Academic journal article Memory & Cognition

Assessing the Role of Different Spatial Frequencies in Word Perception by Good and Poor Readers

Academic journal article Memory & Cognition

Assessing the Role of Different Spatial Frequencies in Word Perception by Good and Poor Readers

Article excerpt

Numerous studies indicate that dyslexic and nondyslexic individuals exhibit different patterns of sensitivity to spatial frequency. However, the extension of this effect to normal (nondyslexic) adults of good and poor reading abilities and the role played by different spatial frequencies in word perception have yet to be determined. In this study, using normal (nondyslexic) adults, we assessed reading ability, spatial frequency sensitivity, and perception of spatially filtered words and nonwords (using a two-alternative forced choice paradigm to avoid artifactual influences of nonperceptual guesswork). Good and poor readers showed different patterns of spatial frequency sensitivity. However, no differences in accuracy of word and nonword perception were found between good and poor readers, despite their differences in spatial frequency sensitivity. Indeed, both reading abilities showed the same superior perceptibility for spatially filtered words over nonwords across different spatial frequency bands. These findings indicate that spatial frequency sensitivity differences extend to normal (nondyslexic) adult readers and that a range of spatial frequencies can be used for word perception by good and poor readers. However, spatial frequency sensitivity may not accurately reveal an individual's ability to perceive words.

A substantial body of psychophysical evidence indicates that the human visual system operates in the spatial frequency domain, responding to visual patterns on the basis of their spatial frequency content. Sensitivity to various spatial frequencies is known to vary systematically (e.g., Campbell & Robson, 1968; Graham, 1989). In particular, the amount of contrast (i.e., contrast threshold) necessary for the perception of static bars whose luminance is modulated sinusoidally about a fixed mean level is known to vary as a function of their spatial frequency. The reciprocal of contrast threshold is contrast sensitivity. Graphical representation of variation in contrast sensitivity over a range of spatial frequencies describes the contrast sensitivity function (CSF). In general, the human visual system is most sensitive to spatial frequencies in the range of 2-6 cycles per degree (cpd), and more contrast is needed for detection of lower and higher spatial frequencies.

Numerous researchers have found that dyslexic and nondyslexic controls exhibit different patterns of sensitivity to spatial frequencies (e.g., Borsting et al., 1996; Cornelissen, 1993; Demb, Boynton, Best, & Heeger, 1998; Evans, Drasdo, & Richards, 1993, 1996; Gross-Glenn et al., 1995; Lovegrove, Bowling, Badcock, & Blackwood, 1980; Lovegrove et al., 1982; Martin, Cornelissen, Fowler, & Stein, 1993; Martin & Lovegrove, 1984, 1988). In particular, studies indicate that dyslexic individuals have reduced sensitivity to certain spatial frequencies and that this reduction tends to be greatest in the low- to midfrequency range (i.e., between 2 and 8 cpd; Borsting et al., 1996; Cornelissen, 1993; Demb et al., 1998; Evans et al., 1993, 1996; Lovegrove et al., 1980; Lovegrove et al., 1982, Experiment 2; Martin & Lovegrove, 1984, 1988; see Skottun, 2000, for a review).

It remains to be seen whether this sensitivity difference between dyslexic and nondyslexic controls extends to normal (i.e., nondyslexic) adult readers. Unfortunately, studies in which spatial frequency sensitivity differences between normal adult readers of good and poor reading ability have been examined are rare, and the issue is far from resolved. However, one possibility is that the sensitivity differences found between dyslexic and nondyslexic controls represent a distinct disorder that is specific to this reading-disabled population (Rutter & Yule, 1975). Alternatively, the reduced sensitivity of dyslexic individuals to various spatial frequencies may represent the lower end of a normal continuum (Au & Lovegrove, 2001; Cornelissen et al. …

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