The Neurobiology of Curiosity

Have you ever heard the saying, “curiosity killed the cat”? While this may be true, curiosity also protects the cat. Curiosity is a highly adaptive characteristic of many animals, from cats to rodents to humans. While rushing into unfamiliar situations without caution can prove to be dangerous, or even fatal, carefully and systematically exploring new environments allows us to gather information to adapt, survive, and thrive. In this way, curiosity is as vital to animal survival as hunger or reproduction.

“Curiosity” as a psychological construct was first coined in 1899 by William James as the “desire to understand what you do not know”. This term was used by the psychologist to describe his observation that children are driven towards novel objects (James, 1899). Igor Pavlov noted similar tendencies by his dogs (yes, those dogs) in 1927 when he noticed them snapping their attention towards passing squirrels (Pavlov, 1929, 2010). A century after these early reports of curiosity, we are still trying to understand how the brain drives our tendency to explore new things.

To understand the neural activity that drives curiosity, scientists study the way animals interact with novel objects – similarly to how Pavlov would observe his dogs’ reactions to squirrels. Exploration of new objects or new environments can be evolutionarily adaptive for animals in the wild that are often faced the exploit vs. explore dilemma. This can be described as animals choosing to exploit a resource they are familiar with to receive a quick reward, like food or shelter, or explore something they are unfamiliar within the off chance it is even more beneficial to them (Addicott et al, 2017). This could become a life or death situation when the resources they previously exploited unexpectedly disappear or become depleted. In the lab scientists have found that when a food reward is unpredictable, rats are more likely to use sensory dominated strategies than memory dominated strategies when foraging (Jackson et al, 2020). This means that since they are unsure of the likelihood they will receive a food pellet, rats are using their senses to further explore their environment to search for new sources of food, rather than just remembering where they had previously gotten it. In addition to this behavioral study, biological insights have recently identified a brain region that may be responsible for curiosity-driven investigation of novel objects. Mehran Ahmadlou and colleagues at University College London cleverly repurposed the novel object recognition task in rodents, traditionally used to investigate memory, to uncover that curiosity is coordinated by the brain region called the zona incerta (ZI) (Ahmadlou et al, 2021).

The ZI is a brain region that is largely composed of GABAergic neurons – inhibitory neurons that oppose or damp down activity in the brain to help regulate levels of neuronal activity. In addition to this regulatory role, the ZI has been shown to facilitate a myriad of other processes, including sleep, attention, fear learning, and sensory processing (Power and Mitrofanis, 1999, Wang et al., 2020). However, its role in motivational behavior, including curiosity, is controversial. One reason for this is the use of flawed experimental models: while past studies on curiosity have often relied on tasks where mice are trained to expect a reward, curiosity drives exploration even when there is no expectation for reward (Kidd and Hayden, 2015).

To investigate novelty seeking behavior independent of traditional reward-seeking behavior, Ahmadlou et al. designed a simple task in which the mouse is allowed to freely interact with both a familiar object and a novel object. This free access model revealed two distinct exploratory behaviors, which Ahmadlou’s team termed shallow and deep investigation. Shallow investigation describes when mice approach and sniff an object only fleetingly, while deep investigation is shallow investigation followed by biting (a more committed exploration). The scientists found that deep investigation occurs more frequently with novel objects, a result that supports the hypothesis that exploratory behavior is indeed associated with exposure to novel experiences.

To identify the brain region associated with curiosity, the team of scientists used optogenetics: an exciting technology that harnesses light to selectively activate or deactivate neuronal activity. The team found that optogenetic activation of neurons in the ZI led to increased deep investigation of novel objects by mice. Conversely, inhibiting activity in these ZI neurons reduced the frequency of deep investigation. Measurements of neuronal activity during exploration revealed a modest increase in ZI neurons during shallow investigations, and an even larger one during deep investigations. Together, these experiments suggest that the ZI region of the brain guides exploratory behavior in the mouse, an exciting step forward in understanding the neurobiology of curiosity.

Identifying the ZI as a brain region vital for exploration of novel objects is a step forward in understanding curiosity, but there are many unanswered questions that are open for investigation. One ongoing question in this field is, how do our brains recognize something as new? This is a relevant question in the psychiatric field because several neuropsychiatric conditions (ex. anxiety, phobias) are characterized by aversion to the unfamiliar and may arise from dysregulation of brain activity that recognizes novelty. For example, someone with social anxiety may choose to not go to a party with a lot of strangers if they tend to experience heightened fear and cautiousness when entering new situations. While this avoidance may offer a sense of short-term security, it may ultimately be maladaptive by causing people to miss out on potentially rewarding or beneficial experiences. Further insights into which neurons drive curiosity and shape our interpretation of the world may help uncover novel targets to advance pharmacological interventions for conditions like anxiety.

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Written by Caitlin Goodpaster.

Illustrated by Himani Arora.

Edited by Sean Noah and Abinaya Muthusamy.

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References

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Ahmadlou, M., Houba, J.H.W., van Vierbergen, J.F.M., Giannouli, M., Gimenez, G., van Weeghel, C., Darbanfouladi, M., Shirazi, M.Y., Dziubek, J., Kacem, M., de Winter, F., Heimel, J.A. (2021). A cell type-specific cortico-subcortical brain circuit for investigatory and novelty-seeking behavior. Science 372(6543): eabe9681. DOI: 10.1126/science.abe9681

Jackson, B. J., Fatima, G. L., Oh, S., & Gire, D. H. (2020). Many Paths to the Same Goal: Balancing Exploration and Exploitation during Probabilistic Route Planning. eNeuro, 7(3), ENEURO.0536-19.2020. DOI: 10.1523/ENEURO.0536-19.2020

James, W. (1899). Talks to teachers on psychology–and to students on some of life’s ideals. Metropolitan Books/Henry Holt and Company. DOI: 10.1037/10814-000

Kidd, C., & Hayden, B. Y. (2015). The Psychology and Neuroscience of Curiosity. Neuron, 88(3), 449–460. DOI: 10.1016/j.neuron.2015.09.010

Pavlov, I.P. (1927). Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex. Translated and Edited by G. V. Anrep. Oxford University Press, London, 142.

Pavlov I.P. (2010). Conditioned reflexes: An investigation of the physiological activity of the cerebral cortex. Annals of neurosciences, 17(3), 136–141. DOI: 10.5214/ans.0972-7531.1017309

Wang, X., Chou, X., Zhang, L.I., Tao, H.W. (2020). Zona Incerta: An Integrative Node for Global Behavioral Modulation. Trends in Neurosciences, 43(2): 82-87. DOI: 10.1016/j.tins.2019.11.007