Memory Systems
Before discussing childhood cognitive development, a brief discussion of memory and review of the animal and human research will help to contrast the different types of memory from one another. Memory is not a single entity. It is characterized by many different multiple parallel systems and neural circuits that occasionally interact in convergence zones in discrete regions of the brain, like the thalamus, hippocampal formation, striatum, cingulate cortex, etc. (Damasio, 1992). There is one type of memory called explicit-declarative memory. It supports conscious thought that underlies intentional behavior. It is accessible behaviorally and verbally due to its ability for recall and supports cue/stimulus dependent and independent recognition and recall. It is also fast-learning, flexible, associated with many different sensory modalities with complex multimodal associations, can be generalized across different learning environments, and is mediated by the structural integrity of the hippocampus and surrounding medial temporal lobe structures (Ryan & Cohen, 2003; Squire, 1992; Squire, Knowlton, & Musen, 1993). All these components are important for the development and for the later retrieval of personally meaningful autobiographical event memory.
There is another type of memory called implicit-nondeclarative memory. It is evidenced in nonconscious abilities as in perceptual priming skills, i.e. the ability for matching task-dependent sensory fragments to an image in its presence, and in learning procedural habits or motor sequences and skills. Its learning is characterized as inaccessible, due to its inability for deliberate recall in the absence of a stimulus. It is also modality-specific and harbors sensory data driven and behaviorally inferred qualities. It requires many repetitive learning trials to meet learning performance demands. It is also evidenced during early acquisition learning of emotional conditioning (Roediger, 1990; Schacter, 1990; Squire, 1992; Squire et al., 1993).
When critical components of a specific memory system or specific neural circuitry associated with either memory system are removed or destroyed, later task-related memory is impaired. For instance, when the explicit memory-associated fornix of the hippocampal region is removed in the rodent, deficits in informational processing about changing relationships among environmental cues for resolving feature ambiguity in spatial location have been noted as well as the slowing of learning acquisition (McDonald, Murphy, Guarraci, Gortler, While, & Baker, 1997; McDonald & White, 1995). To compensate for this region’s loss and the animal’s inability for recalling previously learned environmental features, the animal relies on implicit memory associated exteroceptive input from the sensory cortex to learn about immediate nonretrievable perceptual features (Badgaiyan & Posner, 1997) and the dorsal striatal-caudate motor system’s self-generated movements to gain egocentric information about the environment (Kesner, Bolland, & Dakis, 1993). On the other hand, neural inactivation of the implicit memory-associated dorsal striatal region allows for expression of hippocampal-mediated explicit memory processes of place learning as well as the uncontrollable and disruptive tendency for unintentionally retrieving previously learned behavioral sequences in new environments (DeCoteau & Kessner, 2000; Packard & McGaugh, 1996), suggesting that this region (dorsal striatum) is needed for the expression of new behavioral sequences. Another neural circuit that is characterized by implicit memory and emotional conditioning involves the amygdala-ventral striatum. This circuitry plays a crucial role in identifying anticipated cues that are associated with rewarding or aversive outcomes and reward mediating approach or avoidance behaviors respectively needed for facilitating emotional conditioning and learning (Chai & White, 2004; Holahan & White, 2002).
Human lesioning of the medial temporal lobe, including the hippocampal region, suggests that it is critically involved in the later ability for verbally declaring that which had been experienced and learned. For instance both a woman with Korsakoff’s syndrome and amnesics incurring destruction to the diencephalon (involving the thalamus) and medial temporal lobe (involving the amygdala &/ hippocampal region) respectively, were able to acquire early pain and classical eye-blink conditioning (Claparede, 1995; Weiskrantz & Warrington, 1979); however, when asked, they were unable to remember and describe their previous conditioning experiences despite verbal reminders. In fact, a circumscribed lesion of the hippocampus has the capacity for impairing the ability for acquiring and verbally reporting declarative facts about task-related stimulus-context relationships but has no effect on the acquisition of a conditioned autonomic response (Bechara, Tranel, Damasio, Adolphs, Rockland, & Damasio, 1995). A lesion of the amygdala impaired a patient’s acquisition of conditioned autonomic responses of increased skin conductance (Glascher & Adolphs, 2003) to a startling sound but spared the ability for acquiring and verbally reporting task-related declarative facts about task-related stimulus-context relationships (Bechara,et al. 1995). These findings suggest that the amygdala processes material via its interaction with structures tied with autonomic activity; the hippocampus is involved in the ability for gaining and reporting declarative knowledge about stimulus-context relationships.
According to the above-noted lesioning research each memory system interacts with one another, has a temporally determined contribution, and, in its absence, allows for the expression of other memory systems to meet task dependent demands (Poldrack & Rodriguez, 2004; White & McDonald, 2002). The lesioning research also suggests that any compensatory response by expressed regions will be wholey limited by their inherent regional functional specialization.
References
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