Stimulus feature selectivity in excitatory and inhibitory neurons in primary visual cortex

Abstract

Although several lines of evidence suggest that stimulus selectivity in somatosensory and visual cortices is critically dependent on unselective inhibition, particularly in the thalamorecipient layer 4, no comprehensive comparison of the responses of excitatory and inhibitory cells has been conducted. Here, we recorded intracellularly from a large population of regular spiking (RS; presumed excitatory) and fast spiking (FS; presumed inhibitory) cells in layers 2-6 of primary visual cortex. In layer 4, where selectivity for orientation and spatial frequency first emerges, we found no untuned FS cells. Instead, the tuning of the spike output of layer 4 FS cells was significantly but moderately broader than that of RS cells. However, the tuning of the underlying synaptic responses was not different, indicating that the difference in spike-output selectivity resulted from differences in the transformation of synaptic input into firing rate. Layer 4 FS cells exhibited significantly lower input resistance and faster time constants than layer 4 RS cells, leading to larger and faster membrane potential (V(m)) fluctuations. FS cell V(m) fluctuations were more broadly tuned than those of RS cells and matched spike-output tuning, suggesting that the broader spike tuning of these cells was driven by visually evoked synaptic noise. These differences were not observed outside of layer 4. Thus, cell type-specific differences in stimulus feature selectivity at the first level of cortical sensory processing may arise as a result of distinct biophysical properties that determine the dynamics of synaptic integration.

Figure 1.

Classification of RS (open circles) and FS cells (filled circles). A, Plot of F–I slope against spike width, divided at an action potential width of 0.5 ms, and an F–I slope of 300 Hz/nA. Although the two populations of cells are largely separated, there is some overlap evident in the lower left quadrant. B, Plot of adaptation index (percentage of spikes occurring in the first 50 ms of a 100 ms current pulse) against action potential width. Here the two populations are clearly separated into two quadrants.

Figure 2.

Orientation tuning curves of four representative neurons. In each case, one response to an optimally oriented stimulus (left trace) and one response to a stimulus at the half-width of the tuning curve (right trace) are shown on the left. A, A simple RS cell in layer 4 showed a characteristic modulated V_m and spike response to an optimal drifting grating (left trace). Spike duration at half height was 0.49 ms (left, inset spike). The V_m (blue) and spike (red) tuning curves of the F1 component of the visually evoked response show tuning for orientation (HWHH: V_m, 29.7°; spikes, 13.1°). The tuning of the spike response was markedly narrower than that of the V_m response. B, A complex RS cell in layer 5 showed an unmodulated, sustained response to an optimal drifting grating. Spike duration was 0.50 ms. The V_m and spike tuning curves of the DC component of the visual response were both well tuned for orientation (HWHH: V_m, 32.6°; spikes, 19.9°). C, A representative simple FS cell in layer 4 showed modulated firing in response to an optimal grating. Spikes were characteristically short in duration (0.26 ms) and showed a large, brief AHP. The V_m of the cell showed tuning similar to that of the simple RS cells shown in A, but the spike response of this cell was broader than that of the RS cell (HWHH: V_m, 29.6°; spikes 24.0°). D, A complex FS cell in layer 5 showed a sustained level of fast spiking in response to an optimal grating and V_m and spike responses that were well tuned for orientation (HWHH: V_m, 31.7°; spikes, 20.1°). The spike duration was 0.30 ms.

Figure 3.

Spatial frequency tuning curves of the cells shown in Figure 1. A, The layer 4 simple RS cell was well tuned for spatial frequency at both the V_m (blue) and spike (red) levels (BW: V_m, 2.80 oct; spikes, 1.62 oct). B, Similarly, the V_m and spike responses of the layer 5 complex RS cell were well tuned (BW: V_m, 2.20 oct; spikes, 1.45 oct). C, The V_m response of the simple layer 4 FS cell was similar to that of the layer 4 RS cell shown in A, but the spike response of the FS cell was more broadly tuned than that of the RS cell (BW: V_m, 2.73 oct; spikes, 2.32 oct). D, Tuning curves for the V_m and spike responses of the layer 5 complex FS cell (BW: V_m, 2.24 oct; spikes, 1.62 oct).

