Cortical evoked responses and response decrement in nonretarded and Down's syndrome individuals

Cortical evoked responses were recorded from 66 nonretarded and 66 Down's syndrome subjects. Analyses of single-trial responses from which evoked responses were averaged indicated that visual evoked response amplitude in nonretarded subjects was more dependent on variability than on amplitude of single responses. Response decrement, the tendency for evoked responses to attenuate with repetitive stimulation, was clearly observed in the evoked responses of the nonretarded subjects but was minimal in those of the Down's syndrome group. The findings support the concept that the development of inhibitory processes in Down's syndrome persons' brains may be halted or delayed at an early age.