The present study examined the ability of Type A and Type B subjects to sustain attention during a 40-min visual vigilance task. It was predicted that Type A subjects would perform better than Type B subjects and that the performance of both groups would be related to the frequency of daydreams during the vigil. Type A subjects outperformed Type B subjects with regard to perceptual sensitivity (A') and number of signal detections. Although both groups reported an increase in the number of their daydreams as the vigil progressed, Type A subjects reported fewer daydreams during each period of watch than did Type B subjects. In addition, an inverse relationship was found between the number of signal detections and the frequency of daydreams.

The present study constituted an initial experimental effort to examine the fragmentation characteristics of subjective contours within the photopic and upper scotopic ranges of illumination. Four stimulus factors known to influence the visibility of subjective contours-target luminance, inducing area size and contrast, and contour orientation-were examined. Results indicated that subjective contours are indeed unstable perceptual phenomena. On the average, fragmentation or fading occurred after only 15 sec of observation, and some form of stimulus outage was present for 28 % of the viewing time of each stimulus. Fragmentation latency was significantly shorter and total time in fragmentation longer for diamond than for square contours, and total time in fragmentation varied inversely with inducing-area size. Fragmentation tended to occur in whole units rather than in isolated elements, a result reminiscent of the fading of real contours under impoverished viewing conditions.