Inhibition of cortical responses to Aδ inputs by a preceding C-related response: Testing the “first come, first served” hypothesis of cortical laser evoked potentials

Although laser pulses activate concomitantly Aδ and C fibres, the corresponding brain evoked responses remain strictly limited to the Aδ component, without any potential consistent with C-fibre activation. To investigate whether this phenomenon depends on the order of arrival to the cortex (“first come first served” hypothesis) or is simply explained by A-to-C inhibition and/or lower energy associated with the desynchronised C-fibre input, we devised an experiment where the physiological order of arrival to the cortex was artificially inverted. Following a conditioning C-pulse, the cortical response to a second laser stimulus was significantly attenuated, whether it was Aδ or C. Thus, a C-volley was able to depress the response to a subsequent Aδ stimulus, in support of the “first come first served” hypothesis. However, the conditioning C-fibre stimulus attenuated significantly more a subsequent C-volley than a subsequent Aδ-volley, indicating that the suppression effect does not depend solely on the order of arrival to the cortex, but also on the ratio of energy per unit time conveyed by the successive inputs. This supports the notion that cortical evoked potentials to laser pulses (and probably to other sensory stimuli) reflect networks detecting rapid energy changes relative to a preceding baseline. The output of such networks should depend both on the time elapsed between successive inputs and on the relative energy per unit time conveyed by successive volleys. Such dedicated networks aimed at detecting energy changes may be related to orienting reactions, and can be dissociated from subjective perception.