The highly-significant main effect of trial-type provides evidence for the existence of switching costs, consistent with previous research (Garavan, 1998; Gehring et al., 2003). The considerable effect size for this interaction indicates that the Internal Switching Task (IST) is a powerful tool for measuring switching costs. Researchers may wish to consider using this task for future investigations of attention-switching in many different areas of investigation. There was also a highly significant main effect of condition, again with a very large effect size, indicating that both the meditators and nonmeditators exhibited generally much slower reaction times (RTs) in the affective condition relative to the neutral condition. This is consistent with previous research which found affective material to generally increase RTs (Murphy et al., 1999), and supports the notion that emotive or personally-relevant information initiates processing at a deeper semantic level and thus poses an increased challenge to executive function.
Consistent with Hypothesis 4, the large time by group interaction reflected an overall decrease in the meditators' RTs, relative to the nonmeditators', following the intervention. While there was a small effect size for this interaction, this finding generally supports the notion that mindfulness training enhances executive function (Baer, 2003; Bishop et al., 2004). Hypothesis 5 predicted that this postintervention reduction in the meditators RTs would be even more significant for the affective condition, since Vipassana meditation practice, in addition to enhancing attention-switching, breaks the usual cognitive pattern of reacting defensively or appetitively to affective stimuli. However, this was not supported by the results, which indicated a nonsignificant time by group by condition interaction, with a negligible effect size.
This can be interpreted in two ways. While previous research has postulated that mindfulness training enhances capacity for various executive functions, such as sustained attention, inhibition of elaborative processing, and attention switching (Bishop et al., 2004), perhaps it does not operate upon all of these. Further research is required in order to tease out exactly which mechanisms of executive function are impacted by mindfulness training. Alternatively, it is possible that the failure to detect this interaction effect reflects methodological issues regarding the IST. In particular, a number of participants from both groups indicated that a number of the items in the affective condition were somewhat ambiguous (that is, open to interpretation as whether they reflected positive or negative self-descriptions). Examination of the RTs for the affective condition confirmed that there was discrepant responding on some of the items (for example, words such as "cute" and "alone"). This is despite the fact that the words were drawn from a validated source (Bradley & Lang, 1999). Future studies which replicate the present design could overcome this problem by having participants rate each of the affective words after completion of the affective-condition IST.