Mechanisms involved in the encoding of image motion by the human visual system
The aim of this study is to investigate the processes that underlie image motion detection in human vision. To do this I have investigated motion perception for a wide range of stimulus velocities across the visual field, and have made use of different stimuli. Two mechanisms were revealed at different positions across the visual field as a result of the examination of the temporal properties of the Lower Threshold of Motion (LTM), that is, the lowest velocity that is reliably detected. The results for central vision showed that the LTM is mediated by a code that utilizes the spatial displacement transversed by the stimulus. For peripheral vision, the LTM is mediated by a code that utilizes the velocity or temporal frequency of the stimulus. This raised the question, do these two processes underlie image motion processing at all eccentricities with different sensitivities at threshold? To investigate this question, a wide velocity range was used to assess the ability of the visual system to discriminate different speeds. The temporal and spatial properties of the stimulus were individually disrupted to reveal the critical cues for velocity discrimination. The results show the presence of two processes at all eccentricities. The two processes can be characterised as a displacement code, and a velocity code. Evidence is shown that the velocity code uses 'velocity' information and not solely temporal frequency information. For central vision, the displacement code is most sensitive for short stimulus durations. The duration at which it is most sensitive is inversly proportional to the velocity of the stimulus. The velocity code is most sensitive at long term regions of the visual field. However, the range of velocities to which each mechanism is sensitive changes at different rates across the visual field. This leaves a range of low velocities which are detected only by the velocity mechanism at large eccentricities. Further investigation into the displacement code has revealed that this code can be characterised by an optimal displacement. This is less than the 1/4 of a spatial cycle of the stimulus which is proposed value for quadrature phase. Also luminance contrast was found to be an important parameter of the motion process. The two codes described above could be mediated by two motion areas of the primate visual cortex: the striate and prestriate cortex. From recent single cell studies, the emerging properties of neurons in these two parts of the visual cortex suggest that the displacement code may be mediated by the striate cortex, and the velocity code by the middle temporal area of the pre striate cortex.