YELLOW FILTERS, MAGNOCELLULAR RESPONSES, AND READING

Abstract

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.

Key Words:

• colored filters
• dyslexia
• magnocellular
• reading
• S-cones
• yellow filter

Notes

Although this is not a settled issue several authors have emphasized the beneficial effects of blue filters, or blue overlays, on reading performance (Lehmkuhle, 1993; Solan et al., 1997; Solan et al., 1998; Williams et al., 1992). This runs counter to the reasoning of Ray et al. (2005) according to which blue filters should increase inhibitory S-cone input to the magnocellular system causing reduced magnocellular activity and impaired reading performance (that is, of course, provided reading performance depends on magnocellular responsivity, as claimed by Ray et al., 2005).

It seems that Ray et al. (2005) themselves are not fully convinced that S-cones actually inhibit magnocellular activity. For instance, they point out that this hypothesis is controversial and they down-play it by stating “… M cells are PROBABLY inhibited by s-cone input” (emphasis added) (Ray et al., 2005, p. 290). For this reason Ray et al. (2005) offer a “fall-back” hypothesis, namely that the yellow filter should have the effect of changing the L- /M-cone balance. They suggest that poor readers have “an overly strong L-cone input to the surrounds of their m[agnocellular]-cells”. If this were the case, the way to boost magnocellular activity would be to use a filter that would reduce the stimulation of the L-cones (or reduce the stimulation of the L-cones relative to that of the M-cones). As Ray et al. (2005, pp. 291–292) acknowledge, their yellow filter accomplishes the opposite. The yellow filter would, in this regard, have an effect similar to that of red filters (only less pronounced), which, according to Ray et al. (2005, p. 286) reduce magnocellular activity. (The problems of relying on red stimuli selectively to influence the magnocellular system have been discussed by Skottun, 2004a.) It appears that Ray et al. (2005) are proposing two contradictory views with regard to the effect of yellow stimuli on the magnocellular system.