Figure 2. Sample records showing three types of responses to IR.

A, excitatory ‘on’ (n = 18), B, inhibitory ‘off’ (n = 35), and C, mixed inhibitory-excitatory (n = 7). Exponential fits (red lines) illustrate the time course of afferent responses. A, an excitatory afferent, increased its firing rate from 63 spikes s⁻¹ to 124 spikes s⁻¹ in response to IR (50 pps, 228 μJ) with an onset time constant τ_e≈ 3.28 s and recovered to pre-stimulus rate with time constant τ_er≈ 3.9 s. B, inhibitory afferent, reduced its firing rate from 82 to 8 spikes s⁻¹ in the presence of IR (100 pps, 358 μJ); onset time constant τ_i≈ 3.44 s and recovery time constant τ_ir∼ 1.37 s. C, mixed afferents, initial inhibition followed by excitation. In a subset of afferents excitation eventually exceeded inhibition leading to an increased discharge rate near the end of the stimulus (e.g. black, associated time constants were τ_mi≈ 0.29 s, τ_me≈ 3.7 s and τ_mr≈ 3.49 s), while in other afferents inhibition dominated during IR but excitation dominated during the recovery period (e.g. green). D and E, example graded dose–response relationship from 122 to 1107 μJ pulse⁻¹ at 100 pps. The change in firing rate (i.e. the difference between the afferent's background firing rate and the firing rate with IR stimulation) with IR radiant energy per pulse is shown in E. For this afferent, the IR-induced change in firing rate occurred in a sigmoidal fashion with radiant energy (red curve).