Vision Res.(16) Lie
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 others.
Lie’s 1979 paper (see Ref. below) describing a comparison of ‘area-threshold functions for the detection and resolution of single targets presented at various retinal eccentricities’ is an extension of the earlier investigations conducted by Aulhorn and Johnson et.al.. Although Lie’s investigations were based on these earlier studies, the procedure used was different in three respects:
- The circles and squares used by the previous investigators were replaced by square targets placed in one of the two positions illustrated below, the orientation being selected at random on each occasion.
- Both thresholds were determined by a stepwise variation of target size for fixed levels of target contrast. Target size was varied until either the detection or the resolution threshold was reached.
- The viewing time was unlimited.
Above: The test targets used by Lie (1979).
Only one (normal-sighted and extensively trained) observer participated under all experimental conditions. In addition, ‘control data’ were collected from three (less trained) observers. As in Aulhorn and Johnson et.al.’s investigations, observers viewed the targets using only one eye.
The shape of the resulting area-threshold function for detection was found to differ from that for recognition.
Lie identified two important differences between the detection and resolution curves. These were that
- As the stimulus size decreased below a certain minimum, resolution thresholds could not be obtained - but a corresponding limitation was not found for detection thresholds.
- The number of linear segments having different slopes which must be fitted to the data points to achieve a good fit to the data was larger for the resolution curves than for the detection curves.
Lie considered the implications of his findings in terms of the accepted theory of spatial summation within retinal receptive fields as described by Johnson et.al.. He stated that this criterion theory may explain why resolution thresholds cannot be obtained when the stimulus area is decreased below a certain minimal size. However, the other shape differences between the detection and resolution curves, such as different number of ‘slope shifts’, i.e. the linear segments mentioned above, suggest that the relationship between the detection and resolution properties of the visual system is more complex than the criterion theory implies. It was further suggested that these ‘slope shifts’ indicate transitions from one receptive field size to another.
For more about this see: I.Lie, "Visual Detection and Resolution as a function of Retinal Focus", Vision Res. Vol.20, pp.967-974 (1980).
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.