Nearly every morning for the past four weeks, I go to Le Pain Quotidien on Rue de Charonne at 9:05, sit at the long wooden table, order “deux oeufs BIO à la coque et un café crème avec lait de soja” (two organic soft-boiled eggs and a soy latte), and enjoy a nice breakfast before my first class begins at 10. After Dr. Shreckengost’s morning class, I take a leisurely walk from the Accent Center to Le Bar à Soupes, also on Rue de Charonne, wait outside the restaurant until the owner opens the door at 12:00pm, and order one of the six soups of the day “à emporter” (take away).
Le Pain Quotidien and La Bar à Soupes in relation to the Accent Center
Both the waitress at the Le Pain Quotidien and the owner of Le Bar à Soupes identify me as a regular customer. I no longer need to order when I go to breakfast because the waitress knows I order the same thing every morning. The soup bar owner even gave me a Soup Stamp Card to fill up and receive a free soup after 10 purchases! Many friends on my study abroad trip have quickly surmised from my daily routines that I am a “creature of habit,” so I began to wonder how we form habits.
My filled-out Soup Card!
Do I keep going back because of the comforting food and friendly faces or is there another underlying, physiological reason? What differentiates a habit from a choice?
First, let’s start with the definition of habit. From a scientific perspective, habits are “a form of goal-directed [automatic] behavior” (Aarts and Dijksterhuis, 2000). Goals can activate habitual actions. Take an example of biking to school every day: once someone develops this habit, the activation of the goal, going to class, may automatically elicit a habitual response, biking.
Now that we understand what habits are, let’s examine how habits may actually form in our brains!
In a study conducted by Wang et al. (2011), researchers investigated the mechanisms underlying learning and forming habits. Prior research revealed the importance of the neurotransmitter known as dopamine (DA) in habit learning. In both human and rodent models, an impaired ability to form habits correlated to degeneration of dopaminergic neurons. Wang et al. (2011) examined the importance of DA even further by looking at the impact of specific receptors on DA neurons. The researchers bred mice that lacked N-methyl-D-aspartate receptors (NMDARs) on their DA neurons (NR1 mice). NMDARs are glutamate (a type of neurotransmitter) receptors found in nerve cells that play a crucial role in synaptic plasticity—the ability of neuronal synapses to change shape or function in response to activity – and memory function. Wang et al. then hypothesized that the NR1 mice, lacking NMDARs on their DA neurons, will have impaired habit learning as compared to normal, wild type (WT), control mice.
First, researchers tested if WT mice differed from NR1 mice in various abilities other than habit learning, such as fine motor movements, balance, general endurance, anxiety levels, and object recognition. The WT and NR1 mice did not differ in their abilities, proving NR1 mice to be suitable models for testing differences in habit learning. To test the impact of the NMDAR deletion on DA neurons cellular processes, the researchers recorded both WT and NR1 mice DA neuron activity in response to the dopamine receptor agonist apomorphine through electrodes inserted into the ventral tegmental area, an area of the brain that serves as the origin of dopaminergic cell bodies. The researchers discovered reduced activity in the NR1 mice, indicating the NR1 mice DA neurons did not exhibit as much activity in the presence of dopamine or dopamine agonists as compared to WT controls. Additionally, the NR1, as compared to WT, DA neurons showed decreased responses during paradigms testing reward-predicting cues over three days, showing that the NR1 mice exhibited greatly lowered bursting responses, known as DA neuron blunting, and that NMDARs play a crucial role in dopaminergic cell activation.
Figure 8 – Wang et al.
The researchers designed these Water-Filled Zigzag Maze-Based Habit Tasks to more obtain more direct measurements of habit learning by eliminating the competition factor between “spatial” and “habit” memory systems the NR1 deletion may have skewed. You can see a significant difference habitual learning ability between NR1 mice (blue) and WT mice (green).
To investigate habit learning deficits in NR1 mice, the researchers tested both NR1 and WT mice in a lever-pressing operant-conditioning task. This task, pressing a lever to obtain food, can transform goal directed responses into habitual responses through extensive training. Over time, mice typically become desensitized to outcome devaluation, the decrease in reward value. Wang et al. (2011) discovered that the NR1 mice showed a significant difference in lever presses from Non-Devalued to Devalued outcomes, suggesting that the NR1 mice failed to develop the lever pressing habit, and their actions remained goal directed. The researchers continued to test habit learning through various maze-like paradigms and discovered that the NR1 mice showed habitual learning, but not spatial memory, impairments as compared to WT controls.
Wang et al. (2011) studied the importance of DA neuron NMDARs in habit learning and formation both on a behavioral and a cellular level, providing various methodological analyses for their findings. Their control experiments testing the differences in behavior for NR1 and WT mice provided clear evidence that the researchers could use the NR1 mice to only test behavioral differences in habit learning. Additionally, the researchers discovered an interesting phenomenon exhibited in some DA neurons – decreased and then rapidly increased responses to negative experiences – suggesting the importance of NMDARs extend beyond habit formation of reward stimuli and may apply to habitual learning through the reward of escaping negative stimuli (Wang et al., 2011).
The findings of this study not only provide the scientific community with better understanding of the crucial role NMDARs play in habit formation but also lay the groundwork for future researchers to better understand the underlying mechanisms of habit disorders, such as Obsessive-Compulsive Spectrum Disorders. Who knows, maybe even executives at Starbucks rely, in part, on the science behind how and why people form habits to boost their sales and marketing efforts.
Getting lunch at Le Bar à Soupes – yum!!!
For me, my taste buds thanks my DA neurons NMDARs for their role in my behavior to seek yummy foods at the same places every day. Based on the information in Wang et al.’s study and the Aarts and Dijksterhuis (2000) definition, I am not sure if my daily trips to Le Pain Quotidien and Le Bar à Soupes have fully transitioned to a “habit” as, without the delicious food and friendly faces, I would likely seek other rewards. While habits are the result of reflexive and automatic behavior (Dolan and Dayan, 2013), I still make the reflective decision to seek the reward my reliable breakfasts and lunches provide me. I find comfort in routine, especially amid a busy schedule in an unfamiliar country. While many people use the colloquial phrase “creature of habit,” I think of myself as more of a creature of conscious goal-directed behavior… who likes consistency in my rewards 🙂
References:
Aarts H, Dijksterhuis A (2000) Habits as knowledge structures: automaticity in goal-directed behavior. Journal of Personality and Social Psychology 78(1): 53-63
Wang LP, Li F, Wang D, Xie K, Wang D, Shen X, Tsien JZ (2011) NMDA Receptors in Dopaminergic Neurons Are Crucial for Habit Learning. Neuron 72(6): 1055-1066.
Dolan RJ and Dayan P (2013) Goals and Habits in the Brain. Neuron 80(2): 312-25.
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