Figure 4. Effects of dark rearing and tuning of inhibitory neurons. A, Average global OSI of spiking responses of excitatory neurons (Ex n; black) and FS inhibitory neurons (FS n; red) at ST1 and ST2 of normal development and under dark rearing (DR, tested at ST2). N = 25, 17, 21, 12, 15, 13 from left to right. Error bars indicate SEM. *p < 0.05; **p < 0.05; ***p < 0.001; ANOVA with post hoc test. Inset, Example spike waveforms of recorded FS neurons. B, Example excitatory and inhibitory synaptic responses in a dark-reared animal at P25. Calibration: 100 pA (exc)/270 pA (inh), 200 ms. Bottom, Normalized synaptic tuning curves. C, Average global OSI (top) and global OSI (bottom, see Materials and Methods) for excitation (black) and inhibition (red) at ST1 and ST2 of normal development and under dark rearing. n = 13, 14, 11, respectively. Error bars indicate SEM. ***p < 0.001, ANOVA with post hoc test. D, Average peak conductance of cycle-averaged responses to optimal orientation. **p < 0.01, ANOVA with post hoc test. n = 13, 14, 11. E, Average firing rate of FS neurons at the preferred and orthogonal orientations. Error bars indicate SD. **p < 0.01, ANOVA with post hoc test. n = 12, 15, 13. F, Proposed circuit models. Top, A simple feedforward circuit can explain the normal development of OS. FS neurons and their target layer 4 excitatory neuron receive a similar set of thalamic input, thus they exhibit similar preferred orientations. Strengthening of thalamocortical drive onto FS neurons (as indicated by the thickening of arrows) elevates their Vm responses, resulting in a broadening of their spiking responses (spike threshold is indicated by a horizontal dotted line). Bottom, Under dark rearing, the thalamocortical drive is not strengthened. The connections from inhibitory neurons preferring other orientations (marked by a different shade, which then receive different sets of thalamic input) are unselectively enhanced.