Phantom contour
[3] At high temporal frequencies (20 Hz), the alternating frames are perceived as one non-flickering image, where the individual dots are no longer visible, while simultaneously creating the illusion of a distinct border, dividing the top and bottom halves of the display.
Perceived borders similar to the phantom contour, observed via luminance contrasts, were reported in 1987, when Livingstone and Hubel analyzed various aspects of vision, and linked them to the magno- and parvocellular subsystems.
[2] Rogers-Ramachandran and Ramachandran tested whether or not this preference for peripheral stimuli in the magnocellular cells would have an effect on phantom contour perception.
As was predicted, sparsely spaced objects, which degrade the perception of the contours in central vision, were more easily perceived when subjects adjusted their fixation from 0 to 5 degrees eccentricity.
[3] Insight into the connection between this illusion and temporal-frequency processing could help us understand underlying mechanisms responsible for certain types of dyslexia (learning impairments in one's reading ability).
[9] Children with dyslexia possess a lower flicker frequency threshold compared to non-dyslexics when the phantom contour images are achromatic (lacking in color).
[2] Ramachandran and Rogers-Ramachandran compared using equiluminance contours on these tasks to using a psychophysical “scalpel” to separate the visual pathway subsystems based on their functional roles.
[2] When the stimuli used to present phantom contours consist of adjacent dark and light achromatic horizontal stripes (squarewave gratings), variations in spatial and temporal frequency can be examined.
[1] When analyzing flicker-generated forms, Quaid and Flanagan[5] noted that, as stimulus size increased, phase-contrast thresholds for these phantom contours decreased.
Surface characteristics are defined as the perception of the absolute temporal phase of the flickering images, which becomes apparent at lower frequencies (5–7 Hz).
[6] Quaid and Flanagan recommend looking to the dorsal stream for illusory contour processing, claiming that motion-defined-forms may be detected, even with a magnocellular deficit.
[5] They also point out that the dorsal stream mediates low contrast, high temporal frequency stimuli, as well as motion, making it a viable candidate for processing this illusion.
They claim the inability of dyslexic children to process stimuli that change rapidly or occur briefly may be unrelated to the magnocellular system.
Ultimately, they conclude that further research is needed to back the theory that phantom contours and surface characteristics are caused by the magnocelluar and parvocellular systems, respectively, and, therefore, it would be wise not to limit one's research of phantom contours and surface characteristics exclusively to the analysis of how the magno- and parvocellular systems function.
