Discrimination of line orientation in humans and monkeys

Orientation discrimination, the capacity to recognize an orientation differ ence between two lines presented at different times, probably involves cort ical processes such as stimuli encoding, holding them in memory, comparing them, and then deciding. To correlate discrimination with neural activity i n combined psychophysical and electrophysiological experiments, precise kno wledge of the strategies followed in the completion of the behavioral task is necessary. To address this issue, we measured human and nonhuman primate s' capacities to discriminate the orientation of lines in a fixed and in a continuous variable tusk. Subjects have to indicate whether a line (test) w as oriented to one side or to the other of a previously presented line (ref erence). When the orientation of the reference line did not change across t rials (fixed discrimination task), subjects can complete the task either by categorizing the test line, thus ignoring the reference, or by discriminat ing between them. This ambiguity was avoided when the reference stimulus wa s changed randomly from trial to trial (continuos discrimination task), for cing humans and monkeys to discriminate by paying continuous attention to t he reference and test stimuli. Both humans and monkeys discriminated accura tely with stimulus duration as short as 150 ms. Effective interstimulus int ervals were of 2.5 s for monkeys but much longer (>6 s) in humans. These re sults indicated that the fixed and continuous discrimination tasks are diff erent, and accordingly humans and monkeys do use different behavioral strat egies to complete each task. Because both tasks might involve different neu ral processes, these findings have important implications fur studying the neural mechanisms underlying visual discrimination.