Lack of inhibition in Parkinson's disease: evidence from a lexical decision task

Electrophysiological alterations during action semantic processing in Parkinson's disease

Assessments of action semantics consistently reveal markers of Parkinson's disease (PD). However, neurophysiological signatures of the domain remain under-examined in this population, especially under conditions that allow patients to process stimuli without stringent time constraints. Here we assessed event-related potentials and time-frequency modulations in healthy individuals (HPs) and PD patients during a delayed-response semantic judgment task involving related and unrelated action-picture pairs. Both groups had shorter response times for related than for unrelated trials, but they exhibited discrepant electrophysiological patterns. HPs presented significantly greater N400 amplitudes as well as theta enhancement and mu desynchronization for unrelated relative to related trials. Conversely, N400 and theta modulations were abolished in the patients, who further exhibited a contralateralized cluster in the mu range. None of these patterns were associated with the participants' cognitive status. Our results suggest that PD involves multidimensional neurophysiological disruptions during action-concept processing, even under task conditions that elicit canonical behavioral effects. New constraints thus emerge for translational neurocognitive models of the disease.

Neurocognitive correlates of semantic memory navigation in Parkinson's disease

Cognitive studies on Parkinson's disease (PD) reveal abnormal semantic processing. Most research, however, fails to indicate which conceptual properties are most affected and capture patients' neurocognitive profiles. Here, we asked persons with PD, healthy controls, and individuals with behavioral variant frontotemporal dementia (bvFTD, as a disease control group) to read concepts (e.g., 'sun') and list their features (e.g., hot). Responses were analyzed in terms of ten word properties (including concreteness, imageability, and semantic variability), used for group-level comparisons, subject-level classification, and brain-behavior correlations. PD (but not bvFTD) patients produced more concrete and imageable words than controls, both patterns being associated with overall cognitive status. PD and bvFTD patients showed reduced semantic variability, an anomaly which predicted semantic inhibition outcomes. Word-property patterns robustly classified PD (but not bvFTD) patients and correlated with disease-specific hypoconnectivity along the sensorimotor and salience networks. Fine-grained semantic assessments, then, can reveal distinct neurocognitive signatures of PD.

Reading Difficulties in Parkinson's Disease: A Stepped Care Model for Neurovisual Rehabilitation

BACKGROUND

People with Parkinson's disease (PD) frequently experience reading difficulties. Little is known about what functional impairments distinguish people with PD with and without reading difficulties and how these should guide rehabilitation.

OBJECTIVE

To provide concrete advice for an efficient stepped care model for reading difficulties in PD, based on extensive functional assessments.

METHODS

This study included 74 people with PD in a neurovisual rehabilitation setting who underwent assessment of visual, visuoperceptual, and cognitive functions. Outcomes were compared between those with frequent (RD+; N = 55) and infrequent reading difficulties (RD-; N = 19). Aids and advice provided during rehabilitation were registered.

RESULTS

Only a few functions appeared to distinguish RD+ and RD-. Visual functions (i.e., contrast sensitivity, g = 0.76; reading acuity, g = 0.66; visual acuity, g = 0.54) and visuoperceptual functions (i.e., visual attention, g = 0.58, visual motor speed, g = 0.56) showed significant worse scores in RD+ compared to RD-. Aids and advice applied consisted mainly of optimizing refraction, improving lighting, and optimizing text size and spacing.

CONCLUSION

The test battery showed significant differences between RD+ and RD-on only a few tests on visual and visuoperceptual functions. The applied aids and advice matched well with these impairments. Therefore, we recommend a stepped care model, starting with a short test battery on these functions. If this battery indicates functional impairments, this can be followed by standard aids and advice to improve reading. Only in case of insufficient effect additional testing should take place.

