Dependence of Panum's fusional area on local retinal stimulation

The range of fusion in the human binocular vision system has been shown by many research groups to be capable of extension. Although each of these research groups tested the extension of fusion under different conditions, the majority agreed that an extension of fusion is possible, and most have noted a variety of variables that contribute to the phenomenon. The extension of fusion may be interpreted in at least two ways. The first is that the range over which fusion is possible is enlarged, and the second is that the range is shifted. Therefore a study of human binocular fusional ranges was performed to clarify the nature of the extension of Panum's fusional area. The following results were obtained: (1) A shift of the fusional area occurs when one of a pair of fused retinally stabilized images is moved into disparate locations on the retinas. Results for one observer demonstrate a corresponding small enlargement of the fusional area. Thus the extension of the fusional area not only involves the recruitment of retinal locations into the fusional area that are not normally in the area but also involves the loss from the fusional area of certain retinal locations that are normally within it. (2) A continually present stimulus fuses over a wider range with a flashed stimulus than with another continually present stimulus.