· Link Some Neurons ·

This is the third of the blog posts where I am going to summarize, explain and share my thoughts on some of the influential scientific papers that are related to learning, a subject that I am very passionate about. My aim in these blog posts is to present the thought process, the idea and the technicalities of these papers in a comprehensive and simple way.

In this blog post, the main subject will be the foundational work of The Organization of Behavior by Donald Hebb.

Donald O. Hebb (1949) – The Organization of Behavior (1949)

Unlike the works I have examined in my previous blog posts, The Organization of Behaviour is a book rather than a paper. It is a crucial work with foundational ideas that challenges the Behavorist view of the time. Several concepts in Hebb’s work, synaptic plasticity, cell assemblies, phase sequences, the role of experience in perception, importance of early learning, and the idea that neural connections strengthen with experience are still relevant today.

Since the book covers a wide range of topics, I’ll start with bullet points to highlight its key ideas before diving into a more detailed discussion. This way, I hope to give a clear overview of Hebb’s main contributions before exploring their significance.

• Hebb attempts to develop a comprehensive theory of behaviour grounded in the physiology of the nervous system to bridge the gap between neurophysiology and psychology.
• He proposes a physiological theory of thought that can apply to both animals and humans.
• He introduces the concepts of “cell-assemblies” and “phase sequences” as fundamental units of neural activity and thought processes.
• He challenges the traditional stimulus-response psychology, arguing that behavious is not solely series of responses to environmental stimuli and emphasizes the importance of central processes.
• He explores the role of learning in perception (via cell-assemblies and phase sequences), and challenges the idea that perception is solely based on innate sensory organization. In other words, he argues that perception is not solely result of innate structures but is significantly influenced and refined by learning and experience which shifts the emphasis to the plasticity of the brain and its adaptability to the environment through mechanisms like cell-assemblies and phase sequences.
• “Neurons that fire together, wire together.” Hebb predicts NMDA receptors, key molecular mechanisms for synaptic plasticity and learning, many years in advance of its discovery.

To give a short information about the idea Hebb challenges: Innate sensory organization is the idea that our ability to perceive relies on pre-determined neural structures, the built-in architecture of the brain and nervous system that processes sensory inputs in fixed ways. This means ability to perceive does not depend on experience and learning, instead it relies on biologically programmed mechanisms. Hebb challenges this.

In The Organization of Behaviour, Hebb aims to explain complex psychological phenomena like thought, perception and learning through neural mechanisms. Behavorist views of the time treats brain as a black-box and instead focuses on observable stimuli and responses. Hebb emphasizes the importance of central brain processes, arguing that these internal activities play a crucial role in shaping behaviour.

Learning in Hebb’s Theory

A significant part of the book is dedicated to understanding learning, and he addresses the topic accross several chapters . One of his key ideas is that learning is not just about forming connections between stimuli and responses but about building neural structures, specifically cell-assemblies, that represent experiences. These cell assemblies are formed through repeated stimulation, leading to a slow development of diffuse structures in the cortex and diencephalon. Once formed, these assemblies can act briefly as closed systems and are the basis of more complex cognitive processes. These assemblies can also deliver facilitation to other such systems, and this central facilitation is a prototype for “attention”.

Think of a cell assembly as a group of people in a meeting. When they talk to each other often, their connections strengthen, making it easier for them to communicate in the future.

To exemplify, when you see a dog, different groups of neurons respond to the dog’s shape, color, smell, and movement. If you repeatedly see dogs, these neurons begin to fire together, strengthening their connections and forming a cell assembly, a network of neurons that collectively represent the concept of “dog.” Once formed, activating part of the assembly (e.g., hearing a bark) can trigger the whole network, allowing you to recognize a dog even if you only get partial sensory input like a bark. This mechanism provides a physiological basis for concepts, memory, and even imagination.

Hebb further elaborates on the concept of “phase sequences”. A phase sequence is a series of cell assembly actions, that is, a sequence of thoughts. These sequences are the neural basis of the thought process, and the links between these sequences, also established through learning, make up our understanding and anticipation of the world. Each assembly action may be aroused by a preceding assembly, by a sensory event, or normally by both.

Hebbian Learning

Hebb’s famous phrase “Neurons that fire together, wire together” is about the essence of synaptic plasticity, the ability of neurons to strengthen their connections through repeated activity. The behaviorist view of Hebb’s time saw learning as merely forming associations between stimuli and responses, Hebbian learning suggests that experience rewires the brain, physically altering its structure over time which is fascinating.

