Encoding of emotional memories depends on amygdala and hippocampus and their interactions.
Richardson MP, Strange BA and Dolan RJ;
Commented by , 19 Apr 2004
Background
The amygdala and the hippocampus play a key role in memory functions The amygdala, in particular, is important in modulating hippocampal activity during encoding of emotional items (ref. 1).
Understanding the relationship between the amygdala and the hippocampus during memory encoding may help to predict the impact of surgery procedures on the memory of patients with temporal lobe epilepsy (TLE).
Aim
To assess the impact of pathology on memory performance and encoding-evoked activity in patients with TLE.
Methods
Functional magnetic resonance (fMRI) recordings during encoding of neutral and emotional stimuli (a set of 288 words) were obtained in 16 right-handed TLE patients with left hippocampal sclerosis (with or witout amygdala involvement) and in 12 healthy controls.
Subjects underwent subsequently a word recognition task, and memory findings were correlated with imaging data obtained during encoding.
Results
The severity of left hippocampal pathology predicted memory performance for both neutral and emotional items, whereas the severity of left amygdala pathology predicted memory for emotional items alone.
Hippocampal activity during encoding of emotional items correlated with the degree of left amygdala pathology. Conversely, amygdala activation during emotional encoding correlated with left hippocampal pathology. The severity of pathology in the left hippocampus correlated positively with encoding-related activity in the right hippocampus.
Comment
Everyone knows that we remember emotional events much better than neutral ones. The role of the amygdala in this process is well established (ref. 2). Overall, this study provides supportive evidence that the effects of amygdala activation during the encoding of emotional stimuli are expressed through modulation of hippocampal activity in regions involved in verbal encoding (ref. 3).
While specific impairment in the recall of emotional items had been demonstrated before in patients with isolated amygdala damage (ref. 4), this seems to be the first time that a correlation between memory impairment and severity of pathology has been demonstrated in TLE.
Another interesting finding of this study is the reciprocal dependence that exists between the amygdala and the hippocampus. This observation, which is in line with recent experimental findings (ref. 5), provides the first demonstration of modulation of amygdala activity by the hippocampus in humans.
A third important observation of the study was the remarkable functional plasticity of inter-hemispheric connections. In this group of TLE patients with relatively good memory performance, the severity of left hippocampal sclerosis appeared to correlate with the degree to which verbal memory was reorganized in the right hemisphere, as assessed by pattern of brain activation during encoding.
Interestingly, the distribution of encoding activity between the left and right hippocampus and the right and left amygdala predicted the recognition accuracy for neutral and emotional items respectively.
TLE surgery carries a significant risk of memory deficits. Studies such this suggest that fMRI might be used to predict memory outcomes after surgery. In fact, it is possible that the degree of functional reorganization evidenced by fMRI preoperatively could reflect the ability of the brain to compensate for the consequences of surgical resection.
Use of fMRI to localize memory functions in the individual person might also be used to tailor the extent of resection to minimize memory loss. Prospective studies are eagerly awaited to test the validity of these hypotheses.
References
1. Cahill L., McGaugh JL. Mechanisms of emotional arousal and lasting declarative memory. Trends Neurosci. 1998; 21:294-9
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2. Dolan RJ. Emotion, cognition and behaviour. Science 2002; 298;1191-4
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3. McGaugh JL, Cahill L, Roozendaal B. Involvement of the amygdala in memory storage: interaction with other brain systems. Proc. Natl. Acad Sci. USA 1996; 93:13508-14 (free access to full text article)
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4.Adolph R, Cahill L, Schul R, Babinsky R. Impaired declarative memory for emotional material following bilateral amygdala damage in humans. Learn. Mem. 1997; 4:291-300
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5. Seidenbecher T, Laxmi TR, Stork O, Pape HC. Amygdalar and hippocampal theta rhythm syncronization during fear memory retrieval. Science 2003; 301:846-50
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