The purpose of this investigation was to explore possible methods of isolating the pupillary response to focused light. Two methods were used; one employed flash stimuli of various sizes on a dark background and the other employed a masking technique. An infrared photographic technique was used for recording changes in pupil diameter in response to the stimulus conditions used in this study.
When using the 51° flash stimulus, it was hoped that enough photoreceptors would be included in the area of focused light to ellicit a pupillary response before the stray light would become intense enough to produce a response.
In the masking technique the retina was adapted to a centrally fixated stimulus for four to six seconds and then a one-second flash stimulus 15° in diameter or less was superimposed upon it. The purpose of the adapting stimulus was to mask the pupillo-motor effect of the stray light from the flash stimulus.
Data for the flashes of various sizes on a dark background support the hypothesis that stray light is the main determinant of the pupillary response in this situation. However, when a 51° flash stimulus was used, a response to focused light was evident at low levels of luminance.
In general the masking technique was successful in masking the pupillary response to stray light when used with flashes of low luminance.
The visual and pupillary thresholds were shown to be about the same in this investigation.
It is recommended that the masking technique used in this study be used to study summation and the pupillo-motor sensitivity of different parts of the retina.
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