Measurement of the Horizontal Extent of Panum's Area by a Method of Constant Stimuli

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Abstract
It has long been accepted that simultaneous stimulation of any pair of corresponding points in the retinas will give rise to a single perceived image. Many experiments also suggest that around each point there is a region wherein fusion is still possible with the corresponding point in the other retina. The extent of this region of fusion, sometimes called Panum's Area, has not often been measured in the central fovea, and there has not been very good agreement between the results. For these reasons, a new determination has been made by using a method of constant stimuli, or ‘forced-choice’. The observer made judgments on a flashed test spot which was variable in depth. The percentage of replies ‘ spot single ’ was plotted against the parallax at which the test spot had appeared. This technique allowed measurements to be made in the fovea, and was found to give repeatable results, with good agreement between three observers. The test spot was 1·5′ in subtense and was presented at various visual angles from 0° to 6°. In the fovea, the region of single vision was found to be 15′ to 20′ in extent. A vertical line 45′ in height was also used, and with this the extent of the area was found to be 25′. The results appeared to be unaffected by the duration of the appearance of the test spot at least for exposures greater than 0·01 sec. The figures obtained are slightly greater than the average for previous results, but fall within their spread.

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