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It has been proposed that magnocellular deficits cause the reading problems in dyslexia. However, how magnocellular deficiencies are supposed to cause these problems is unclear. Recently it has been proposed that reading performance is limited by the L-/M-cone inputs to the magnocellular system. However, as explained in this review, this is problematic for a number of reasons. Particularly difficult is the linking of L- and M-cone sensitivity specifically to the magnocellular system. [ABSTRACT FROM AUTHOR]

Schizophrenia, it has been proposed, is associated with deficits in the magnocellular part of the visual system. In support of this suggestion, it has been claimed that schizophrenic subjects have abnormal backward masking. However, if this abnormality is to be linked specifically to magnocellular defects, then it must be specific to backward masking, and not, also effect, for example, forward masking. We examined this issue by reviewing the studies of masking in schizophrenic subjects. We find: (i) Most studies (56 out of 67) of backward masking have researched only backward masking. This makes it impossible to determine if the abnormalities found in these studies are exclusively confined to backward masking. (ii) Of those studies (11) that have included both forward and backward masking conditions, the majority found some degree of abnormality under both forward and backward masking conditions. It is concluded that the evidence for linking the abnormalities found in those with schizophrenia specifically to backward masking, rather than masking in general, or more general visual impairments, is at present relatively weak. Given the rationale for using backward masking as a test of magnocellular sensitivity, research in this area does not point to a deficit specific to the magnocellular system. [ABSTRACT FROM AUTHOR]

It has been found that visual search is slower with isoluminant color stimuli than with luminance stimuli. This has been interpreted to mean that the magnocellular system plays a vitally important role in visual search. We here propose that this observation can be more parsimoniously understood in terms of color and luminance rather than in terms of the magno-and parvocellular systems. According to this interpretation, the slower search under isoluminant condition is a consequence of the fact that visual processing in general is slower, and that spatial vision is poorer, under isoluminant color conditions relative to luminance conditions. We also point out that in order to draw meaningful conclusions with regard to the underlying neural substrate, one needs to equate the luminance and color stimuli for discriminability. [ABSTRACT FROM AUTHOR]

The central tenet of the magnocellular deficit theory of dyslexia is that dyslexia is caused by a magnocellular deficit. A number of investigators have found deficiencies in visual coherent motion perception among dyslexic readers. These deficiencies have been attributed to magnocellular deficits, which means that they directly reflect the cause of dyslexia. However, similar perceptual deficiencies have been found in association with autism, Williams's syndrome, hemiplegia, and schizophrenia. These findings appear to undermine at least one of the following claims: (1) that a magnocellular deficit is the cause of dyslexia, and (2) that coherent motion is a reliable test of magnocellular sensitivity. [ABSTRACT FROM AUTHOR]

It has been suggested that yellow filters may increase magnocellular responsivity. This suggestion was, in large part, based on the assumption that the S-cones inhibit the magnocellular system. However, the evidence invoked to justify this assumption is only indirect. A previously reported direct electrophysiological investigation of this issue has found that S-cone input to the magnocellular system actually sum with L-and M-cone inputs. Therefore, the notion that yellow filters enhance magnocellular responses by reducing inhibition from S-cones cannot be maintained. [ABSTRACT FROM AUTHOR]

In some “flickering” spot stimuli, two kinds of percepts can be seen at different frequencies: phantom contours and “surface characteristics.” It has been proposed that their perception may be used to evaluate the magnocellular and parvocellular systems. This, however, is problematic. First, the difference in temporal frequencies associated with the two percepts is large compared to that between the temporal tuning of magno- and parvocellular neurons. Second, the lack of absolute temporal phase information in the case of surface characteristics does not fit with parvocellular neuron behavior. Third, the low temporal threshold of surface characteristics suggests cortical mechanisms rather than parvocellular ones. And fourth, the relationship between the two percepts and color may reflect parvocellular responses not magno- and parvocellular ones. [ABSTRACT FROM AUTHOR]