Required reading- the need for attention

Method used

• In order to examine whether both types of change blindness might be due to the same attentional mechanism, and whether this mechanism might also lead to change blindness under more normal viewing conditions, we developed a flicker paradigm.

Flicker paradigm

• In this paradigm, an original image A repeatedly alternates with a modified image A', with brief blank fields placed between successive images (Fig, 1). Differences between the original and modified images can be of any size and type. (In the experiments presented here, the changes were chosen to be highly visible,) The observer freely views the flickering display and hits a key when the change is perceived.

• To prevent guessing, we ask the observer to report the type of change and describe the part of the scene that was changing

To test if the participants build an image

And because the stimuh are available for long stretches of time and no eye movements are required, the paradigm also provides the best opportunity possible for an observer to build a representation conducive to perceiving changes in a scene.

To test for the influence of higher level factors

we divided changes further according to the degree of interest in the part of the scene being changed

Interest was determined via an independent experiment in which five naive observers provided a brief verbal description of each scene;

• Central interests (CIs) were defined as object or areas mentioned by three or more observers

• marginal interests (Mis) were objects or areas mentioned by none

Experiment 1- predictions and expectations

• Experiment 1 examined whether the basic flicker paradigm could indeed induce change blindness

• If insufficient viewing time were the reason for the change blindness found in the brief-display experiments, we expected changes in this experiment to be seen within at most a few seconds of viewing

• If saccade-specific mechanisms were responsible for the change blindness found in the saccade experiments, we expected changes in this experiment to be easy to see simply by keeping the eyes still

• But if the failures to detect change in the previous paradigms were due to an attentional mechanism, we expected changes under these flicker conditions to take a long time to see

Experiment 1- Results

• The results of Experiment 1 (Fig, 3a) show a striking effect:

  • Under flicker conditions, changes in Mis were extremely difficult to see

  • Changes in CIs were noticed much more quickly

  • To confirm that the changes in the pictures were indeed easy to see when flicker was absent, the experiment was repeated with the blanks in the displays removed. A completely different pattern of results emerged: Identification required only 1.4 alternations (0,9 s) on average, showing that observers noticed the changes quickly. No significant differences were found between Mis and CIs for any type of change, and no significant differences were found between types of change (p > ,3 for all comparisons).

Experiment 2 - what might have gone wrong in experiment 1

• One explanation for the poor performance found in Experiment 1 might be that old and new scene descriptions could not be compared because of time limitations.

• The images in Experiment 1 were displayed for only 240 ms, which may have interfered with consolidation, and thus with the ability to compare successive images.

Experiment 2- added tweaks

• the blanks between pairs of identical images were "filled in" by replacing them with an 80-ms presentation of the "surrounding" images

• Thus, instead of presenting each image for 240 ms, followed by a blank for 80 ms, and then presenting it again for another 240 ms, we presented images without interruption for 560 ms (240 -i- 80 -i- 240) at a time

• If memory processing were the limiting factor, the longer display of the images in this experiment should have allowed consolidation to take place, and so caused the changes to be much more easily seen

Experiment 2- results

• however, show that this did not occur (consolidation).

• Although there was a slight speedup for Ml changes, this was not large; indeed, response times for Mis and CIs for all three kinds of change were not significantly different from their counterparts in Experiment 1.

• Note that these results also show that the temporal uncertainty caused by the repeating images in Experiment 1 does not affect performance greatly; Pairs of identical 240-ms images separated by 80 ms have much the same effect as a single image presented for 560 ms

Experiment 3- another possible limiting factor

• Another possible explanation for the occurrence of change blindness under fiicker conditions is that the fiicker reduces the visibility of the items in the image to the point where they simply become difficult to see

Experiment 3 - added tweaks

• To examine this possibility, in Experiment 3 we repeated Experiment I, but with a verbal cue (single word or word pair) placed in a white rectangle for 3 s at the beginning of each trial.

• In the completely valid condition, cues were always valid. If visibility is Indeed the limiting factor, no large effect of cuing should have occurred—the target would simply remain difficult to find. Otherwise, performance should have been greatly sped up by valid cues, and relatively unaffected (or even slowed down) by invalid ones

Experiment 3 - Results

• valid cues always caused identification of both MI and Cl changes to be greatly sped up.

• This speedup was significant for both the partially valid condition {p < .001 for MI; p < .03 for Cl) and the completely valid condition (p < .001 for both MI and Cl).

• Indeed, in the completely valid condition, the difference in response times for Mis and CIs declined to the point where it was no longer significant

• that this latter result indicates that the faster performance for CIs in Experiment 1 is unlikely to be due to the simple salience of their features: Such a near-equality of search times would hardly be expected if the CIs contained features salient enough to preferentially catch the attention of observers

• In contrast to valid cues, invalid cues caused a slight slowdown in performance (although this was not found to be significant).

• Taken together, then, these results show that observers could readily locate a cued target under flicker conditions, thereby demonstrating that visibility was not a limiting factor.

Conclusion

• he preceding experiments show that under flicker conditions, observers can take a surprisingly long time to perceive large changes in images of real-world scenes. This difficulty is due neither to a disruption of the information received nor to a disruption of its storage. It does, however, depend greatly on the significance of the part of the scene being changed, with identification being faster for structures of central interest than for those of marginal interest

• Visual perception of change in an object occurs only when that object is given focused attention

• In the absence of focused attention, the contents of visual memory are simply overwritten (i.e., replaced) by subsequent stimuli, and so cannot be used to make comparisons.

• The results presented here are also related to studies finding (Mack, Tang, Tuma, Kahn, & Rock, 1992; Neisser & Becklen, 1975; Rock, Linnett, Grant, & Mack, 1992) that attention is required to explicitly perceive a stimulus in the visual field.