Rapid eye movements in sleep are related to visual imagery of semantic contents but not to abstract objects

There is a controversial debate as to whether rapid eye movements (REM) in sleep are related to visual imagery during dreamy states supporting the notion that they may be involved in memory consolidation. Alternatively, they might just represent eye movements with the eyes closed. This study used kinematic parameters (velocity profiles) to compare REM with eye movements in awake volunteers with the eyes closed while they repeated their own visual exploration of previously explored visual targets immediately prior recordings.

Polysomnographic recordings including eye movement recordings by EOG were performed in the sleep laboratory. Ten healthy subjects (range 18–41) participated in the study. Visual targets were presented as visual objects (dots), stationary images (pictures) and videos. Prior to sleep, with the head stationary and the eyes open, subjects were asked to look at the dot objects consecutively (paradigm #1) and to explore stationary images (paradigm #2) and videos (paradigm #3). Subsequently they were asked to explore the same stimuli while imagining them under two experimental conditions: with the eyes open in complete darkness (condition #1) and with the eyes closed (condition #2). After the recordings subjects slept in the sleep laboratory.

We compared the relationship between amplitude and peak velocity (main sequence) of rapid eye movements (saccades) under the two experimental conditions during visual exploration of the three paradigms.

Results are twofold: (i) Main sequence of REMs did not differ from eye movements with the eyes opened during imagery exploration of stationary visual images and videos (paradigms #2,3). (ii) In contrast, velocity of REMs were slower than voluntary saccades with the eyes open in darkness to remembered non-semantic visual objects (paradigm #1; p=0.014).

Interestingly, REMs resemble the velocity profiles of large saccades (>10 deg) during imagined visual exploration of previously seen stationary and dynamic scenes (paradigms #2,3). According to these velocity parameters, REMs reflect an active state similar to awake conditions. This can be taken as some indirect evidence for their involvement in visual imagery during dreaming, with a possible role in memory consolidation. Unlike these semantic objects, saccades to imagined non-semantic (non-purposeful) objects are much fast, suggesting a significant different neural control than that of sleep-related REMs.