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INTRODUCTION
Cognitive and neuroimaging studies have demonstrated an association between the frontal lobes and cognitive set-shifting (e.g., McDonald et al., 2005; Moll et al., 2002; Stuss et al., 2001). One of the most commonly used measures of set-shifting is the traditional Trail Making Test (TMT), which has become a standard neuropsychological assessment instrument (Army Individual Test Battery, 1944; Brown & Partington, 1942; Partington & Leiter, 1949). The TMT has been shown to be sensitive to the presence of brain injury in general, and frontal lobe injury in particular (Demakis, 2004; Johnstone et al., 1995; Lange et al., 2005; Stuss et al., 2001).
Some researchers have suggested isolating the "executive" component of Part B of the traditional TMT from its perceptual and motor demands by using either a difference score, which subtracts Part A from Part B, or by using a ratio score of Part B over Part A (Arbuthnott & Frank, 2000; Lamberty et al., 1994). However, others have found that these methods did not add to the diagnostic utility of the test (Lange et al., 2005; Martin et al., 2003). One possible reason is that Part A and Part B of the test have somewhat different perceptual and motor demands, such as different numbers of stimuli and different trail lengths (Arnett & Labovitz, 1995; Gaudino et al., 1995) and that other cognitive skills may be tapped by Part B but not Part A (e.g., letter sequencing). Another possible reason is that brain injury may not lead to a true set-shifting deficit above and beyond a more basic sequencing deficit. The question arises then, whether frontal lobe patients' difficulty on the TMT is because of a set-shifting deficit per se or to a more basic visuo-motor sequencing deficit. For example, Martin et al. (2003) did not find disproportionate impairment in set-switching beyond a sequencing impairment on the TMT, in a sample of patients with traumatic brain injuries (TBI).
This study explored whether frontal lobe damage leads to simply a sequencing deficit or a higher-level switching deficit. This was investigated with a new version of the TMT. The Trail Making Test of the Delis-Kaplan Executive Function System (D-KEFS; Delis et al., 2001) was created in part to isolate set-shifting from other component skills such as letter...