doi:10.1038/nindia.2013.30 Published online 27 February 2013
Our everyday life is often a series of habits played out under slightly different circumstances. Habits are what make or break us. It is with this realization that we try to cultivate certain habits over others and yet we understand very little of how they are formed, evaluated or changed.
Habit formation is a field of study at the interface of cognitive psychology, neuroscience and neuropsychology. From being unconscious to automatic to implicit, the very definition of habits have changed and evolved with time. Scientists now define habits as sequential, repeated behaviours that result from a specific trigger or stimulus and once released, get completed without conscious awareness — something like reflexively reaching for the switch even when there is no power.
Evolutionarily, habits free up 'mental bandwidth' and allow us to switch from an exploratory, outcome-dependent behaviour to a more focused, exploitative behaviour. Imagine driving your car every day, the way you drove on the first day with an acute awareness of everything around you — it sure will be a big mental drain to do everything the way you did it first!
The process of habit forming begins as we start exploring our environment, learn responses from it and form action plans. This leads to the 'exploitation' phase where out behaviour becomes more automated and less flexible, almost reflexive. Thus, it all begins with neural flexibility in the learning-promoting and habit-promoting sites. Then a 'habit' is formed when the balance tips between the two competing pathways — that of conscious learning and unconscious execution.
The basal ganglia (a cluster of neurons located below the cortex) was first speculated to be involved in habit formation by Mishkin and colleagues1 due to its early evolutionary and developmental origin. Its extensive anatomical connections with the cortex also provided a mechanism to associate the sensory inputs to the motor outputs. Classical studies2,3,4,5 in patients and animals with specific brain lesions and chemical inactivation also identified the basal ganglia as the brain region critical for the storage and recall of implicit memories or habits.
Since the formation of a habit involves a tip in the balance between competing neural circuits for flexibility and rigidity, previous studies have tried to localize brain activity during the learning phase and the automated phase. These studies identified different segments of the striatum (a part of the basal ganglia) to be involved in the two stages. However, the shift in strategy from goal-directed learning to the 'habitual' was still unexplained.
In any given situation, the brain is forced to choose between multiple action possibilities. So it seemed that a 'higher' level decision centre must exist to arbitrate among these available options. A possible candidate for this function was the infralimbic cortex (ILC) in mice or the medial prefrontal cortex in humans. The ablation or pharmacological manipulation of ILC was shown to reverse a habitual behaviour into an outcome-sensitive state6,7 . While, these studies suggest a role for the ILC in promoting habit formation, anatomically, the ILC is connected to flexibility promoting regions; and this dual connectivity further implicated the ILC as the 'higher order' decision centre.
However, little could be done from a neuroscience perspective to modulate these brain areas and to show a cause-effect relationship because the necessary scientific tools to engineer, manipulate and record from brain areas in awake rodents were lacking.
Recent developments in optogenetics (genetically encoded light-sensitive ion channels, allowing activation or inhibition of neurons with a flash of light), optical technology and stereotaxic surgeries have enabled a long-term study and manipulation of brain regions in rodents. Scientists have now used these technologies to uncover some fascinating truths about habit formation and recall8.
Here, rats were trained to run down the long-arm of a T-maze and then turn left or right depending on audio instructions to receive a reward for the correct turns. The rats were over-trained to the point that they turned to the instructed side even when they were punished with a nauseous chemical instead of the usual reward — in other words, they were habituated. The scientists then used light to inhibit target neurons in the ILC of these overtrained rats during the maze runs and found that blocking the ILC blocked the acquired habit of turning to the instructed side and allowed the rats to run in a more purposeful, outcome dependent way — thus breaking the 'bad habit'.
However, when this exercise was repeated over and over with a blocked ILC, the rats acquired a new habit of running to the other (non-devalued) side. At this point, the scientists continued inhibiting the ILC and found something remarkable after around two weeks of perturbation of the newly acquired habit (of turning to the non-devalued side) — the rats suddenly switched their behavior and returned to their old habit of running to the instructed side (with the noxious chemical). Inhibiting the ILC, while performing the 'newer' habit thus unmasked the old habit, which was not erased but just maintained latently.
These results clearly show that contrary to our understanding, habitual behaviours are under constant 'online' supervision despite their seeming automaticity. Further, from the perspective of evolutionary adaptation, in a competition between old habits and new, the newer habits (which might carry adaptive value) are favoured. Thus, the ILC seems to perform this vital function of maintaining the newer habit on top of an old one.
This suggests that our habits are stored elsewhere in the brain and the ILC only functions as an executive controller in selecting for the newest applicable habit.
This result lends some credence to the old saying — "old habits die hard" — because as we now see, they do not really die, they just sink to the bottom only to resurface at the earliest given opportunity. Where and how these habits are learned and stored, when suppressed and how accessed by the ILC are fascinating questions that need more elaborate experiments.