Impaired extra-dimensional shift performance in medicated and unmedicated Parkinson's disease: evidence for a specific attentional dysfunction

Incidental and intentional recall in Parkinson's disease: an account based on diminished attentional resources

The recall of common objects and their spatial location was examined in 65 patients with Parkinson's disease (PD) under conditions in which available attentional resources were manipulated by secondary task demands. PD patients were impaired at item recall particularly under intentional learning conditions but were unimpaired at recall of spatial location. These findings were similar in newly diagnosed, untreated cases as well as patients who had suffered with the disease for an average of 9.6 years. Test performance was not improved by levodopa therapy, despite it benefiting motor control, and was not impaired by anticholinergic medication. Item recall correlated significantly with other memory measures (particularly tasks of working memory) but only weakly with indices of physical disability and traditional frontal-lobe measures. Spatial recall, by contrast, correlated with memory quotient but no other cognitive measure and depression and disease duration failed to correlate significantly with performance on either recall task. These results are attributed to a deficit in attentional resources in PD that impairs performance most markedly for tasks and conditions that make the greatest demands upon effort.

Visuospatial memory deficits at different stages of Parkinson's disease

Groups of patients with idiopathic Parkinson's disease (PD), either medicated or unmedicated, were compared with matched groups of normal controls on a computerised battery of tests designed to investigate spatial working memory, visuospatial recognition memory and learning. The medicated PD patients were subdivided into those with mild and severe clinical disability on the basis of Hoehn and Yahr ratings, thus making three groups of PD patients in all. In a test of spatial recognition memory, a significant impairment was only evident in those PD patients who were medicated and had severe clinical symptoms (Hoehn and Yahr stage III-IV). In contrast, none of the three patient groups were impaired in a complementary test of visual pattern recognition memory. Whilst all three patient groups performed well in a test of simultaneous visual matching to sample, medicated patients (MED PD) with severe clinical symptoms were significantly impaired when a short (0-12 sec) delay was introduced. In a test of paired associates learning requiring both visual pattern and visuospatial memory, deficits in learning and memory were only evident in the severely impaired MED PD group. In contrast, in a test of spatial working memory known to be sensitive to frontal lobe damage, significant impairments were found in both groups of medicated PD patients and particularly in those patients with more severe clinical symptoms. Taken together, the results suggest that there are multiple memory impairments in PD which may differentially depend on the clinical severity of the disease.

The relationship between visuospatial ability and perceptual motor function in Parkinson's disease

To assess the contribution of visuoperceptual function to complex visuomotor responding in Parkinson's disease, 14 patients with idiopathic PD and 12 normal controls matched for age, education and general intellectual function were administered a visual tracing task. No difference was found between the groups on two visuoperceptual tests, the Benton Line Orientation test and a test of trajectory judgement. However, patients were significantly impaired in tracing a sawtooth design when two consecutive angles of the sawtooth were occluded. This impairment occurred in reproducing the basic form of the stimulus and not with accuracy of fine detail. These results suggest higher-order perceptual motor dysfunction independent of any breakdown in basic visuoperceptual processing or loss of fine motor control. It is concluded that Parkinsonian patients are unable to use sensory information accurately to plan and execute complex or new movements.

Executive function in Parkinson's disease: set-shifting or set-maintenance?

In order to distinguish impairment in set-shifting in Parkinson's disease (PD) from inability to inhibit distraction by stimuli that compete for attention, 18 nondemented patients with idiopathic PD and 13 normal controls equated for age and education, were administered the Odd-Man Out (OMO) test and the Stroop Color-Word Test. PD patients were significantly impaired on the OMO test but showed no evidence of interference during the Stroop test. Analysis of error patterns during the OMO test indicated that the requirement to repeatedly switch rules, rather than the requirement to maintain steady responding between rule switches, was responsible for impaired OMO performance. It is concluded that the OMO test is fundamentally a test of set shifting, rather than a test of set maintenance in PD. In addition, analysis of a larger sample of PD patients revealed a significant positive relationship between number and severity of extrapyramidal signs and error production on the OMO, and between the latter and global mental function. These relationships were independent of each other, suggesting that impairment in set-shifting function in PD may arise from pathology of the fronto-striatal system independently of changes in cognitive ability.

