The effect of Parkinson's disease on interference control during action selection

Basal ganglia structures comprise a portion of the neural circuitry that is hypothesized to coordinate the selection and suppression of competing responses. Parkinson's disease (PD) may produce a dysfunction in these structures that alters this capacity, making it difficult for patients with PD to suppress interference arising from the automatic activation of salient or overlearned responses. Empirical observations thus far have confirmed this assumption in some studies, but not in others, due presumably to considerable inter-individual variability among PD patients. In an attempt to help resolve this controversy, we measured the performance of 50 PD patients and 25 healthy controls on an arrow version of the Eriksen flanker task in which participants were required to select a response based on the direction of a target arrow that was flanked by arrows pointing in the same (congruent) or opposite (incongruent) direction. Consistent with previous findings, reaction time (RT) increased with incongruent flankers compared to congruent or neutral flankers, and this cost of incongruence was greater among PD patients. Two novel findings are reported. First, distributional analyses, guided by dual-process models of conflict effects and the activation-suppression hypothesis, revealed that PD patients are less efficient at suppressing the activation of conflicting responses, even when matched to healthy controls on RT in a neutral condition. Second, this reduced efficiency was apparent in half of the PD patients, whereas the remaining patients were as efficient as healthy controls. These findings suggest that although poor suppression of conflicting responses is an important feature of PD, it is not evident in all medicated patients.

Inefficient response inhibition in individuals with mild cognitive impairment

Individuals diagnosed with mild cognitive impairment (MCI) show primary deficits in memory and are at increased risk for developing Alzheimer's disease (AD). In light of recent evidence that executive cognitive deficits are common in AD and may be detectable in individuals diagnosed with MCI, we extend these findings to the investigation of response inhibition, an essential aspect of executive cognitive control. Twenty MCI patients and 20 healthy controls (HC) completed an arrow version of the flanker task [Eriksen, B. A., & Eriksen, C. W. (1974). Effects of noise letters upon the identification of target letters in a non-search task. Perception & Psychophysics, 16, 143-149] in which participants responded to a target arrow surrounded by distractors (i.e., flankers) that signaled a same (congruent) or a conflicting (incongruent) response. Reaction time (RT) increased in both groups when flankers signaled an incongruent response, but more so among MCI patients. MCI patients taking a cholinesterase inhibitor showed smaller flanker interference effects than those not taking this medication. Analysis of the flanker effect as a function of the entire RT distribution indicated that MCI patients show increasing interference at the slowest segments of the distribution, a finding that implicates deficient inhibition of the incongruent response [Ridderinkhof, K. R. (2002). Activation and suppression in conflict tasks: Empirical clarification through distributional analyses. In W. Prinz & B. Hommel (Eds.), Common mechanisms in perception and action. Attention & performance, Vol. XIX (pp. 494-519). Oxford: Oxford University Press]. These results suggest that deficits in response inhibition are detectable in MCI patients and merit further investigation as to whether these changes aid prediction of which MCI patients convert to AD.

Activation of conflicting responses in Parkinson's disease: evidence for degrading and facilitating effects on response time

Response selection often occurs in a context of competition among conflicting responses. According to recent models, the basal ganglia may play an integral role in resolving this competition by focusing the selection and inhibition of responses. We hypothesized that basal ganglia dysfunction produced by Parkinson's disease (PD) disrupts selection among conflicting responses. Using a version of the Eriksen flanker task, we tested the specific prediction that individuals with PD would experience greater response interference when distractors in the visual field activate a response that conflicts with the target response. In addition, we investigated whether greater response interference induced by these distractors could actually reduce normal response time costs in PD when the task required production of the response opposite the target. Compared to 16 healthy controls (HC), 16 individuals with PD showed an exacerbated slowing when target and distracting stimuli corresponded to conflicting responses. No group differences occurred when targets and distractors corresponded to the same response. Furthermore, the slowing induced by the distractors was reduced in both groups, but more so in PD, when execution of a response opposite the target response (i.e. incompatible response) was required. Moreover, among individuals with PD, the magnitude of the interference produced by the distractors was related to clinical ratings of bradykinesia. These findings are consistent with the hypothesis that basal ganglia dysfunction due to Parkinson's disease disrupts processes that resolve response conflict.

