Robust habit learning in the absence of awareness and independent of the medial temporal lobe

Abstract

Habit memory is acquired subconsciously and slowly, by trial-and-error. It is more easily studied in animals than in humans, because of our strong tendency to acquire information as conscious (declarative) knowledge. Yet our capacity for unconscious learning is a vital aspect of the human condition, facilitating many routine tasks. Now it can be confirmed that humans do have a robust capacity for habit learning. Two patients with large medial temporal lobe lesions and profound amnesia were asked to acquire a task that is ordinarily learned by conscious memory. They learned gradually, in the way that monkeys learn the same task, and without being aware of what was being learned. The knowledge was rigidly organized, and performance collapsed when the task format was altered. Habit memory is thought to involve slowly acquired associations between stimuli and responses and to depend on the basal ganglia1. Habit memory has been well studied in experimental animals but is poorly understood in humans because of their strong tendency to acquire information as conscious (declarative) knowledge. Here we show that humans have a robust capacity for gradual trial-and-error learning that operates outside awareness for what is learned and independently of the medial temporal lobe. We tested two patients with large medial temporal lobe lesions and no capacity for declarative memory. Both patients gradually acquired a standard eight-pair object discrimination task over many weeks but at the start of each session could not describe the task, the instructions or the objects. The acquired knowledge was rigidly organized, and performance collapsed when the task format was altered.