Reverse Visually Guided Reaching in Patients with Parkinson’s Disease

In addition to motor symptoms such as difficulty in movement initiation and bradykinesia, patients with Parkinson’s disease (PD) display nonmotor executive cognitive dysfunction with deficits in inhibitory control. Preoperative psychological assessments are used to screen for impulsivity that may be worsened by deep brain stimulation (DBS) of the subthalamic nucleus (STN). However, it is unclear whether anti-Parkinson’s therapy, such as dopamine replacement therapy (DRT) or DBS, which has beneficial effects on motor function, adversely affects inhibitory control or its domains. The detrimental effects of STN-DBS are more apparent when tasks test the inhibition of habitual prepotent responses or involve complex cognitive loads. Our goal was to use a reverse visually guided reaching (RVGR) task, a hand-based version of the antisaccade task, to simultaneously measure motor performance and response inhibition in subjects with PD. We recruited 55 healthy control subjects, 26 PD subjects receiving treatment with DRTs, and 7 PD subjects receiving treatment with STN-DBS and DRTs. In the RVGR task, a cursor moved opposite to the subject’s hand movement. This was compared to visually guided reaching (VGR) where the cursor moved in the same direction as the subject’s hand movement. Reaction time, mean speed, and direction errors (in RVGR) were assessed. Reaction times were longer, and mean speeds were slower during RVGR compared to VGR in all three groups but worse in untreated subjects with PD. Treatment with DRTs, DBS, or DBS + DRT improved the reaction time and speed on the RVGR task to a greater extent than VGR. Additionally, DBS or DBS + DRT demonstrated an increase in direction errors, which was correlated with decreased reaction time. These results show that the RVGR task quantifies the benefit of STN-DBS on bradykinesia and the concomitant reduction of proactive inhibitory control. The RVGR task has the potential to be used to rapidly screen for preoperative deficits in inhibitory control and to titrate STN-DBS, to maximize the therapeutic benefits on movement, and minimize impaired inhibitory control.

Altered Inhibitory Mechanisms in Parkinson's Disease: Evidence From Lexical Decision and Simple Reaction Time Tasks

INTRODUCTION

Although the motor signs of Parkinson's disease (PD) are well defined, nonmotor symptoms, including higher-level language deficits, have also been shown to be frequent in patients with PD. In the present study, we used a lexical decision task (LDT) to find out whether access to the mental lexicon is impaired in patients with PD, and whether task performance is affected by bradykinesia.

MATERIALS AND METHODS

Participants were 34 nondemented patients with PD, either without (off) medication (n = 16) or under optimum (on) medication (n = 18). A total of 19 age-matched control volunteers were also recruited. We recorded reaction times (RTs) to the LDT and a simple RT (control) task. In each task, stimuli were either visual or auditory. Statistical analyses consisted of repeated-measures analyses of variance and Tukey's HSD post hoc tests.

RESULTS

In the LDT, participants with PD both off and on medication exhibited intact access to the mental lexicon in both modalities. In the visual modality, patients off medication were just as fast as controls when identifying real words, but slower when identifying pseudowords. In the visual modality of the control task, RTs for pseudowords were significantly longer for PD patients off medication than for controls, revealing an unexpected but significant lexicality effect in patients that was not observed in the auditory modality. Performances of patients on medication did not differ from those of age-matched controls.

DISCUSSION

Motor execution was not slowed in patients with PD either off or on medication, in comparison with controls. Regarding lexical access, patients off medication seemed to (1) have difficulty inhibiting a cognitive-linguistic process (i.e., reading) when it was not required (simple reaction time task), and (2) exhibit a specific pseudoword processing deficit in the LDT, which may have been related to impaired lateral word inhibition within the mental lexicon. These deficits seemed to be compensated by medication.

The implementation of expectancy-based strategic processes is delayed in normal aging

The present research examined if the time needed to implement expectancy-based strategic processes is different in younger and healthy older adults. In four experiments participants from both age groups performed different strategic priming tasks. These included a greater proportion of incongruent (or unrelated; 80%) than of congruent (or related; 20%) trials. With this procedure performance is worse for congruent (less frequent) than for incongruent (more frequent) trials, thus demonstrating that the relative frequency information can be used to predict the upcoming target. To explore the time course of these expectancy-based effects, the prime-target SOA was manipulated across experiments through a range of intervals: 400, 1000 and 2000 ms. Participants also performed a change localization and an antisaccade task to assess their working memory and attention control capacities. The results showed that increases in age were associated with (a) a slower processing-speed, (b) a decline in WM capacity, and (c) a decreased capacity for attentional control. The latter was evidenced by a disproportionate deterioration of performance in the antisaccade trials compared to the prosaccade ones in the older group. Results from the priming tasks showed a delay in the implementation of expectancies in older adults. Whereas younger participants showed strategic effects already at 1000 ms, older participants consistently failed to show expectancy-based priming during the same interval. Importantly, these effects appeared later at 2000 ms, being similar in magnitude to those by the younger participants and unaffected by task practice. The present findings demonstrate that the ability to implement expectancy-based strategies is slowed down in normal aging.

