Temporal characteristics of line orientation identification

The hypothesis that identification of line orientation is based on different mechanisms-a detector mechanism at large orientation differences and a computational one at small orientation differences-was tested in three experiments. The first two experiments compared reaction time and time of complete temporal summation (t(c)) in two tasks, line detection and line orientation identification. Identification at orientation differences 15 degrees or more was similar to detection in several respects, suggesting that it was accomplished according to the principle of ``labeled lines.'' In agreement with the initial hypothesis, identification at differences smaller than 15 degrees had a slower time course and could not be explained by the ``labeled lines'' principle. Experiment 3 explored the orientation acuity as a function of exposure duration and stimulus energy. Energy could not completely substitute for time in providing high orientation acuity, a result suggesting the involvement of neurophysiological mechanisms of large time constants.