Discrimination of Line Orientation in Humans and Monkeys

Abstract

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.

A role for the ventral premotor cortex beyond performance monitoring

“…From then on the decision process continued as in the CD task. The most parsimonious explanation is that S1 is retrieved from LTM and maintained in WM (11,16,17). The behavioral results show that the monkeys' performance in the 2 tasks is close to their psychophysical thresholds and is based on the comparison of the 2 stimuli, regardless of whether they were recently shown or retrieved from LTM ( Fig.…”

“…Monkeys were trained to perform in 3 tasks, already described in detail: continuous discrimination, control task (12,17), and fixed discrimination with implicit reference (FDIR, (11,17)) (SI Methods). Here we report on the same neurons studied in the CD and FDIR (n ϭ 105) (Fig.…”

“…Most decisions are made by comparing recent events with current ones. This is what happens in tasks where monkeys are trained to decide on the difference between 2 sensory stimuli (S1 and S2) showed sequentially and separated by a short interval: the continuous discrimination (CD) task (11,(16)(17)(18). This has revealed the role played by several cortical areas in decision making (4,5,11,(19)(20)(21), including the participation of the ventral premotor cortex (PMv) in reporting outcomes and in integrating previous choices with their consequences (12).…”

“…To evaluate the consequences of these decisions the information about the retrieved sensory evidence has to be available together with the information about previous choices and their outcomes. This process can be studied with the Fixed Discrimination with Implicit Reference task (FDIR), a variant of the CD task, in which S1 was implicit and monkeys had to recover its correct orientation from long-term memory (LTM) and use its internal representation in the following trials (11,17).We hypothesized that for the PMv to play a role in decisionmaking, single neurons had to combine short-term or long-term memorized sensory events with current ones. Moreover, as both the sensory information used to take the decision and the outcomes can play a role in learning and shaping future behavior (12), we also hypothesized that this information has to be available after the behavioral response.…”

“…To evaluate the consequences of these decisions the information about the retrieved sensory evidence has to be available together with the information about previous choices and their outcomes. This process can be studied with the Fixed Discrimination with Implicit Reference task (FDIR), a variant of the CD task, in which S1 was implicit and monkeys had to recover its correct orientation from long-term memory (LTM) and use its internal representation in the following trials (11,17).…”