August 24, 2021
Written by: Claudia Lopez-Lloreda
Octopuses, some of the most neurologically complex creatures of the ocean, have never been regarded as particularly advanced in terms of feeling pain and emotions. Although they have the cells required to sense pain, called nociceptors, researchers weren’t sure what that meant: did they actually feel pain and experience the emotional response that comes along with it?
In new research published in iScience, Robyn Crook, a neuroscientist from the University of San Francisco, showed that octopuses’ capacity to sense pain goes beyond what scientists thought before: they feel pain not just physically but also emotionally.
The feeling of pain is divided into two aspects: the discriminative aspect, meaning the details of the physical pain like the location or intensity, and the affective aspect, meaning the negative emotional state that comes along with pain. Although it is clear that vertebrates and higher order animals experience both of these, it was not clear if invertebrates did. Researchers wanted to determine if invertebrate animals, particularly octopuses, can experience the affective aspect of pain.
To do this, they induced pain in octopuses by injecting a toxic chemical substance. They measured how much the animals learned to avoid the injection using a test called conditioned place preference. When the octopuses are injected with the chemical in a specific location, they learn to avoid that environment associated with the pain. The researcher also found that octopuses groomed the area where the painful chemical had been injected, sometimes grooming the injury site for up to 20 minutes. With these observations, the researchers concluded that octopuses go through a lasting experience of pain.
Would the octopuses then learn to seek out environments associated with less pain? To test this, scientists paired the harmful chemical with an analgesic that numbed the pain and provided pain relief. When the researcher administered the analgesic after the painful acid injection, the octopuses preferred that environment over the environment in which they received the painful stimulus without the analgesic since it represented a relief from pain. Interestingly, they only found this preference for the side in which octopuses received the analgesic only when they received it in combination with the acid, but not when they just received the analgesic. The researchers point out that because octopuses learned to avoid pain and seek out pain relief, that means that they recognize pain as an inherently negative experience and change their behavior based on that affective experience. This suggests that the octopuses may experience negative emotions with pain that they learn to avoid.
The brain patterns the researchers observed also supported the idea that the octopuses were actively and consciously experiencing the different aspects of pain. By measuring the electrical activity of the octopus’s nervous system, they found that injection with the painful chemical revved up the electrical activity of the brachial connectives, which connect the body to the central brain. According to the authors, this electrical activity possibly provides information about the painful stimulus, such as the location, to the brain. This suggests that the pain message was being sent from the periphery to the central brain, revealing a connection that had not been seen before in octopuses in the context of pain processing. Interestingly, administering the painful chemical along with the anesthetic, which provided pain relief, silenced the electrical activity in this part of the brain.
This new study could change how scientists’ study nervous system processing in animals. First, it shakes the long-held belief that most of the processing of neural messages in octopuses happens in the periphery. Second, it brings up a question about how the pain experience evolved: if lower order animals like octopuses experience the negative emotional response that comes along with pain, this could mean it arose early during evolution and would suggest that other invertebrates are also capable of feeling the affective aspect of pain.
Additionally, the new study could bring up questions of whether octopuses or other animals are capable of other higher-order cognitive processes which could potentially alter the ethical guidelines of animal research. As of today, researchers are free to experiment on octopuses, without any guidelines on humane care. Studies like these may shape ethical considerations of how animal research can and should be done in the future.
Images
Cover image. From Morten Brekkevold on Flickr.
References
- Crook, R. J. (2021). Behavioral and neurophysiological evidence suggests affective pain experience in octopus. IScience, 24(3), 102229. https://doi.org/10.1016/j.isci.2021.102229
- Alupay, J. S., Hadjisolomou, S. P., & Crook, R. J. (2014). Arm injury produces long-term behavioral and neural hypersensitivity in octopus. Neuroscience Letters, 558, 137–142. https://doi.org/10.1016/j.neulet.2013.11.002
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