THE PERCEPTION OF COLOUR 



633 



Didelphys. Among insectivores, only the hedgehog, Erinaceus, has 

 been examined ; in experiments wherein brightness -differences were 

 inadequately controlled, Herter and Sgonina (1933-34) suggested that 

 this animal could see yellow as distinct from grey, but on the evidence 

 this conclusion seems unjustified. 



Among the rodents a considerable number of species has received 

 experimental attention. The rat, with its nocturnal habits, its practical 

 absence of cones and complete absence of a Purkinje phenomenon as 

 measured pupilloscopically or electroretinographically, would not be 

 expected to possess a colour sense. Training experiments with spectral 

 lights (Watson and Watson, 1913) or coloured papers (Munn, 1932 ; 

 Coleman and Hamilton, 1933 ; Muenzinger and Reynolds, 1936) have 

 verified this expectation. The work of Walton (1933), Walton and 

 Bornemeier (1938) and Cain and Extremet (1954), however, suggested 

 the opposite conclusion — that this animal could make choices on the 

 basis of hue-discrimination particularly between red and green ; but 

 this view is unique. Similarly negative results were obtained by 

 training experiments in domestic ynice (Yerkes, 1907 ; Trincker and 

 Brendt, 1957) but again, one investigator, Hopkins (1927), claimed 

 that 1 mouse in 7 could distinguish red from white. Wild mice were 

 investigated by Salzle (1936) ; one variety, the European field-mouse, 

 Ajjodeinus, showed no evidence of colour vision, but the red-backed 

 vole, Clethrionomys, could, in his view, discriminate between red and 

 green, although not between green, yellow and blue. In the reports 

 of these experiments, however, the control of the intensity of stimula- 

 tion is vague. A similar criticism applies to the study of the guinea-pig 

 by Sgonina (1936) who assumed that this animal had the same apprecia- 

 tion of brightness as he himself ; but in well-controlled experiments 

 using the optomotor reaction, Trincker and Berndt (1957) obtained 

 different responses with red, yellow, green and blue. Completely 

 negative results were obtained in the rabbit by Washburn and Abbot 

 (1912) and in the porcupine by Sackett (1913). The evidence therefore 

 points to the conclusion that these rod-rich nocturnal rodents, all of 

 which show a low sensitivity to red light, have achromatic vision. 



It would be expected that the highly diurnal squirrel with its cone- 

 rich retina would be in a different case. Here the evidence is confusing. 

 In experiments in which brightness was considered analogous to 

 its appreciation by the human experimenter, Colvin and Burford 

 (1909) and Salzle (1936) concluded that the European tree-squirrel, 

 Sciurus vulgaris, could discriminate hues ; but in more adequately 

 controlled work Locher (1933) found that one squirrel out of three 

 could with great difficulty be trained to differentiate yellow and 

 light green ; all other colours were equated with different shades of 

 grey. Meyer-Oehme (1956), on the other hand, claimed that squirrels 



Hedgehog 



Por: 



eupiiie 



