Chapter 8 – Memory: Varieties and Mechanisms
Memory is the series of processes whereby the nervous system acquires information from
new experiences, retains this information over time, and eventually uses it to guide behavior
and plan future actions. This definition point to three basic memory phases shared by all
forms of memory:
1. Encoding consists of the processes whereby experiences can alter the nervous
system. These alterations, known as memory traces, are believed to involve primarily
changes in the strength and/or number of synaptic connections between neurons.
2. Storage is the retention of memory traces over time.
3. Retrieval is the accessing of stored memory traces, which may lead to change in
behavior and sometimes is associated with the conscious experience of
remembering.
Learning can be described as gradual changes in behavior as a function of training. In this
second meaning, the term learning refers to the combined effect of all encoding, storage,
and retrieval in gradually enhancing the performance of a particular task.
Memory systems can be defined as groups of memory processes and associated brain
regions that tend to interact to mediate performance over a class of similar memory tasks.
Working memory mediates the maintenance and manipulation of information only for a few
seconds or minutes. In contrast, long-term memory systems mediate the retention of
information for longer periods.
Long-term memory is divided into declarative and nondeclarative types. Declarative memory
(explicit memory) refers to conscious memory for events (episodic memory) and facts
(semantic memory). Nondeclarative memory (implicit memory) refers to memories that are
expressed through performance independently of consciousness.
The most telling evidence supporting the distinction between working memory and
declarative memory has been provided by cases of amnesia. This term is typically applied to
severe memory loss due to brain damage. The memory loss may affect information acquired
after the damage – anterograde amnesia – or information acquired before the lesion –
retrograde amnesia.
Because of anterograde amnesia, patients with bilateral damage to the medial temporal
lobe cannot form new memories for events.
,The fact that patients with amnesia can maintain a normal conversation implies that working
memory is spared, since conversing requires remembering what was said during the last few
seconds or minutes.
A key feature of medial temporal lobe amnesia is that it impairs declarative memory, as
exhibited in tests, or recall and recognition but not in nondeclarative memory tasks such as
skill learning and priming. For example, H.M. was able to learn to trace geometrical figures
projected in a mirror, even though he could not remember the training sessions.
Thus, both major distinctions in the taxonomy are supported by clinical cases. Whereas
medial temporal lobe damage tends to impair declarative but not working memory, left
temporoparietal damage can disrupt working memory while sparing declarative memory. In
addition, medial temporal lobe damage tends to disrupt declarative memory while sparing
nondeclarative memory functions, whereas occipital lobe damage may impair
nondeclarative memory without altering declarative memory performance.
Nondeclarative memory
All forms have in common the facts that they are expressed through performance and are
independent of conscious awareness. Despite these shared properties, the major forms of
nondeclarative memory are very different from each other:
- Priming is a change in the processing of a stimulus due to a previous encounter with
the same or related stimulus, in the absence of conscious awareness about the first
encounter. Priming can be measured as an increase in the probability of generating a
particular stimulus, increases in processing speed and changes in eye movement
patterns. The requirement that participants be unaware that they are using
information from the first session is critical.
- Skill learning is a gradual improvement in performance due to repeated practice.
- Conditioning consists of simple responses to associations between stimuli
Depending on the relationship between the stimulus that generates the priming effect (the
prime) and the stimulus eliciting that effect (the target), priming can be classified as direct or
indirect. In direct priming, also called repetition priming, prime and target stimuli are the
same; in indirect priming they are different.
The most typical form of indirect priming is semantic priming, in which the prime and
the target are semantically related. Depending on the type of cue used in the test, direct
priming can be further classified as perceptual or conceptual. In perceptual priming, the test
cue and the target are perceptually related, as in the word fragment completion test. In
conceptual priming, the test cue and the target are semantically or associatively related. Just
as perceptual priming reflects prior processing of the perceptual aspects of a stimulus,
conceptual priming reflects prior processing of its conceptual aspects.
Although priming is usually associated with a reduction in activity (repetition suppression), in
some conditions it can be associated with the opposite effect: an increase in activity called
repetition enhancement. For stimuli that have pre-existing representations representations,
such as familiar faces, priming could reflect a modification of stored representations. But for
stimuli without pre-existing representations, priming might require the creation of new
representations.
, Skill learning appears to depend on the interaction between the neocortex and subcortical
structures such as the basal ganglia.
Complex skills require fast and accurate responses, efficient processing of sensory stimuli,
and the abstraction of stimulus-response patterns. In most real-world skills, these motor,
perceptual, and cognitive operations are closely intertwined.
Perceptual skill learning refers to improvements in processing perceptual stimuli that are
identical or similar to stimuli that have been repeatedly encountered. For example, this type
of skill learning underlies our ability to understand spoken and written language.
Cognitive skill learning refers to problem-solving tasks in which subjects are required to use
various cognitive skills to solve a task.
In classical conditioning, an innate reflex is modified by associating its normal triggering
stimulus with an unrelated stimulus that reliably predicts the trigger; the unrelated stimulus
eventually will trigger the original response by virtue of this association. In classical
conditioning it’s important to distinguish between delay and trace conditioning. The
difference is that in delay conditioning, the CS is still present when the US starts, and the two
terminate at the same time; in trace conditioning however, there is a brief time interval
between the end of the CS and the start of the US. Thus, in trace conditioning the CS must
leave some kind of memory trace in the nervous system in order for as CS-US association to
be established, whereas this is not the case in delay conditioning.
Pavlov’s experiment: The dog’s innate reflex was salivation (the unconditioned
response, or UR) in response to the sight/smell of food (the unconditioned stimulus, US). The
association was elicited in the animals by repeatedly pairing the sight/smell of food with the
sound of a bell (the conditioned stimulus, CS). Such conditioning, also called the conditioned
reflex, was considered established when the CS by itself elicited salvation (the conditioned
response, CR), even without the presentation of food.
In operant conditioning, the probability of a behavioral response is altered by associating the
response with a reward (or in some instanced a punishment).
In both classical and operant conditioning, it usually takes a number of CS-US training trials
for the conditioning to become established – a process called acquisition. Furthermore, if the
conditioned animal performs the critical response but the US is no longer provided, the
conditioned response gradually disappears – a phenomenon called extinction.
Cellular mechanisms of memory
At the cellular and molecular levels, all the forms of memory seem to depend on changes in
neural connectivity and the relative strength of synaptic transmission.
The term engram to refer to the physical and biochemical changes underlying memory
storage in the brain. Two related questions about engrams, or memory traces, have long
fascinated neuroscientists: where are they located in the brain, and how are they formed?
The modern view is that engrams are defined primarily by the connectivity of the
brain regions originally involved in processing the relevant category of information. That is,
memory traces for visual information are stored mostly in visual cortices. If engrams consist
of networks of neurons, then they are formed by a process whereby neurons that once
functioned relatively independently begin to work together by virtue of strengthened
connections between them.
