When a flash a memory makes: Memorability of pictures in an RSVP task

What is it we remember, and why? Research in cognitive psychology has provided a broad and often very reliable sketch of the variables that determine memory performance. For example, recall of words is better when word repetitions are spaced rather than massed. To learn the Lithuanian word for cookie, you are better off spreading apart repetitions of sausainis rather than crowding them together (“sausainis – sausainis – sausainis – …”). The spacing effect is sufficiently strong for it to be a main component of a technique that doubles the learning rate for the acquisition of Lithuanian from 15 words an hour to 30 and that we blogged about here.

We also know that memory recall is better for items presented at the start or end of a list than for items in the middle. Few people order a beer from the middle of the list that a waitress recites (“Miller, Heineken, Bud, Bud Light, Beck’s, …”) when you ask her what’s on tap. This serial-position effect, like the spacing effect, is so strong that it replicates pretty much anywhere—from the classroom to the lab or the pub.

Although these variables are powerful, they predict memory performance at the level of stimulus ensembles rather than individual items. Words buried in the middle of a list are recalled worse than those at the ends, irrespective of which items are assigned to the positions. In fact, experiments usually randomize those items and collect data from many trials precisely because we are not interested in particular items but broad principles of memory.

There is, however, another question we can ask about memory: which particular items are remembered better than others? Is sausainis a more memorable Lithuanian word than, say, palepstis? Can we predict the memorability of specific items from their attributes?

Those questions have been tackled in a line of recent research that examined and compared the memorability of more than 2,000 pictures. The figure below shows a sample of those pictures, taken from a study by Phllip Isola and colleagues, and separates them by their memorability.

A crucial aspect of studies into the memorability of items is that noise and random variability must be differentiated from systematic idiosyncratic differences. After all, any set of stimuli will produce varied responses across items, so how do we know what is noise and what are stable properties of individual stimuli?

In this instance, we can be fairly confident that the differences between the memorable items on the left of the figure, and the forgettable items on the right, result from more than just random variability: the difference is consistent across observers and across retention intervals ranging from 36 seconds to 40 minutes. Moreover, memorability can be predicted on the basis of high-level properties of the scenes. Scenes are more memorable if they include people, interiors, foregrounds, and human-scale objects. In contrast, exteriors, wide-angle vistas, backgrounds, and natural scenes tend to be less well remembered. Intriguingly, low-level properties such as hue, saturation, or luminance, or the number of objects in a picture play no role.

A recent article in the Psychonomic Bulletin & Review took this research one step further by asking what role differences in perceptibility play in determining memorability. Are pictures remembered better when they are easily and readily perceived?

To shed light on this issue, researchers Nico Broers, Mary Potter, and Mark Nieuwenstein presented pictures of known but varied memorability to participants in a rapid serial visual presentation (RSVP) task, in which stimuli are flashed up briefly in rapid succession. The presentation duration was varied from only 13 ms to 360 ms—so from 1/75th of a second to about a third of a second. 1/75th of a second is rapid indeed.

The sequence of events in the experiment by Broers and colleagues is shown in the figure below:

People saw 6 pictures followed by a single item for which they had to decide whether it was among the original 6 (respond yes) or a new item (no). The critical memory item always appeared in position 2, 3, 4, or 5, and it was either of high or low long-term memorability as determined by earlier research.

The results are shown in the figure below for two experiments that differed only in the range of presentation durations. Performance is represented by d’, a statistical measure that provides a bias-free measure of performance.

In both experiments, performance increased with presentation duration. This result is unsurprising because a brief flash (1/75th of a second) is barely sufficient to recognize a scene let alone remember it. The more interesting result is that highly memorable pictures were remembered better in the RSVP task than less memorable pictures at any presentation duration. Moreover, for those memorable pictures, performance increased over duration sooner and at a steeper rate (at least in Experiment 1) than for the less memorable pictures.

At first glance, one might be tempted to dismiss this result as either circular or unsurprising. After all, if pictures are memorable, why is it surprising that they are remembered better?

What is surprising about the results is that they link memorability to the information that can be extracted in a single sweep of a scene, in 1/75th of a second. This is before any top-down feedback (e.g., direct eye movements to a critical object) can kick in that might enhance processing and extract more meaning from the picture.

As Broers and colleagues put it: “In short, the present results suggest that there is a strong link between the speed of understanding a picture and the likelihood of remembering it. What you are more likely to remember, you may also be more likely to see.”

This result is particularly intriguing in light of the findings from earlier research, mentioned above, that low-level features of a picture, such as hue and saturation or luminance, are not related to long-term memorability. So whatever happens in the first brief sweep of an image is already going beyond the surface-feature level and is extracting meaning and structure from the image—provided that structure involves people, interiors, foregrounds, and human-scale objects.

Article focused on in this post:

Broers, N., Potter, M. C., & Nieuwenstein, M. R. (2017). Enhanced recognition of memorable pictures in ultra-fast RSVP. Psychonomic Bulletin & Review. DOI: 10.3758/s13423-017-1295-7.

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