Effect of background brightness on the visual contrast threshold of the Japanese common squid [Todarodes pacificus]

The visual contrast threshold of Japanese common squid Todarodes pacificus was studied using contrast test boards newly equipped with on-off switches. This improved apparatus enabled us to judge exactly the jet-hit response of the squid under very low ambient lights. Background brightness was maintained at four light levels, the quantum irradiance ranging from 2.3x10 super(-1) mu Em super(-2)/s (equivalent to about 4.1 lx) to undetectable levels (darkness). The apparent contrast values of the test board ranged from 33.6 to zero (0). At the light levels of 2.3x10 super(-1), 1.8x10 super(-3), 8.0x10 super(-5) mu Em super(-2)/s and at dark, the contrast threshold values of the squid were estimated to be 0.007, 0.008, 0.014, and 0.025, respectively, increasing with decreasing irradiance. At dark, the threshold was approximately four-fold that obtained previously at 1.0 lx. These experiments suggest that the background brightness by which the contrast threshold of the squid is influenced is less than 1.8x10 super(-3) mu Em super(-2)/s.