On-line anosognosia: Unawareness for chorea in real time but not on videotape delay

In hemiplegics, anosognosia (unawareness of deficit) rests on a mismatch between expected and actual movement: a feedback hypothesis emphasizes sensory deficits or neglect, a feedforward hypothesis postulates impaired intention to move. Anosognosia for other problems is less studied. The authors report a man without sensory deficits who was unaware of choreiform movements, except on videotape delay. The authors believe that a feed-forward mechanism underlies his "on-line" unawareness.

Divergent findings regarding negative priming in Parkinson's disease: A comment on Filoteo et al. (2002) and Wylie and Stout (2002)

This commentary discusses divergent findings in 2 articles published in this issue of Neuropsychology. The studies used negative priming (NP) to probe the associations between basal ganglia function and cognition in Parkinson's disease (PD) and tested different predictions about NP in PD. Different NP tasks were used, and although the subject samples appeared to have similar clinical features, results were quite different. This commentary, written jointly by the authors of the 2 studies (J. V. Filoteo, L. M. Rilling, & D. L. Strayer, 2002; S. A. Wylie & J. C. Stout, 2002), describes a process by which their disparate results may be used to facilitate the design of new studies that may determine how specific features of NP tasks lead to different findings in PD. The results are a more systematic account of how task features, such as specific response demands, interact with the response selection processes that are implemented by the basal ganglia.

Enhanced negative priming in Parkinson's disease

In the ignored repetition paradigm, negative priming (NP) is defined as the increase in response time that occurs when the current target stimulus served as a distractor stimulus in the previous trial. In this study, 25 Parkinson's disease (PD) participants and 17 age-matched healthy controls (HCs) were tested using a touchscreen version of the ignored repetition task that allowed response time to be partitioned into response initiation and response execution segments. In both groups, NP effects were stronger in the response execution than in the response initiation segments. The most striking result was that the PD group showed larger NP effects overall than the HC group. In PD, clinical ratings of bradykinesia, but not tremor, were related to larger NP effects. Results indicate that in PD, disruption of dopamine neuromodulation diminishes response efficiency when action must be directed toward previously ignored information.

Divergent findings regarding negative priming in Parkinson's disease: A comment on Filoteo et al. (2002) and Wylie and Stout (2002)

This commentary discusses divergent findings in 2 articles published in this issue of Neuropsychology. The studies used negative priming (NP) to probe the associations between basal ganglia function and cognition in Parkinson's disease (PD) and tested different predictions about NP in PD. Different NP tasks were used, and although the subject samples appeared to have similar clinical features, results were quite different. This commentary, written jointly by the authors of the 2 studies (J. V. Filoteo, L. M. Rilling, & D. L. Strayer, 2002; S. A. Wylie & J. C. Stout, 2002), describes a process by which their disparate results may be used to facilitate the design of new studies that may determine how specific features of NP tasks lead to different findings in PD. The results are a more systematic account of how task features, such as specific response demands, interact with the response selection processes that are implemented by the basal ganglia.

Is the age-complexity effect mediated by reductions in a general processing resource?

The rate of information processing, as revealed in measures of reaction time, slows with advancing age and this slowing is most evident as processing complexity increases. This phenomenon, known as the Age-Complexity effect, has been attributed to general changes in the speed of processing that affect all components of processing indiscriminantly, both within and across tasks in a particular processing domain. That the slowing is thought to be task- and process-independent has led to the additional inference that it reflects reductions in a general processing resource. On the basis of converging evidence identified in a review of both behavioral and chronopsychophysiological studies, we argue that the slowing induced by older age is not generalized, but rather is both task-dependent and process-specific and, as such, cannot be explained in terms of a diminished general processing resource. We close by speculating that elements of the age-induced slowing can be interpreted within the context of the cognitive-energetical model.