The effects of eccentricity and stimulus magnification on simultaneous performance in position and movement acuity tasks

We presented two tasks, spatial interval discrimination and displacement detection, simultaneously in the same location at various eccentricities. The subject was to solve (i) only the spatial interval task; (ii) only the displacement task; or (iii) both tasks simultaneously. With 500 msec stimulus duration, and using the method of spatial scaling, the E2 value (the eccentricity at which stimulus size has to be doubled to maintain performance level) was found to be 0.17-0.39 deg for spatial interval discrimination and 1.0-1.2 deg for displacement detection. These values remained unaffected whether the subject solved one task or two tasks simultaneously. This finding was confirmed using a shorter, 50 msec stimulus duration. As there is no interference between tasks, the mechanisms solving the tasks appear to be functionally independent i.e., operating in parallel at all eccentricities.