Memory echoes
Rapid-eye movement (REM) or “active” sleep is widely thought to play a role in transforming experiences into memories in humans. A detailed recollection of events – the who, what and when – we experience during waking hours is often categorized as episodic memory. When these memories are formed (consolidated) during sleep, the events may be “replayed” in dreams.
“[Episodic memory] is one of the most sophisticated memory systems an animal can develop,” Obenhaus said. Re-experiencing and dreaming are high-level functions of memory that not all animals experience.
The patterns activated during an octopus’s sleep may be due to its memory replay. But it could also be tied to simpler forms of memory, like motor learning. Some researchers believe that sleep states help to establish the right wiring for motor and somatosensory systems. As an animal’s nervous system develops, neural connections have been found to form during sleep, building pathways for learning new movements.
Dynamic design
In octopuses, the expression of skin pigment is a motor action, powered by so-called chromatophores. Motor neurons contact these ring muscle cells throughout the cephalopod’s body and direct them to contract and expand to spread pigment.
It’s possible, then, that unconscious skin pattern expression is a byproduct of some motor learning, running in the background during REM sleep.
Because sleep is a dynamic process, with pigment cells firing and limbs twitching during REM, it’s hard to disentangle motor learning from memory consolidation, Obenhaus said. “The replay hypothesis may be part of the bigger picture.”
For example, Obenhaus believes that octopuses might add to their repertoire of skin designs as they mature. “It’s kind of a scaffold, and it can be expanded through age,” Obenhaus said. That “scaffold” seems to be pre-programmed, or wired, into the octopus’ neural and motor systems, with the capacity for enrichment later in life.
Adding further complexity, if these learned patterns are tied to an event – something one octopus experiences that’s different from another – that would suggest that the animal’s personal history is played back during sleep, lending support for the replay hypothesis.
Abstract dreaming
What’s missing, Obenhaus said, is the right paradigm to directly address the replay hypothesis of active sleep in cephalopods.
Octopuses are difficult to train (“They’re like underwater cats,” Obenhaus joked), many are solitary creatures, so their only pattern displays are related to camouflage and survival.
Cuttlefish, however, are charismatic cephalopods. When they interact, their skin blushes with designs unique to the interaction, visibly distinguishing a social scenario from a survival one.
“The cuttlefish are so behaviorally extravagant that you can get so much from that very intricate language that they’re displaying on their skin,” Obenhaus said.