Fig. 1. Excitatory and inhibitory synaptic strengths control the gain and threshold of the neuronal input-output function.

A) Topology of the simulated feed-forward inhibitory circuit.

B) Sample voltage responses of the Ex unit at different input intensities (see text), for a particular combination of Ex→Ex and Inh→Ex synaptic weights (number 2 in panel D). Voltage traces were colored gray after the peak to ease the visualization of overlapping lines.

C) I/O function of the Ex unit in panel B, obtained by plotting the action potential probability versus the EPSP slope of the voltage traces (in bins, see text and Experimental Procedures).

D) Parameter scan of the excitatory and inhibitory synapse space. At each coordinate an I/O function was determined for the corresponding Ex→Ex and Inh→Ex synaptic weights. The numbers in the foreground depict the individual I/O functions plotted in panel E. Top: the gain (inverse) of each I/O function is plotted in color (range: [0.09–1.10] ms/mV). Hot colors depict an I/O function with a shallow slope, while cold colors depict an I/O function with a very sharp slope. Black depicts coordinates in which the inhibitory synapses were so strong that the Ex unit never fired. In gray the Ex unit fired occasionally, but not yielding enough points to be fitted with a sigmoid. Bottom: as above, but plotting the threshold of the same I/O curves (range: [10–20] mV/ms). Hot colors depict I/O functions with high threshold while cold colors depict I/O functions with low threshold. The dashed arrow highlights that a single I/O function is defined by two properties (gain and threshold).

E) Sample individual I/O functions. The gain and threshold of these sigmoids are highlighted in the corresponding plots in panel D by the corresponding numbers.