Human long-term memory and the brain: what and where?
Human long-term memory is thought to comprise two distinct components: episodic and semantic memory. Episodic memory refers to our store of personal events from the past, while semantic memory is the term used to describe our knowledge about objects, people and the meanings of words. These two types of memory are often affected after brain damage and studies in patients provide, therefore, a unique opportunity to investigate the relationship between, and the neural underpinnings of, human long-term memory. In this talk I will describe how neuropsychological studies have contributed to three emerging views about the organisation of long-term memory. First, I will touch upon the nature of the relationship between episodic and semantic memory, in particular new results that challenge the assumption that amnesic patients cannot acquire new semantic information (Vargha-Khadem et al., 2001) and that patients with semantic memory impairment cannot lay down new episodic memories (Graham & Hodges, 1997). Second, I will discuss recent work in amnesic cases that is contradictory to the long-standing view that the medial temporal lobe plays only a temporary role in the consolidation of episodic and semantic memories. Instead, it is possible that the hippocampus, or more broadly the medial temporal lobe, may be critical for the retrieval of episodic memories throughout the lifetime (Nadel & Moscovitch, 1997). Finally, I will discuss whether the medial temporal lobe can be considered part of a single unitary memory system or, as is seeming more likely, whether different regions within this structure are differentially specialised for the processing of perceptual and spatial information (Gaffan, 2002). Gaffan, D. (2002). Against memory systems. Philosophical Transactions of the Royal Society of London, Series B – Biological Sciences, 357, 1111-1121. Graham, K.S., & Hodges, J.R. (1997). Differentiating the roles of the hippocampal complex and the neocortex in long-term memory storage: Evidence from the study of semantic dementia and Alzheimer’s disease. Neuropsychology, 11, 77-89. Graham, K.S., Simons, J.S., Pratt, K.H., Patterson, K. & Hodges, J.R. (2000). Insights from semantic dementia on the relationship between episodic and semantic memory. Neuroopsychologia, 38, 313-324. Nadel, L., & Moscovitch, M. (1997). Memory consolidation, retrograde amnesia and the hippocampal complex. Current Opinion in Neurobiology, 7, 217-227. Vargha-Khadem, F., Gadian, D.G., Watkins, K.E., Connelly, A., Van Paesschen, W. & Mishkin, M. (1997). Differential effects of early hippocampal pathology on episodic and semantic memory. Science, 277, 376-380.