participants. In order to assess how brain function is altered in the autism spectrum, perceptual switching should be investigated in the participants with autism using a neuroimaging methodology.

Authors: Heath, J

• Publication date: 2010
• Type of award: MSc
• Level of award: Masters
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: structure from motion; ambiguous; bistable; internal switch; MEG; magnetoencephalography; autism
• Subjects: Medical Sciences