Figure 1. Orientation selectivity in normally reared kittens. Data are shown for postnatal days indicated. (a), (c) and (e): orientation polar maps. Color and brightness indicate preferred orientation and orientation selectivity. White curves delineate functionally defined area 17. Scale bars, 2 mm. Color code is placed below e. (b), (d) and (f): orientation histograms. Height of bins indicates the relative size of cortical domains preferentially responding to stimulus orientations. Horizontal lines indicate the relative size of iso-orientation domains for a uniform orientation representation. The same bin is doubly depicted at 0u and 180u for symmetric representation. Cortical coordinate placed on c: A, anterior; P, posterior; R, right; and L, left. (g): the relative size of cortical domains preferentially responding to vertical orientation (red dots) and horizontal orientation (blue dots) plotted against the postnatal day of optical imaging repeated in 23 normally reared kittens. Two curves were obtained using Stineman’s smoothing procedure. doi:10.1371/journal.pone.0005380.g001 online). A one-way repeated-measures ANOVA on the relative sizes of cortical domains for 6 stimulus orientations supports this observation (p = 0.0001). The relative size of domains representing 0u was significantly larger than that of the other orientations, except for 150u (p,0.0001 for 60u and 90u; p = 0.0002 for 120u; p = 0.0005 for 30u; ANOVA, post-hoc analysis by Tukey’s HSD test). This indicates that orientation representation is biased toward the horizontal orientation for very young normal kittens. This is analogous to the innate bias toward the contralateral eye, as has been known in ocular dominance [27]. Cat Orientation Plasticity The horizontal bias diminished at P51 (Fig. 1d), and shifted to a weak vertical bias at P84 (Fig. 1f). The sample-averaged orientation histogram across 15 normally reared kittens of P33– 84 showed a weak bias toward vertical orientation (Fig. S1b online). A one-way repeated measures ANOVA on the relative sizes of cortical domains indicates non-uniform orientation distribution (p = 0.0001). The following pairs showed significant differences: 90u and 30u, 90u and 150u, and 90u and 0u (p = 0.0011, p,0.0001 and p = 0.014, respectively; post hoc analysis by Tukey’s HSD test), but the other pairs did not show significant differences. Figure 1g shows the relative domain size for 90u (red open circles) and for 0u (blue open squares) repeatedly measured in 23 normally reared kittens, as a function of the postnatal day of optical imaging. The red and blue curves were obtained by a smoothing procedure to show slight but significant age-dependent changes of orientation selectivity in normal kittens; the horizontal bias was prominent before P35, but thereafter it was taken over by the vertical bias. Goggle rearing Here, we first describe the results of two weeks of goggle rearing (GR). Measurements in goggle-reared kittens were completed within 5 hrs after the removal of the goggles. Immediately after the 2-week GR started at P16–P17, the over-representation of horizontal orientation was exclusive (Figs. 2g and h), whereas that of vertical orientation was somewhat moderate (Figs. 2a and b). For GR that started at P27–P29, the over-representation of vertical orientation also became nearly exclusive (Figs. 2c and d). The representation of horizontal orientation was still predominant, although that of unexposed orientations appeared (Figs. 2i and j). The imbalance between vertical and horizontal orientations at P16–P17 may reflect, at least partly, the horizontal bias detected in normal kittens (Figs. 1a and b). When 2-week GR was started at P49–54, the induced over-representation of the exposed orienta- tion, either vertical or horizontal, diminished drastically (Figs. 2e, f, k and l). A close examination of Figs. 2e and f revealed that the effect of vertical GR was reversed to a slight under-representation of the vertical orientation. Figure 3 shows which changes in the orientation representation during 1-w GR also sufficiently modify orientation selectivity if adequately timed. In Fig. 3a, single-orientation maps of a kitten at P28 after normal rearing show a patchy organization for stimulus orientations (0u and 90u). After 1-w vertical GR from P31 to P38 in the same kitten, patchy organization became more prominent for the exposed orientation (90u), and the domain size increased particularly around the posterior part of the visual cortex. In contrast, patchy organization for the unexposed orientation (0u) almost disappeared. To quantify these changes, profiles of response strengths were measured along the line (7.2 mm long) drawn anteroposteriorly in the single-orientation maps (0u: Fig. 3b; 90u: Fig. 3c) and compared between the data from before and after GR. Figure 3d depicts the net decrease or increase of response strengths caused by GR. Our results show that vertical GR decreases response strengths at 0u by 21.56102461.261025 (s.e.) per pixel on average (Figs. 3b and d), whereas those at 90u increase by 1.16102461.561025 (s.e.) (Figs. 3c and d). This implies that, in local response strengths, the suppression occurring for the unexposed orientation is comparable with or even larger than the enhancement occurring for the exposed orientation. Such suppression and enhancement (Fig. 3d) together account for the marked over-representation of the exposed orientation, observed in the orientation polar maps (Figs. 2a, c, g and i). PLoS ONE | www.plosone.org 2 April 2009 | Volume 4 | Issue 4 | e5380