The Suppression of Irrelevant Semantic Representations in Parkinson's Disease

The impairment of lexical-semantic inhibition mechanisms in Parkinson's disease (PD) remains a source of contention. In order to observe whether people with PD are able to suppress irrelevant semantic information during picture naming, the present study employed an object-based negative priming paradigm with 16 participants with PD and 13 healthy controls. The task required participants to name a red target image while ignoring a superimposed, green distractor image. The semantic relationship between the distractor image and the target image of the subsequent trial was manipulated, such that the distractor image was identical, semantically related, or semantically unrelated to said target image. The PD group and the control group were slower in naming a target image that had previously served as a distractor image, relative to naming a target image that was unrelated to the previous distractor image. Thus, a negative priming effect was present in both groups. Furthermore, no significant difference in the magnitude of this effect was observed between the control and PD groups. When considered in the context of existing literature surrounding negative priming in PD, these results suggest that inhibition is subserved by multiple, domain-specific mechanisms and that the inhibitory processing of visual-semantic stimuli is intact in PD.

Computerized testing in Parkinson's disease: Performance deficits in relation to standard clinical measures

OBJECTIVE

This study assessed deficits associated with Parkinson's disease (PD) at two time points separated by 1 year using a computerized neuropsychological battery, and determined interrelationships with conventional clinical measures of cognitive functioning (Montreal Cognitive Assessment; MoCA) and motor impairment (Part III of the Unified PD Rating Scale; UPDRS), as well as other factors known to influence cognitive dysfunction in PD.

METHOD

Participants included 37 with PD and 47 controls. Linear mixed-effects models were developed for each computerized task.

RESULTS

Results showed that the PD group performed worse than controls on all of the computerized tasks at both time points. In contrast, MoCA scores differed between PD and controls only at follow-up. However, the MoCA detected decline over the year in the PD group, whereas only one of the computerized tasks did. In both groups, higher MoCA scores predicted better performance on some but not all of the computerized tasks. Surprisingly, UPDRS-rated motor impairment did not predict performance on any of the computerized tasks, and aside from older age, which predicted poorer performance on all but one task, the other factors-education, affective and impulsivecompulsive symptoms, sleep quality, dopaminergic medication-generally had no relationship with performance on the computerized tasks.

CONCLUSIONS

The presence of performance deficits for all of the computerized tasks in the PD group compared to controls, but not for the MoCA at initial testing, indicates that the computerized battery was better able to detect deficits. However, in contrast to the MoCA, the current results call into question the suitability of the computerized battery as measured here for tracking decline.

Lexical ambiguity resolution during sentence processing in Parkinson's disease: An event-related potential study

Event-related potentials (ERPs) were recorded to investigate lexical ambiguity resolution during sentence processing in 16 people with Parkinson's disease (PD) and 16 healthy controls. Sentences were presented word-by-word on computer screen, and participants were required to decide if a subsequent target word was related to the meaning of the sentence. The task consisted of related, unrelated and ambiguous trials. For the ambiguous trials, the sentence ended with an ambiguous word and the target was related to one of the meanings of that word, but not the one captured by the sentence context (e.g., 'He dug with the spade', Target 'ACE'). Both groups demonstrated slower reaction times and lower accuracy for the ambiguous condition relative to the unrelated condition, however accuracy was impacted by the ambiguous condition to a larger extent in the PD group. These results suggested that PD patients experience increased difficulties with contextual ambiguity resolution. The ERP results did not reflect increased ambiguity resolution difficulties in PD, as a similar N400 effect was evident for the unrelated and ambiguous condition in both groups. However, the magnitude of the N400 for these conditions was correlated with a measure of inhibition in the PD group, but not the control group. The ERP results suggest that semantic processing may be more compromised in PD patients with increased response inhibition deficits.

Time Course of Lexicalization During Sentence Production in Parkinson's Disease: Eye-Tracking While Speaking

PURPOSE

Growing evidence suggests that sentence formulation is affected in Parkinson's disease (PD); however, how speakers with PD coordinate sentence planning and speaking remains unclear. Within 2 competing models of sentence production, this study examined whether speakers with PD show advanced buffering of words to minimize disfluencies and increased demands during speech or whether they plan one word at a time, compromising accuracy and fluency of speech.

