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Joseph Malpeli, Incheol Kang, Rachel Reem, Amy Kaczmarowski; Scotopic contrast sensitivity: Cat versus human. Journal of Vision 2005;5(8):188. doi: 10.1167/5.8.188.
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© ARVO (1962-2015); The Authors (2016-present)
Here we estimate the contributions of optical and neural factors to scotopic sensitivity differences between cats and humans. A central laser spot was fixated at trial start (for cats, a prerequisite for trial progression). Targets were upward drifting (temporal frequency = 4 Hz) Gabor functions of fixed width (SD = 1.5 deg) and various spatial frequencies (0, 1/8, 1/4, 1/2, 1, 2 or 4 cyc/deg) presented 8 deg left or right of center, with onset and offset occurring smoothly over 500 msec. A correct response was defined as an eye movement toward the target for cats, or a correct button press for humans. Humans were instructed to fixate the laser spot throughout the trial, and to always respond. Cats had 5 sec to respond, and the target stayed on until the response was made. For humans, targets appeared for 1.25 sec, an interval encompassing most cat response latencies. Stimulus contrast was adjusted following a modified staircase procedure, and thresholds estimated from psychometric functions. Pupils were measured by digital photography at each adaptation level. As expected, cats had better dim-light vision than humans for low spatial frequencies over a wide range of contrast: in terms of ambient light levels, cats were approximately 0.80 log units (a factor of 6.2) more sensitive for 0 and 1/8 cyc/deg. For spatial frequencies above 1/2 cyc/deg, humans were more sensitive. However, most, if not all, of the cats' advantage in dim light was optical: their larger exposed pupil areas, reflective tapeta, and shorter focal lengths would improve sensitivity by about 0.32, 0.16 and 0.26 log units, respectively. When these optical elements are factored out of the data, the cat's advantage for low spatial frequencies in the scotopic range shrinks to 0.06 log units (a factor of 1.15), which is well within the range of cumulative errors in our measurements. We conclude that at 8 deg eccentricity, the retinas of the two species have similar intrinsic sensitivity. Supported by NIH EY02695.
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