4 -T 1 3 2 2 1 0
Controls Severe Very severe Head injury
Figure 6.4. Group differences in accuracv of judgements of elapsed time during interrupted information processing task
(plots show means, standard errors and standard deviations).
6.4 Discussion
The discussion of the experimental results will address firstly the neuropsychological deficits demonstrated by the subjects. The profile of time estimation performance will then be reviewed within the context of the underlying neuropsychological problems. In general, the studies confirmed that information processing deficits and, to a lesser degree, spelling difficulties are associated with increasing severity o f brain injury. The fact that at least one statistically significant relation was demonstrated between the rates of false negative errors and spelling test scores (by severely head-injured subjects for the no-error condition) suggests that the lexical decision task was indeed sensitive to spelling ability, at least for those subjects with most difficulties.
There was in fact no difference in accuracy between the controls and head injured subjects in the no error lexical decision task, but there was a significant difference in timing accuracy between the two head injury groups. This suggests that only the most severely brain injured subjects had impaired awareness o f time for the two minute duration of the relatively easy no error lexical decision task. By contrast, the error version of the task appears to elicit a more consistent trend towards increasingly inaccurate time estimation with increasing severity of brain injury (see table 6.4). In particular the very severe head injury group appear to be clearly deficient in time awareness, yet the discrepancies did not reach statistical significance. Nevertheless amongst the very severely injured there was a significant correlation between the number of false negative errors in the error condition and estimated duration which suggests that subjects who failed to identify incorrectly spelled words were less accurate in their time estimation. In contrast to attentional models of psychological time which predict an improvement in timing accuracy with less attention to task, this finding suggests that attentional decrements associated with brain injury may undermine both task performance and temporal monitoring (see also chapter 4).
With regard to the non-interrupted information processing task, the clear differences in information processing ability across the groups were associated with a trend towards
less accurate timing in the verbal estimation task (see table 6.3). In the time production task there was a significant difference in timing across all the groups but when controls were compared with the head injured subjects as a whole they were no more accurate. However, the fact that the severe group were significantly more accurate than the very severe group again suggests that only the most severely brain injured subjects experienced impaired temporal monitoring during performance of the two minute task.
No single task was estimated as significantly different in duration when it was performed under interrupted conditions than when completed continuously. However, the interrupted lexical decision task revealed a trend for increasingly inaccurate judgements of duration to be associated with increasing severity of brain injury. Yet again the very severe head injury group demonstrated a significant relation between the number of false negative errors and estimated duration, this time suggesting that subjects who omitted to identify salient stimulus items were also more likely to judge the task as shorter than their more accurate counterparts. Clearly therefore subjects do not pay less attention to task because they are heeding the time. It appears that very severely head injured subjects are failing to attend to both task and to time rather than due to an information processing bias towards time or task demands as current attentional accounts of psychological time require.
In the interrupted information processing task there was a significant difference in time estimation accuracy between the controls and head injured subjects which was broadly consistent with but statistically more robust than the trend from the interrupted lexical decision task. This result partially vindicates the original experimental hypothesis concerning the nature of task interruptions of concurrent timing ability. It will be recalled that there were significant differences between controls and head injured subjects in information processing speed, but whereas non-interrupted tasks showed a tendency for the only most severely brain injured subjects to perform less accurately than controls, the interrupted information processing task demonstrated a clear difference between head injured and non-head injured subjects. To a lesser extent the lexical
decision task demonstrated this as well though the discrepancies were not significant. Therefore, although it was stated above that no group exhibited significantly less accurate timing with the provision of tasks interruptions, the interrupted tasks (principally the information processing task) were the only tasks to distinguish between head injured and control subjects. It seems plausible that under ‘normal’ experimental conditions when subjects are allowed to complete tasks without interference from interruption, then only a very severe brain injury is likely to be associated with poor time estimation, as a result of generalised attentional decrements. In contrast, when tasks are made somewhat more ecologically realistic by introducing periodic interruptions then the first signs appear that (while task completion continues apparently unaffected) the incidental task of concurrent timing starts to suffer and time estimation accuracy deteriorates.
Although these results are encouraging there are both methodological and conceptual grounds why some of the findings suggest only trends and did not reveal unequivocally significant relationships. Perhaps the most obvious reason is that the sample size was inadequate given the sample variability to demonstrate more robust data. In addition the nature o f the task interruptions may have been inadequate to disturb timing processes more severely. Certainly, in the terms of a central executive system the action of the switching-off of a timer in response to a signal occurring at regular intervals does not require much cognitive mediation and could conceivably be executed by a routine action programme with minimal attentional demands. In fact, this is the impression gleaned from observing the control subjects perform the task. Head-injured participants, by contrast, often found the periodic “bleeping” disconcerting and somewhat irritating. This qualitative difference in task performance was not always fully reflected in the results but it seems plausible that difficulties in switching attention by head-injured subjects may have even greater effects on psychological time than are evident in the present study. Such effects might be observed in the context of changes in task demands of a more attention-demanding and cognitively challenging nature. These modifications are addressed in chapter 7.
The plausibility of a cognitive neuropsychological account o f the data appears to run counter to the internal clock framework, as the latter has typically entailed common timing mechanisms for both perception and action (Wing & Kristofferson,1973; Vorberg & Hambuch,1984). Thus some influence of concurrent motor acts on estimated task duration might well have been expected from an internal clock. However, such an effect might be avoided if time is represented at multiple levels throughout the cognitive system. An activation model which encompasses this notion was proposed by MacKay (1982, 1987) in which timing is the third component (after content and order) of cognition. MacKay’s is a dynamic model in the sense that timing is distributed throughout the system, not imposed at one level (see also Summers & Bums, 1990). Within such a framework it is entirely plausible that higher-level experience of duration may be unaffected by changes in temporal aspects of motor programming. These issues raises concerns about the testability of internal clock versus general cognitive accounts of psychological time. In the final section of the thesis (chapter 14) the data from the present study is considered along with results from other chapters in more detailed discussion of these issues.