Do octopuses replay memories when their skin changes during sleep?

Octopus laqueus（圖片來源：Pophale A et al，採用 CC BY 4.0 授權）

Octopuses have long been regarded as among the most intelligent invertebrates. Yet until recently, relatively little was known about how they sleep. A neurological study of the Octopus laqueus has revealed that these animals undergo a striking sleep cycle in which calm resting periods are repeatedly interrupted by short bursts of intense activity lasting only tens of seconds.

These sleep cycles occur regularly and form a rhythmic two-stage pattern consisting of quiet sleep (QS) and active sleep (AS). During quiet sleep, the octopus closes its eyes and rests flattened against the bottom of the aquarium. Its skin typically appears pale yellow or white, and the body remains almost completely still. In experimental observations, this state lasted about 50.5 ± 16.4 minutes each time.

Between these long resting intervals, the animal suddenly enters active sleep. During this phase, the octopus rapidly changes the patterns and colors across its skin while its eyes move, the body twists, and breathing becomes faster and more irregular. These episodes last roughly 75 ± 28 seconds. Although this state is clearly different from normal wakeful behavior, both the neural activity and certain behavioral features resemble those seen while the animal is awake.

https://facebook.com/61568286094421/videos/1280656703431586

Changes During Sleep in Octopus laqueus（圖片來源：Pophale A et al，採用 CC BY 4.0 授權）

To determine whether these bursts truly represent a sleep state rather than brief awakenings, researchers tested the animals' responsiveness. When the octopuses were in either quiet sleep or active sleep, they reacted very weakly to mild mechanical stimulation compared with when they were awake. In addition, when octopuses were temporarily awakened during sleep, a compensatory effect appeared: the frequency of active sleep increased during the following two days. If an active sleep episode was interrupted, the next one occurred sooner than usual. These findings show that the behavior is under physiological regulation and represents a genuine stage of sleep rather than wakeful activity.

One of the most remarkable features of active sleep is the wide variety of skin patterns that appear during these short bursts. High-resolution video combined with neural-network image analysis revealed that each episode follows a unique trajectory of changing patterns without a fixed sequence. However, across different individuals there exists a shared repertoire of patterns, and these patterns strongly overlap with those displayed while the octopus is awake.

This observation suggests that the brain of the octopus may be replaying previously experienced patterns during sleep, in a process somewhat analogous to dreaming. In vertebrates, neural circuits can reactivate during sleep and replay patterns of activity associated with earlier experiences. The skin patterns of octopuses are directly controlled by neural signals, meaning that these visible changes may provide an external window into the activity of the sleeping brain.

Why would octopuses produce such dramatic skin displays during sleep? One possible explanation is that the nervous system is rehearsing control over the skin-pattern system, much like how motor skills in vertebrates are refined during sleep. Another possibility is that these episodes represent memory replay, similar to the hippocampal replay observed in mammals during sleep when past experiences are reactivated and integrated.

Neural recordings provide additional clues. Using Neuropixels probes, researchers recorded local field potentials (LFPs) from the octopus brain. During active sleep, activity in the superior frontal lobe and vertical lobe—regions associated with learning and memory—increased markedly, and the frequency range resembled that seen during wakefulness. The superior frontal lobe displayed activity across a broad range of frequencies including oscillations around 30 Hz, while the vertical lobe generated large low-frequency waves.

During quiet sleep, overall brain activity was reduced. However, the superior frontal lobe occasionally produced oscillations between 12 and 18 Hz, similar to the sleep spindles observed in mammals. These events indicate that neural communication related to memory processing may still occur during this quieter stage of sleep.

Interestingly, these sleep characteristics show strong parallels with vertebrate sleep architecture. Quiet sleep resembles non-rapid eye movement (NREM) sleep, while active sleep shares several features with rapid eye movement (REM) sleep, including wake-like neural activity accompanied by body and eye movements. Yet octopuses and vertebrates diverged more than 500 million years ago. Because the brain structures involved are not evolutionarily homologous, these similarities are interpreted as convergent evolution—independent solutions that arose in very different lineages to support complex nervous systems and cognition.

The discovery that octopuses possess a two-stage sleep cycle raises a broader question about the nature of sleep itself. If animals separated by such vast evolutionary distances show comparable sleep structures, it may suggest that two-stage sleep represents a general strategy used by sophisticated nervous systems to process information, maintain neural circuits, and consolidate memory.

In octopuses, the rapidly shifting patterns across the skin offer something unique among animals: a visible trace of what the brain may be doing during sleep. Through these fleeting displays, scientists may be witnessing the outward signs of neural processes—perhaps even something resembling dreams—playing out across the body of a sleeping cephalopod.

Author: Shui-Ye You

Reference:

Pophale A et al. (2023). Wake-like skin patterning and neural activity during octopus sleep. Nature.