METHOD

Participants described 3 computer-displayed pictures using the sentence "the A and the B are above the C." Name agreement (codability) was varied to be high (clock) or low (sofa/couch) for each object position (A, B, C), affecting difficulty of lexical selection. Participants' gaze durations to each object were recorded.

RESULTS

Speakers with PD showed incremental word-by-word planning, retrieving only the first lexical item (A) before speech onset, similar to controls. However, they produced greater word-finding errors and disfluencies compared to controls for the low-codable pictures, but not for high-codable pictures.

CONCLUSIONS

These findings suggest that by following word-by-word incremental production, speakers with PD compromise fluency and accuracy of speech to a greater extent than healthy older speakers and that PD is associated with impaired inhibitory control during lexical selection.

Reading comprehension in Parkinson's disease

PURPOSE

Although individuals with Parkinson's disease (PD) self-report reading problems and experience difficulties in cognitive-linguistic functions that support discourse-level reading, prior research has primarily focused on sentence-level processing and auditory comprehension. Accordingly, the authors investigated the presence and nature of reading comprehension in PD, hypothesizing that (a) individuals with PD would display impaired accuracy and/or speed on reading comprehension tests and (b) reading performances would be correlated with cognitive test results.

METHOD

Eleven adults with PD and 9 age- and education-matched control participants completed tests that evaluated reading comprehension; general language and cognitive abilities; and aspects of attention, memory, and executive functioning.

RESULT

The PD group obtained significantly lower scores on several, but not all, reading comprehension, language, and cognitive measures. Memory, language, and disease severity were significantly correlated with reading comprehension for the PD group.

CONCLUSION

Individuals in the early stages of PD without dementia or broad cognitive deficits can display reading comprehension difficulties, particularly for high- versus basic-level reading tasks. These reading difficulties are most closely related to memory, high-level language, and PD symptom severity status. The findings warrant additional research to delineate further the types and nature of reading comprehension impairments experienced by individuals with PD.

Evidence inhibition responds reactively to the salience of distracting information during focused attention

Along with target amplification, distractor inhibition is regarded as a major contributor to selective attention. Some theories suggest that the strength of inhibitory processing is proportional to the salience of the distractor (i.e., inhibition reacts to the distractor intensity). Other theories suggest that the strength of inhibitory processing does not depend on the salience of the distractor (i.e., inhibition does not react to the distractor intensity). The present study aimed to elucidate the relationship between the intensity of a distractor and its subsequent inhibition during focused attention. A flanker task with a variable distractor-target stimulus-onset asynchrony (SOA) was used to measure both distractor interference and distractor inhibition. We manipulated the intensity of the distractor in two separate ways, by varying its distance from the target (Experiment 1) and by varying its brightness (Experiment 2). The results indicate that more intense distractors were associated with both increased interference and stronger distractor inhibition. The latter outcome provides novel support for the reactive inhibition hypothesis, which posits that inhibition reacts to the strength of distractor input, such that more salient distractors elicit stronger inhibition.

Identity-based inhibitory processing during focused attention

Recent data suggest that the operating principles governing inhibition of distractor-related activity may deviate from dominant models describing inhibitory processing during selective attention. Here we aimed to gain a better understanding of these data in order to determine whether they actually defy premises of current models. In addition to providing evidence against noninhibitory accounts of the data (see especially Experiment 6), the results support three main novel findings that challenge current theories. First, the data provide evidence that inhibition overpowered excitation from ongoing external input (Experiments 1-4), which suggests that inhibitory control processes are more powerful than current models indicate. Second, negative effects emerged even when targets appeared alone (Experiment 5), which suggests that selection does not play an essential role in triggering inhibitory processing. Third, relatively early distractor-related activity was affected, which supports a role for inhibition prior to action control (Experiment 3). These findings suggest a need to revise current models describing inhibition of distracting information during selective attention.

