The full significance of these categorizations is still being explored. The categorical purity of semantic memory is open to question, as is the notion of a strict dichotomy between semantic and episodic memory. Most importantly, a separate anatomic basis for these systems of memory has not been clearly established (see the following text). Further interesting derivative issues regarding the neuropsychology of memory in relation to brain diseases can be found in the review by Kopelman. Among these is the degree to which a disparity between retrograde and anterograde memory can be detected in certain diseases. He also points out the subtle distinctions between recall and memory by recognition.
Neuropsychologists have further subdivided memory and suggested that there are corresponding anatomic regions for specific categories (see Table 20-4). Some of these more complex subtypes have been alluded to above and others are simply restatements of the act of registration. Furthermore, it is not surprising that the participation of certain areas of the brain not primarily involved in memory function, particularly the language and visuospatial areas, is required for the performance of most memory tasks. Among the special modules of memory, the notion of a working memory has both clinical and neuropsychologic credibility. This relates to the capacity to register and attend to a task, and there is little question that it is a measurable form of memory. Several regions of the brain must be active during tasks of working memory, including the hippocampi and dorsal thalamus, but lesions of the dorsolateral prefrontal cortex most specifically impair the skill. The original work of Goldman-Rakic may be referred to for discussion of the mechanisms that underlie working memory.
Finally, there are reasons, based mainly on the neuroanatomic and functional imaging studies discussed later, to view episodic memory for spatial and topographic information in a particular way. Certainly, the recollection of personally experienced events can be dissociated to some degree from the memory of the topographic arrangement of the scene in which these memories were formed, but often these two elements are inextricably bound in one experience. More salient may be a disproportionate degradation of learned topographic and directional information compared to learned semantic material; such a dissociation can be found, but only in relative terms, in patients who have injuries to their right hippocampus, whereas semantic material is dependent more on the left hippocampus (see later).
Anatomic Basis of the Amnesic Syndrome
Two anatomic structures are of central importance in memory function: the thalamus (specifically the medial portions of the dorsomedial and adjacent midline nuclei) and the hippocampal formations of the medial temporal lobes including their associated structures (dentate gyrus, hippocampus, parahippocampal gyrus, subiculum, and entorhinal cortex). Discrete bilateral lesions in these two main regions derange memory and learning disproportionate to all other cognitive functions, and even a unilateral lesion of these structures, especially of the dominant hemisphere, can produce a lesser degree of the same effect. These two main structures are linked by the mammillothalamic tract (tract of Vicq d’Azyr) with a single synapse in the mamillary bodies. The clinical–anatomic relationships that bear on this subject are discussed by Aggleton and Saunders and in the monograph on Wernicke-Korsakoff syndrome by Victor et al.