Nondeclarative memory (or implicit memory) is one of the two categories of long term memory. Declarative memory or explicit memory are those long-term memory (LTM) related to autobiographical facts and events or experience (episodic memory) and contextual knowledge (semantic memory) (see.post “types of memory” for more detail about declarative memory). Nondeclarative memory distinguish from explicit memory in three ways:

1. nondeclarative or implicit memory operates outside of awareness i.e. it does not require consciousness and focused attention as being emphasized by declarative memory;
2. the retrieval and influence are implicit, usually only observable by changes in behavior or habits;
3. functionally independent of declarative memory and the medial temporal lobe is not involved.

The most easily understood and perhaps most widely studied nondeclarative implicit memory is procedural memory – the type of memory give us the skill for “procedural (motor) tasks” such as how to play music instruments or how to do surfing or diving. Procedural memories are affected by the part of the brain known as striatum (the subcortical part of the forebrain, the major input station of the basal ganglia system) and cerebellum. There are several other nondeclarative memories with unique characteristics and depends on different and specific brain circuits.

Priming defines a category of implicit memory that is associated with the same or related stimuli we encountered before. Basically, priming is the ability to constitute an unconscious response to the information with increased speed and accuracy to which we have been encountered before. There are a few forms of priming. Two main categories are perceptual priming and conceptual priming. Perceptual priming  refer to the priming for different kinds of sensory perceptions (visual, auditory and etc.). They are mediated by the corresponding cortex of the brain for each specific sensory type. Conceptual priming facilitates processing of the meaning of a stimulus or enhances the access to a concept.  One example of priming is:  if a person was shown study list of words including the word “table”, and then is asked to complete a word starting with “tab”, the probability that he or she will answer “table” is greater than if they are not shown (primed) the study list. In the research field, the “perceptual identification task” is used to identify and evaluate the perceptual priming. The participant was exposed to a series of words on the computer screen for a flash of 34 milliseconds, then the participant is asked to identify each flashed words. Then the result is compared to that of the participants or same individual who was first shown a list of words containing the flashed words before the “perceptual identification task”. The probability or accuracy of identifying the flashed words increases if the participants was shown the word list before, even though they themselves report that they were simply guessing the flashed words. This suggests that the participant are not aware of the influence by the study list. Similar phenomenon  can be observed for semantic representations. Conceptual priming (facilitated processing of the meaning of a word) can be demonstrated by “category exemplar generation task” which will not be detailed here.

Habit memory is a type of nondeclarative implicit memory belong to the category of non-associative learning. With habit memory we have the capacity to learn and retain new information unconsciously, retaining so-called habit memory even when conscious or declarative memory is absent. This type of memory require reflex pathways as well as basal ganglia for function which give us the ability to respond to habitualized stimuli formed through habitual repetition and slow accumulation of knowledge between a stimuli and a response in the past. The acquisition and retrieval of habit memory can be identified and evaluated using “probabilistic classification task” – an experimental test procedure designed to measure the habit memory.  Combined with the techniques of neuroimaging which can measure what part of the brain in activated during “probabilistic classification task”, researchers has observed the increased basal ganglia activation and decreased medial temporal lobe activities, a result consistent with the conclusion that implicit memories are functionally separated and independent of explicit memory which is an active “learning process” instead of “passive learning”. For this reason, habit memory may be a trainable technique for helping people whose conscious explicit learning or declarative memories are impaired. The process by which new behaviors become automatic is habit formation. Real life examples of habit memory include those classified as nervous habits such as nail-biting, sniffling.

In contrast to non-associative learning (habit memory), associative learning (conditioned association) form another category of nondeclarative implicit memory. A brief definition for associative learning is the process of acquisition, retention and expression of a response or behavior through association between two stimuli (for classical conditioning) or between a behavior and a stimulus which is the consequence of such behavior (for  operant conditioning). The two forms of associative learning are classical conditioning  and operant conditioning. Classical conditioning (or respondent conditioning) is the learning of an involuntary reflexive response to a neutral stimuli through association of this neutral stimuli or so called conditioned stimuli with its natural stimuli (also called unconditioned stimuli). Classical conditioning is the most basic and simplest form of associative learning first described by behavioral psychologist Pavlov. A simple example of classical conditioning is the eyeblink response. Eyeblink response to air puff (the unconditioned stimuli) occurs naturally and automatically, When the participant was presented each time before air puff with a tone sound (the conditioned stimuli), then human has the ability to acquire or learn to respond (the “learned blink” or “conditioned response”) to the tone sound in the absence of the unconditioned stimuli (the air puff) eventually.

Operant conditioning (or instrumental conditioning) is distinguished from classical conditioning  in that operant conditioning deals with the modification (increase or decrease) of “voluntary behavior” or operant behavior. The terminology was coined by behavioral psychologist B.F. Skinner. In the case of operant conditioning, the association of the learning occurs between the behavior itself and the consequence of such behavior. When the consequence of the behavior is associated with behavior, such behavior or the probability or frequency of the behavior is either increased or decreased. The increase of the behavior as a result of operant conditioning is called “reinforcement” while the decrease of the behavior is called “punishment”. Examples in experimental and real-world setting help to clarify the concept. One simple example of operant conditioning is in the classroom setting: each time a student raise hand to ask a question, he or she receives a praise from the teacher. Such association of “raise hand and ask question” with the consequence of “receiving praise” will reinforce or increase the frequency of student’s behavior of “raise hand and ask a question”. Under certain situations, in order to reduce or remove an undesired behavior, association can be made between the behavior and the “punishment”. For example, if the student will be told and punished with losing their good grades if they talk out of turn or interrupt in the class. This consequence of the behavior acts as the stimuli to a decrease potential disruptive behavior.

The classical and operant conditioning are applicable to many areas of real life situations. Two major areas of application of classical and operant conditioning is in the skeletal musculature reflex responses and the emotional responses such as phobia (fear), anxieties. These two functional aspects of classical conditioning are medicated by the different areas of brain during the acquisition stage. Amygdala has the role in forming conditioned associative learning related to emotional responses. Cerebellum is required for acquiring classical conditioning of skeletal musculature reflex responses. Patients with cerebellar lesion failed to acquire new conditioned eyeblink response. Cerebellar circuits are not essential for the storage and expression of the acquired conditioned responses, however.