Peripheral vision and pattern recognition: a review - strasburger - main

Peripheral_Vision.doc
4.2.2 Spatial-frequency characteristics of letter identification
To understand mechanisms underlying letter recognition, the concept of perception as a noiselimited
process (Barlow, 1977; Legge & Foley, 1980; Pelli, 1981) has been applied to letter
recognition (Sperling, 1989, Parish & Sperling, 1991, Solomon & Pelli, 1994, Majaj, Pelli,
Kurshan, & Palomares, 2002). Parish and Sperling (1991) embedded band-pass filtered
versions of the 26 letters of the alphabet in identically filtered Gaussian noise and averaged
performance over these letters. Observers used best (42% efficiency) spatial frequencies of 1.5
cycles per letter height over a 32:1 range of viewing distances. Solomon and Pelli (1994)
presented the 26 letters unfiltered but masked by high- or low-pass noise. Unlike Parish and
Sperling (1991) they obtained filters of about 3 cycles per letter from both high- and low-pass
data, and an observer efficiency of about 10%. Object spatial frequencies are now often used to
characterize filtered letters. However, Petkov and Westenberg (2003) showed that the spectral
specification in terms of cycles per letter rather than cycles per degree in Solomon and Pelli’s
(1994) was misleading. Indeed, in the latter study letter stroke width had covaried with letter
size. Conventional spatial frequency in cycles per degree therefore may still be the most
appropriate measure for the recognition of letters as well as of the non-symbolic patterns to
which Petkov and Westenberg had extended their study.
Performance levels for letter identification in central and peripheral vision were directly
compared by Chung, Legge, and Tjan (2002). They found spatial frequency characteristics of
letter recognition to be the same in the two viewing conditions. Chung and Tjan (2009) used
similar techniques to study the influence of spatial frequency and contrast on reading speed, in
the fovea and at 10° eccentricity. At low contrast, speed showed tuning effects, i.e., there was
an optimum spatial frequency for reading. The spatial-frequency tuning and scaling properties
for reading were rather similar between fovea and periphery, and closely matched those for
identifying letters, particularly when crowded.
4.2.3 Contrast thresholds for character recognition
First measurements of contrast thresholds for peripheral form recognition were performed with
the Tübinger perimeter using a diamond vs. circle discrimination task (Aulhorn, 1960, 1964;
Aulhorn & Harms, 1972; Johnson, Keltner, & Balestrery, 1978; Lie, 1980), and by Fleck (1987)
for characters displayed on a computer terminal. Herse and Bedell (1989) compared letter- to
grating-contrast sensitivity at 0°, 5°, 10°, and 15° in two subjects on the nasal meridian.
Eccentric viewing resulted in a larger sensitivity loss for letters than for gratings. In their Fig. 6,
they plotted log contrast sensitivity versus hypothetical spatial frequency, using the rule-ofthumb
relation cpd = 30/MAR, and obtained a linear dependency. If the abscissa is converted
back to the actual data, hyperbolic functions similar to those in Figure 13 result.
Strasburger et al. (1991) reported the first extensive contrast-threshold measurements for
characters where retinal eccentricity and stimulus size were varied independently so as to
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