We investigate the limitations on the ability to detect when a target has changed, using Gabor targets as simple quantifiable stimuli. Using a partial report technique to equalise response variables, we show that the log of the Weber fraction for detecting a spatial frequency change is proportional to the log of the number of targets, with a set-size effect that is greater than that reported for visual search. This is not a simple perceptual limitation, because pre-cueing a single target out of four restores performance to the level found when only one target is present. It is argued that the primary limitation on performance is the division of attention across multiple targets, rather than decay within visual memory. However in a simplified change detection experiment without cueing, where only one target of the set changed, not only was the set size effect still larger, but it was greater at 2000 msec ISI than at 250 msec ISI, indicating a possible memory component. The steepness of the set size effects obtained suggests that even moderate complexity of a stimulus in terms of number of component objects can overload attentional processes, suggesting a possible low-level mechanism for change blindness.

Visual memory for spatial frequency in stationary Gabor targets was measured as a function of interstimulus interval (ISI 2 or 8 s), number of targets (2, 3, or 4), and pre-cueing versus post-cueing of the critical target. The width of the Gabor and the initial and final spatial frequency were varied to eliminate confounding variables such as number of cycles, apparent contrast, and initial or final spatial frequency. A signal-detection analysis was employed, in which the subject was required to determine whether the spatial frequency of a selected Gabor target was the same or different over the ISI. The data were analysed by plotting d' as a function of spatial- frequency difference. The results show only small and inconsistent decreases in d' as a function of the ISI. There was little change in d' in any condition except certain 3 and 4 target post-cueing conditions, where its magnitude was halved. It was expected that pre-cueing should be superior to post-cueing because, although the Gabor displays are the same, effectively only one pre-cued target needs to be retained over the ISI. However, the expected effect did not always occur. In some spatial-frequency conditions, discrimination performance was maintained even for the most difficult condition (4 targets, 8 s ISI, post-cueing). It is concluded that there is a durable representation of stimulus spatial frequency in visual memory. Under optimum conditions, 2 – 4 targets can be remembered for 8 s with minimal change in d'.