Further analysis of the cognitive effects of tetrahydroaminoacridine (THA) in Alzheimer's disease: assessment of attentional and mnemonic function using CANTAB

Results of a placebo controlled cross-over trial (N = 89) of the anticholinesterase drug THA as a treatment for dementia of the Alzheimer's type (DAT) are reported, with reference to previous trials of the drug and the cholinergic hypothesis of aging and dementia. Using computerised tests sensitive to specific aspects of memory and attention, evidence is found for improvements in attentional function rather than memory, in patients with mild to moderate DAT. Although these improvements were significant, they were small and restricted to certain tests of attentional function. Nevertheless, they add to the growing body of evidence that the cholinergic system is involved in the control of attentional processes. The results will be relevant to future investigations into the therapeutic effects of enhancement of the cholinergic system in DAT sufferers.

Haloperidol affects stimulus-dependent strategies and not reward-dependent strategies

In order to investigate effects of haloperidol on response strategies, rats were confronted with an unsolvable discrimination problem after being injected with haloperidol (0.1 mg/kg, IP) or its solvent. The results showed that haloperidol-treated rats displayed systematic behaviour to a lesser degree than the control rats. Haloperidol did not affect reward-dependent response strategies. However, haloperidol did affect stimulus-dependent response strategies. Haloperidol-treated rats showed more response strategies based on visual or auditory cues, and less spatial response strategies than control rats. It is concluded that this differential effect of haloperidol is the consequence of the different nature of the stimuli (discretely present visual and auditory stimuli versus constantly present spatial stimuli).

Cognitive strategies and hypothesis testing during discrimination learning in Parkinson's disease

This study investigated the nature and extent of impairments in the use of hypotheses and cognitive strategies in medicated subjects with idiopathic Parkinson's disease (PD) and matched control subjects. PD subjects did not differ from controls in solving one- or two-dimensional discrimination learning problems, but showed impairment on four-dimensional problems which did not appear to be attributable to memory deficits. They achieved fewer correct solutions, used fewer hypotheses, and were less likely to use appropriate lose-shift strategies following negative feedback. The pattern of findings was similar to those previously reported for subjects with frontal lobe lesions.

A specific form of cognitive rigidity following excitotoxic lesions of the basal forebrain in marmosets

The effects of N-methyl-D-aspartate-induced lesions of the basal forebrain were studied on performance of a series of visual discrimination tests that examined a range of cognitive functions in the marmoset. These included the ability to attend to the various dimensional properties of stimuli and to use just one of these properties in order to solve a discrimination (intra-dimensional shift); to switch attention from one dimension to another (extra-dimensional shift); to learn the reinforcement value of specific exemplars within a dimension (new learning); and to relearn their reinforcement value following reversal of the reward contingencies (serial reversals). Lesions of the basal forebrain did not impair the ability either to attend selectively to the dimensional properties of the stimuli or to switch attention from one dimension to the other. However, the lesion did affect various aspects of associative learning including a transient impairment of new learning and a marked disruption of serial reversal learning. The reversal deficit could be characterised as a tendency to perseverate on the previously correct stimulus and as a failure to to show the formation of a reversal learning set. In addition, the lesion prevented disruption of performance of a well-learned discrimination when novel exemplars from the irrelevant dimension were introduced (probe test). It is suggested that the functional effects of the basal forebrain lesion reflect impaired learning of stimulus-reward associations and behavioural rigidity. The finding, however, that there was no effect of the lesion on attentional set-shifting suggests that any loss of inhibitory control was specific to the level of stimulus-response or stimulus-reward associations, inhibitory control at the level of attentional selection remaining intact. The similarity of the effects of damage to the basal forebrain to those seen following damage to the orbitofrontal cortex and the amygdala are discussed in the context of the close anatomical and functional relationships that exist among these three structures.

