The brain learns to recognize familiar objects at two very different speeds: the rapid adaptation that occurs within minutes of seeing something new, and the gradual consolidation that builds over days into durable recognition. A study published July 4 in Nature Communications shows these two timescales are driven by fundamentally different mechanisms in the primate visual cortex, settling a long-standing question in systems neuroscience.
The work, by Krithika Mohan, Ulisses Pereira-Obilinovic, and David Freedman at the University of Chicago, together with Nicolas Brunel at Duke University, used electrophysiological recordings from the inferotemporal (IT) cortex of two macaque monkeys performing a familiarity-learning task. The IT cortex is the brain region responsible for visual object recognition.
Two timescales, two mechanisms
The researchers tracked neural spiking activity in IT cortex both within single experimental sessions (minutes) and across multiple sessions (days). They found that familiarity learning at the two timescales produces distinct neural signatures, pointing to different underlying plasticity rules.
At the fast timescale, within a single session, neural responses to repeated stimuli showed rapid decay, accompanied by a rise in spontaneous activity. At the slow timescale, across sessions, averaged neural responses decreased gradually as stimuli became more familiar.
The team used a recurrent neural network model endowed with plasticity rules inferred from the neural data. The model successfully reproduced the observed dynamics only when both types of plasticity were included, synaptic plasticity for the slow, long-term changes and intrinsic plasticity (changes in the electrical excitability of the neuron itself) for the fast, short-term adaptation.
This dual-mechanism framework reconciles conflicting results in the prior literature. Some earlier studies had reported decreased neural firing as stimuli became familiar; others found increased activity. The new results show these apparently contradictory findings reflect different timescales operating simultaneously in the same cortical circuits.
“For many types of learning, we engage with new information over different timescales,” the researchers note. “This study shows that the primate brain uses parallel mechanisms, each appropriate to its timescale, rather than a single, slower process.”
The findings offer a new framework for understanding how immediate sensory experience transitions into durable memory, a fundamental question in cognitive neuroscience.