One commonly reported difficulty in psychiatric populations involves set-shifting, the abovementioned process of moving attentional focus between internal objects (Gehring et al., 2003; Garavan, 1998). This may underlie the tendency to ruminate, a symptom commonly reported among psychiatric populations (Treynor et al., 2003). While recent research has suggested that rumination may in fact have adaptive as well as maladaptive aspects, and could represent a method of coping with negative mood (Treynor et al., 2003; Wells & Matthews, 1994), it is often focused on negative affect and seemingly beyond the consciousness control of those who experience it. It thus may represent a failure of executive function.
Research into set-shifting has historically employed Card-Sorting (Moritz et al., 2002; Davis & Nolen-Hoeksema, 2000), Trail-Making (Paradisio, Lamberty, Garvey & Robinson, 1997), dichotic listening (Hughdahl et al., 2003), Stroop (Schatzberg et al., 2000) and Dot-Probe (Bradley, Mogg & Lee, 1997) tasks. These, however, have only managed to assess the external domain, in the sense that they examine the capacity of individuals to shift the focus of their attention between various external stimuli.
The abovementioned research by Garavan (1998) and Gehring and colleagues (2003) expanded this line of investigation to incorporate the internal domain. They found evidence of switching effects, suggesting that executive function is a limited capacity system also when focusing on internal representations. Research by Murphy and colleagues (1999) has further suggested that this switching effect becomes even more pronounced when it includes affective material. In this study, which utilised a go/no-go task designed to assay internal attentional-shifting, manic and depressed patients responded to target words of either positive or negative affective tone while inhibiting responses to words from the other affective category. Both groups demonstrated response biases for emotional stimuli: depressed patients for "sad" words, and manic patients for "happy" words. This suggests that emotive or personally-relevant information may pose an increased challenge to attentional control, presumably because it initiates processing at a deeper semantic level.
From this perspective, rumination, worry, and related phenomena may reflect a particular difficulty with set-shifting in the internal domain of executive function. This may reflect a disorder of executive function fundamentally different to those that pertain to the external domain, such as attention-deficit disorder. The former arguably represent a self-focus, while the latter reflect an environmental one. Rumination, depression and anxiety may all be related to dysfunction in this internal domain. This is in fact supported by evidence which links these disorders to high levels of self-consciousness (Brown & Ryan, 2003; Treynor et al., 2003).