The main finding of this study was that attentional control is deficient in MHD, evident primarily through a heightened error rate on the SART. This behavioural deficit was corroborated by abnormalities of P300 characteristics.
SART error rate
As expected, MHD made more errors of any type than the other groups, indicative of defective attentional mechanisms. Unexpectedly, this was not caused by a high rate of No-go errors, but by significantly more Go errors. The only study using the SART in HD did not report about the type of errors made [
21]. Even though they used different Go/No-go paradigms other studies also report on attentional deficits in manifest HD as measured by more Go errors for manifest HD compared to controls, but all have concomitantly also found more No-go errors, contrary to our findings [
27,
36]. Our findings are partly in line with studies using the SART in other brain disorders with known attentional deficits. Schizophrenic patients have also been found to largely make Go errors and not No-go errors [
7]. However, patients with traumatic brain injury made significantly more errors of both types, with stronger evidence for No-go than Go errors [
3,
31]. So, our findings cannot be easily attributed to attentional deficits alone as earlier findings in other studies were not replicated. Therefore, they were further investigated in reaction time pattern analyses.
Reaction time patterns surrounding No-go errors
As the SART is likely to provoke No-go errors due to the repetitive nature of the task and the rarity of No-go stimuli, we further investigated the significant amount of Go errors made by the early HD group. Examination of these Go errors in MHD revealed that most were made directly following a No-go error. Analysis of the reaction time patterns for trials directly preceding and following a correct No-go trial (correctly withholding response to a three) revealed identical patterns for the three groups, although the MHD group reacted significantly slower than the PMHD or control groups. Prior to correct responses to a No-go trial RT was relatively stable for all three groups. Directly after such a correctly withheld response, RT was noticeably shorter. This speeding most likely represents action anticipation that is evoked by the repetitive nature of the SART. This primes the motor response; after having correctly withheld the response at the No-go trial, the response to the next Go trial is more quickly accessed, resulting in a quicker response [
37]. Although the MHD group reacted slower, the general pattern was the same as for the other groups. This suggests that the cause of slowing is due to motor disturbances and not to different cognitive processing.
Interestingly, a different pattern emerged concerning incorrect No-go responses, i.e. when participants incorrectly pressed the space bar in response to a 3. For all three groups the trials directly preceding such a No-go error showed a shortened RT. We hypothesize that this pre-error speeding could mean that the task was performed fairly automatically, with less attentional control, eventually resulting in an error [
31]. Remarkably, this pre-error speeding was more prominent in MHD than in the other two groups. This could indicate that subjects with MHD can sustain attention less well than the other groups. After such a No-go error RT returned to the pre-error level almost immediately for the PMHD and control groups, but not for the MHD group, showing a dramatic post-error slowing.
One proposition for this is that a No-go error induces MHD subjects to slow down in response time in the hope of making fewer errors. This is an unconscious cognitive strategy known as ‘speed accuracy trade-off’ (SAT): low speed allows high accuracy [
33]. That healthy controls performing the SART use this SAT strategy has also been put forward by Helton and colleagues [
18]. At first glance one would then expect that subjects who choose ‘accuracy over speed’ would make fewer errors, but this was not the case. A more likely explanation is that there is an intrinsic deficit of attentional control in MHD. This is seen in the obvious drop in RT trials preceding an error. This could possibly reflect a drop in attentional control, in turn causing the No-go error. The post-error RT pattern shows that the PMHD and control groups are able to return to the task immediately and perform on pre-error level. The pattern of the MHD subjects, however, reflects a difficulty in recovery; it takes this group several trials to return to pre-error performance. This difficulty could be due to the realization of having made an error, i.e., the response evaluation, causes confusion; the subsequent quick return to a Go trial adds to this confusion, leading to a slower return to pre-error performance. Alternatively, this post-error slowing does not reflect cognitive confusion, but could be indicative of an inability to switch from a No-go to a Go response, and thus from inhibiting the response to activating it. Together with the fact that directly following a No-go error significantly more Go errors are made in the MHD group than in the other two groups we speculate that attentional and inhibitory deficits are the probable causes of inadequate attentional control in MHD. Adding to this theory of impaired attentional control we found, on further analysis, that in the trial directly following a No-go error trial, the early group made significantly more go-errors (8%) than both the premanifest (0.5%) and control groups (3%). Similar results in a task-switch and stop-signal task in MHD have been reported [
2]. Post-error slowing was interpreted in that study as task-switch cost and a deficit in the ‘inhibition of the just-performed response’ respectively. The authors attributed these phenomena to deficient inhibition. These explanations are not mutually exclusive in that early HD subjects can use the speed accuracy trade-off strategy to avoid making further errors, but that their cognitive abilities are deficient and they cannot use this strategy successfully.
Even though constructs such as attention and inhibition are not directly measurable and can only be derived from secondary measurements, we hypothesize that the RT pattern around No-go errors in the MHD group seems to reflect a cognitive rather than a motor process as subjects with early HD are able to respond in the same manner as PMHD and controls in correctly withheld No-go trials, albeit slower. This similar pattern for all groups demonstrates that it is not a No-go trial per se that elicits a deviant reaction from early subjects. The problem seems to lie purely in the fact that an error was made.
P300 amplitude and latency
As stated before, P300 amplitude is hypothesized to reflect the amount of attentional capacity that is being allocated to a stimulus [
30]. If so, then P300 amplitude would be lower for incorrectly performed No-go trials. This was indeed the case for the MHD group, confirming a lowered attentional control during presentation of No-go stimuli. Our findings correspond well to those of Beste et al. [
4] and Jurgens et al. [
21]. Münte et al. [
28] also reported lowered P300 amplitude; however, not in the context of a Go/no-go task.
P300 latency is thought to be linked to the speed of attentional processing [
30]. In accordance with Münte et al. [
28], P300 latency was significantly longer in MHD compared to the other groups for Go trials. This implies a low speed of attentional processing during Go trials is lessened for MHD. Together with a lowered attention during No-go trials this strengthens our hypothesis that the disturbed pattern observed surrounding No-go errors is of a cognitive rather than a motor nature.
Premanifest HD results
PMHD did not exhibit any attentional or inhibitory deficits. Explanations for this are that no attentional control deficits are yet present or that subtle changes in attentional control capacity are already present in PMHD, but that they are still too subtle to be measured with this method. Possibly these deficits gradually worsen and are better picked up in subjects closer to expected onset. This interpretation seems plausible as both SART and P300 data did show a nonsignificant trend towards worse performance in the premanifest group. The only reverse pattern concerned SART error rate, where PMHD subjects made fewer errors than controls. We hypothesize that this reflects a high motivation. Clinical experience suggests that PMHD subjects are highly motivated to perform to their best on the tests, as they may wish to prove that there they are still in the premanifest phase.
Practical implications and limitations
Patients with HD may experience more distress from the decline of their cognitive functions rather than the presence of motor disturbances. The results from this study indicate that patients with HD experience difficulties with recovering after an error and maintaining attentional control for a longer period, which adds to the knowledge about cognition in HD and could have implication for daily care.
A limitation to the present study is the relative small number of subjects in the PMHD group and therefore having less statistical power. This could have obscured possible subtle differences from controls.
We conclude that there is an attentional control deficit in MHD. MHD subjects are cognitively not able to directly resume task requirements after having made an error and that they need more time to return to pre-error performance level. No attentional control deficits were found for the PMHD group.