Representational Drift in Neurons of the Visual Cortex

Article: Neurons in visual cortex of the brain ‘drift’ over time
Source: Washington University in St. Louis
Published: August 27, 2021

Single-neuron episodic activity with distinct
episode-specific rate variations across weeks

It was previously thought that neurons of the visual cortex reliably encode information from sensory stimuli. However, experiments in mice found evidence of "representational drift" of neuronal activity, known to occur in other areas of the brain, to also happen in the visual cortex (V1). Furthermore, while flexibility in neuronal activity is expected in response to changes in learning or experience, what was surprising was that neurons in the visual cortex exhibited drift even when presented with the same stimulus across time. The experiments involved showing mice a short 30-second movie clip on a loop, while the activity of hundreds of neurons were recorded in their primary visual cortex using two-photon calcium imaging. These viewing sessions were repeated weekly for up to seven weeks. Analysis of the neuronal population activity showed that individual neurons did not respond the same way to the visual stimulus, i.e., the exact scene at specific moments in the film, when the mouse watched the film one week as compared with another week. Specifically, the single-neuron activities consisted of individual spiking episodes, and it is these spiking episodes that showed distinct variations across weeks. In other words, the long-term stability of single-neuron responses varied across the weeks. As the first author of the study argues, “What is somewhat unexpected is that even when there is no learning, or no experience changes, neural activity still changes across days in different brain areas.” Although an implicit variable that is difficult to dissociate is the learning and experience changes that occur across the weeks beyond the time periods when activity is recorded in the lab, underlying the concept of representational drift is the thought that representations, or identities, of stimuli are expected to remain stable across the life of the organism. Discovering that neuronal encoding of stimuli varies across time could point to compensatory mechanisms in downstream brain areas to maintain stable representations.

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Xia J, Marks TD, Goard MJ, et al. "Stable representation of a naturalistic movie emerges from episodic activity with gain variability." Nature Communications.  12:5170. 27 August 2021. https://doi.org/10.1038/s41467-021-25437-2