Temporal characteristics of line orientation identification

The hypothesis that identification of line orientation is based on differen t mechanisms-a detector mechanism at large orientation differences and a co mputational one at small orientation differences-was tested in three experi ments. The first two experiments compared reaction time and time of complet e temporal summation (t(c)) in two tasks, line detection and line orientati on identification. Identification at orientation differences 15 degrees or more was similar to detection in several respects, suggesting that it was a ccomplished according to the principle of "labeled lines." In agreement wit h the initial hypothesis, identification at differences smaller than 15 deg rees had a slower time course and could not be explained by the "labeled li nes" principle. Experiment 3 explored the orientation acuity as a function of exposure duration and stimulus energy. Energy could not completely subst itute for time in providing high orientation acuity, a result suggesting th e involvement of neurophysiological mechanisms of large time constants.