September 28, 2021
Written by: Catrina Hacker
Imagine the last magazine or picture book that you read. It’s likely that some vivid memories come to mind of certain pages and pictures. Maybe you can even describe details about these pictures like what people are wearing, what objects are in the background and other specifics. But could you describe every image with the same detail? The answer is probably no. While we have an incredible capacity to remember large numbers of images, we are not equally likely to remember every image. What makes some of these images more memorable than others? Through large-scale studies of human memory, scientists have uncovered one important piece of the answer to this question: image memorability.
Humans are remarkably good at remembering what they see. After seeing as many as 10,000 images, subjects in one study were estimated to be capable of remembering 6600 of them1. Visual memories are also highly specific. When subjects were asked which of two very similar images they had seen before (Figure 1) they could nearly always distinguish the novel from the previously viewed image2. These studies show that people have a high capacity for incredibly specific visual memory.
While we remember a lot of what we see with incredible detail, we do not remember everything. Remarkably, there is high convergence between people for what images are easily remembered3. In other words, we all tend to remember and forget the same images. How likely an image is to be remembered is called its memorability, with high memorability images more likely to be remembered than low memorability images (Figure 2).
What makes high memorability images different from low memorability images is hard to describe. Surprisingly, people are very bad at predicting what images are more memorable than others. One group of scientists tried to explain memorability by coming up with words to describe images and seeing if more memorable images shared some descriptors that less memorable images didn’t. They came up with 127 labels to either describe images (using words like open, cluttered, funny, or frightening) or the content of images (using labels like people, walking, sitting, woman, or famous place). Even with 127 possible descriptors, those labels still couldn’t fully capture the differences in memorability between images3. That indicates that there is something else about images that can’t easily be described with words that makes some easier to remember than others.
Despite our inability to articulate exactly what makes high memorability images more memorable than low memorability images, image memorability reliably predicts human memory performance across different populations. Not only are the same images more or less memorable across different groups of subjects, but scientists are also able to train computer models that can accurately predict the memorability of an image4. These models are able to predict memorability because they can learn complex patterns of features that can’t be described with words. Whereas scientists trying to explain memorability were limited to words that they could think of to describe images, computer models are able to learn descriptions of images that are too complicated to be described in words. Recent models have even been trained to take some images and change them to make them more or less memorable5.
What’s going on in the brain that makes some images more easily remembered than others? The answer may lie in brain regions traditionally associated with recognizing images. One particular brain region called Inferior Temporal Cortex (ITC) is known to play an important role in helping us recognize the things that we see. It is thought that patterns of activity across thousands of neurons in this region produce complex neural codes that distinguish images. Recent work showed that high memorability images produce a stronger response in ITC than low memorability images6. This means that, on average, neurons in this region fire more in response to a high memorability image than to a low memorability image. This enhanced response then increases the probability that an image will be remembered.
While image memorability helps to explain why we remember some things more than others, it doesn’t give a complete answer to the question. Several studies have shown that the context in which an image is shown has an impact on its memorability. For example, one study showed that it’s much harder to remember a specific lighthouse when you’ve recently seen many other lighthouses than when it’s the first lighthouse you’ve seen in a while7. Other things like attention and emotional state also likely have strong impacts on how likely we are to remember an image. Despite these other factors, image memorability is still a powerful tool for predicting what people are and are not likely to remember. This information could be used in many beneficial ways, including for education. Understanding image memorability can allow teachers to generate more memorable lessons, public health experts to produce more memorable fliers and PSAs, and government officials to make important announcements more memorable.
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