Summation of target and mask metacontrast stimuli.

Three experiments are reported; they show the following: (i) Metacontrast occurs at high photopic conditions (250 cd m-2); at this luminance the function peaks at stimulus asynchronies of approximately 40 ms (compared with 100 ms at lower luminances). (ii) The strength of metacontrast is contrast-independent, down to contrast very near detection threshold. (iii) Detection thresholds for the combined target and mask presentation are lower than those for the mask alone, implying summation between target and mask. Summation is greatest at the same asynchronies which yield maximum metacontrast masking. The experiments are taken as evidence in favour of a fusion explanation for metacontrast. An argument is made which implicates temporally units of the motion detection system as being instrumental in the fusion process.