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Research into Visual Perception conducted by Aulhorn
In 1960 Aulhorn investigated variations in the sensitivity of different
parts of the retina by presenting a square and a circular target to
observers in a random order.
The use of this pair of stimuli
made possible
simultaneous investigations into both human visual contrast thresholds
and visual acuity. Aulhorn argued that a square and circle of the same
area and illumination must have the same contrast and therefore, the
observer detection threshold for each shape must be the same. Having
eliminated the influence of the contrast in this way, it is suggested
that the
symbols
can be correctly recognized only if the observer is able to resolve
the difference between the outlines of the shapes. The distance which
must
be resolved is labelled (ra - ri)
in the diagram below, which is compared with the dimensions of other
symbols used to measure human
visual acuity,
such as the Landolt ring (far right in diagram below).
Diagram comparing (ra - ri) with
the dimensions of other symbols used to measure human
visual acuity,
such as the Landolt ring (far right).
A few years later (1964), Aulhorn described a similar but more sophisticated
approach to the same problem, this also employed a square and a circle
to test human visual acuity. In this case, the square and circle used
had the same surface area, and the square was oriented so that its
diagonals were vertical and horizontal. The value of the visual acuity
demonstrated
by
the ability to distinguish the square from the circle was said
to correspond to a ‘form criterion’. The size of this ‘form
criterion’ may
be defined by the diameter of the a small circle which fills out the
corner of the test square so that it touches two of its sides plus
the largest
circle which could be inscribed into the square, as shown below:
 
Illustration of the shapes (square and circle) used and the ‘form
criterion’ described.
Comparing Aulhorn’s two methods of
measuring visual acuity using a circle and square, presented at random.
The distance
ra - ri (proposed 1960,
illustrated top diagram) is a longer length than the diameter
of the small circle (proposed 1964, illustrated lower diagram).
Aulhorn concluded that the smaller the test symbol, the higher the threshold
luminance for both detection and recognition. She also stated that
the difference between the two thresholds is greater for recognition than
for perception, resulting in an increased difference between the
curves when the size of the stimulus is decreased. However, the most
important conclusion from this work is that : “ The necessary difference
in luminance for the recognition of a test symbol (visual acuity) is
the same as that for the perception of a test surface which is the
size of the symbol’s form criterion.” (from Aulhorn's summary
in English of her previous work - published in German).
Therefore the probability of human recognition of one form
is equal to the probability of human detection of another, smaller
form, which may be defined in terms of the first.
Many
authors have made remarks to the effect that ‘recognition is the
detection of details’, e.g. Bennett (1967), Thomas
(1985).
For further details about the work summarised here, see Aulhorn's
Research Papers.
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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