SYNAPTIC PLASTICITY AND MNEMONIC ENCODING BY HIPPOCAMPAL FORMATION PLACE CELLS - Synaptic Plasticity: New Research - page 331

processing of visual information according to immediate behavioral requirements (Crist et al. ,
2001). The findings on orientation plasticity that involve spike timing-dependent rules
demonstrate that these changes can be long-term and can continuously influence vision
(Schoups et al. , 2001; Fu et al. , 2002; Sur et al. , 2002). Asynchronous visual stimuli flashed
at two orientations (Yao & Dan, 2001; Yao et al. , 2004) have been found to induce rapid
shifts in orientation tuning, suggesting a functional relevance for the cortical modifications
(Fig 10). The dependence on the spiking sequence and interval has been demonstrated in
visual cortical slices (Sjostrom et al. , 2001; Froemke & Dan, 2002) and in vivo (Yao & Dan,
2001; Fu et al. , 2002). Examining how visual cortical neurons adapt their response properties
to patterned stimulation or to perceptual learning, and how the capacity for adaptive changes
is mapped onto the cortex, is fundamental for understanding neuronal mechanisms of memory
formation in general. Spike-timing dependent rules functionally demonstrated in visual cortex
can be used to define experience-dependent asymmetric expansion of hippocampal place
fields (Mehta et al. , 1997; Mehta et al. , 2000).
9. O VERVIEW : I NTEGRATION OF THE S YNAPTIC P LASTICITY AND
P LACE F IELD M NEMONIC F UNCTIONS
LTP and LTD are believed to underlie memory processes on a cellular level (Bliss &
Collingridge, 1993; Bear, 1996; Martin et al. , 2000; Lynch, 2004). However, a significant
challenge is precisely determining how LTP and LTD relate to experience-dependent place
cells plasticity. During exploratory behaviour, exploration-associated complex-spike firing
neurons in the hippocampus are shown to form assemblies of cells with similar spatial
responses (O'Keefe, 1976). Together with spike timing-dependent plasticity (Levy & Steward,
1983), high-frequency spikes may provide a mechanism by which cell assemblies encode the
same part of the environment (Lisman, 1997). Naturalistic and interventional experiments on
place cell plasticity together with synaptic plasticity research and computational models of
hippocampal function integrate the ideas of how place fields process experience-related
mnemonic function. Place cell plasticity has different aspects which can be can be
summarized as follows:
(1) Place cell activity is capable of stable representation of particular environment for
continuous time even after the removal of place field-controlling sensory cues
(Muller & Kubie, 1987) (O'Keefe & Speakman, 1987; Save et al. , 2005). This
learning process is related to short-term synaptic plasticity and is believed to involve
working memory mechanisms.
(2) In a stable environment place fields undergo with experience asymmetrical
expansion (Mehta et al. , 1997; Mehta et al. , 2000). The change of firing rate and
firing field size with time is also a form of plasticity which is expressed by the theta-
related phase precession. The mechanisms responsible for this field development are
believed to be common with spike-timing dependent plasticity rules.
(3) Place fields are able incrementally to discriminate different contexts and to undergo
alterations with time due to discrete environmental cues. Therefore it is proposed that
place cells appear to be a neural substrate for long-term incidental learning (Lever et