Verified answer

HM's ability to form long-term procedural memories was intact

thus he could

for example

learn new motor skills

despite not being able to remember learning them

in other words

he had implicit memory

but severe anterograde amnesia, i.e. not explicit memory

he could remember the motor skills involved in the task

without remembering having done it before

so implicit memory is part of procedural memory

Procedural memory is a type of long-term memory and, more specifically, a type of implicit memory. Procedural memory is created through "procedural learning" or, repeating a complex activity over and over again until all of the relevant neural systems work together to automatically produce the activity. Implicit procedural learning is essential to the development of any motor skill or cognitive activity.

The dorsolateral striatum is associated with the acquisition of habits and is the main neuronal cell nucleus linked to procedural memory. Connecting excitatory afferent nerve fibers help in the regulation of activity in the basal ganglia circuit. Essentially, two parallel information processing pathways diverge from the striatum, both acting in opposition to each other in the control of movement, they allow for association with other needed functional structures. One pathway is direct while the other is indirect and all pathways work together to allow for a functional neural feedback loop. Many looping circuits connect back at the striatum from other areas of the brain; including those from the emotion-center linked limbic cortex, the reward-center linked ventral striatum and other important motor regions related to movement. The main looping circuit involved in the motor skill part of procedural memory is usually called the cortex-basal ganglia-thalamus-cortex loop.

B