Old dog, new tricks? Probing motion processing, dyslexia and visual search with the Ternus display
The Ternus display is a well-established moving visual stimulus, with a bistable appearance in certain configurations which has traditionally been explained in terms of low- and high-level motion processing. As well as its use within the psychophysics community as a motion stimulus, it has also appeared in the literature more recently as a probe for phonological dyslexia in connection with the magnocellular deficit hypothesis. I will present evidence that low-level motion processing does not, in fact, play any role in the analysis of the Ternus display. Rather, such stimuli appear to be mediated solely by a high-level process, sensitive to spatial form. This implies that the Ternus display is an inappropriate tool for examining dyslexia, even if one subscribes to the magnocellular deficit hypothesis. To confirm this, dyslexic and control populations were tested: the Ternus display failed to discriminate between them. Attempts to characterise the processing which underlies perception of the Ternus display revealed surprisingly large thresholds across a number of parameters (orientation, spatial frequency, contrast). These are taken to be the signature of a process which is not sensitive to just noticeable differences, but rather to significant or meaningful change. These thresholds were used to define the difference between targets and distractors in a visual search task. Performance on the search task was inefficient below these thresholds and efficient above them, suggesting that similar processing might occur for both Ternus and visual search displays. This implies that the Ternus display might be a useful surrogate for certain visual search paradigms.