The present results are inconsistent with progressive automatization being the cause of time on task effects on mind wandering. Firstly, there is no consistently mapped stimulus response association to automatize in a lecture (e.g. Shiffrin & Schneider, 1977; Logan, 1988) and, more importantly, even if lecture processing could be automatized, it would have been automatized prior to our experiment. The progressive automatization account requires automatization within the task (otherwise no change in mind wandering would be found). Thus, an interpretation in terms of progressive automatization freeing up resources which can be used to mind wander is difficult to maintain in a lecture context. This leaves open the possibility that while mind wandering, individuals process stimuli in an automatized manner. In contrast, McVay and Kane's (2010) executive failure account of mind wandering provides a ready interpretation of the present results. If sustaining attention on the lecture demands executive control and the resources required for control deplete as time on task increases (Parasuraman, 1979), then the likelihood that participants will report mind wandering should also increase as time on task increases. Smallwood et al.'s (2007, 2008; Smallwood, 2011) cascading inattention account can also explain the present results. In this account, mind wandering early on in the lecture impairs the formation of a situation model disrupting the ability of the lecture to hold an individual's attention. The negative feedback loop between mind wandering and disruption to the situation model over time would lead to both an increase in mind wandering over time and a decrease in comprehension. Future work distinguishing between these accounts would provide further insight into fluctuations in attention over time. However, the executive control and cascading inattention accounts likely provide similar predictions (or at least could provide similar predictions) in a number of contexts (e.g. if executive control failures can be viewed as both a cause of situation model impairments and are influenced by the quality of a situation model).
One potential issue with the executive failure account is the assumption that participants prioritize the tasks in the way that they are instructed by the experimenter (i.e. that participants want to pay attention to the lecture). If they do not, then an off task thought (i.e. what we have defined as mind wandering) might not be a “failure” at all (Baars, 2010). With this caveat in mind, it is important to consider accounts of the increase in mind wandering as a function of time in terms of how the motivational state of the participant might change with increases in time on task. For example, recent research on the relation between affect and mind wandering has demonstrated that negative moods lead to an increase in mind wandering (Smallwood, Fitzgerald, Miles, & Phillips, 2009). In addition, research on vigilance tasks and lectures has demonstrated that increasing time on task is associated with increases in frustration (Galinsky, Dember, & Warm, 1993; Warm et al., 2008). This negative affective state, which takes hold as time on task increases, could erode the participant's motivation to stay on task thus leading to an intentional withdrawal of resources from the primary task (Jeffries, Smilek, Eich, & Enns, 2008). For example, participants might withdraw attention in order to focus on less aversive thoughts in order to improve mood or relieve frustration (Smallwood, Fitzgerald, et al., 2009; Smallwood, Nind, & O'Connor, 2009). While in this account the mind wandering remains a result of attentional disengagement, it differs in that the disengagement is caused by the regulation of affective state rather than the failure of executive control. While the former account has focused on mood causing mind wandering, recent work by Killingsworth and Gilbert (2010) suggest that the causal direction may be reversed (i.e. mind wandering decreases mood). Thus, the increase in mind wandering as a function of time in lectures (or the mechanism underlying it) may cause an individual's mood to sour. Interestingly, the role of an individual's affective state (e.g. motivation) in mind wandering has been largely ignored until recently (Kane et al., 2007; Killingsworth & Gilbert, 2010; McVay, Kane, & Kwapil, 2009). Issues concerning affective state (and others) could be investigated by asking participants to provide information about the contents of their “off-task” thoughts and/or their level of interest in the topic. For example, Smallwood, Nind, et al. (2009) demonstrated a prospective (i.e. future thinking) bias in off-task thought that was modulated by level of interest in the material.