Explicit and Implicit Memory Network

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Declarative or explicit memory refers to what can be recalled consciously and reported. This form of memory includes episodic memories, which are personal experiences, images and everyday events with their rich contexts and recreation over time, and semantic memories, which refer to the recall of factual knowledge about the world and general information about our surrounds. Declarative memory stands in contrast to implicit or procedural memory, which cannot be overtly reported, such as how one types on a keyboard after training. With nondeclarative memory, past experience influences current behavior, even though we do not consciously recollect the details of what was learned. Procedural memory for skills, as discussed earlier, as well as habits and biases, depends especially upon the cerebellum and neostriatum. Knowledge about the qualities of items that place them in the same category can also be acquired implicitly, so that even an amnesic patient can learn the pattern that classifies items.310

Declarative learning depends upon limbic and diencephalic structures. These networks, especially the parahippocampal regions and hippocampus, receive new sensory information regarding facts and events and sketch a memory trace until it is bound onto the remote cortical modules that consolidate the memory.311 Then, with effortful recollection or inferences drawn from personal experience, a person can tap into large-scale networks of memories. For example, episodic memory mediates a synthesis of all the auto trips a person takes around a city. Even without looking at a paper map, one can make navigational inferences while driving that are built upon past spatial (streets that run east to west) and visual memories (relationships to a previously passed supermarket and theater) and other personal experiences (a conversation with a friend about the intended target) to arrive at a new location. The heart of declarative memory, then, is the process of making associations and the ability to retain relational information over time.

The hippocampal region is not required for immediate recall. The prefrontal cortex manages working memory and neighboring areas are involved in the maintenance and manipulation of ongoing working memory. The amygdala is a center that mediates emotional memory and modulates the strength and persistence of memories in other memory systems. Prospective memory has a declarative component (what needs to be recalled) and a temporal or contextual component (when or where the intention or action is to be carried out). Prospective recall involves a frontal lobe network that enables a person to carry out an intention after a delay. Cerebral trauma often impairs this function. Other specialized areas of the brain allow for rich and stable new associations. Memories of faces and complex visual patterns, for example, are encoded in the inferotemporal cortex, those for words in the midtemporal cortex, memory tasks related to spatial relationships in the superior parietal cortex, and multimodal memory tasks in the posterior parietal and pre-frontal cortices.

Priming, an implicit learning strategy, facilitates recognition by using a cue, such as the first letters of a word. The cue biases a subsequent response in the correct direction toward recall. For example, a whole word is recalled after a subject is given only the first syllable of the word. Priming is not sensitive to associations with other knowledge. Priming systems handle information about physical form and structure, rather than about the meanings and associative properties of objects and words.312 Priming strategies rely on perceptual representations stored by modality-specific memory subsystems, such as those that process word forms and visual objects. Each hemisphere, in fact, may store different representations. For example, changing the font of letters did not affect word-stem priming when fragments of a word were presented to the left hemisphere, but it did impair recognition when presented only to the right hemisphere.313 Priming is a valuable strategy for working with amnestic subjects following a traumatic brain injury, herpes encephalitis, and posterior cerebral artery infarcts that damage the hippocampus. The medial temporal lobe is not needed for successful priming or for sequence learning.314

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