Dr. Andrew Rider

My PhD examined global motion perception from multiple Gabor arrays under the supervision of Professor Alan Johnston and Professor Peter McOwan. Because moving objects are often larger than the receptive fields of motion sensitive cells in primary visual cortex, the visual system must integrate motion signals from disparate areas of the visual field in order to extract the correct movement of objects and of our own movement through the world. To examine how the visual system does this I employed mathematical modelling and psychophysical techniques to assess human perception of arrays of drifting Gabors (sinusoidal gratings windowed by Gaussians). These stimuli are ideal for this task as they provide ambiguous motion signals to the visual system and they are localised in space.

I found that observers could accurately disambiguate these motions when they were consistent with a global translation, rotation, expansion, contraction or spiral motion (a rotation plus expansion or contraction). I also found that the motion in these Gabors can distort our perceptions of spatial position. Moreover, this distortion is dependent on the global, unambiguous motion (rather than the local, ambiguous motions) which is at odds with previous explanations of motion-induced position-shifts being a low level phenomenon. Finally, I constructed arrays of Gabors that are consistent with a range of global motions including rigid translation and an infinite number of rotations. Measuring human perceptions of these arrays showed that we can extract several global solutions depending on where they are presented in the visual field. This result is not explicable in light of existing models of motion perception and suggests we extract visual motion in a more flexible and parallel manner than previously thought.

My next destination is the School of Electronic Engineering and Computer Science. Queen Mary, University of London as a Research Assistant.

 

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