Fig. 1. Gain modulation and threshold shift. (A) The `gain' of a sensory neuron is the slope of the function relating firing rate to stimulus intensity. The gain associated with Function 1 decreases with increased intensity. The differential response R₁ to a small variation in stimulus S decreases as the stimulus itself increases. Changing the shape of the curve (2) can be described as gain modulation. The response R₂ to the same S and hence the gain is smaller at the given stimulus intensity. Shifting the response along the intensity axis (3) is better described as a shift in threshold. (B) Higher gain may limit the range of stimuli that can be described by a sensory neuron. Neurons described by Functions 1 and 2 both have a maximum firing rate R_max. The gain for Neuron 1 is larger than that for Neuron 2 for stimulus intensities smaller than S₀. Neuron 1 cannot unambiguously describe stimulus intensities larger than S₀, because its firing rate has saturated and its gain has dropped to zero.