Amnesia

Explanation for Preserved Memory Function in Amnesia

A number of studies have shown that amnesia is not usually an all-pervasive impairment, but tends to impair certain specific memory functions whilst leaving others intact. These findings have provided the basis for several different theories of amnesia which are summarized below:

  1. Declarative memory is impaired in amnesics but procedural memory remains intact (Cohen and Squire, 1980).
  2. Explicit memory is impaired in amnesics but implicit memory remains intact (Graf et al., 1984).
  3. Context recollection is impaired in amnesics but familiarity judgments are unimpaired (Huppert and Piercy 1976; Mandler, 1989).

All three of these theories are supported by extensive research evidence. However, there is substantial overlap between these theoretical approaches, and efforts have therefore been made to find a common underlying factor which might make it possible to combine them into a single general theory.

One such approach (Cohen and Squire, 1980) argues that the impaired functions (i.e., declarative memory, explicit memory and context recollection) all require the conscious retrieval of memory, which can therefore be classified as declarative in the broadest sense.

In contrast, the preserved functions (i.e. procedural memory, implicit memory and familiarity judgments) all involve memories which can be demonstrated without conscious awareness, and which can therefore be generally described as non-declarative.

Cohen (1997) therefore argues that the declarative/non-declarative distinction can provide the basis of a general theory of organic amnesia which to a large extent subsumes other related theories.

(NB: It should be noted that Cohen has adopted a slight change in terminology for this more broadly based theory, preferring to use the terms declarative/non-declarative memory in order to allow for the inclusion of concepts such as implicit memory which cannot easily be categorized as procedural in nature.)

Mandler (1989) has emphasized the role played in these memory functions by automatic and controlled processes. He suggests that the memory functions which are impaired all require the use of consciously controlled processes, whereas the preserved functions can all apparently operate at an automatic level.

Mandler therefore concludes that organic amnesia is essentially ‘a disease of consciousness’. In his view amnesics have lost the ability to carry out consciously controlled retrieval and are thus forced to rely mainly upon unconscious automatic processes.

It has also been argued (Cohen and Eichenbaum, 1993; Cohen, 1997) that declarative memory involves the learning of associative connections and relationships between memories (such as linking two memories together, or linking an item with its context).

In contrast, non-declarative memory seems to be restricted to the strengthening of a single response mechanism (such as a motor skill or the level of activation and familiarity of a word).

An important consequence of this distinction is that declarative memory can be applied in a flexible way to fit novel situations, whereas non-declarative responses tend to occur in an inflexible manner as a reaction to one particular stimulus situation.

Cohen (1997) suggests that the hippocampus Opens in new window performs the associative binding function of declarative memory, whereas non-declarative memory involves the cortex and cerebellum. This view has received some support from MRI brain imaging studies.

Until recently the episodic/semantic distinction provided the basis of a widely accepted theory of amnesia, which proposed that episodic memory was impaired in amnesics whilst semantic memory remained intact (Warrington, 1979). However, this theory has lost much of its credibility because it has now been established that both episodic and semantic memories are usually impaired in amnesics. This finding lends weight to the view (Cohen, 1984) that episodic and semantic memory are both forms of declarative memory.