Spontaneous saccadic eye movements are slower than saccades to other imagined stimuli – and even slower than REMs

There is a controversial debate whether rapid eye movements in sleep (REMs) are meaningful (functional) saccades or are due to spontaneous eye muscle contractions. We therefore compared saccadic eye movements with closed eyes to previously seen visual stimuli (dots, static images, videos), auditory stimuli, during math calculations and spontaneous saccades to REMs.

In trials with visual stimuli subjects were asked to remember and imagine them. In trials with auditory stimuli short audio tapes were played, in trials with math calculations subjects had to count back. Additionally one trial was without detailed instruction recording their spontaneous eye movements (the subjects were told to have a break due to preparations for the next trials). Subsequently, their REMs were recorded at the sleep laboratory. Peak velocities of saccadic eye movements were calculated by main sequence fits (relationship between amplitude and the corresponding peak saccadic velocity).

Saccades to remembered visual stimuli, auditory stimuli or during math calculations were about the same as compared to REMs. Surprisingly spontaneous saccadic eye movements were slower than all other types of saccades – including REMs.

Thus, REMs may be related to exploratory saccadic behaviour in the awake to remembered visual and non-visual stimuli. There is an ongoing debate about whether REMs resemble exploratory eye movements during dreaming. Current imaging methods revealed that during saccades and REMs similar brain areas were activated. Hence it seems likely that REMs reflect dreamy states but we cannot exclude that the oculomotor system and these brain areas are activated in parallel inducing simultaneous eye movements.