Cat Orientation Plasticity Figure 5. Effect of NMDA receptor antagonist on orientation plasticity. (a), (e) and (i): orientation polar maps constructed immediately after concurrent GR and D-AP5 infusion for 1 week in three kittens for different onset ages of GR. (b), (f) and (j): response-strength maps constructed by summing all single-orientation maps in the respective animals. Brightness indicates response strength. White dots in a, e and i, and red dots in b, f, and j indicate locations of infusion centers of D-AP5 in the left-hemisphere or saline in the right hemisphere. (c), (g) and (k), and (d), (h) and (l) show orientation histograms for the D-AP5-treated visual cortex and for the control visual cortex, respectively. Other conventions are as in Fig. 1. doi:10.1371/journal.pone.0005380.g005 PLoS ONE | www.plosone.org 5 April 2009 | Volume 4 | Issue 4 | e5380 Figure 6. Effects of D-AP5 infusion on the sensitivity profile. Sensitivity profiles are shown for control and D-AP5-treated hemi- spheres, separately. For reference, we plot the sensitivity profile for 1- week GR without D-AP5 infusion obtained from 6 kittens. Symbols are defined in the figure. doi:10.1371/journal.pone.0005380.g006 preserve the orientation. Discussion moderate over-representation of the exposed In this study, we applied a combination of head-fixed cylindrical-lens goggles and optical imaging of intrinsic signals to investigate postnatal orientation plasticity in kitten primary visual cortex. By measuring the over-representation of exposed orienta- tion using optical imaging at the end of 1- or 2-week GR, we revealed a brief period exhibiting a sharp increase in the sensitivity of orientation modifiability between postnatal 3–5 weeks (Fig. 4 and Fig. 6). This increase was nearly abolished after 1-week infusion of D-AP5 (Fig. 6). The presently delineated critical period for orientation plasticity overlaps the most sensitive period for ocular dominance plasticity (postnatal 4–5 weeks) [29]. It may be worth mentioning that the method presently applied to kittens would have broad application to various animal species. We recently reported an over-representation of the exposed orienta- tion in goggle-reared young rats [30], in which individual neurons showing orientation selectivity are distributed in the visual cortex without grouping in the form of orientation maps. NMDA-dependent orientation plasticity Neuronal mechanisms underlying the increased responses to the exposed orientation have yet to be investigated, but based upon the wealth of knowledge about cortical synaptic plasticity so far accumulated, the following conjecture may be made. In cortical synapses, activity-dependent LTP could be induced at synapses with postsynaptic NMDA receptors by the coincidence of pre- and