Dyslexia and rapid visual processing: A commentary

Abstract

It has been suggested that dyslexia is the result of a deficit in rapid sensory processing. Several methods have been used to assess this with regards to vision: temporal contrast sensitivity, visual persistence, temporal order judgments, temporal acuity, and coherent motion. Here we examine these methods. We find that several of them—visual persistence, temporal order judgments, and coherent motion—are poorly suited to evaluating the dynamic aspects of vision. In the case of temporal contrast sensitivity and temporal acuity the results from these tests either are conflicting or provide little support for an impairment. As far as vision is concerned there is little evidence for a specifically temporal deficit.

Keywords:

• Temporal processing
• Dynamic stimuli
• Rate processing
• Vision
• Dyslexia

Notes

¹With regard to the linking of “timing” of visual events to the magnocellular system, Stein & Talcott (1999, p. 65) wrote that magnocellular neurons respond to visual transients and are responsible for “the timing” so as to “signal when new events occur.” This conclusion seems problematic given that Levitt, Schumer, Sherman, Spear, and Movshon (2001) found that magnocellular neurons are less transient than are parvocellular neurons. Also, Blakemore and Vital-Durand (1986) found parvocellular neurons with transient responses.

²In their study Slaghuis and Ryan (2006) used drifting gratings and gave the drift rate in cycles per second, which means that the drift rate is equal to the temporal frequency. These authors pointed to deficits for dysphonetic and mixed dyslexic readers at 6 Hz, 12 Hz, and 18 Hz using 1.0 cycles/deg stimuli. These deficits by themselves could indicate a deficit at high (and medium) temporal frequencies. However, such a conclusion would be countered by the finding that the same groups in the cases of 4 cycles/deg and 8 cycles/deg stimuli also showed deficits at 0.75 Hz. Consequently, these results do not point in an unequivocal manner to a dynamic deficit even in the limited case of subjects with dysphonetic and mixed dyslexia. Slaghuis and Ryan (2006) interpreted the findings obtained with 1 cycle/deg gratings as evidence for a magnocellular deficit in dysphonetic and mixed dyslexic readers. However, the reduced sensitivity at the lowest temporal frequency (i.e., 0.75 Hz) in the case of 4 cycles/deg and 8 cycles/deg stimuli undermines this interpretation (Skottun & Skoyles, 2007a).

³ Hood & Conlon, 2004, did not study dyslexic readers but measured the correlation between reading performance and temporal order judgment in an unselected group.