THE HIPPOCAMPAL SYSTEM AND THE DECLARATIVE-PROCEDUAL MEMORY
DISTINCTION IN ANIMALS
- Research aims of animal models
- Neuropsychological aim
- Permits the investigator to maintain very strong control of the learning
experiences of the subjects and,
- permits systematic studies to be performed to delineate precisely the critical
structures
- Comparative aim
- Search for cross-species commonalities and differences in the functional role of
the structures
- Inquire about the kinds of mechanisms that have evolved to support memory in
the species with different environmental challenges and behavioural demands.
- Neurobiological mechanisms
- Allow systematic exploration of these structures' physiological properties and
functional organization with methods not amenable for use with humans
- Single unit recording
- Neuroanatomical and neurochemical studies
- Limitations and special challenges associated with the study of declarative memory in animals
- Lack of verbal declaration
- Distinctions between episodic vs. semantic, explicit vs. implicit memory is not
immediately clear
- Visual recognition memory in nonhuman primates
- The set of behavioural tests
- delayed non-match-sample task (DNMS), see Fig. 2-15
- Ss are exposed to an object once and then,
- after a delay,
- asked to recognize the object by indicating the unfamiliar one in a
two-choice presentation
- Monkeys with experimental lesions of the entire medial temporal area (see Fig. 2-14)
- intact STM but impaired LTM, see Fig. 2-16
- Retrograde amnesia
- intact capacities for skill acquisition task
- Structures of the medial temporal lobe critical to supporting declarative memory
- amygdala is not important
- damage limited to the hippocampus has only modest effect in monkeys (but
bigger effect in humans)
- damage to the perirhinal and parahippocampal region can produce the full
pattern of the amnesic deficit, see Fig. 2-17, 2-18, 2-19, 2-20, 2-21.
- Cognitive processing in rodents (see: Memory, amnesia, and the hippocampal system, written
by Cohen, N. J. and Eichenbaum, H. B. Cambridge, MA: MIT Press, 1993. On reserve in
Health Sciences Library WL 102 .C6785m 1993, pp.154-160; pp.109-126 )
- O'Keefe and Nadel's (1978): hippocampus mediates cognitive map (the establishment
of an organized neural representation of the physical environment)
- Rats with hippocampal system damage are severely impaired in many form of
spatial exploration and learning.
- Place learning: Morris water-maze task, see Fig. 22
- rats are trained to find a hidden escape platform submerged just below the
surface in a pool of cloudy water
- Hippocampal system damage (see Fig. 2-14) impair the ability to lean the
location of the escape platform
- Place learning (a detailed description)
- Water maze, developed by Morris (1981), see Fig. 2-22
- Tank
- water
- platform
- extra maze cues
- Experiment 1
- Demand of the task
- The animal swims around until it to find the platform to escape
- standard version of the task, variable-start-location
- Results, escape latency
- Across the trial, normal rats come to locate the platform
increasingly rapidly, and eventually swim directly to the platform,
produce very short latency.
- Hippocampal system damage
- Impaired ability to learn, not able to swim directly to the
platform, maintain long escape latency
- but demonstrate the ability to learn and retain the procedures
of the task, and to generate and use adaptive strategies
- Experiment 2
- Demand of the task for task version 2
- Constant starting location
- Results
- H damage did not prevent rats from learning
- Interpretation
- Variable-start-location
- The need of animals to build a representation of the position of the
platform in relation to the various visual cues arrayed in the room,
independent of particular swimming routes,
- and to flexibly express this stored information regardless the start
location.
- The characteristics of the hippocampal-dependent declarative
memory system: representational flexibility and relationality
- Constant-start-location
- Lack of the demand for flexibility and relationality in the standard
test
- Emphasis on the representation of spatial relations among the
distal cues is eliminated
- learning a rigid approach trajectory guided toward a particular cue
or cue complex
- Procedural memory systems
- Hippocampus place cells
- These neurons fire only when the rat is in a particular location in its environment
and firing is independent of the rat's orientation or ongoing behaviour.
- Several lines of evidence indicate that place cell activity reflects an encoding of
the spatial relationships between physical stimuli in the environment
- place fields move in concert with rotation of salient visual cues, see Fig.
2-23
- they scale with enlargement of all features of the environment, see Fig.
2-24
- they are altered when the spatial arrangement of cues is disrupted
- Place cell activity can be related to the rat's memory about its spatial
location, see Fig. 2-25
- Relational processing is not exclusively spatial, see Fig. 2-26