L-dopa withdrawal in Parkinson's disease selectively impairs cognitive performance in tests sensitive to frontal lobe dysfunction

A group of ten patients with idiopathic Parkinson's disease (PD) was tested on a series of automated tests of learning, memory, planning and attention whilst either on or off L-dopa medication. Controlled withdrawal of L-dopa interfered with aspects of performance on three of the tests that had previously been shown to be sensitive to frontal lobe dysfunction; a spatial working memory task, the Tower of London planning test, and a visual discrimination paradigm that also included intra- and extra-dimensional shift tests of selective attention. More specifically, errors were increased in the spatial working memory test, and both the accuracy and latency of thinking were impaired. Thinking time was significantly slowed following L-dopa withdrawal, even though the possible contaminating effects on motor slowing were fully controlled by a yoked control procedure. Nine out of ten patients reached a further stage of the visual discrimination, set-shifting paradigm when on, rather than off, L-dopa medication. Spatial span was also impaired off medication, but there were no effects of L-dopa withdrawal on tests of pattern and spatial recognition memory, simultaneous and delayed matching to sample or visuospatial conditional associative learning. Comparisons with a large control group confirmed previous findings that PD is associated with deficits on the majority of these tests. The results are discussed in terms of the fronto-striatal, dopamine dependent nature of some of the cognitive deficits found in PD, but the apparent dopamine-independent nature of deficits in other aspects of cognitive functioning, notably in tests of visual recognition memory and associative learning.

Extra-dimensional versus intra-dimensional set shifting performance following frontal lobe excisions, temporal lobe excisions or amygdalo-hippocampectomy in man

Attentional "set" shifting was assessed in a group of 20 neurosurgical patients with localized excisions of the frontal lobes, a group of 20 patients with unilateral temporal lobe lesions and a group of 11 patients who had undergone amygdalo-hippocampus removal. These three patient groups were compared with groups of both young (age-matched) and elderly normal control volunteers on a computerized test of visual discrimination learning involving both an intra- and an extra-dimensional shift. The frontal lobe group were selectively impaired in their ability to shift response set to a previously irrelevant dimension but not to shift attention to new exemplars of a previously relevant dimension. A similar pattern was observed in the elderly group of normal control volunteers. By comparison, both the temporal lobe patients and the amygdalo-hippocampectomy patients were unimpaired in their ability to perform either shift, although both groups had significantly prolonged selection latencies at the extra-dimensional shift stage of the task. These data are compared to previous findings from patients with idiopathic Parkinson's disease and are discussed in terms of a specific attentional set shifting deficit following frontal lobe damage.

Sparing of attentional relative to mnemonic function in a subgroup of patients with dementia of the Alzheimer type

Patients with dementia of the Alzheimer type (DAT) received two tests of visual selective attention, together with tests of spatial and visual recognition memory and visuospatial conditional learning previously used to show deficits early in the course of DAT. One set of attentional tests compared visual discrimination learning along intra- and extra-dimensional shifts, using a "total change" design. In the 12 DAT patients capable of attempting the extra-dimensional shift (subgroup 1), performance was equivalent to that of controls. This subgroup was also unimpaired at simple and compound discrimination learning and reversal and an intra-dimensional shift. They were as accurate as controls on a visual search task requiring matching of stimuli on two dimensions with variable numbers of alternatives, but were significantly impaired in the tests of recognition memory and learning. By contrast, the other 13 patients showed marked impairments in the attentional tasks. This subgroup was also significantly worse than subgroup 1 in performance on the visual recognition and conditional learning tasks, and showed greater severity on most of the clinical ratings of dementia. The sparing of attentional shifting in patients early in the course of DAT is contrasted with the impairments previously described in patients with Parkinson's disease with only mild or absent memory loss. The implications of this double dissociation of deficits for understanding the neural bases of the cognitive deficits in these two neurodegenerative diseases are discussed and their significance for the staging of DAT is considered.