Response preparation in Parkinson's disease: automatic vs. controlled processing

Using a finger cuing paradigm, we investigated response preparation in Parkinson's disease (PD). The central question was whether PD individuals are differentially affected by preparatory cues that specify a more automatic response set configuration (that induces within-hand preparation) as opposed to a more controlled one (that induces between-hands preparation). Reaction times (RTs) and error rates were measured in 20 non-demented individuals with PD and 20 healthy control participants with a long and short preparation interval (500 ms and 2000 ms). RT benefits and/or costs were measured for cues indicating a within- and between-hands motor preparatory set. Overall, RTs were significantly longer, and errors more frequent, for PD participants than for control participants. More importantly, in comparison with control subjects, PD individuals showed a significant deficit in between-hands preparation but not in within-hand preparation. Furthermore, longer preparation intervals slowed down cued RTs of the control participants, but not those of the PD individuals. Together, these findings suggest that whereas automatic response preparation processes are spared in PD, controlled response preparation processes operate at a slower rate and/or are delayed in time.

Acute nicotine enhances strategy-based semantic processing in Parkinson's disease

Nicotinic mechanisms may play a role in the cognitive deficits of Parkinson's disease (PD). Recently, on a cognitively demanding strategy-based priming task, nicotine selectively affected controlled semantic processing in young adult non-smokers as reported by Holmes et al. (International Journal of Neuropsychopharmacology 11, 389-399, 2008). Such controlled semantic processing is compromised in PD. This study investigated the effects of acute transdermal nicotine on controlled semantic processing in non-smokers with PD (n = 10) and non-smoking matched controls (n = 16) using a strategy-based semantic priming paradigm. Transdermal nicotine patches (7 mg/24 h) were administered in a double-blind, placebo-controlled, crossover design. Participants were instructed to expect target words from specified semantic categories based on the primes, while unexpected targets were also presented. Priming conditions included those concurring with trained expectations (expected-related and expected-unrelated), those which did not (unexpected-related and unexpected-unrelated), and neutral-baseline conditions. Controls evidenced significant expectancy effects (i.e. reaction-time differences for expected vs. unexpected conditions) under both drug states. An expectancy effect was not evident for PD under placebo due to a lack of reaction-time slowing for unexpected conditions. However, under nicotine an expectancy effect was present for PD at a level comparable to controls. Overall the findings indicate that nicotine can improve impaired controlled semantic processing in PD possibly via enhanced expectancy or inhibitory mechanisms.

Famous faces demand attention due to reduced inhibitory processing

People have particular difficulty ignoring distractors that depict faces. This phenomenon has been attributed to the high level of biological significance that faces carry. The current study aimed to elucidate the mechanism by which faces gain processing priority. We used a focused attention paradigm that tracks the influence of a distractor over time and provides a measure of inhibitory processing. Upright famous faces served as test stimuli and inverted versions of the faces as well as upright non-face objects served as control stimuli. The results revealed that although all of the stimuli elicited similar levels of distraction, only inverted distractor faces and non-face objects elicited inhibitory effects. The lack of inhibitory effects for upright famous faces provides novel evidence that reduced inhibitory processing underlies the mandatory nature of face processing.

Visual spatial cognition in neurodegenerative disease

Visual spatial impairment is often an early symptom of neurodegenerative disease; however, this multi-faceted domain of cognition is not well-assessed by most typical dementia evaluations. Neurodegenerative diseases cause circumscribed atrophy in distinct neural networks, and accordingly, they impact visual spatial cognition in different and characteristic ways. Anatomically-focused visual spatial assessment can assist the clinician in making an early and accurate diagnosis. This article will review the literature on visual spatial cognition in neurodegenerative disease clinical syndromes, and where research is available, by neuropathologic diagnoses. Visual spatial cognition will be organized primarily according to the following schemes: bottom-up/top-down processing, dorsal/ventral stream processing, and egocentric/allocentric frames of reference.

Decreased semantic competitive inhibition in Parkinson's disease: Evidence from an investigation of word search performance

Aberrant semantic competitive inhibition has been reported in Parkinson's disease (PD). Whether PD-related alterations cause an increase or a decrease in lateral inhibition, however, remains unclear. Accordingly, the present study aimed to examine semantic inhibition during lexical-semantic processing in non-demented people with PD. Twenty-two people with PD and 18 matched controls completed a computerized word search task in which both the relationship between the background items and the target (related or unrelated) and the search type (open e.g., any dog or closed e.g., collie) were manipulated. It was hypothesized that decreased semantic inhibition would be evidenced by abnormally short response times for open searches among words related to the target, while increased inhibition would lead to abnormally long response times. Analysis of the results revealed that control participants performed open searches faster for unrelated vs related word lists. In contrast, the PD group recorded similar response times regardless of background items. Hence, the present findings are consistent with the notion of decreased semantic competitive inhibition in PD and suggest that an impaired ability to inhibit unwanted information during lexical retrieval may underlie observed deficits on semantic tasks such as verbal fluency.

