Repeating the protocol, but intermixing unpaired stimulations in which only the postsynaptic neuron was stimulated cancelled the potentiation despite having applied the exact same 10 paired trials ( figure 2 b). Applying 10 of these paired stimulations, they found strong potentiation of the synapse ( figure 2 a). used a standard STDP protocol, with the presynaptic neuron stimulated 5–10 ms prior to the postsynaptic neuron. Other experiments have refined our understanding of the consistency required for synaptic plasticity to take place. Temporal asymmetry of spike-timing-dependent plasticity. As Hebb had predicted, causation is thus the key to synaptic plasticity.įigure 1. If the two neurons simply fire together, the inevitable temporal jitter would make the presynaptic neuron sometimes fire just before and sometimes just after the postsynaptic neuron, and potentiation and depression would annul each other over time, leading to no substantial net STDP. By contrast, if the presynaptic neuron is stimulated just after the postsynaptic neuron the synapse is depressed. When an excitatory synapse connects onto an excitatory neuron, if the presynaptic neuron is stimulated 40 ms or less prior to the postsynaptic neuron, the synapse is potentiated. Experiments in which two connected neurons were stimulated with various stimulus onset asynchronies evidenced an asymmetric window of STDP ( figure 1). In the 1990s, neurophysiologists laid the foundation for our modern, neurophysiological understanding of Hebbian learning based on STDP. This paraphrase should thus be read with a pinch of salt. Temporal precedence, rather than simultaneity, is the signature of causality and would indicate that ‘one took part in firing the other’. at the same time, the firing of one cannot cause that of the other. While mnemonic, this summary bares the risk of obscuring the importance of causation in Hebb's actual work: if two neurons literally fire together, i.e. Carla Shatz (but not Hebb himself) has paraphrased his principle in a rhyme: ‘what fires together, wires together’. He writes not that two neurons need to fire together to increase the efficiency of their connection but that one neuron needs to repeatedly (consistency) take part in firing (causality) the other. A careful reading of Hebb's principle reveals his understanding of the importance of causality and consistency. The term Hebbian learning derives from the work of Donald Hebb, who proposed a neurophysiological account of learning and memory based on a simple principle: ‘When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased’ (p. Finally, we argue that vicarious activations also occur in somatosensory and emotional cortices and that the same Hebbian learning rules could explain the emergence of mirror-like neurons in these brain regions. We explore how this refined understanding of Hebbian learning helps us understand how mirror neurons emerge and suggests how mirror neurons become a form of active predictive mind reading. We define what modern neuroscience understands by Hebbian learning based on spike-timing-dependent plasticity (STDP). Here, we explore a mechanistic perspective of how such mirror neurons could emerge during development. Such selectivity is unlikely to be genetically preprogrammed. Monkeys have mirror neurons that respond to the sound and vision of crumpling a plastic bag and human premotor cortices respond to sounds like the hiss of opening a Coca-Cola can. Here, we do not address the question of their function, but rather explore how they could develop. Twenty years after their discovery, the function of mirror neurons is still heatedly debated. The discovery of mirror neurons provides neuroscientific evidence for what we call vicarious activations: the neural substrates of our own actions are vicariously activated while witnessing the actions of others through vision or sound.
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