Vision Res.(12) Aulhorn

Ways of Seeing Visual Perception: Physiology, Psychology and Ecology Theories of Vision from Al-Kindi to Kepler (Chicago History of Science & Medicine)
Ways of Seeing Visual Perception: Physiology, Psychology and Ecology Theories of Vision from Al-Kindi to Kepler (Chicago History of Science & Medicine)

Research into Visual Perception conducted by Aulhorn

In 1960 Aulhorn investigated variations in the sensitivity of different parts of the human retina by presenting a square and a circular target to observers in a random sequence.

The use of this pair of stimuli enabled simultaneous investigations to be made 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, she suggested that the symbols could be correctly recognized only if the observer were 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).

Above: 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, which also used a square and a circle to test human visual acuity. In this case, the square and circle had the same surface area. The square was oriented so that its diagonals were vertical and horizontal. The value of the visual acuity needed in order 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 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:


Above: Illustration of the shapes (square and circle) used and the ‘form criterion’ described.

It is interesting to compare 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 subsequently 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 some of Aulhorn's Research Papers, incl. e.g.

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This section includes summaries of historial research and theories of human visual perception of simple two-dimensional objects. For more about the human visual system see The Eye, Parts of Eye, Eye & Vision Disorders, Ophthalmological Procedures.

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