Comparison of smooth pursuit eye movement deficits in multiple system atrophy and Parkinson's disease

Because of the large overlap and quantitative similarity of eye movement alterations in Parkinson's disease (PD) and multiple system atrophy (MSA), a measurement of eye movement is generally not considered helpful for the differential diagnosis. However, in view of the pathophysiological differences between MSA and PD as well as between the cerebellar (MSA-C) and Parkinsonian (MSA-P) subtypes of MSA, we wondered whether a detailed investigation of oculomotor performance would unravel parameters that could help to differentiate between these entities. We recorded eye movements during sinusoidal pursuit tracking by means of video-oculography in 11 cases of MSA-P, 8 cases of MSA-C and 27 cases of PD and compared them to 23 healthy controls (CTL). The gain of the smooth pursuit eye movement (SPEM) component exhibited significant group differences between each of the three subject groups (MSA, PD, controls) but not between MSA-P and MSA-C. The similarity of pursuit impairment in MSA-P and in MSA-C suggests a commencement of cerebellar pathology in MSA-P despite the lack of clinical signs. Otherwise, SPEM gain was of little use for differential diagnosis between MSA and PD because of wide overlap. However, inspection of the saccadic component of pursuit tracking revealed that in MSA saccades typically correct for position errors accumulated during SPEM epochs ("catch-up saccades"), whereas in PD, saccades were often directed toward future target positions ("anticipatory saccades"). The differences in pursuit tracking between PD and MSA were large enough to warrant their use as ancillary diagnostic criteria for the distinction between these disorders.

Space-based but not object-based inhibition of return is impaired in Parkinson's disease

Impairments in certain aspects of attention have frequently been reported in Parkinson's disease (PD), including reduced inhibition of return (IOR). Recent evidence suggests that IOR can occur when attention is directed at objects or locations, but previous investigations of IOR in PD have not systematically compared these two frames of reference. The present study compared the performance of 18 nondemented patients with PD and 18 normal controls on an IOR task with two conditions. In the "object-present" condition, objects surrounded the cues and targets so that attention was cued to both a spatial location and to a specific object. In the "object-absent" condition, surrounding objects were not presented so that attention was cued only to a spatial location. When participants had to rely on space-based cues, PD patients demonstrated reduced IOR compared to controls. In contrast, when objects were present in the display and participants could use object-based cues, PD patients exhibited normal IOR. These results suggest that PD patients are impaired in inhibitory aspects of space-based attention, but are able to overcome this impairment when their attention can be directed at object-based frames of reference. This dissociation supports the view that space-based and object-based components of attention involve distinct neurocognitive processes.

Lexical-semantic inhibitory mechanisms in Parkinson's disease as a function of subthalamic stimulation

Inhibitory control may be affected by Parkinson's disease (PD) due to impairment within the non-motor basal ganglia-thalamocortical circuits. The present study aimed to identify the effects of chronic stimulation of the subthalamic nucleus (STN) on lexical-semantic inhibitory control. Eighteen participants with PD who had undergone surgery for deep brain stimulation (DBS) of the STN, completed a picture-word interference (PWI) task and the Hayling test in on and off stimulation conditions. The results of PD participants were compared with 21 non-neurologically impaired control participants. PD participants performed no differently from controls on the PWI task, and no significant differences between on and off stimulation conditions were revealed, therefore suggesting that PD participants are not impaired in lexical-semantic interference control. In contrast, in the off stimulation condition, PD participants had significantly delayed reaction times and increased errors on the inhibition section of the Hayling test compared with the STN stimulation condition and control participants. These results suggest that PD patients are impaired in aspects of inhibitory control that are dependent on behavioural inhibition (such as the suppression of prepotent responses) and selection from competing alternatives without the presence of external cues. Furthermore, STN stimulation acts to restore these behavioural inhibitory processes.

