1. Summary
At its core, learning from experience means keeping a trace of past events or outcomes inside the body and letting it change what you do next. A nervous system is a powerful way to do this fast, selectively and in complex ways, but it isn't required for the simplest kind of learning, called habituation (getting used to a harmless, repeated stimulus). Slime molds have no nerve cells, yet they have been shown to react less and less to a harmless irritant that keeps being repeated, and to regain their reaction after a rest. [1]
2. The question: what do we actually call "learning"?
A weaker response to a stimulus doesn't automatically mean learning. Numb sense organs, tired muscles or damage can also make a response fade. To show habituation, you need to check that the response comes back once the stimulus stops, and that the organism still reacts to a new stimulus.
Learning from experience, in the broad sense, includes habituation, sensitization (becoming more reactive), classical conditioning, operant conditioning (learning from rewards and punishments), spatial learning and social learning. The more complex the learning, the more it depends on neural circuits that link sensory input, memory, the valuing of outcomes and the choice of action.
3. The evidence: sorting life by learning ability
| Group | Nervous system | How well learning from experience is supported |
|---|---|---|
| Mammals, birds, fish, etc. | Central nervous system | Conditioning, spatial and social learning, and problem solving are clearly shown |
| Insects, spiders, crustaceans, mollusks, nematode worms | Nervous system | Even small nervous systems can support associative learning and behavior updates |
| Jellyfish, sea anemones, etc. | No centralized brain, but a nerve net | Habituation is shown, and associative learning has been demonstrated in some species [2] |
| Sponges, placozoans, etc. | No typical neurons | They adjust their behavior, but strict evidence of learning is limited |
| Slime molds | No nervous system | Stimulus-specific habituation is fairly well supported [1] |
| Plants, fungi, bacteria | No nervous system | They show history-dependent responses and priming, but calling it "learning" is debated |
| Viruses | No independent senses or behavior | Usually not treated as individuals that learn |
The lines in this table aren't absolute. The verdict changes depending on where you draw the border between "memory," "adaptation" and "learning."
4. How it works: is human learning animal learning too?
The foundation clearly is. A dog that drools at just the words it keeps hearing before dinner is showing classical conditioning, an association between food and a sound. And when you feel on edge about the mirror or the space behind you for a while after a horror movie, that looks like learning from experience too, a mix of a threat you saw on screen, fear spreading to similar situations, and your imagination predicting what might happen. [3]
What sets humans apart is that we can learn without experiencing things firsthand: from other people's reactions, from language, from video, from rules and from simulating the future in our heads. Language compresses information and culture shares it, and both sit on top of the animal-style associative learning underneath.
5. Common misconceptions
- "Without a nervous system, nothing can be learned." Non-neural organisms show at least changes that resemble habituation and depend on their history.
- "Any change in response counts as learning." You have to tell it apart from fatigue, poisoning, damage, development and evolutionary adaptation.
- "Human reason can override animal-style learning." Even when your head knows you're safe, a conditioned automatic reaction can fire first.
- "Reduced activity in animals is the same as human depression." What we can observe is depression-like behavior; we can't confirm subjective feelings such as guilt or hopelessness.
6. In practice: understanding why the body won't move in depression
In human depression, people can experience psychomotor retardation, a slowing of thinking, speech and the start of voluntary movement. Several networks are involved, including the frontal cortex, the basal ganglia, the motor system and the reward system, so it isn't simply weak willpower. [4]
Rodents also show less exploring, social avoidance, a weaker response to rewards and lower activity after chronic stress or social defeat. In research, though, this is treated not as "depression itself" but as a depression-like state that imitates part of the symptoms. [5]
From an evolutionary angle, it's possible that damping activity when danger can't be escaped, or when you're worn out, helps in the short term. But human depression can't be neatly explained away as a "survival mode." It's a multifactor condition in which genetics, stress, sleep, inflammation, physical illness and social environment all interact.
7. Conclusion
The minimal form of learning from experience is flexible information processing that stores a trace of the past and changes the next response. A nervous system is the specialized hardware for it, and even advanced human reasoning rests on animal-style learning circuits. At the same time, we shouldn't assume the same inner experience or the same illness just because two behaviors look alike.
References (5)
- Boisseau RP, Vogel D, Dussutour A. “Habituation in non-neural organisms: evidence from slime moulds.” Proceedings of the Royal Society B (2016). PMCID: PMC4855389 pmc.ncbi.nlm.nih.gov
- Botton-Amiot G, et al. “Associative learning in the cnidarian Nematostella vectensis.” PNAS (2023). PMCID: PMC10068830 pmc.ncbi.nlm.nih.gov
- Debiec J, Olsson A. “Social Fear Learning: From Animal Models to Human Function.” Trends in Cognitive Sciences (2017). PMCID: PMC5507357 pmc.ncbi.nlm.nih.gov
- Bennabi D, et al. “Psychomotor Retardation in Depression: A Systematic Review of Diagnostic, Pathophysiologic, and Therapeutic Implications.” BioMed Research International (2013). PMCID: PMC3830759 pmc.ncbi.nlm.nih.gov
- Song J, et al. “Animal models for the study of depressive disorder.” CNS Neuroscience & Therapeutics 27(6):633–642 (2021). DOI: 10.1111/cns.13622. PMCID: PMC8111503 pmc.ncbi.nlm.nih.gov
