Abstract
Brain dedicated single photon emission computed tomography (SPECT) was used to compare the neuroactivation produced by the cued recall of response words in a set of studied word pairs with that produced by the cued retrieval of words semantically related to unstudied stimulus words. Six of the 12 subjects scanned were extensively trained so as to have good memory of the studied pairs and the remaining six were minimally trained so as to have poor memory. When comparing episodic with semantic retrieval, the well-trained subjects showed significant left medial temporal lobe activation, which was also significantly greater than that shown by the poorly trained subjects, who failed to show significant medial temporal lobe activation. In contrast, the poorly trained subjects showed significant bilateral frontal lobe activation, which was significantly greater than that shown by the well-trained subjects who failed to show significant frontal lobe activation. The frontal activations occurred mainly in the dorsolateral region, but extended into the ventrolateral and, to a lesser extent, the frontal polar regions. It is argued that whereas the medial temporal lobe activation increased as the proportion of response words successfully recalled increased, the bilateral frontal lobe activation increased in proportion to retrieval effort, which was greater when learning had been less good.
Highlights
It is well known that lesions of the medial temporal lobes (MTL) and the prefrontal cortex (PFC), whether left- or right-sided or bilateral, disrupt the acquisition of memories about episodes and facts and the retrieval of memories about some episodes and less well-learned facts
The demands of the episodic memory task involved retrieval of both semantic and episodic information. This design ensured that while both tasks were similar in the cognitive demands they made with respect to verbal output and semantic processing, the episodic memory task would have involved additional retrieval of contextual information linked to the studied word pairs
The first CBF contrast involved a within group comparison of the activation pattern produced by episodic retrieval compared to semantic retrieval, for the high and low performance groups separately, This contrast produced a left inferior medial temporal lobe activation (BA36, Z = 3.01) in the high performance group and a bilateral dorsolateral frontal activation (BA 46/9/10, Z = 3.91 and 3.58) and a left-sided orbito-frontal activation (BA 11/47, Z = 3.04) in the low performance group
Summary
It is well known that lesions of the medial temporal lobes (MTL) and the prefrontal cortex (PFC) (see [30]), whether left- or right-sided or bilateral, disrupt the acquisition of memories about episodes and facts and the retrieval of memories about some episodes and less well-learned facts (see [15]). Retrieval of well-established factual memories seems to be immune to the effects of either MTL or PFC lesions (see [28]). It remains unclear, what particular role, if any, these structures play in the input processes of en-. Many workers believe that the MTL consolidates and stores (at least for a while) an index that links together the components of episodes (and possibly facts) that are represented in different neocortical sites (for example, see [29]). One possibility is that the MTL consolidates and stores at least temporarily an index that represents in memory certain kinds of associations [6, 15]. There is extensive neuroimaging evidence that the MTL is activated during encoding [9,31] and that this activation is greater when encoding successfully produces later memory [3,35], and that MTL activations are greater when inter-item associations rather than items are encoded [10,11,16,17,18]
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