Intact encoding, impaired consolidation in procedural learning in Parkinson's disease

Idiopathic Parkinson's disease (IPD) patients and matched healthy participants were compared on a non-motor procedural task involving semantically related inverted word pairs, and 3 months later to determine the extent of skill consolidation. IPD patients were found to acquire new procedural skills necessary to read these inverted words, thus indicating that they are not impaired in all types of procedural learning. However, results on post-tests 3 months later, revealed significant group differences with IPD subjects showing little off-line learning relative to the controls. This suggests that a dopamine (DA)-deafferented neural system is not consolidated in the same way that a normally DA-innervated system is, and that impaired maintenance of procedures and routines may place IPD patients in a situation of constant relearning of embedded strategies in motor and non-motor domains.

Impaired inhibitory oculomotor control in patients with Parkinson's disease

A hallmark of voluntary control is the capacity to inhibit or change partially prepared responses, an ability thought to be compromised in patients with Parkinson's disease (PD). To test this hypothesis in relation to oculomotor control, PD patients and age-matched controls performed a redirect task in which they were instructed to cancel a partially prepared saccade on some random fraction of trials. Using a race model framework, the time it takes to cancel a saccade, the target switch reaction time (TSRT), was estimated for PD and control subjects. While saccadic reaction times of control and PD subjects were similar, the average TSRT in PD subjects was 139 ms, and was significantly greater than the TSRT in controls, which was 113 ms. These results support the hypothesis that poor voluntary control exhibited by PD patients in a variety of complex behaviors may be caused by impaired inhibitory control as a result of basal ganglia dysfunction.

The ubiquity of memory retrieval deficits in patients with frontal-striatal dysfunction

OBJECTIVE AND BACKGROUND

Previous studies have shown that patients with frontal-striatal dysfunction demonstrate improved performance on tests of recognition memory relative to free recall memory, suggesting deficits in retrieval processes. Not all studies, however, have indicated that all patients with frontal-striatal dysfunction display this profile. In this study, we examined the ubiquity of this "retrieval deficit" profile in a relatively large sample of patients with Parkinson disease (PD) or Huntington disease (HD).

METHODS

Participants included 150 patients with PD and 65 patients with HD. Patients were classified as demonstrating a retrieval deficit or not based on a comparison of their standardized performances on the Recognition Discriminability and Long-Delay Free Recall indices from the California Verbal Learning Test.

RESULTS

Results indicated that 1) a retrieval deficit was more prevalent in patients with HD than PD, 2) this group difference emerged only in patients with at least a mild level of global cognitive impairment, and 3) even when the profile did emerge more frequently in patients with HD, it was present in only 44% of the patients.

CONCLUSIONS

These findings suggest that not all patient groups with frontal-striatal dysfunction display a retrieval deficit profile, but in groups that do (ie, patients with HD), it is more likely to appear in individuals with greater cognitive impairment.

Unimpaired negative but enhanced positive priming in Parkinson's disease: Evidence from an identity and a location priming task

Mechanisms of selective attention are frequently reported to be impaired in Parkinson's disease (PD). Fundamental to selective attention is attending to relevant information and, concurrently, ignoring irrelevant information. Both processes can be assessed by positive priming (PP) and negative priming (NP) tasks, respectively. Unlike previous studies, in the present experiment, two separate identity- and location-based priming tasks were applied to 48 PD patients and 48 sex- and age-matched healthy controls. Results indicated that identity and location PP were reliably enhanced in PD patients compared to controls. Both groups showed significant location NP of almost identical magnitude but no identity NP. However, there was evidence for a positive functional relationship between severity of bradykinesia and identity NP. Furthermore, with increasing depression scores, location NP was enhanced in PD patients but not in controls. These findings suggest that disturbed selective attention associated with PD is due to changed mechanisms mediating attention to relevant information rather than due to mechanisms involved in inhibition of irrelevant information.

Cortico-striatal contributions to category learning: Dissociating the verbal and implicit systems

To better characterize the neuropsychological mechanisms of implicit and verbalizable category learning, the author studied weather prediction task (WPT) and information integration task (IIT) performance in patients with Parkinson's disease (PD) and healthy older and younger adults. Both older adults and patients with PD were impaired on the WPT, but only patients were impaired on the IIT, suggesting the 2 tasks rely on dissociable systems. Whereas the IIT appeared to rely on implicit processes, results suggest WPT classification depends on explicit processes. Awareness of underlying structure, hypothesis testing ability, and working memory capacity were all related to accuracy on the WPT but not the IIT. The variability commonly noted in WPT performance may reflect individual differences in hypothesis testing ability.