EFFECT OF BACKGROUND BRIGHTNESS ON THE VISUAL CONTRAST THRESHOLD OF THE JAPANESE COMMON SQUID

The visual contrast threshold of Japanese common squid was studied usi ng contrast test boards newly equipped with on-off switches. This impr oved apparatus enabled us to judge exactly the jet-hit response of the squid under very low ambient lights. Background brightness was mainta ined at four light levels, the quantum irradiance ranging from 2.3 x 1 0(-1) mu Em(-2)s(-1) (equivalent to about 4.1 Ix) to undetectable leve ls (darkness). The apparent contrast values of the test board ranged f rom 33.6 to zero (0). At the light levels of 2.3 x 10(-1), 1.8 x 10(-3 ), 8.0 x 10(-5) mu Em(-2)s(-1) and at dark, the contrast threshold val ues of the squid were estimated to be 0.007, 0.008, 0.014, and 0.025, respectively, increasing with decreasing irradiance. At dark, the thre shold was approximately four-fold that obtained previously at 1.0 1x. These experiments suggest that the background brightness by which the contrast threshold of the squid is influenced is less than 1.8 x 10(-3 ) mu Em(-2)s(-1).