Thus, a noise stimulus circumvents the threshold nonlinearity, resulting in a spiking receptive field map that is comparable to that recorded directly from Vm responses (Mohanty et al., 2012). Threshold is also likely to provide the explanation for why pharmacological blockade of GABAA-mediated inhibition broadens orientation tuning in cortical cells (Sillito, 1975). Blocking inhibition appears to increase the overall excitability of cortical neurons such that previously ineffective stimuli on the edges of the spike-rate tuning curve become suprathreshold (Katzner et al., 2011). Up to now, we have considered receptive field properties in the spatial

Anti-diabetic Compound Library purchase domain—that two stimuli of different orientations suppress one another, that orientation tuning is contrast invariant, selleck chemicals and that the width of orientation tuning is narrower than predictions based on the receptive field map. Here we consider three temporal aspects of simple cell responses that also fail to emerge from the simplest forms of the feedforward model. First, simple cells do not respond well to rapidly changing stimuli. Compared to LGN cells, the preferred temporal frequencies (TFs) of simple cells are lower by a factor of

2 (Hawken et al., 1996 and Orban et al., 1985). Here, temporal frequency refers to the number of bars of the drifting grating that pass over the receptive field in each second. Compare, for example, the TF tuning of the LGN cell in Figure 6A (black) and the simple cell in Figure 6C (black). The peaks of the tuning curves are shifted relative to one another, as are the TF50 values first (arrows; the frequency at which

the response amplitude falls to 50% of its peak). Note that the simple cell’s Vm responses (Figure 6B) fall somewhere between the LGN and the simple cell’s spike responses (Figures 6A and 6C). This mismatch in preferred TF between LGN and cortex does not represent a nonlinearity; a linear, low-pass RC filter could shift the peak frequency of a simple cell’s output relative to its input. The second temporal feature of simple cells is that the preferred TF in simple cells decreases almost 2-fold with decreasing stimulus contrast (Albrecht, 1995, Carandini et al., 1997, Hawken et al., 1996, Holub and Morton-Gibson, 1981 and Reid et al., 1992). Compare, for example, the black and gray tuning curves in Figure 6C. This property does represent a nonlinearity: the transformation between stimulus and response changes with stimulus strength (contrast). One element that surely contributes to the mismatch in preferred TF between simple cells and their synaptic input from the LGN is the membrane time constant, τ. Together, the membrane input resistance R and membrane capacitance C form a linear low-pass filter with a time constant τ = RC, which lies near 15 ms for most simple cells ( Anderson et al., 2000). The frequency at which such a filter attenuates its input by a factor of 2 (f3dB = 1/2πτ) is about 11 Hz.