Humans have a fascinating ability to recreate events in the mind’s eye, in exquisite detail. Over 50 years ago, Donald Hebb and Ulrich Neisser, the forefathers of cognitive psychology, theorized that eye movements are vital to our ability to do so. 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 their theory.
It could help research in everything from human biology to robotics. For example, it could shed new light on the connection between eye movements, mental imagery and dreaming.
We can only process information from a small part of our visual field at a time. We overcome this limitation by constantly shifting our attention through eye movements. Eye movements occur in sequences of fixations and saccades. Fixations occur three to four times per 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 connects visual details (eg, faces and objects). It 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 viewed images of scenes and objects, such as a cityscape and vegetables on a kitchen counter. After a short pause, they were asked to recall the images as completely as possible while looking at a blank screen. They rated the quality of their memory 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 participants’ scanpaths, their sequences of eye movements, both as they inspected the images and as they recalled them.
We showed that scanpaths during memory retrieval were related to participants’ memory quality. When the participants’ scanpaths most closely reproduced the way their eyes moved when looking at the original image, they did their best during the recollection. Our results provide evidence that actual replay of an eye movement sequence stimulates memory reconstruction.
We analyzed different characteristics of how participants’ scanpaths progressed in space and time, such as the order of fixations and the direction of saccades. Some scanpath features were more important than others, depending on the nature of the memory being searched. For example, the direction of eye movements was more important when recalling details of how pastries were placed next to each other on a table than when recalling the shape of a rock formation. These differences can be attributed to different memory demands. Reconstructing the precise layout of the pastries is more demanding than reconstructing the rough layout of a rock formation.
Episodic memory allows us to mentally travel through time to relive past experiences. Previous research has established that we tend to replicate gaze patterns from the original event we are trying to recall and that gaze locations during memory retrieval have important consequences for what you remember. These results are all about 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 whole image viewed during episodic recall. Our study showed that the way scanpaths unfold over time is key to recreating experiences in our minds.
One step forward
The results could be important for research in cognitive neuroscience and human biology and in fields as diverse as computer science and image processing, robotics, workplace design, as well as clinical psychology. Indeed, they provide behavioral evidence for a critical link between eye movements and cognitive processing that can be leveraged for treatments such as brain injury rehabilitation. For example, eye movement desensitization and reprocessing (EMDR) is a well-established psychotherapeutic treatment for post-traumatic stress disorder (PTSD).
In this therapy, the patient focuses on the trauma and engages in bilateral eye movements, which is associated with a reduction in the vividness and emotion associated with remembering the trauma. But the mechanisms underlying the therapy are not yet fully understood. Our study shows a direct link between eye movements and human memory systems, which may be a critical piece of the puzzle.
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