Procedural memory · Grey Matter

Procedural memory is the memory of how to do things (ride a bicycle, type, tie a knot, play a scale) that shows itself in performance and cannot be fully put into words.

Procedural memory. Procedural memory is the memory of how to do things (ride a bicycle, type, tie a knot, play a scale) that shows itself in performance and cannot be fully put into words.

It is non-declarative: it improves with practice whether or not the learner remembers the practice. Its circuits are mostly outside the medial temporal lobe. The basal ganglia learn which actions to chain and which to repeat, from feedback and reward, and turn them into habits; the cerebellum calibrates the timing and accuracy of each movement against its predicted outcome; the motor cortex and premotor areas store the refined patterns of the most practised movements.

It survives the loss of the hippocampus. The patient H.M. improved day after day at tracing a star seen only in a mirror, while remembering none of the earlier sessions.

Its failures follow its circuits. Parkinson's disease, which strikes the basal ganglia, and cerebellar damage both impair learning new skills, in different ways (sequences and habits in one, calibration and timing in the other).

Practice moves a skill from effortful to automatic. Early learning engages the prefrontal cortex and attention; as the skill becomes procedural, prefrontal involvement falls and the skill runs in parallel with other thought.

Questions: What part does the cerebellum play in learning a new skill? The cerebellum learns the fine calibration of a movement: the force, timing and trajectory that make it land where intended. Each error is reported to its Purkinje cells by climbing fibres from the inferior olive, and those signals adjust the synapses that produced the error, so the internal model of the arm, the tool or the instrument improves with practice. The basal ganglia learn which actions to chain and repeat, and the cerebellum how to execute each one smoothly, which is why cerebellar damage makes adapting to new conditions (prism glasses, a heavier racket) particularly hard. How does a deliberate action become an automatic habit? At first an action is goal-directed: it depends on the prefrontal cortex and the caudate (the associative striatum), and it changes as soon as its outcome stops being wanted. With repetition, and with dopamine marking each success, control shifts toward the putamen (the sensorimotor striatum), which chunks the sequence into a single unit triggered by its context. The habit then runs with little attention and keeps running even when its reward has lost value, which makes habits efficient and also hard to break. How can someone with amnesia get better at a skill without remembering practising it? Skills are stored by circuits that do not need the hippocampus: the basal ganglia, the cerebellum and the motor cortex adjust with every repetition, whether or not the repetition is remembered. In the early 1960s the patient H.M. traced a star seen only in a mirror over several days and improved steadily, while each day he said he had never done the task before. The dissociation showed that memory is several systems, one that knows that something happened and others that change how something is done.