Figure 7. Effects of duration and timing of GR. The relative size of cortical domains preferentially responding to the exposed orientation is plotted against the postnatal day of optical imaging. Symbols are indicated in the figure. Plots obtained from identical kittens are linked by solid lines. Numerical figures indicate onset ages of GR. doi:10.1371/journal.pone.0005380.g007 postsynaptic activities [31]. Such LTP could occur at those synapses to visual cortical neurons optimally activated by the exposed orientation. The NMDA receptor dependence of the GR- induced over-representation of exposed orientation (Figs. 5 and 6) supports this view. The NMDA receptor is composed of obligatory NR1 subunits and other NR2 subunits such as NR2A and NR2B. Because D-AP5 effectively blocks NMDA receptors containing NR2A subunits more than those containing NR2B subunits [32], it may be that NR2A subunits contribute more to GR-induced modification of orientation selectivity than NR2B subunits. Indeed, it has been suggested that signaling through an NR2A-PSD95 complex is important for the maturation of orientation selectivity by visual experience in mice [33]. It has also been shown that activation of NR2A-containing, but not NR2B-containing, NMDA receptors, induces activity-depen- dent dendritic protein synthesis for synaptic changes in cultured hippocampal neurons [34]. Because NR2A subunits are persistently expressed in the superficial layers from postnatal 1 week up to adulthood in cats [35], the brief age-dependence of orientation modification may require a gating mechanism of the downstream signaling of NR2A-containing NMDA receptors to determine a basic structure of orientation maps through LTP induction associated with dendritic protein synthesis. NMDA-independent orientation plasticity The decreased responses to unexposed orientations for GR, as shown in Fig. 3, are also one of the major causes of the over- representation of the exposed orientation [11]. Close examination to the D-AP5-infused kittens reveals that, in the control hemisphere, response strengths to the unexposed orientations 0u, 30u or 150u are comparable to those to any orientations in the D- AP5-infused hemisphere (Fig. S2 online). This observation suggests Cat Orientation Plasticity that the decrease of responses to unexposed orientations does not involve NMDA receptor function and possibly is mediated by a process such as NMDA-independent LTD. One possible mech- anism of the reduction of responses to unexposed orientations may be activity-dependent LTD induced by the activation of cannabinoid receptors located at presynaptic sites [36,37], but the elucidation of the mechanism requires further investigation. Modifiability of orientation selectivity after the critical period The presently revealed brief critical period for orientation modifiability (Fig. 4) contrasts with the long tail phase of ocular dominance plasticity [29]. This discrepancy suggests different neural mechanisms of ocular dominance and orientation plasticity. In this regard, it may be worth noting that the over-representation of the exposed orientation shifts over to under-representation after the critical period for orientation plasticity (Fig. 4). There may be opposing forces that expand and contract the area of cortical domains representing the exposed orientation in early life. If it is the case, the end of the critical period may be determined by the age at which the contraction dominates the expansion, resulting in the rapid decrease of the sensitivity profile for orientation plasticity different from the long tail of the sensitivity profile for ocular dominance plasticity. In accordance with the above-mentioned under-representation of the exposed orientation, decreased neuronal responses to the exposed orientation have been shown in unit recording experi- ments. An early experiment on adult cats performed by Creutzfeldt and Heggelund revealed a decrease in the number of neurons optimally responding to vertical orientation exposed in a striped environment for 1 hr, twice a day during 2 weeks [38]. As speculated by these authors, adaptation-dependent response modification may be a mechanism of orientation plasticity after the end of the critical period. A recent experiment has shown a similar form of orientation-dependent adaptation in the orienta- tion tuning curves of neurons, which rapidly changed in adult cats when one orientation was presented more frequently than the other orientations [39]. However, the time scale of such adaptation is distinct in the order of that of the under- representation observed in the present study. A question remains on whether common mechanisms of adaptive response reduction work in wide range of time scales. On the other hand, it has been reported that intracortical microstimulation induced the expansion of an iso-orientation domain around the stimulation site in adult cats [40]. In addition, the pairing of single-orientation visual stimulation and the injection of depolarizing current to visual cortical neurons shifted preferred orientations towards the exposed orientation in adult cats as well as in kittens [41]. These findings seem to be contradictory to the under-representation of the exposed orienta- tion observed in the present study. Considering that our goggle- reared cats after the end of the critical period simply experienced a single orientation in the freely moving condition, the forced stimulation to the cortex may uncover the suppression of synaptic plasticity that would be latent in the adult visual cortex. Persistence of modified orientation selectivity Goggle rearing for 1 or 2 weeks modifies orientation preferences to nearly match the exposed single orientation, but such changes quickly diminish upon returning kittens to normal rearing (Fig. 7). Extension of GR for 4 weeks or longer is required for obtaining a persistent over- representation of exposed orientation, which was preserved at least 4 weeks after the kittens were returned to normal vision (Fig. 7). Very recently, Ohzawa et al. (SFN Abstract 2007-A-109015) performed single-unit recordings in cats that were reared with our PLoS ONE | www.plosone.org 6 April 2009 | Volume 4 | Issue 4 | e5380