I. Neural substrates
II. Types of Learning
A. Conditioning
1. Explicit Cue Conditioning
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a) Of Fear
- contextual fear conditioning requires the hippocampus; lesions of the hippocampus block the increase in freezing in the presence of the context, but not the explicit cue (1, 2).
- not surprising that hippocampus may be involved in contextual fear conditioning, because of extensive data showing the necessity of this structure in the processing of spatial information, which may well be required to remember the place where prior shocks occurred (3).
Questions to answer:
1) How does contextual conditioning differ from compound conditioning? Is contextual conditioning simply a form of compound conditioning in which the compound is not clearly specified? Is difference between explicit cue conditioning and contextual conditioning an absolute or a theshold difference in terms of complexity of the cue, and how does this relate to structures involved?
References:
1) Kim, J. J., & Fanselow, M. S. (1992). Modality-speicific retrograde amnesia of fear. Science, 256, 675-677.
2) Phillips, R. G., & LeDoux, J. E. (1992). Differential contribution of amygdala and hippocampus to cued and contextual fear conditioning. Behavioral Neuroscience, 106, 274-285.
3) Davis, M. (1996). Differential roles of the amygdala and bed
nucleus of the stria terminalis in conditioned fear and startle
enhanced by corticotropin-releasing hormone. In T. Ono, B. L.
McNaughton, S. Molotchnikoff, E. T. Rolls, & H. Nishijo (Eds.),
Perception, memory, and emotion: Frontiers in neuroscience (pp.
525-548). Oxford: Elsevier.
3. Compound Conditioning
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Learning is often impaired when animals are required to learn tasks that can be acquired only using nonspatial information. (3) Tolman (1948) argued that animals form cognitive maps (i.e., internal representations of spatial information) of their environments. (1) Following Tolman, O'Keefe and Nadel (1978) proposed that the hippocampus is the brain site critical for the formation of such spatial cognitive maps. (2)
Although evidence suggests that the hippocampus might not be involved specifically in the formation of spatial cognitive maps (e.g., see Shapiro & Olton, 1994; 4), it has been shown (see review by Jarrard, 1991; 5) that lesions to the hippocampal system impair an animal's ability to solve a wide variety of spatial tasks, such as:
i) learning to navigate to an escape platform in the widely used Morris water maze (Morris et al., 1982; 6)
ii) remembering the location of places where rewards were received previously (McDonald & White, 1994; 7)
iii) learning associations between spatial information and emotional
(fear) states (i.e., contextual learning;
Kim, Rison, & Fanselow, 1993; 8)
It has been proposed that place cells represent the cellular substrate of the spatial cognitive map. (2) Electrophysiological studies recording activity of hippocampal pyramidal neurons (place cells) indicate that in the awake, freely behaving animal, individual place cells are active only when the animal is in restricted regions of the environment (the "place field") and remain virtually silent when the animal is in all other locations (9, 10).
Animals exploring the space of a novel environment show a rhythmic oscillation in hippocampal activity in the 5- to 10-Hz range (the theta rhythm). Changes in synaptic strength can be produced by this endogenous activity and are thought to be necessary for storing information about space. Synaptic plasticity in the theta frequency range may regulate hippocampal place cells, the pyramidal neurons (in the CA3 and CA1 subfields of the hippocampus) whose activity is correlated with the animals' location in the environment. [11]
CaMKII is a serine-threonine protein kinase that is restricted to the forebrain. It is expressed in the neurons of the neocortex, hippocampus, amygdala, and the basal ganglia. CaMKII may be a molecular substrate of memory. [11] Targeted disruption of the CaMKII gene produces deficits in long-term potentiation (LTP) and severely impairs performance on hippocampal-dependent memory tasks. Mayford et al. (11) demonstrated that expression of an activated calcium-independent CaMKII transgene resulted in a loss of hippocampal LTP in response to
References:
1) Tolman, E. C. (1948). Cognitive maps in rats and man. Psychological Review, 42, 189-208.
2) O'Keefe, J., & Nadel, L. (1978). The hippocampus as a cognitive map. Oxford, England: Clarendon Press.
3) Matthews, D. B., Best, P.J., White, A. M., Vandergriff, J. L., & Simson, P. E. (1996). Ethanol impairs spatial cognitive processing: New behavioral and electrophysiological findings. Current Directions in Psychological Science, 5, 111-115.
4) Shapiro, M. L., & Olton, D. S. (1994). Hippocampal function and interference. In D. L. Schacter & E. Tulving (Eds.), Memory systems 1994 (pp. 87-117). Cambridge, MA: MIT Press.
5) Jarrard, L. E. (1991). On the neural bases of the spatial mapping system: Hippocampus vs. Hippocampal formation. Hippocampus, 1, 236-239.
6) Morris, R. G. M., Garrud, P., Rawlins, J. N. P., and O'Keefe, J. O. (1982). Place navigation impaired in rats with hippocampal lesions. Nature, 297, 681-683.
7) McDonald, R. J., & White, N. M. (1994). Parallel information processing in the water maze: Evidence for independent memory systems involving dorsal striatum and hippocampus. Behavioral and Neural Biology, 61, 260-270.
8) Kim, J. J., Rison, R. A., & Fanselow, M. S. (1993). Effects of amygdala, hippocampus, and peri-aqueductal gray lesions on short- and long-term contextual fear. Behavioral Neuroscience, 107, 1093-1098.
9) O'Keefe, J., & Dostrovosky, J. (1971). The hippocampus as a spatial map: Preliminary evidence from unit activity in the freely-moving rat. Brain Research, 34, 171-175.
10) Thompson, L. T., & Best, P. J. (1990). Long term stability of hippocampal unit activity recorded from freely behaving rats. Brain Research, 509, 299-318.
11) Mayford, M., Bach, M. E., Huang, Y., Wang, L, Hawkins, R. D., Kandel, E. R. (1996). Science, 274, 1678-1683.
III. Areas for study:
- long-term potentiation.
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