Biomedical subjects

George Mather

Publications and source records attributed to George Mather.

4 recordsLinked to original sources

PubMed2006

Two-stroke: a new illusion of visual motion based on the time course of neural responses in the human visual system.

A sequence of static images presented in rapid succession can create a powerful impression of visual movement, a fact exploited by the visual media (television and cinema) and by animators. A new illusion of movement called "two-stroke" is described, in which repeated presentation of a two-frame pattern displacement can create an impression of continuous forward motion, without the inclusion of any additional pattern displacements. The illusion can be explained by a biphasic temporal impulse response that modifies the stimulus delivered to motion energy sensors. It offers a basis for further research on temporal and motion responses in the visual system as well as a tool for animators and graphic artists to create consistent apparent movement from minimal external stimulation.

Humans↗

PubMed2005

No effect of spatial phase randomisation on direction discrimination in dense random element patterns.

Two computational strategies have been proposed for motion analysis in the human visual system. Energy-based schemes involve detection of spatiotemporal Fourier energy in the frequency components comprising a moving pattern. Edge-based schemes track shifts in the position of local edges in the pattern over time. This paper describes a stimulus manipulation, spatial phase randomisation, that acts as a diagnostic test for the involvement of energy-based processes, and describes the results of two experiments which apply the manipulation to random element patterns. Both experiments compared direction discrimination performance in patterns before and after the spatial phase of their components was randomised in the Fourier domain. For dense patterns, there was no effect of phase randomisation on the maximum displacement supporting reliable direction discrimination, indicating that energy-based responses were dominant. For sparse patterns, a significant effect of phase randomisation was obtained, indicating a greater role for edge-based responses.

Discrimination, Psychological↗

PubMed2004

Combining depth cues: effects upon accuracy and speed of performance in a depth-ordering task.

Two experiments investigated how the number of available depth cues affected the speed and accuracy of depth-ordering judgements. A series of textured tiles was presented on a computer monitor, with relative depths defined by combinations of contrast, blur and interposition. Subjects were required to move a mouse pointer inside each tile in turn, starting with the tile that appeared nearest, clicking on each. Accuracy of depth-ordering was much higher than chance in all conditions, though performance using the interposition cue alone was worse than in all other conditions. The only difference in reaction time in different cue conditions was in the time elapsed before the first-click. Subjects responded substantially faster when three depth cues were present (0.84 s) than when only one depth cue was present (1.41 s). The improvement in reaction time with cue numerosity is consistent with probability summation between cues extracted by independent processes.

Cues↗

PubMed2002

Blur discrimination and its relation to blur-mediated depth perception.

Retinal images of three-dimensional scenes often contain regions that are spatially blurred by different amounts, owing to depth variation in the scene and depth-of-focus limitations in the eye. Variations in blur between regions in the retinal image therefore offer a cue to their relative physical depths. In the first experiment we investigated apparent depth ordering in images containing two regions of random texture separated by a vertical sinusoidal border. The texture was sharp on one side of the border, and blurred on the other side. In some presentations the border itself was also blurred. Results showed that blur variation alone is sufficient to determine the apparent depth ordering. A subsequent series of experiments measured blur-discrimination thresholds with stimuli similar to those used in the depth-ordering experiment. Weber fractions for blur discrimination ranged from 0.28 to 0.56. It is concluded that the utility of blur variation as a depth cue is constrained by the relatively mediocre ability of observers to discriminate different levels of blur. Blur is best viewed as a relatively coarse, qualitative depth cue.

Adult↗