Brain, Vol. 126, No. 9, 1915-1916,
September 2003
© 2003 Guarantors of Brain
doi: 10.1093/brain/awg256
Editorial |
Editorial
Unravelling the cognitive architecture of transient global amnesia
Wessex Neurological Centre,Southampton General Hospital, Department of Psychology, University of Southampton, Southampton, UK 1 Department of Psychology, University of Aberdeen, Aberdeen, UK
Transient Global Amnesia (TGA) is a common condition which is uncommonly studied. The reason for this is simple: like a beautiful sunset, by the time you have recovered from your feelings of wonder and have decided to examine the phenomenon in detail, nature has taken it away as quickly as she gave it. The scientific study of TGA therefore requires well-planned logistics and lots of patience. Since TGA represents a microcosm of the brain’s ability to evolve and resolve a lesion, it provides an opportunity to study mechanisms of recovery of function. In view of the density of the anterograde amnesia that accompanies TGA, it also permits researchers to highlight dissociations that may occur in the field of human memory disorder. Since most patients make a complete recovery from TGA, there is the added bonus that patients may in one sense act as their own controls for their memory test performance during TGA, thus circumventing the problem of individual differences in premorbid memory functioning.
In this issue of Brain, Quinette et al. (2003
) have assembled a unique set of data that allows us to unravel features of the cognitive architecture of TGA, and in particular the status of working memory and of executive functions. The concept of working memory has now been an active one for over 20 years, bringing together the traditional fields of attention and short-term memory (for recent reviews see Baddeley and Logie, 1999; Logie, 2003). The concept has succeeded in capturing the imagination of researchers for a number of reasons: it has highlighted the active nature of shorter-term memory processes in a variety of cognitive tasks, ranging from language to spatial reasoning to mental discovery (Logie, 2003); it has been relatively amenable to the design of experimental paradigms, both by cognitive scientists and by brain imaging scientists; many of the features of working memory can be readily subject to computer simulation; and some components of working memory can find an analogue in non-human studies, permitting a more fine-grained analysis of basic mechanisms. Studies of neurological patients have pointed to fractionation of components within working memory (e.g. Della Sala and Logie, 2001), and Quinette et al. (2003) were able to provide further support for such fractionation. Thus, they found that elementary components of working memory, such as the phonological buffer, were intact, as were aspects of executive functioning. They did, however, find an impairment in performance on the Brown–Peterson task.
The Brown–Peterson task has a chequered history. It was frequently used in early experimental studies of human memory, and was widely applied in studies of amnesic patients, but research using the paradigm has tailed off in recent years. Baddeley (1997) has attributed this tailing off in interest to the possibility that the task does not reflect ‘anything of basic importance’ (p. 37). The Brown–Peterson task does however have many attractive features, not least of which is its apparent similarity with everyday lapses of memory, such as where a message is readily forgotten after a short period of intervening distracting activity (try asking a conference speaker two or more questions, and he will have forgotten one question after he has answered the first!). However, some insight into its interpretation might be gained if we assume that the distracting activity occupies working memory, thereby preventing rehearsal and leaving little capacity to retain the Brown–Peterson preload. It is then a reasonable conclusion that whatever material remains after a filled delay has been retained by some system other than working memory. As such, delayed recall in this task is more likely to reflect loss of information from long-term memory (that may be phonological or semantic traces), and therefore it may not be testing a capacity limitation of working memory. This interpretation would merit further empirical test, but it is consistent with the Quinette et al. finding that working memory is largely intact in TGA, and would explain why performance on the Brown–Peterson task is impaired by what appears to be a temporary deficit of long-term memory. Moreover, the clear dissociation between impaired long-term memory function and intact working memory (including intact executive functions) reinforces the concept of working memory as being a system that is quite distinct from long-term memory, in contrast with recent arguments to the contrary (e.g. Ruchkin et al., 2003). Perhaps we should focus on the practical utility of the task in clinical and research settings, its reliability, and its specificity for particular forms of memory impairment. Therefore recent attempts to develop norms for the test are to be welcomed (Morrow and Ryan, 2002).
A further important observation by Quinette et al. was the selective impairment of recollection during the acute recovery phase of TGA. The awareness of subjects when they make memory judgements has formed a key feature of recent formulations of episodic memory (Tulving, 2002), and while ‘remember/know’ distinctions have been applied in many neurological settings, the demonstration by Quinette et al. is one of the first in an acute recovery-of-function setting. Since TGA is generally associated with hippocampal dysfunction, their observations highlight the role of medial temporal lobe structures in ‘mental time travel’.
As well as unravelling the cognitive architecture of TGA, we need also to unravel its anatomical and pathophysiological architecture. Designing eloquent experimental cognitive and imaging paradigms for use in an acute setting such as TGA is a daunting task. The next stage will be to combine approaches such as those employed by Quinette et al. with imaging procedures. However, the ambiguity with regard to the cognitive functions associated with the Brown–Peterson task and also with the N-Back task highlight the need to ensure that how participants perform a task is well understood at a cognitive theoretical level before it is employed in brain imaging studies (Logie et al., 2003), whether these relate to resting levels of physiological integrity (Kapur and Kopelman, 2003) or to functional imaging procedures (c.f. LaBar et al., 2002).
References
Baddeley AD. Human Memory. Theory and Practice. Revised Edition. Hove: Psychology Press; 1997.
Baddeley AD, Logie RH. Working memory: The multiple component model. In: Miyake A, Shah P, editors. Models of Working Memory, New York: Cambridge University Press, 1999, p28–61.
Della Sala S. Logie RH. Theoretical and practical implications of dual-task performance in Alzheimer’s disease. Brain 2001; 124: 1479–1481.
LaBar KS, Gitelman DR, Parrish TB, Mesulam M-M. Functional changes in temporal lobe activity during transient global amnesia. Neurology 2002; 58: 638–641.
Kapur N, Kopelman MD Advanced imaging procedures and human memory disorder. Br Med Bull 2003; 65: 61–81.
Logie RH. Spatial and visual working memory: A mental workspace. To appear in: Irwin D, Ross B, editors. Cognitive Vision. The Psychology of Learning and Motivation, Vol 42. Academic Press, 2003, in press.
Logie RH, Venneri A, Della Sala S, Redpath T, Marshall I. Brain activation and the phonological loop: the impact of rehearsal. Brain and Cognition 2003, in press.
Morrow LA, Ryan C. Normative data for a working memory test: The four word short-term memory test. Clinical Neuropsychologist 2002; 16: 373–80.
Quinette P, Guillery B, Desgranges B, Sayette V, Viader F, Eustache F. Working memory and executive functions in transient global amnesia. Brain 2003; 1917–1934.
Ruchkin DS, Grafman J, Cameron K, Berndt RS. Working memory retention systems: A state of activated long-term memory. Behavioral and Brain Sciences 2003, in press.
Tulving E. Chronesthesia: conscious awareness of subjective time. In: Stuss DT, Knight R, editors. Principles of Frontal Lobe Function. Oxford: Oxford University Press; 2002, p311–325.
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