A phantom contour is a type of illusory contour. Most illusory contours are seen in still images, such as the Kanizsa triangle and the Ehrenstein illusion. A phantom contour, however, is perceived in the presence of moving or flickering images with contrast reversal. The rapid, continuous alternation between opposing, but correlated, adjacent images creates the perception of a contour that is not physically present in the still images. Quaid et al. have also authored a PhD thesis on the phantom contour illusion and its spatiotemporal limits (University of Waterloo) which maps out limits and proposes mechanisms for its perception centering around magnocellularly driven visual area MT (see also Quaid et al., 2005 on www.pubmed.com).
Example
One example of this illusion involves stimuli consisting of two similar frames, with uniform grey backgrounds: in one frame, the top half contains white dots and the bottom half contains black dots. A second frame contains the reverse of the first frame, in which the white dots of the first frame are replaced with black dots and the black dots are replaced with white dots. The rapid alternation between these two frames reverses the polarity of the dots, while keeping their positions static. 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. This perceived border is a phantom contour illusion.
History 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. Vilayanur S. Ramachandran and D.S. Rogers-Ramachandran's subsequent research, however, helped pare down and solidify the concept of phantom contours. Their research was also the first use of “texture borders” to induce this illusion. Ramachandran and Rogers-Ramachandran's research has since led to several more papers on the topic, with varying conclusions regarding the underlying mechanisms responsible for these illusions, as well as considerations for this illusion's potential link to dyslexia. See Theories of dyslexia.
Magnocellular pathway One theory suggests that temporal-frequency processing in the magnocellular pathway, an anatomical pathway that originates in the retina, goes through the lateral geniculate nucleus, and ends in the primary visual cortex, may be connected to the appearance of this illusion (Skottun and Skoyles debated this link in 2006). The magnocellular pathway is contrast-sensitive, sensitive to motion, and also sensitive to flashing black-to-white edges. Livingstone and Hubel looked at lateral geniculate cells in both the magno- and parvocellular layers, and found responses to luminance contrasts to be much stronger in magnocellular cells. These cells also had better spatial and temporal resolution. Additionally, the magnocelluar pathway is activated more by peripheral vision, in contrast to the parvocelluar pathway, which is activated more by central vision. 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. This supports the idea that magnocellular cells are responsible for phantom contour perception under such conditions.
Clinical use Ramachandran and Rogers-Ramachandran proposed that the phantom contour illusion could be used to test whether or not a person's magnocellular pathway is functioning properly, as well as provide a means for analyzing the role and function of the magnocellular system in general. Loss of magnocellular function can be found in the early stages of glaucoma. 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). Deficits in rapid visual processing have been seen in dyslexics and are believed to be linked to deficits in the magnocellular pathway. Differential sensitivities to temporal-frequency processing may play a role in both the perception of phantom contours, as well as certain reading impairments. Sperling et al. found that children with phonological dyslexia (a deficit related to coding meaning of soundsystems of language) showed a decreased ability in perceiving phantom contours, and thus, may be experiencing a magnocellular deficit. Additionally, based on rigorous testing of the dyslexics’ reading abilities, which were compared to their inability to process phantom contours, they found a negative correlation between this magnocellular deficit and reading ability, suggesting a link between magnocellular deficits and orthographic processing (storing patterns of letters in the visual processing system). This is consistent with the theory that some dyslexic people may have motion perception deficits. The pan-sensory deficit hypothesis with regard to dyslexics, states that a deficit in processing rapidly changing stimuli may be a congenital deficit in magnocellular or magnocellular-like processing.
Variations
Achromatic vs. chromatic images Children with dyslexia possess a lower flicker frequency threshold compared to non-dyslexics when the phantom contour images are achromatic (lacking in color). However, when presented with similar images to the black and white dot images mentioned above, but using equiluminant (a.k.a. isoluminant) color, in which the luminance of the colors is the same but the hue is not, the illusion disappears for non-dyslexics as well. Adding a luminance difference as small as 10% between the colors, however, re-activates the illusion. This finding suggests that the parvocelluar pathway, which is sensitive to color, is not responsible for this illusion. The magnocelluar pathway, in contrast, is believed to be insensitive to color. 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.
… excerpt ends here. Continue reading the full article.
