Sorting Through Stimuli

November 14, 2018

Stanford biologists found that neurons in the Paraventricular Thalamus (PVT) are activated by a variety of behaviors, including reward, aversiveness, novelty, and surprise. The paraventricular thalamus is a relay station connecting the brainstem and hypothalamic signals.

The thalamus is composed of several anatomically and functionally distinct subnuclei. Among them, the PVT is particularly situated for integrating information applicable to behavioral relevance. The PVT is not directly connected with the sensory cortex but is reciprocally connected with regions involved in top-down control, such as the prefrontal and insular cortex. The PVT also receives extensive inputs from the hypothalamus and brainstem, which convey signals about motivational arousal and homeostatic states.

Location of the thalamus Photo Credit: Database Center for Life Science

In order to observe the activation of neurons in the PVT, the experimenters infected the PVT neurons with adeno-associated virus (AAV) which expressed a genetically encoded Ca2+ indicator. They then used fiber photometry to record population Ca2+ signals in the PVT of the head-fixed mice across days of associative learning. To create learning conditions, the mice were water not given water but exposed to the odors one signalled an air puff and the other indicated water. Initially, both stimuli robustly activated the PVT, but whereas PVT responses to free water remained consistent, odor-evoked responses were rapidly diminished.

During the air-puff phase, two-thirds of PVT neurons responded to both odors while an additional 30 percent were activated only by the odor signaling water. In other words, during this phase the PVT responded to both good and bad outcomes, but there was greater response to good. During the electric-shock phase, however, the balance shifted. Almost all PVT neurons responded to the shock, while about three quarters of them responded to both good and bad outcomes. A similar shift happened when mice had had their fill of water. Now that water mattered less to the mice, the PVT was less responsive to water and more responsive to air puffs, meaning it became more responsive to bad outcomes and less so to good ones. Taken together, the results showed the PVT tracks what was most important in the moment -- the good outcome when that outweighed the bad, and vice versa.

This study showed the importance of the PVT and how it can sort through different types of stimulus to decide what is most important for an animal. This can point to more studies occuring on the PVT to help with many behavior based disorders like addiction. By stimulating or suppressing the PVT, animals can relearn the connections between a stimulus and the reward that is given. If the PVT is suppressed when a drug addict sees their choice of drug, they may not have the craving for the high or reward because that will not be seen as important anymore. There will be many more studies on the PVT because of its impact on learning and behavior.

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