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- ItemMaking context memories independent of the hippocampus(Cold Spring Harbor Laboratory Press, 2009) Lehmann, Hugo; Sparks, Fraser T.; Spanswick, Simon C.; Hadikin, Crystal; McDonald, Robert J.; Sutherland, Robert J.We present evidence that certain learning parameters can make a memory, even a very recent one, become independent of the hippocampus. We confirm earlier findings that damage to the hippocampus causes severe retrograde amnesia for context memories, but we show that repeated learning sessions create a context memory that is not vulnerable to the damage. The findings demonstrate that memories normally dependent on the hippocampus are incrementally strengthened in other memory networks with additional learning. The latter provides a new account for patterns of hippocampal retrograde amnesia and how memories may become independent of the hippocampus.
- ItemObject/context-specific memory deficits associated with loss of hippocampal granule cells after adrenalectomy in rats(Cold Spring Harbor Laboratory Press, 2010) Spanswick, Simon C.; Sutherland, Robert J.Chronic adrenalectomy (ADX) causes a gradual and selective loss of granule cells in the dentate gyrus (DG) of the rat.Here, we administered replacement corticosterone to rats beginning 10wk after ADX. We then tested them in three discrimination tasks based on object novelty, location, or object/context association. Only during testing of the object/context association did ADX rats demonstrate deficits. These findings add to a body of evidence that the hippocampus is necessary when contextual information is important.We also confirm that memory deficits after chronic adrenalectomy are not a result of loss of corticosterone per se.
- ItemA novel animal model of hippocampal cognitive deficits, slow neurodegeneration, and neuroregeneration(Hindawi Publishing, 2011) Spanswick, Simon C.; Lehmann, Hugo; Sutherland, Robert J.Long-term adrenalectomy (ADX) results in an extensive and speciﬁc loss of dentate gyrus granule cells in the hippocampus of adult rats. This loss of granule cells extends over a period of weeks to months and ultimately results in cognitive deﬁcits revealed in a number of tasks that depend on intact hippocampal function. The gradual nature of ADX-induced cell death and the ensuing deﬁcits in cognition resemble in some important respects a variety of pathological conditions in humans. Here, we characterize behavioural and cellular processes, including adult neurogenesis, in the rat ADX model.We also provide experimental evidence for a neurogenic treatment strategy by which the lost hippocampal cells may be replaced, with the goal of functional recovery in mind.
- ItemSuppression of neurotoxic lesion-induced seizure activity: evidence for a permanent role for the hippocampus in contextual memory(Public Library of Science, 2011) Sparks, Fraser T.; Lehmann, Hugo; Hernandez, Khadaryna; Sutherland, Robert J.Damage to the hippocampus (HPC) using the excitotoxin N-methyl-D-aspartate (NMDA) can cause retrograde amnesia for contextual fear memory. This amnesia is typically attributed to loss of cells in the HPC. However, NMDA is also known to cause intense neuronal discharge (seizure activity) during the hours that follow its injection. These seizures may have detrimental effects on retrieval of memories. Here we evaluate the possibility that retrograde amnesia is due to NMDAinduced seizure activity or cell damage per se. To assess the effects of NMDA induced activity on contextual memory, we developed a lesion technique that utilizes the neurotoxic effects of NMDA while at the same time suppressing possible associated seizure activity. NMDA and tetrodotoxin (TTX), a sodium channel blocker, are simultaneously infused into the rat HPC, resulting in extensive bilateral damage to the HPC. TTX, co-infused with NMDA, suppresses propagation of seizure activity. Rats received pairings of a novel context with foot shock, after which they received NMDA-induced, TTX+NMDAinduced, or no damage to the HPC at a recent (24 hours) or remote (5 weeks) time point. After recovery, the rats were placed into the shock context and freezing was scored as an index of fear memory. Rats with an intact HPC exhibited robust memory for the aversive context at both time points, whereas rats that received NMDA or NMDA+TTX lesions showed a significant reduction in learned fear of equal magnitude at both the recent and remote time points. Therefore, it is unlikely that observed retrograde amnesia in contextual fear conditioning are due to disruption of non-HPC networks by propagated seizure activity. Moreover, the memory deficit observed at both time points offers additional evidence supporting the proposition that the HPC has a continuing role in maintaining contextual memories.
- ItemMental rotational ability is correlated with spatial but not verbal working memory performance and P300 amplitude in males(Public Library of Science, 2013) Christie, Gregory J.; Cook, Charles M.; Ward, Brian J.; Tata, Matthew S.; Sutherland, Janice; Sutherland, Robert J.; Saucier, Deborah M.This study investigated how both sex and individual differences in a mental rotation test (MRT) influence performance on working memory (WM). To identify the neural substrate supporting these differences, brain electrical activity was measured using the event-related potential technique. No significant sex differences were observed in a test of verbal WM, however males were significantly faster than females to respond to probe stimuli in a test of spatial WM. This difference was no longer significant after controlling for differences in MRT score, suggesting that rotational ability mediates performance in the spatial memory task for both sexes. A posterior P300 was observed in both tasks as participants encoded information into memory, however the amplitude of the P300 correlated with RT in the spatial task but not in the verbal task. Individual differences in the MRT also correlated with RT and with the amplitude of the P300, but again only in the spatial task. After splitting the analysis by sex, partial correlations controlling for MRT revealed that for males, individual differences in rotational ability completely mediated the correlation between the P300 and RT in the spatial task. This mediating effect was not observed for the female participants. The results therefore suggest a relatively stronger association in males between innate mental rotational ability, spatial memory performance, and brain electrophysiological processes supporting spatial memory.