Tripartite Neuron Circuit Governs Representation Consistency in the Mouse Visual Cortex

Article: Behind the scenes, brain circuit ensures vision remains reliable
Source: Picower Institute at MIT
Published: September 8, 2021

PV (red) and SST (green) inhibitory neurons combine
to regulate the reliability of image representation by
excitatory neurons in the mouse visual cortex

Visual processing in the brain is accompanied by a lot of noise as different groups of neurons in the visual cortex (V1) are stimulated by the same scene. Neuroscientists at MIT explored how the brain compensates for this neuronal noise to produce fidelity in visual representation. The researchers used optogenetics and two-photon microscopy to observe the brain activity of mice watching movies. They discovered that in addition to groups of excitatory pyramidal neurons that respond when images appear, two groups of inhibitory neurons work in a circuit to enforce reliability behind the scenes. In particular, low representation reliability corresponded with high activity among parvalbumin-expressing (PV) inhibitory neurons and low activity among somatostatin-expressing (SST) neurons. High representation reliability corresponded with low PV activity and high SST activity. When excitatory activity became unreliable, SST activity followed PV activity.

The PV neurons inhibit excitatory activity to control their gain, that is, to prevent them from becoming saturated amid a flood of incoming images. However, gain suppression comes at the cost of making the representation of images less reliable. Meanwhile, the SST neurons can inhibit the activity of PV neurons, the SST neurons kicking in when excitatory activity has become unreliable. This tripartite neuron circuit was then modeled with computer software. Once they learned how the circuit worked, the researchers could control the inhibitory cells to influence how consistently the excitatory neurons represented images. For example, increasing PV activity made any existing reliability less reliable, and increasing SST activity made unreliable activity more reliable. Importantly, because SST inhibitory neurons work by inhibiting PV neurons—in other words, SST neurons increase reliability by inhibiting inhibition—they would not be able to enforce reliability without PV neurons. It is an inherent tripartite circuit, due in part to differences in how these two cells form synapses. (SST neurons form inhibitory synapses at neuron dendrites while PV neurons form inhibitory synapses at neuron somas.) The authors remark, "[I]t is the co-operative dynamics between SST and PV [neurons] which is important for controlling the temporal fidelity of sensory processing." They add that the circuit also interacts and receives input from other higher order brain areas, for example, to concentrate on a particular scene with volitional attention.

My rating of this study: ⭐⭐⭐

Rikhye RV, Yildirim M, Hu M, et al. "Reliable Sensory Processing in Mouse Visual Cortex through Cooperative Interactions between Somatostatin and Parvalbumin Interneurons." Journal of Neuroscience.  41(42):8761-8778. 20 October 2021. https://doi.org/10.1523/JNEUROSCI.3176-20.2021