{'fraction_non_alphanumeric': 0.01804979253112033, 'fraction_numerical': 0.006639004149377593, 'mean_word_length': 5.236739974126778, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Systems of Memory in the Human Brain Minireview

Daniel B. Willingham University of Virginia Charlottesville, VA 22903 neurologically intact subjects (Corkin, 1984). In studying HM and other amnesic patients, it also became clear that memory is not always associated with intention to remember nor with awareness that one has learned. The influence of prior training or study may be apparent through task performance rather than through a report of what is consciously remembered. Examples of such tasks include motor skill learning and repetition priming tasks. In a priming task, the subject is asked to identify or complete a word or picture, and subjects do so more quickly or accurately if the stimulus was viewed a short time ago. HM shows normal motor skill learning and normal priming effects (Corkin, 1984). A straightforward interpretation of this pattern of performance is that explicit memory has been devastated in HM, but his ability on the other types of memory tests is unimpaired because these tests tap other memory systems that are undamaged. Much of the evidence supporting the multiple-system view of memory comes from patients with focal lesions like HM. The multiple-system view has been adopted, in part, because it appears that individual memory systems can be selectively disrupted while leaving other memory systems undisturbed. The conclusion that the damaged structure mediates the lost memory function is not straightforward, however, because the lesion may disrupt fibers of passage, alternative brain structures may be recruited to compensate for damaged tissue, or the subject may have learned to adopt different strategies to complete a task that normally relies on the damaged structure. Such concerns are to some extent assuaged when studies using functional imaging methods (Positron Emission Tomography or functional Magnetic Resonance Imaging) implicate the same brain structures in a memory system that lesion studies implicate. Functional imaging studies have their own problems of interpretation, but they do not overlap with the problems of lesion studies. In imaging studies one may see activation in a parallel system that is not crucial to the function of interest. There are also theoretical and practical difficulties in using the subtraction technique that is typically used to establish neural areas of activation. A combination of lesion and imaging studies provide converging evidence for all of the memory systems described here. Separate Memory Systems Explicit memory is associated with a conscious awareness that one is remembering and is usually tested by directly querying the individual, as in a typical recognition or recall test. Such memory relies on a set of structures in the medial temporal lobe, particularly the hippocampus, entorhinal and perirhinal cortex, and dienchephalon (Squire, 1992). Lesions to these structures produce explicit memory deficits in humans and nonhuman primates (although defining explicit memory in nonhuman animals is a matter of some controversy because the definition usually includes reference to the subject's awareness), and functional imaging studies show activation in these structures during encoding and recall of explicit memories (Squire et al., 1992). A useful distinction in thinking about explicit memories is that between episodic and semantic memories The last 20 years have seen a bedrock change in how psychologists think about memory, largely due to findings from neuropsychology. Until the early 1970s, psychologists viewed the memory system of the human brain as unitary. Today, human memory is more often viewed as a confederation of systems, each serving a different function, each subserved by different neural structures, and each capable of operating independently (see Schacter and Tulving, 1994, for a number of perspectives). What used to be called simply "memory" is today often referred to as explicit (or declarative) memory: memory that is associated with awareness and with intention to recall. Notably, other forms of memory are not associated with awareness. While one may not be aware that one has learned, the influence of prior experience will be apparent from task performance. Initially, researchers referred to all forms of unconscious memory as implicit (or procedural), but today these forms of memory are separated into distinct systems. The number of memory systems is a matter of ongoing debate, but there is relative agreement on the separability of the six systems listed in Table 1. These systems are differentiated in terms of their function and in terms of their neural substrate. It should be noted that these neural substrates are not necessarily the locations in which memory representations are stored but are areas thought to be critical to the normal functioning of the