You move an individual from a bright-lit room to a dark room and test his detection threshold to a dim light as a function of time. You decide to use a very small target stimulus (i.e. 0.5 degrees in size). Which of the above graphs would most likely be the result?
Perceptual Function / Color Vision
No
U
B
The classic dark adaptation curve utilizes a 420 nm light stimulus AND a large target (e.g. 10 degrees in diameter) so that the "cone thresholds" and "rod thresholds" are readily apparent. The use of a 420 nm light stimulus or a large target results in the curve seen in the image below (answer choice A).
However, if the light stimulus is instead 650 nm and/or a very small target size is used (e.g. 0.5 degrees), then the dark adaptation curve resembles the one depicted in answer choice B. This occurs because the testing conditions now isolate only the cone threshold responses. This graph shows that there is no bimodal distribution of light sensitivities as a function of time. In other words, only the cone thresholds persist at this wavelength. With a very small target size, only the cones are being tested since only the fovea is being stimulated (i.e. no rods in the fovea).
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