Cat Orientation Plasticity Figure 2. Orientation selectivity in kittens continuously exposed to vertical- or horizontal orientation for 2 weeks. Data obtained by starting/ending GR at the indicated postnatal days are illustrated similarly to Fig. 1a–f. doi:10.1371/journal.pone.0005380.g002 Critical period of orientation selectivity Figure 4 illustrates the time profile of the sensitivity for the modification of orientation selectivity as revealed by 2-week GR in 18 kittens. Here, we quantified the sensitivity as the relative size of the cortical domains for the exposed orientation, which is normalized by the representation bias in normal kittens (Fig. 1g) at respective days of optical imaging (see legend of Fig. 4). The sharp enhancement obtained for vertical- and horizontal-orienta- tion exposures are generally consistent with each other, except for a discrepancy at 2–3 postnatal weeks. Taken together with the horizontal bias observed in normal kittens as mentioned above, this discrepancy in the sensitivity profiles may suggest that the innate bias toward horizontal orientation is inherent to neuronal mechanisms that determine orientation selectivity. The critical period can be defined as the postnatal period during which 2-week GR applied (started and/or ended) effectively causes the modification of orientation selectivity. Thus defined, the critical period starts 2 weeks after birth and lasts for 6 weeks. Figure 4 also shows that the critical period is followed by a late phase of under-representation of the exposed orientation for GR starting between P55 and P151. We also observed the case of an adult cat reared with vertical goggles from P396 for one month; this cat showed a small relative size of cortical domains representing the exposed orientation (0.11) for 90u. Therefore, continuous single-orientation exposure after the critical period, even in adulthood, leads to the under-representation of the exposed orientation. NMDA receptor antagonist blocks modifiability of orientation selectivity To test whether NMDA receptors contribute to the critical period of orientation selectivity, we infused the left visual cortex with an NMDA receptor antagonist, D-(-)-2-Amino-5-phospho- nopentanoic acid (D-AP5), by an osmotic minipump system [28]. In three kittens, we carried out 1-week concurrent GR and minipump infusion. In the kitten starting GR and D-AP5-infusion at P24, the darkness in the orientation polar map in the D-AP5-infused hemisphere (Fig. 5a lower) indicates an overall reduction of orientation selectivity. This contrasts with the almost full representation of the exposed vertical orientation in the non- infused (control) right hemisphere (Fig. 5a upper). The response- strength maps in Figs. 5b, f and j show that, although visual responses completely disappeared within the diameter of 1–2 mm from the site of D-AP5-infusion (red dots), low levels of visual responses were preserved outside of this unresponsive region. In orientation histograms, the D-AP5-infused hemisphere showed a weak over-representation of vertical orientation (Fig. 5c), as contrasted to the virtually full representation in the control hemisphere (Fig. 5d). Similar results were obtained in the kitten with GR and D-AP5 infusion starting at P30 (Figs. 5e–h). In the other kitten started at P37 at the late critical period (Fig. 4), D-AP5 was virtually ineffective (Figs. 5i–l). Figure 6 illustrates the sensitivity profile of orientation selectivity for 1-week GR. The critical period defined for 1-week GR (grey dots) rapidly ended before P40, earlier than the end of the critical period defined for 2-week GR (Fig. 4). The ineffectiveness of D- AP5 infusion at P37 on the induction of over-representation of the exposed orientation seems consistent with the end of the critical period for 1-week GR. The sensitivity was drastically suppressed in the D-AP5-infused hemispheres of kittens goggle-reared from P24 and P30 (blue dots), whereas the D-AP5-untreated hemispheres still exhibited large sensitivity (yellow dots) comparable to that for goggle-reared kittens without D-AP5 infusion (grey dots). PLoS ONE | www.plosone.org 3 April 2009 | Volume 4 | Issue 4 | e5380