Figure 4.

Population tuning characteristics for orientation and spatial frequency. A, Plot of V_m versus spike orientation tuning width (HWHH) for cells in all cortical layers. Both RS (open circles; n = 108) and FS (filled circles; n = 61) cells predominantly fell below the unity line (dashed line), indicating broader V_m than spike tuning. For clarity, several cells with extremely broad V_m or spike tuning are not shown. As highlighted by the overlapping histograms on each axis, there were no differences in V_m or spike tuning between RS and FS cells when all cells were included. B, Plot of V_m versus spike spatial frequency tuning bandwidth. Both RS (n = 76) and FS (n = 39) cells mostly fell below the unity line, indicating a sharpening of tuning in the transformation from V_m activity to spike output. As shown by the accompanying histograms, there were again no differences in the distribution of V_m or spike tuning when cells in all layers were included. C, Within layer 4, there was no difference in V_m orientation tuning between RS (n = 31) and FS (n = 26) cells. However, as shown by the population means (red symbols) and accompanying histograms, FS cells showed significantly broader orientation tuning of spike responses. D, There was no difference in V_m spatial frequency tuning between layer 4 RS (n = 21) and FS (n = 16) cells. However, as highlighted by the population means and histograms, FS cells showed significantly broader spike tuning for spatial frequency.

Figure 5.

A power-law relationship between mean V_m depolarization and firing rate fails to predict the broad spike response tuning of layer 4 FS cells. For each layer 4 cell, a set of power-law function parameters relating V_m and firing rate were obtained from visually evoked activity. Those parameters were then used with averaged V_m activity to predict spike output during presentation of stimuli of varying orientations. A, For an example simple layer 4 RS cell (top), the predicted spike response tuning (gray) agreed well with the observed spike tuning (black). In contrast, the predicted spike tuning for an example simple layer 4 FS cell (bottom) was narrower than the observed spike response tuning. B, The RS cell values for observed versus predicted tuning (open circles) fell along the unity line, indicating close agreement. However, the values for FS cells (filled circles) predominantly fell below the line, indicating broader observed than predicted tuning. C, Predicted and observed spike tuning curves for spatial frequency for the two cells shown in A. As before, the prediction generated by power-law fits agreed well with the observed RS cell tuning, but underestimated the spatial frequency tuning bandwidth of the FS cell. D, Plot of predicted versus observed spatial frequency tuning. RS cell data fell mostly along the unity line, but FS cell data fell predominantly below the line.

Figure 6.

Tuning of V_m fluctuations matches spike response tuning in layer 4 cells. A, Synaptically driven V_m fluctuations were measured during presentation of stimuli of varying orientations. The V_m response (black), spike response (red), and V_m fluctuation response (V_m SD; blue) tuning curves for a representative RS (top) and FS (bottom) cell in layer 4 were well tuned for orientation. Example V_m traces showing visually evoked V_m fluctuations in each cell are shown to the left. Particularly in the FS cell, the V_m SD tuning curve agreed well with the spike response tuning curve. B, The HWHH of the spike response, V_m response, and V_m SD response tuning curves were measured for each cell. Each cell's spike HWHH is plotted against the V_m HWHH (black circles) and against the V_m SD. HWHH (blue circles). For both RS (top) and FS (bottom) cells, the values for the V_m SD tuning width fell closer to the unity line than did the values for the V_m tuning width. C, Plot of the HWHH of the V_m SD response against that of the V_m response. Both RS (open circles) and FS cells (filled circles) fall above the line. D, Bar graphs show the ratios of V_m and V_m SD orientation tuning width to spike response orientation tuning width. A value of 1 indicates a good match between tuning curves. For both RS (open bars) and FS (filled bars) cells, the ratio of V_m to spike response tuning width was significantly >1 (RS, 2.20 ± 0.18; FS, 1.61 ± 0.08). In contrast, the ratio of V_m SD to spike response tuning was not significantly different from 1 for either population of cells (RS, 1.27 ± 0.21; FS, 1.06 ± 0.07). E, The BW of each cell's spike response is plotted against the V_m BW and the V_m SD BW. For both RS (top) and FS (bottom) cells, the V_m SD values fell closer to the unity line than did the values for mean V_m. F, Plot of the BW of the V_m SD response against that of the V_m response. Again, both RS and FS cells fall mostly above the unity line. G, Ratios of V_m and V_m SD tuning BW to spike response tuning BW for RS and FS cells. Again, whereas the V_m to spike response ratios were significantly >1 (RS, 1.86 ± 0.16; FS, 1.68 ± 0.11), the V_m SD to spike response ratios were not significantly different from 1 for either RS or FS cells (RS, 1.21 ± 0.15; FS, 1.05 ± 0.04). **p < 0.001. Error bars indicate SEM.

