a, Somatic whole cell recording from a layer 2/3 pyramidal neuron, exhibiting robust visually-evoked spiking (3 sweeps overlaid; stimulus duration indicated by grey bar) and b, subthreshold responses (50 ms windows around spikes were blanked, and the same results were obtained when blanking window was decreased to 20 ms) which were orientation tuned as evidenced by c, spikes rasters, and d, polar plots of maximal depolarization. e, Hyperpolarization decreased the amplitude of the stimulus-evoked membrane potential modulation, and decreased its tuning selectivity. Hyperpolarization decreased f, Vm modulation amplitude and g, orientation selectivity index across the population. To investigate the mechanisms involved in the dendritic spike contributing to orientation tuning, whole cell somatic patch clamp recordings were performed with 1 µM MK-801 in the pipette solution. h, Orientation tuning selectivity progressively decreased during the recording as MK-801 diffused into the cell and blocked NMDA channels. In these example traces, the response to the preferred orientation decreased from early in the recording to late in the recording. i, Across the population, subthreshold orientation tuning was strongly inhibited at late time points in the recording, compared to early time points. j, In control recordings, with no blockers in the pipette solution, orientation tuning selectivity was not significantly different (P > 0.05, two sample t-test) from the early period of MK 801 recordings. Furthermore, tuning did not significantly change during long recordings.