Intro to our experiment

                     Since the early 20th century humans have been trying to better understand how their more primitive animal relatives perceive the world around them. For example, Ada W. Yerkes (1910) ran a visual discrimination experiment on raccoons. Yerkes had daylight reflect off of colored papers and the raccoons were able to make this distinction even though their natural state in the wild would rarely call upon them do make such a discrimination. Yerkes also ran a study at the same time on monkeys and their color vision. In this study the researchers decided to use spectral light instead by using a system of lenses and mirrors. The rays of light were passed through a large prism and reflected upon a screen in view of the monkey. The results were a little different, the monkeys failed to react to the red light, which raises the question on whether the red end of the light spectrum may not have the same stimulating effect upon animal eyes as it does on humans.

           In the early to mid ‘30’s this research was pushed forward with the use of lab rats in studies by John Slate and Norman Munn as well as a separate study by William Walton. In the Slater and Munn’s (1932) study, A note on brightness vision in the white rat, three rats were trained to discriminate between a color of high intensity and a color of low intensity. Once this was mastered, the criterion of correct responses was set at 80%; none of the rats were able to meet this percentage.

          In the a follow up study, Walton (1933) analyzed previous experiments testing color vision and tried to get to the underlying problems for why those experiments (including the Slater/Munn study) did not put out significant results. He set out to prove that rats and other animals do see in color. To this end he performed his experiment, Color vision and color preference in the albino rat, by pairing colors in six combinations, red-green, blue-yellow, green-yellow, red-yellow, and green-blue; two rats were trained on each combination. The results of the experiment indicated that the albino rat exhibits a preference for blue and green as opposed to red and yellow. The researchers controlled their experiment by allowed 50% of their trials to be with the trained color for food, red, as the darker of the two colors presented and the other 50% of the trials with red as the lighter color. If the animal succeeded in these tests the conclusion was drawn that it could distinguish between hues. The color pairs which were most easily learned were the farthest apart on the spectrum, for example blue-yellow was much easier for the rats than blue-green.

                  That same year, T. B. Coleman and W. F. Hamilton (1933) published their experiment Color blindness in the rat, in the Journal of Comparative Psychology. This experiment used Hooded Chinese rats and showed that the rats could be conditioned to brightness differences by using colored cars. However, the rats could not distinguish between colors whose brightness are equal to the rats eye; thus deeming their vision similar to that of a colorblind man.

                   Animal research into color vision was not contained to the thirties however, research continued into the ‘60’s with Ducker’s (1964) study on color vision in mammals. According to this research animals need two different spectral sensitivities to perceive color. In order to test for this Ducker by including many shades of grey with color because discriminations can be made on cues other than hue, such as brightness.

               In even more animal research, this time using horses, Gudrun Geisbauer, Ulrike Griebel, Axel Schmid, and Brian Timney (2004) tested brightness discrimination and neutral point testing. Horses were used to see if they could discriminate colors using the two choice dimensions. Two horses were trained to discriminate 30 shades of grey varying in low to high relative brightness. Their ability to distinguish shades of grey was poor. This study was later followed up by T. L. Blackmore, T. M. Foster, C. E. Sumpter and W. Temple, (2008) in their important study, An investigation of color discrimination with horses (Equus caballus). This study took Gudrun et. al. research to the next level by not only using grey as a neutral stimulus but seeing if horses could distinguish between that from color. The researchers set up an 85% correct response criterion for the horses. The results showed that horses could tell the different between blue and grey easily, had a harder time discriminating between green and grey as well as yellow and grey. Also they were unable to tell any difference between red and grey. Similar studies have been run with manatees (Griebel and Schmid, 1996) with similar results.

             For this study, researchers chose to utilize spectral light, similar to the Yerkes study, so that the animal (white rat) would be unable to ignore the stimulus. Unfortunately the technology was not available for creating “grey” light as a neutral stimulus. So, instead of testing for color vision per say, researchers tested for simple discriminations. This could be either differences in the brightness or hue. To do this, researchers utilized the RYB color model, which is made up of red, yellow and blue, also known as the primary colors. Since this research would not be testing for color directly the researchers decided to use Slater and Munn’s criterion of 80% from their 1932 study in which they tampered with the brightness level of the colors. In their experiment the rats were unable to meet this criterion. Researchers hypothesize for this experiment that they would be able to discriminate at this high of a level because the light was not to be manipulated in any way.


 Blackmore, T. L.; Foster, T. M.; Sumpter, C. E.; Temple, W. (2008). An investigation of color discrimination with horses (Equus caballus). Behavioral Processes, 78, 387-396.

Coleman, T. B., Hamilton, W. F. (1933). Color blindness in the rat. Journal of Comparative Psychology, 15, 177-181. Ducker, G. (1964) Color vision in mammals. J. Bombay National History Society. 61, 572-586.

Geisbauer, G. Griebel, U., Schmid, A., & Timney, B. (2004). Brightness discrimination and neutral point testing in the horse. Canadian journal of zoology, 82, 660-670.

Slater, J. E.; Munn, N. L. (1932). A note on brightness vision in the white rat. Journal of Comparative Psychology, 13, 273-277.

Walton, W. E. (1933). Color vision and color preference in the albino rat. II. The experiments and results. Journal of Comparative Psychology, 15, 373-394.

Yerkes, Ada W. (1910). Review of “Visual discrimination in raccoons” and “Some experiments bearing upon color vision in monkeys”. Journal of Educational Psychology, 1 ,360-362.


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