A study by Ingrid Olson and colleagues in the Journal of Neuroscience examines feature memory (memory about objects and locations) and relational memory (memory about objects' relations to each other and locations) in patients with medial temporal lobe amnesia. I'll let them describe why:
The dissociation between long and short-term forms of memory in medial temporal lobe amnesia has been used to provide the linchpin evidence for the psychological distinction between short-term or working memory and long-term memory (Wickelgren, 1968; Baddeley and Warrington, 1970; Cave and Squire, 1992). Here, we show that the MTL is important for at least one type of WM: memory for relationships or conjunctions. The results of experiment 1 showed that amnesics have intact WMs for single features at delays of up to 8 s. The fact that feature memory, memory for locations or objects, did not differ between amnesic and control groups allowed us to assess whether amnesic patients had a disproportionate deficit in relational memory. Indeed, when required to remember an object in a location over an 8 s delay period, their memory was dramatically impaired. Additional analyses revealed that this deficit was closely linked to damage in the hippocampus per se and not surrounding structures.
They apply this finding in support of the "relational processing theory" of memory:
The relational-processing theory proposes that the MTL system is important for encoding into memory what has been described as contextual (Hirsh, 1974), configural (Sutherland and Rudy, 1989), or relational (Cohen and Eichenbaum, 1993) information. The underlying theme of these proposals is that the MTL is important for binding together different elements of a memory trace (Eichenbaum, 1999).
I'm noting this because of the finding that relational memory appears to be compromised in some cases where feature memory is intact. The press release (Science Blog) from U Penn makes it clear that this functional distinction may be more primary than the long-term vs. short-term memory distinction:
"While 'long-term' memory and 'short-term' memory have been useful distinctions for us, they may not exist in the same way for the brain," Olson said.
The researchers believe that a more useful distinction would be between feature memory and conjunction memory the ability to remember specific things versus how they are related. In that regard, the hippocampus serves like the brain's switchboard, piecing individual bits of information together in context.
"The hippocampus is another part of our evolving view of the nature of memories and consciousness," Olson said. "Our memories are not the static, permanent things we would like to think and, even in healthy people, these connections can erode or become muddled, leading to false memories or illnesses like post-traumatic stress disorder."
I find it interesting because there may be logical reasons to suppose that these relational memories would require a different kind of processing than memories about the properties of objects and places -- as Gregory Bateson pointed out, relationships are of a different logical type than properties.
But further, relational memory must be fundamental to social interactions in mammals, and primates in particular. I wonder to what extent different aspects of human social behavior are parsed into feature vs. relational memories. Any aspect of human sociality emerges from the relations between two or more people, but we have a way of ascribing some of these as properties of the people themselves -- for instance, a person who repeatedly treats other people rudely (emerging by relations) can be described and remembered as "rude" (inherent property). That kind of "categorizing" seems fundamental to human thought, and it may build off these basic systems of memory.
Olson IR, Page K, Moore KS, Chattergee A, Verfaellie M. 2006. Working memory for conjunctions relies on the medial temporal lobe. J Neurosci 26:4596-4601. DOI link