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The Brain : Learning and Memory
CHAPTER 1. MEMORIES ARE MADE OF THIS HOW MEMORIES ARE FORMED THE BRAIN CONSTANTLY SIMMERS WITH ACTIVITY AS DIFFERENT GROUPS OF NEURONS, DRIVING OUR DIFFERENT THOUGHTS OR PERCEPTIONS, DRIFT IN AND OUT OF ACTION LIKE FIREFLIES IN A FOREST. A memory is the reactivation of a specific group of neurons. But what allows a specific combination of neurons to be reactivated over any other combination of neurons? The answer is synaptic plasticity. This term describes the persistent changes in the strength of connections – called synapses – between brain cells. These connections can be made stronger or weaker depending on when and how often they have been activated in the past. Active connections tend to get stronger, whereas those that aren’t used get weaker and can eventually disappear entirely. This gives added meaning to the term use it or lose it! Not many features of the brain are more important than synaptic plasticity. Changing the strength of existing synapses, or even adding new ones or removing old ones, is critical to memory formation. But there is also evidence that another type of plasticity, not directly involving synapses, could be important for memory formation. In some parts of the adult brain, such as the important memory structure known as the hippocampus, brand new neurons can be created in a process called neurogenesis. Studies in older mice have shown that by increasing neurogenesis in the hippocampus, memory can be improved. In humans, exercise has been shown to increase the volume of the hippocampus – suggesting new neurons are being created – and at the same time improve performance in memory tasks. Memory Pathways A GOOD ANALOGY for memory formation is the way foot traffic creates a path along a stretch of grass. The more a patch of grass is trampled as people pass along it, the clearer the path becomes and the easier it is to follow – it’s as if a ‘memory’ of all the walking has been created. The same thing happens in the brain. The more a neural pathway is activated, the stronger the synaptic connections along the way become. Then, when a thought enters our head – say, a tropical beach – we recall related experiences or knowledge, such as putting on sunscreen and the feel of sand, as our minds funnel our thoughts along well- established neural pathways.