Orientation perception in rhesus monkeys (Macaca mulatta)

It was previously demonstrated that monkeys divide the orientation continuum into cardinal and oblique categories. However, it is still unclear how monkeys perceive within-category orientations. To better understand monkeys' perception of orientation, two experiments were conducted using five monkeys. In experiment 1, they were trained to identify either one cardinal or one oblique target orientation out of six orientations. The results showed that they readily identified the cardinal target whether it was oriented horizontally or vertically. However, a longer training period was needed to identify the oblique target orientation regardless of its degree and direction of tilt. In experiment 2, the same monkeys were trained to identify two-oblique target orientations out of six orientations. These orientations were paired, either sharing the degree of tilt, direction of tilt, or neither property. The results showed that the monkeys readily identified oblique orientations when they had either the same degree or direction of tilt. However, when the target orientations had neither the same degree nor direction of tilt, the animals had difficulty in identifying them. In summary, horizontal and vertical orientations are individually processed, indicating that monkeys do not have a category for cardinal orientation, but they may recognize cardinal orientations as non-obliques. In addition, monkeys efficiently abstract either the degree or the direction of tilt from oblique orientations, but they have difficulty combining these features to identify an oblique orientation. Thus, not all orientations within the oblique category are equally perceived.