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

Peripheral_Vision.doc
Rumelhart, 1981); d) “prothesis and methathesis”, adding non-existent letters to a word on the
left or right (rare); e) shortening of the perceptual image in a certain area in the visual field (p.
65–70); f) assimilation of details to the perceived whole; g) false cognitive set, e.g. the impact of
prior knowledge of font and letter case, and whether the syllables are meaningful or not. Four of
these phenomena (a, b, e, f) are related to or underlie the crowding effect as we conceive it
today – as the impairment of discriminating detail or recognizing a pattern in the presence of
other details or patterns. Some are reflected in formal theories of pattern recognition (a, b1, c,
g). The others are still awaiting integration into future theories.
The phenomenon of crowding was probably familiar to ophthalmologists soon after the
introduction of acuity measurements but was first explicitly described by the Danish
ophthalmologist Ehlers (Ehlers, 1936, p. 62; Ehlers, 1953, p. 432 7 ). Ehlers noted, in the context
of normal reading and use of letter acuity charts, that there are visual, non-cognitive difficulties
of recognizing letters among other letters in eccentric vision. He also observed that the number
of letters recognized is independent of angular letter size at varying viewing distance (p. 62).
Stuart and Burian (1962) later referred to the phenomenon described by Ehlers as the
“crowding effect”.
Further early work on the crowding effect was carried out by Davage and Sumner (1950) on the
effect of line spacing on reading. Müller (1951) used a matrix of 15×15 Snellen E’s, and Prince
(1957, p593) somewhat airily mused that “there is a psychological element which obviates the
known laws of optics in the recognition of patterns”.
Averbach and Coriell (1961) started modern research on both crowding (which they called
lateral masking) and spatial visual attention. They used Sperling’s (1960) iconic memory
paradigm but controlled visual attention within a row of letters by marking one with an enclosing
circle (Figure 19a) – a spatial cue or probe in modern terms (they called it a circle indicator).
They also used a pointing line which they referred to as bar marker and which later became
known as a symbolic cue. Both markers had the desired attention-attracting effect. However,
the circle, unlike the bar, also had the effect of decreasing perceptual performance. Averbach
and Coriell thus discovered contour interaction and motivated Flom’s well-known work which
was published shortly thereafter (1963a; 1963b) (Figure 19b).
The crowding effect is highly important for the understanding of amblyopia and eccentric vision,
where it is particularly pronounced, whereas it is small and often seems to be absent in normal
foveal vision. It is therefore surprising that it was first quantitatively described in normal foveal
7 "When one is testing amblyopic children with isolated letters or E’s, the visual acuity recorded is often much
better than with the ordinary test chart. If the visual field is crowded with letters, the area of the visual field in which
the letters can be recognized narrows. This is very easy to demonstrate, as I showed at the Congress of Scandinavian
Ophthalmologists in 1936."
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