TYPES OF MEMORIES - CLINICOPATHOLOGICAL DATA

Synaptic modifications are probably the basis of the memory processes that take place in the central nervous system. They have been studied in Aplysia or in hippocampal slices. How these minute alterations of t he synaptic strength are integrated in larger neural systems is still poorly understood. in man, hippocampal lesions, when bilateral, cause a deficit in anterograde episodic memory. The loss of previously acqui red memories (retrograde amnesia) is limited. Procedural memory is spa red. Young patients with hippocampal lesions remain able to learn how to read or to write (abilities that belong to semantic memories). Reco rdings obtained with intracerebral electrodes have shown that some neu rons of the hippocampus act as ''place cells''. They fire when the ani mal is in a specific place of the experimental maze, an observation th at suggests that the hippocampus acts as a map that may also be viewed as a context indicator (a ''cognitive map''). Computer models have be en devised to test the hypothesis that the hippocampus recorded the ma p of the activated synapses at a particular moment in time. This patte rn of activity could secondarily be transferred to the isocortex durin g a process known as consolidation. The frontal lobe plays a role in a ttention, which greatly influences the memory process. It also plays a role in the various strategies that are used to recall a memory and i n the analysis of the quality of the recall (metamemory). An asymmetry has been shown by the PET-scan: the left frontal lobe is activated du ring acquisition, and the right one during recall. The ability to inte grate one's own memories in one's own history and consciousness (self- awareness or ''autonoesis'') also depends on the activity of the prefr ontal region. The loss of acquired memories (retrograde amnesia) is mo st often observed in cases of large lesions of the anterior part of th e temporal lobe. Partial amnesias are difficult to separate from possi bly localized deficits of a cognitive function (some types of aphasia may be considered as an amnesia of words). Subcortical amnesias are ca used by diencephalic lesions; the topography of the critical structure s is still discussed: mamillary bodies and mamillo-thalamic tract or d orsomedial nucleus of the thalamus. The amygdaloid nucleus, the fronta l lobe and the dorsomedial nucleus of the thalamus belong to a network of connections that could be involved in emotions. It could be respon sible for the emotional flavor of a memory. Basal ganglia could play a role in procedural memory, but experimental or clinicopathological co nfirmations are still scarce. Finally, the involvement of the choliner gic innervation in the memory processes has been discussed: it could b e direct, or according to more recent data, related to its role in att ention.