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Research into Visual Perception conducted by Johnson et.al.
Following the failure of the early investigators into visual perception
of simple two-dimensional shapes to reach conclusions consistent with
each other, and to explain their findings systematically,
experiments of the same form were still being undertaken many
years after Helson
and Fehrer’s account of the rôle of form in perception.
See for example, work by Blackwell, Casperson,
and Aulhorn.
In 1978 Johnson, Keltner & Balestrery investigated
the effects of target size and eccentricity on visual detection and
resolution. The resulting
paper (see bottom of this page) is among the most frequently cited
of all the papers in this field. The Tübingen perimeter [68] was
used to investigate the ‘functional
characteristics of peripheral vision’.
The stimuli consisted of five circle and square target pairs of varying
sizes. The square of each target pair was approximately 7% greater
in area than the
circle . This difference in area corresponds to the ‘minimum angle of
resolution’ relationship
between the circle and square, illustrated below (i.e. the distance
from the outer edge of the circle to the corner of the square is 0.15 times
the size of the diameter of the circle).
Johnson et.al. ’s circle and square.
This method of measuring the visual acuity necessary for distinguishing
between two objects is related to Aulhorn’s
earlier approaches to the same problem. It can be shown that if squares
of the same size
are used to compare Johnson et.al.’s method (using an appropriate
circle - smaller than the square) with Aulhorn’s form criterion
approach (that depended on the diameter of the small circle
which just
fitted into one corner of the square, touching the two edges of the
square and its inscribed circle), then the resulting measurements differ
by only
6% .
However, Johnson et.al.’s method has the advantage of using
a function of the actual shapes presented in the experiments, rather
than the more abstract constructs employed by Aulhorn.
Johnson et.al.’s experimental method involved presenting a random
sequence of six circle and square targets of fixed size, to observers.
The observers reported which target had been shown on each trial, and ‘guessed’ when
they were uncertain. Luminance of the targets was varied until the observer
could consistently discriminate at least five out of six targets (83%
correct).
Johnson’s results indicated that detection sensitivity is greater
for larger targets than for smaller ones and that target size and eccentricity
have a greater effect on resolution thresholds than on detection thresholds,
even at the fovea.
Generally, the results obtained were consistent with previous studies.
For further information about this work see:
C.A.Johnson, J.L.Keltner & F.Balestrery, "Effects of Target Size
and Eccentricity on Visual Detection and Resolution",
Vision
Res.,
Vol.18, pp.1217-1222 (1978).
This Section consists of short summaries of historial research and theories
into human visual perception of simple two-dimensional objects (these
are extracted and summarised from a Ph.D. Thesis [67] dated
1996).
For more general information about the human visual system see
the sections about:
The
Eye; Parts
of Eye; Visual
Disorders;
Ophthalmological
Procedures.
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