Object memory and change detection: dissociation as a function of visual and conceptual similarity

Semantic Relations between Visual Objects Can Be Unconsciously Processed but Not Reported under Change Blindness

Change blindness—the failure to detect changes in visual scenes—has often been interpreted as a result of impoverished visual information encoding or as a failure to compare the prechange and postchange scene. In the present electroencephalography study, we investigated whether semantic features of prechange and postchange information are processed unconsciously, even when observers are unaware that a change has occurred. We presented scenes composed of natural objects in which one object changed from one presentation to the next. Object changes were either semantically related (e.g., rail car changed to rail) or unrelated (e.g., rail car changed to sausage). Observers were first asked to detect whether any change had occurred and then to judge the semantic relation of the two objects involved in the change. We found a semantic mismatch ERP effect, that is, a more negative-going ERP for semantically unrelated compared to related changes, originating from a cortical network including the left middle temporal gyrus and occipital cortex and resembling the N400 effect, albeit at longer latencies. Importantly, this semantic mismatch effect persisted even when observers were unaware of the change and the semantic relationship of prechange and postchange object. This finding implies that change blindness does not preclude the encoding of the prechange and postchange objects' identities and possibly even the comparison of their semantic content. Thus, change blindness cannot be interpreted as resulting from impoverished or volatile visual representations or as a failure to process the prechange and postchange object. Instead, change detection appears to be limited at a later, postperceptual stage.

Visual long-term memory and change blindness: Different effects of pre- and post-change information on one-shot change detection using meaningless geometric objects

To clarify the relationship between visual long-term memory (VLTM) and online visual processing, we investigated whether and how VLTM involuntarily affects the performance of a one-shot change detection task using images consisting of six meaningless geometric objects. In the study phase, participants observed pre-change (Experiment 1), post-change (Experiment 2), or both pre- and post-change (Experiment 3) images appearing in the subsequent change detection phase. In the change detection phase, one object always changed between pre- and post-change images and participants reported which object was changed. Results showed that VLTM of pre-change images enhanced the performance of change detection, while that of post-change images decreased accuracy. Prior exposure to both pre- and post-change images did not influence performance. These results indicate that pre-change information plays an important role in change detection, and that information in VLTM related to the current task does not always have a positive effect on performance.

Relative salience affects the process of detecting changes in orientation and luminance

This study tests the effect of relative saliency on perceptual comparison and decision processes in the context of change detection in which distinct visual mechanisms process two features (e.g., luminance and orientation). Townsend and Nozawa's (1995) systems factorial technology was used to investigate the process architecture and stopping rule when deciding whether luminance or orientation of a Gabor patch had changed. Experiment 1 found individual differences in decision strategies when we did not control relative saliency. One group of participants adopted co-active processing, and the other group adopted serial self-terminating processing to detect the change signals. When Experiment 2 eliminated the relative saliency, all but one observer adopted parallel processing and followed a self-terminating rule. These results support the relative saliency hypothesis and highlight the fact that observers adopt different change-detection strategies for two features, especially when relative saliency exists between the two feature dimensions.

Is a pre-change object representation weakened under correct detection of a change?

We investigated whether a pre-change representation is inhibited or weakened under correct change detection. Two arrays of six objects were rapidly presented for change detection in three experiments. After detection, the perceptual identification of degraded stimuli was tested in Experiments 1 and 2. The weakening of a pre-change representation was not observed under correct detection. The repetition priming effect was observed for a pre-change object and the magnitude was equivalent to the effect for a post-change object. Under change blindness, repetition priming for a pre-change representation was observed when detection did not require report of location in Experiment 1 and was not observed when location was required to be reported in Experiment 2. The results of Experiment 3 showed that a pre-change representation was recognized at a higher rate under correct detection than under change blindness, reflecting a stronger rather than a weaker pre-change representation in the former context.