Snoozing Cephalopods May Wear Their Dreams on Their Skin
A dog napping on the couch often twitches its feet, flicks its ears and thumps its tail. Watching this behavior, it’s easy to say the dog is lazily pantomiming a vivid dream: He’s chasing a squirrel; she’s running to greet her human. &Բ;
An octopus napping in its tank isn’t too dissimilar. Its many arms stretch, curl and unfurl, as if sensing and grabbing its way through a dream state. &Բ;
But unlike sleeping pets, cephalopods do something extra. &Բ;
In addition to arms that snap and twitch, their skin is just as active, blooming with colors that blot, swirl and fade from one design to the next – a phenomenon directly controlled by their brains. &Բ;
What neural processes are activating these sleepy displays? Is the octopus wearing its dreams, re-experiencing the world through tattoos on its skin?
, a researcher at the Kavli Institute for Systems Neuroscience and Whitman Fellow at the Ůֱ, is following a hunch: He believes that these sleep-induced pattern activations are echoes of the animal’s life experiences, played back through memory during sleep.
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. &Բ;
“[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. &Բ;
It’s possible, then, that unconscious skin pattern expression is a byproduct of some motor learning, running in the background during REM sleep. &Բ;
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. &Բ;
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. &Բ;
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. &Բ;
This presents an opportunity to test the replay hypothesis. If cuttlefish have specialized designs during mating rituals for example, Obenhaus can use their interactions to watch if these experiences are echoed during sleep. &Բ;
For example, Obenhaus can set up experiments in which a cuttlefish sees an opponent only through its right eye, causing the animal’s right side to flood with dark pigment. After repeated exposure, the replay hypothesis would predict that the same color pattern is played back during REM.
A direct comparison between wakeful and sleeping patterns is difficult, Obenhaus said. In fact, the patterns look nothing like the ones they exhibit while awake: “It’s these clouds of [pigment] activations, and they all kind of intermingle. It's like the most abstract dream you can think about,” he said. &Բ;
Still, his observations tell Obenhaus that there’s something going on in cephalopods that’s unique to sleep and memory. His research is an exercise in patience, fueled by a passion for understanding an octopus's mind. With time, perhaps these abstract dreams will reveal themselves.