Time-course of masked response priming and inhibition in Parkinson's disease

Parkinson's disease patients have enhanced interference effects arising from the conflict between competing responses, as probed in various 'conflict tasks'. The possibility that this is due to an inhibitory deficit received recent support from a masked response priming task [Seiss, E., & Praamstra, P. (2004). The basal ganglia and inhibitory mechanisms in response selection: Evidence from subliminal priming of motor responses in Parkinson's disease. Brain, 127, 330-339]. The added information from a masked priming task is that the introduction of a delay between presentation of prime and target stimuli reveals an inhibition of the covert response activation induced by the masked prime stimulus. This inhibition results in a reversal of normal priming effects, such that performance is better with incompatible than with compatible prime-target pairs. We previously found that this reversal is attenuated in Parkinson's disease, when tested at a prime-target delay of 100 ms, thus revealing deficient inhibition of covert response activation. The present study was undertaken to investigate the time course of the underlying inhibition process, using five prime-target ISIs between 0 and 200 ms. While we reproduced the attenuation at ISI 100 ms, the time course information revealed that the rate of change of the compatibility effect over ISIs from 0 to 200 ms was identical for patients and controls. This result indicates that the inhibition underlying the reversal of masked priming effects is normal in Parkinson's disease.