Human beings have a fascinating ability to recreate events in their mind’s eye, in exquisite detail. More than 50 years ago, Donald Hebb and Ulrich Neisser, the forefathers of cognitive psychology, theorized that eye movements are vital to our ability to do this. They pointed out that we move our eyes not only to receive visual sensory information, but also to recall information stored in memory. Our recent study provides the only academic evidence to date for his theory.
It could help research in everything from human biology to robotics. For example, it could shed new light on the link between eye movements, mental images, and dreams.
We can only process information from a small part of our visual field at a time. We overcome this limitation by constantly changing our focus of attention through eye movements. Eye movements develop in sequences of fixations and saccadic movements. Fixations occur three to four times a second and are the brief moments of focus that allow us to sample visual information, and saccades are the rapid movements from one fixation point to another.
Although only a limited amount of information can be processed at each fixation point, a sequence of eye movements ties together visual details (for example, faces and objects). This allows us to encode a memory of what we can see as a whole. Our visual sampling of the world, through our eye movements, determines the content of the memories our brain stores.
A trip down memory lane
In our study, 60 participants were shown images of scenes and objects, such as a cityscape and vegetables on the kitchen counter. After a short break, they were asked to recall the images as completely as possible while looking at a blank screen. They rated the quality of their recall and were asked to select the correct image from a set of very similar images. Using state-of-the-art eye-tracking techniques, we measured the participants’ scanning paths, their eye movement sequences, both when they inspected the images and when they recalled them.
We show that scanpaths during memory retrieval were connected to the quality of participants’ memories. When the participants’ scanpaths more closely replicated how their eyes moved when looking at the original image, they performed better during recall. Our results provide evidence that actual replay of a sequence of eye movements enhances memory reconstruction.
We analyzed different features of how the participants’ exploration tours progressed in space and time, such as the order of fixations and the direction of saccades. Some features of the scanning path were more important than others, depending on the nature of the memory sought. For example, the direction of eye movements was more important when remembering the details of how cakes were placed next to each other on a table than when remembering the shape of a rock formation. Such differences can be attributed to different memory demands. Reconstructing the precise arrangement of cakes is more demanding than reconstructing the crude layout of a rock formation.
Episodic memory allows us to mentally travel back in time to relive past experiences. Previous research established that we tend to reproduce gaze patterns from the original event we are trying to remember and that gaze locations during memory retrieval have important consequences for what you remember. All of these findings relate to static gaze, not eye movements.
Donald and Ulrich’s 1968 theory was that eye movements are used to organize and assemble “partial images” into a complete image displayed during episodic recall. Our study showed that the way scan paths develop over time is critical to recreating experiences in our mind’s eye.
A step forward
The results could be important for cognitive neuroscience and human biology research and in fields as diverse as computing and image processing, robotics, workplace design and clinical psychology. This is because they provide behavioral evidence of a critical link between eye movements and cognitive processing that can be harnessed for treatments such as brain injury rehabilitation. For example, eye movement desensitization and reprocessing (EMDR) is a well-established psychotherapy treatment for post-traumatic stress disorder (PTSD).
In this therapy, the patient focuses on the trauma and performs bilateral eye movements, which is associated with a reduction in vividness and emotion associated with the memory of the trauma. But the underlying mechanisms of the therapy are still poorly understood. Our study shows a direct link between eye movements and human memory systems, which may provide an essential piece of the puzzle.