Tuned thalamic excitation is amplified by visual cortical circuits

Lien, A.D., Scanziani, M. (2013). Tuned thalamic excitation is amplified by visual cortical circuits. Nature Neuroscience 16(9): 1315-1323.

Recording from L4 of visual cortex while showing random white/black dots and drifting gratings. Separate the thalamic component from the cortical component by activating PV with ChR2, which silences the cortex.

Cells receive on and off thalamic inputs (thalamic neurons that respond to increases and decreases in luminance) that have peaks which are slightly off-center. The orientation selectivity arises because an orientation will activate both the on and off field if situated appropriately.

Interestinyly, the thalamic input's integral is constant regardless of the orientaiton of the stimulus (see H in figure 4 below). The tuning arises only because of the synchronous inputs. The neuron is receiving a bunch of current from all of thalamus (which has no orientation tuning), and its preferred orientation is a result of the two thalamic pathways being activated simultaneously.

Figure 4 Separation of ON and OFF thalamic subfields predicts preferred orientation of thalamic excitation. (a–d) Example recording of EPSCThal which both the ON and OFF receptive fields and the responses to drifting gratings at various orientations were obtained in the same cell. (a) EPSCThal in response to black and white squares. Data are averages of five trials per location. (b) Contour plot of the OFF and ON receptive field maps for the cell shown in a. Each contour represents two z scores. Filled magenta and green circles mark the peaks of the OFF and ON receptive fields, respectively. Dashed black line connects the OFF and ON peaks to define the ON-OFF axis. The preferred orientation predicted from the ON-OFF axis, RF_Pref, is indicated by the small grating. (c) EPSCThal in response to drifting gratings of various orientations (average of three trials per direction). The gray rectangle indicates the visual stimulus (1.7 s) and the blue bars represent LED illumination (2.6 s). (d) Orientation tuning curves of F1Thal (blue) and QThal (gray) in polar coordinates for the responses shown in c. The blue line indicates the preferred orientation of F1Thal (Grating_Pref) and the black dashed line corresponds to RFPref. (e) Data presented as in b and d for three additional cells. Tuning curves on polar coordinates in d and e are normalized to peak response. Outer circle represents peak value. (f) RFPref  plotted against GratingPref (n = 8 cells, 7 mice). The black line represents unity. The dashed lines denote the region in which the difference between RFPref  and GratingPref is less than 30 degrees. The distributions of GratingPref (n = 42 cells, 33 mice) and RFPref  (n = 13 cells, 12 mice) across the population of cells in which either value was measured are shown along the top and right, respectively. (g) Absolute difference in RFPref and GratingPref (∆Pref Ori) (n = 8 cells, 7 mice). Error bar represents ± s.e.m. (h) Diagram of how orientation tuning of F1Thal can arise from spatially offset OFF and ON thalamic excitatory input (t1 = time 1, t2 = time 2). The area of the blue shaded region corresponds to QThal. The difference  between the peak and the trough of EPSCThal corresponds to F1Thal

Then to get the cortical component, they just simply subtract the thalamic component from the total. The cortical component is tuned with the thalamic component, but the Q coming in is now aligned with the preferrred orientation. Essentially suggesting that neurons with similar preferences in cortex wire together more strongtly. 

Figure 6. Tuning of non-thalamic excitatory F1 modulation. (a) Example cell. top, EPSC_Sub in response to drifting gratings of various orientations. The gray rectangle represents visual stimulus (1.5s) and the blue bar represents LED illumination (2.6s). Bottom, F1 modualation of EPSC_Sub. Shown are the cycle average (black) and best-fitting sinusoid (green) at the grating temporal frequency (2 Hz). (b) Orientation tuning curves of _Sub (dotted curve) and F1_Sub (solid curve) for the example cell shown in a. (c) Population tuning curve of Q_Sub (dotted curve) and F1_Sub(solid curve. Left, population tuning curves in which Q_Sub and F1_Sub tuning curves for each cell were equally shifted so that the preferred direction of Q_Sub occurred at 0 degrees (Q_Sub reference). Right, population tuning curves in which Q_Sub and F1_Sub tuning curves for each cell were independently shifted so that preferred direction of Q_Sub and F1_sub both occurred at 0 degrees (self reference). (d) OSI of F1_Sub was plotted against OSI of Q_Sub for all neurons. (e) Distribution of absolute differences in preferred orientation (D Pref Ori) between Q_Sub and F1_Sub. The dark curve represents all cells (n=42). The gray curve represents cells in the top 50th percentile of F1_Sub OSI (n=21). (f, g) Data are presented as in d and e for DSI and absolute differences in preferred direction (D Pref Dir). Filled green markers in d and f denote the OSI and DSI values of the example cell. Data in c-g are from n=42 cells from 33 mice. Error bars represent +- sem.

And they show that the cortical component is closesly tuned with the thalamic component, possibly with a 40ms offset or 30 degree phase delay. 

Thalamus provided about 30% of the charge to cortical neurons.