Figure 7.

A linear threshold model with tuned noise accurately predicts the spike response tuning of layer 4 FS cells. Spike response tuning curve predictions were generated using the linear threshold model alone and in combination with tuned membrane potential noise. A, Predicted and observed orientation tuning curves for an example layer 4 RS (top) and FS (bottom) cell. In each case, the LT model prediction (blue) underestimated the width of observed spike response tuning curve (black), but the LTN model with added tuned noise (red) generated an accurate prediction of tuning curve width. B, For both RS (top) and FS (bottom) cells, the LTN tuning prediction fell closer to the unity line than did the LT prediction. C, Ratios of predicted to observed spike response tuning for orientation (HWHH). A value of 1 indicates good agreement with the observed spike response tuning curve. In each case, the LT prediction (filled bars) was significantly <1 (RS, 0.69 ± 0.04; FS, 0.54 ± 0.04), whereas the LTN prediction (open bars) was not different from 1 (RS, 0.92 ± 0.06; FS, 0.97 ± 0.05). D, Predicted and observed spatial frequency tuning curves for the example cells shown in A. The LT prediction underestimated the actual spike response tuning, but the LTN prediction was a good match. E, Again, for both RS and FS cells, the LTN tuning prediction fell closer to the unity line than did the LT prediction. F, Ratios of predicted to observed spike response tuning for spatial frequency (BW). In each case, the LT prediction was significantly <1 (RS, 0.65 ± 0.04; FS, 0.54 ± 0.03), whereas the LTN prediction was not different from 1 (RS, 0.91 ± 0.02; FS, 0.96 ± 0.03), indicating a good match between the LTN prediction and the observed spike response tuning. **p < 0.001. Error bars indicate S.E.M.

Figure 8.

Synaptically evoked V_m fluctuations are faster in FS than RS cells in layer 4. A, Comparison between the visually evoked V_m fluctuations of an RS (left traces) and an FS (right traces) cell in layer 4. Portions denoted by the dashed red boxes are expanded below. Raw traces are shown on the top, followed by the same traces after spike removal. The third trace shows the rate of V_m change (|dV/dt|), calculated as the absolute value of the first derivative of the recorded trace after spike removal. Note the sharp, fast V_m fluctuations in the FS cell recording, which result in a large mean dV/dt magnitude. B, Overlaid traces of individual spikes from the RS and FS cells shown in A. Spikes have been truncated. The mean dV/dt in the 2 ms leading up to spike threshold, denoted by the dashed red lines, was faster in the FS than the RS cells. C, Biophysical membrane properties are correlated with stimulus feature selectivity in layer 4 neurons. Layer 4 RS cells are shown as open circles and layer 4 FS cells are shown as filled circles. Membrane time constant values were linearly related to the HWHH of the spike orientation tuning of layer 4 cells. Cells with lower membrane time constants exhibited less selectivity for stimulus orientation than cells with higher time constants (r² = 0.47; p < 0.0001).