Discrimination of line orientation in humans and monkeys

Orientation discrimination, the capacity to recognize an orientation difference between two lines presented at different times, probably involves cortical processes such as stimuli encoding, holding them in memory, comparing them, and then deciding. To correlate discrimination with neural activity in combined psychophysical and electrophysiological experiments, precise knowledge of the strategies followed in the completion of the behavioral task is necessary. To address this issue, we measured human and nonhuman primates' capacities to discriminate the orientation of lines in a fixed and in a continuous variable task. Subjects have to indicate whether a line (test) was oriented to one side or to the other of a previously presented line (reference). When the orientation of the reference line did not change across trials (fixed discrimination task), subjects can complete the task either by categorizing the test line, thus ignoring the reference, or by discriminating between them. This ambiguity was avoided when the reference stimulus was changed randomly from trial to trial (continuous discrimination task), forcing humans and monkeys to discriminate by paying continuous attention to the reference and test stimuli. Both humans and monkeys discriminated accurately with stimulus duration as short as 150 ms. Effective interstimulus intervals were of 2.5 s for monkeys but much longer (>6 s) in humans. These results indicated that the fixed and continuous discrimination tasks are different, and accordingly humans and monkeys do use different behavioral strategies to complete each task. Because both tasks might involve different neural processes, these findings have important implications for studying the neural mechanisms underlying visual discrimination.