Skoyles, John R.; (1992) Early processes in letter and word recognition. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Recent work in psychophysics and neurophysiology suggests that vision is divided into two channels magno (transient) and parvo (sustained). What relationship do the two vision channels have to the recognition of written words? Two theories have been suggested: first, that the magno (transient) channel inhibits the visual persistence of words seen in the parvo (sustained) channel; and second, that dyslexics have a defective magno (transient) channel which disrupts this persistent inhibition and causes a low level visual problem seeing words. None of these theories is supported by direct research upon the two channels and reading. Neurophysiologists have developed techniques for selectively blocking the two channels. They are noninvasive and enable the direct investigation of the role of each channel in stimulus processing. These depend upon manipulating the visual characteristics of the stimuli. These techniques were adapted for use in reaction time experiments displayed on a PC fitted with a VGA graphics card. Preliminary research uncovered a previously unreported luminosity artifact which affects the boundaries of images presented on, at least some, PC monitors. A correction was devised to minimise this artifact. The technique for blocking parvo channel stimulus perception while permitting magno channel stimulus perception involves presenting images in counterphase: the magno channel unlike the parvo one can resolve fast alternating images. It was found that counterphase stimuli can be recognised through a process which side steps temporal resolution. Experiments in this thesis show that this process can be blocked by preceding counterphase stimuli by premasks. Lexical decision and letter matching were investigated using magno and parvo blocking techniques. There were indications that word vs nonword classifications and letter length were affected. However other variables such as word frequency and imageability were not. There is evidence in letter matching that positive identity matches between letters were affected by magno channel blocking while negative ones were not.

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