The same brain cell may produce a different number of action potentials, or spikes, in response to the same stimulus depending on what came before. This “adaptation” allows the brain to easily detect rare stimuli, or differentiate between two common stimuli – essential skills for navigating our complex world.
Adaptation is usually put down to brain plasticity — that is, changes in the structure of brain cells or their synapses, the connections that carry messages between the cells. For example, more neurotransmitters may be released at the synapse in response to the same stimulus. Brain plasticity is crucial for processes such as learning and memory.
But it turns out that adaptation is also the natural consequence of the brain’s make-up as a network of billions of cells connected every which way — sequentially, as well as backwards and laterally (see below). That is the conclusion of this work by CIBF’s Maria del Mar Quiroga of Monash University in Melbourne, who conducted the research while at Rutgers University in New Jersey, USA.
Quiroga and her colleagues used a mathematical model of cells in the visual cortex (a brain part that processes information from the eyes) connected laterally and sequentially into networks similar to those in the actual brain. The properties of the model synapses and brain cells were unable to change.
When they fed the model two visual patterns that were similar, but with different orientations (see image on top right) in quick succession, the model’s response to the second pattern was influenced by its response to the first. In other words, the mathematical model had adapted to the input without the luxury of being able to change a physical structure such as a synapse.
The adaptation would equate to an animal perceiving the second image tilted more towards the direction of the first than it actually is.
“It turns out you can explain many adaptation effects with this much simpler model,” says Quiroga. “No physical changes are needed. The connections alone create reverberations that carry information after the stimulus is turned off, and that’s enough for adaptation.”
The model shows that the brain’s unique make-up yields even more flexibility in its response than thought.
The mathematical model is being tested in the real world using psychophysics tests, such as showing people images in quick succession and asking what they see. The role of brain plasticity is minimised by presenting each visual stimulus for a very short amount of time, and other means.
Quiroga, M., Morris, A.P., & Krekelberg, B. (2016) Adaptation without Plasticity. Cell reports, 17(1), 58-68
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