Short Thoughts: I deeply enjoy reading and analyzing scientific works that present ideas which feel intuitive upon reading. One such work is Damasio (1996), which I discussed in a previous post. Similarly, Hebb’s idea that experience strengthens neural connections and shapes perception perfectly exemplifies this.

Hebb attempts to bridge the gap between disparate fields like neurophysiology and psychology. As I aim to maintain my interdisciplinary approach throughout my academical journey, I think that this interdisciplinary approach is incredibly relevant today. Cognitive science increasingly seeks to integrate findings from neuroscience, psychology, and computer science, echoing Hebb’s goal of creating a “cooperative affair” between different fields.

I think Hebb’s challenge of traditional stimulus-response psychology is a crucial perspective for understanding learning, as it shifts the focus from simple associations to complex neural activity which opened up a path for the development of neural networks used in deep learning today. In addition, the principles of Hebbian learning aligns with unsupervised learning. I won’t go into detail of this but to explain shortly: The idea that “Neurons that fire together and wire together” is analogous to data points that cluster together in a graph, as both reflect patterns of similarity and association. This act of pattern recognition does not require external supervision in both Hebbian learning and unsupervised learning.

I was also intrigued with chapter 6 where Hebb emphasizes the significance of early learning. He argues that learning in infancy is quite different from learning at maturity: Early learning is inefficient, and slow. However, it lays the groundwork of later learning, influencing how we perceive the world, solve problems and so on. It’s also particularly related to brain plasticity. The reason learning in infancy involves forming entirely new networks from scratch, unlike mature learning which builds on already established neural pathways. For example, language acquisition is easiest in childhood because brain is still forming flexible connections.

On the philosophical aspect, Hebb’s work is a step towards the monistic view of the mind, arguing that mental activity is ultimately rooted in the physical processes of the brain which aligns with a materalistic approach to philosophy of mind where the mind is seen as a product of the brain rather than a seperate entity. I think his work can be interpreted as an effort to explain “mind” in terms of neural action, it pushes for a scientific understanding of consciousness.

Lastly, despite the fact that Hebb’s theory was groundbreaking, a lot of research has been done since 1949. Modern research has refined and, in some cases, challenged his ideas. While Hebb emphasized experience-driven plasticity, contemporary neuroscience acknowledges that innate neural structures play a crucial role in shaping perception and cognition. Studies in developmental neuroscience suggest that certain cognitive abilities are present from birth, forming a foundation for later learning rather than emerging solely through experience. Additionally, Hebb’s model of cell assemblies, though influential, does not fully account for the distributed nature of memory and cognition. Modern imaging techniques reveal that learning and memory arise from dynamic interactions across large-scale neural networks rather than being confined to localized circuits.

Moreover, Hebbian learning, while foundational, is now understood as just one mechanism among many that shape neural connections. Spike-timing-dependent plasticity (STDP) refines Hebb’s principle by incorporating precise timing between neuron activations, and homeostatic plasticity ensures stability in neural networks, preventing excessive strengthening of synapses. Finally, Hebb’s neuron-centric view overlooked the significant role of glial cells, which are now known to contribute to synaptic plasticity and memory formation. These developments do not diminish Hebb’s contributions but rather highlight the complexity of learning and the evolving nature of neuroscience.

📌 Sources & Further Reading

• Hebb, D. O. (1949). The Organization of Behavior: A Neuropsychological Theory. Wiley.

• Spelke, E. S., & Kinzler, K. D. (2007). Core knowledge. Developmental Science, 10(1), 89-96. Argues that infants are born with core cognitive systems that underlie perception, object recognition, and numerical understanding.

• Pessoa, L. (2014). The Cognitive-Emotional Brain: From Interactions to Integration. MIT Press. Argues that cognitive functions emerge from interactions among large-scale networks, not isolated brain regions.

• Bi, G. Q., & Poo, M. M. (1998). Synaptic modifications in cultured hippocampal neurons: Dependence on spike timing, synaptic strength, and postsynaptic cell type. Journal of Neuroscience, 18(24), 10464-10472. First experimental evidence for spike-timing-dependent plasticity (STDP), refining Hebb’s rule by showing that precise timing of spikes determines synaptic strength.

• Fields, R. D. (2005). Myelination: An overlooked mechanism of synaptic plasticity? Neuroscientist, 11(6), 528-531. Shows that glial cells (particularly oligodendrocytes) contribute to neural plasticity.