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Figure 2. Experience-dependent asymmetrical expansion of place fields. A. The size of the place fields
is gradually increasing with the number of laps. The relative place field size of the 43 place fields
increased significantly and asymptotically by 124% over 17 laps on closed track (Adapted from Mehta
et al., 1997). B. The location of the fields gradually is shifting backwards, towards the direction the rat
is coming from, as the number of laps increased. The location of each place field on each lap was
calculated relative to the center of mass of the corresponding average place field for all 17 trials. The
place field location for 43 place fields is shown as a function of lap number. There was a significant,
asymptotic backward shift in the relative field locations over the 17 laps on the track (adapted from
(Mehta et al., 1997)).
3.2. Environmental manipulations and place fields
One of the first indications that memory modulates place fields is the finding that even
after visual cues are removed, place cells firing persisted (O'Keefe & Speakman, 1987). Place
field reorganization (or remapping) results from a variety of cue manipulations (O'Keefe,
1979; Young et al. , 1994; Cressant et al. , 1997; Shapiro et al. , 1997; Tanila et al. , 1997b).
Plasticity of place cells has been observed as a remapping of either their firing rates or their
receptive fields when cues in an environment (Bostock et al. , 1991), or the shape of an
environment, are changed (Lever et al. , 2002; Fyhn et al. , 2007). Rotation of a cue card
attached to the wall in a cylindrical arena is able to produce a rotation of the place field
keeping the same angular relation as in the original configuration (Muller & Kubie, 1987).
Removal of this cue card results in place field rotation to unpredictable positions. In contrast
manipulations of the cue size did not affect place field. Placing a small barrier over the
location of a previously recorded place field is enough to make the place field disappear.
Doubling the size of the area and wall height is producing place field expansion of some cells
or generation of completely new place fields (Bostock et al. , 1991). Context-specific
responses of place cells emerge when rats are required to distinguish between contexts (Smith
& Mizumori, 2006b, 2006a). The removal of a cue proximal to the place fields reduced their
size, while removal of a distal cue would produce an enlargement of the place field size
(Hetherington & Shapiro, 1997). The effect of visual cue manipulation differs also in the
cases when the animal is present or absent during the manipulation. Place cells did not rotate
their field if the cue was moved in their presence but if the cue was rotated while away, then
the place field would also rotate (Jeffery & O'Keefe, 1999).
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