Executive function

2.6.5.1 Trail Making Test. The Trail Making Test (TMT; Reitan, 1958; Reitan &

Wolfson, 1985) is quick and simple to administer and engages a variety of cognitive abilities, including visual search and scanning, attentional capacity, visuomotor processing speed, set maintenance, and mental flexibility in sequencing and set shifting (Lange, Iverson, Zakrzewski, Ethel-Kingc, & Franzen, 2005; Tombaugh, 2004). It consists of two parts: TMT-A that requires the examinee to sequentially connect 25 numbered circles randomly distributed on a sheet of paper; and TMT-B, where the examinee again connects circles in sequence, but alternating between numbers and letters (i.e. 1-A-2-B-3-C-4-D etc.). Time taken to complete each condition is recorded. A ‘difference’ score, indicating mental flexibility was calculated here by subtracting the TMT-A completion time from the TMT-B completion time (TMT-A − TMT-B = alternating- switch cost).

The TMT has been found to be sensitive to the effects of TBI (e.g. Lange et al., 2005; Reitan, 1958). Burgess, Alderman, Evans, Emslie and Wilson (1998) compared 92 neurological patients (M±SD age = 38.5±15.1) of varying etiologies (59% with head injuries) with 216 nonpatient controls (M±SD age = 46.1±19.8) and reported the following percentages for patients performing at or below the 5% level of the controls: 32.1 for TMT-A, 38.9 for TMT-B, and 25.9 for TMT-A − TMT-B. Recent evidence indicates that the test is also sensitive to the degree of injury severity; Demery, Larson, Dixit, Bauer and Perlstein (2010) showed that moderate/severe TBI patients performed significantly worse on both TMT-A and TMT-B than either mild TBI patients or healthy controls. In addition, patients with mild TBI performed significantly worse than controls on the more complex TMT-B. Hanks et al. (1999) studied the relationship between TMT-B performance and outcome after 6-months of acute rehabilitation, as measured using scales tapping a variety of domains including TBI-related disability and community integration. In their mixed sample of 90 TBI, orthopedic, and spinal cord injury patients, TMT-B performance was found to be a strong correlate of functional outcome. Other previous studies have reported similar associations between the TMT and TBI outcome (e.g. García-Molina, Tormos, Bernabeu, Junqué, & Roig-Rovira, 2012; Little, Templer, Persel, & Ashley, 1996; Ross, Millis, & Rosenthal, 1997).

2.6.5.2 Color-Word Interference test. The Delis-Kaplan Executive Function System

(D-KEFS; Delis, Kaplan, & Kramer, 2001) Color-Word Interference test is based on the famous Stroop (1935) procedure, and is construed as tapping verbal inhibition, set-shifting and cognitive flexibility. Performance is however affected by a variety of additional abilities, including initiation, simultaneous processing, maintaining a set, sustained attention and information processing speed.

Two ‘baseline’ conditions assess the examinee’s ability to perform the basic tasks of naming and reading. The more complex ‘inhibition’ condition presents colour names, which are printed in a conflicting colour (e.g. ‘RED’ printed in green). The examinee has to inhibit the prepotent response of reading out what is written (i.e. the colour name) and to instead name the ink colour. The final ‘inhibition/switching’ condition requires the examinee to alternate between two task sets as cued by the test sheet, that is, to either read the colour name or to name the ink colour. Completion time on each trial is recorded as well as two types of errors: uncorrected

and self-corrected. If the examinee makes three consecutive errors, the examiner provides a prompt to respond as appropriate. This verbal prompt is only provided once during a trial and the stopwatch is kept running throughout. For the purposes of the current research, a contrast score indexing cognitive flexibility was calculated by subtracting the average of the completion times of the two baseline trials (naming and reading) from the time to complete the more complex inhibition/switching trial.

Variants of the Stroop have been widely used in clinical practice to assess the effects of TBI on executive function (e.g. Fork et al., 2005; García-Molina et al., 2012). Goverover, Arango-Lasprilla, Hillary, Chiaravalloti, & DeLuca (2009) compared D-KEFS Color-Word Interference test inhibition/switching performance of 10 patients with TBI (all with positive CT/MRI results or documented loss of consciousness ≥ 24 hours) and 15 demographically matched healthy controls. The patients, all more than a year post-injury, did not show significantly impaired performance relative to the controls. However, it is worth noting that in addition to focusing on only the inhibition/switching condition of the test the sample in the Goverover et al. (2009) study was very small and replication of this finding in a much larger group of TBI patients is needed. The predictive validity of the test relative to outcome after TBI within other domains of function also remains to be determined.

2.6.5.3 Verbal Fluency/letter fluency. Also from theD-KEFS, the Verbal Fluency/letter fluency assesses an individual’s word generation fluency. Like the Color-Word Interference test, this test taps into a variety of processes including task initiation, simultaneous processing, maintaining a set, systematic retrieval of responses, and information processing speed. The examinee is required to generate different words that begin with a particular letter (usually F, then A, and finally S), but that are not proper nouns or numbers. The words are generated under time constraints (one minute per each of the three letters). The outcome variable used here was the total words that the examinee generated across the three letters, excluding any words that were repeated or broke the rules specified above.

Reduced verbal productivity after brain injury has been demonstrated in several studies using the letter fluency task (see Lezak et al., 2004). For example, Strong, Tiesma and Donders (2011) found 65 TBI patients of varying levels of injury severity to score significantly lower than 65 demographically matched healthy controls. The sensitivity of the FAS verbal fluency

procedure for neurological impairment screening puroses has been questioned, however (Burgess et al., 1998). In the Burgess et al. (1998) sample of 92 neurological patients of mixed etiologies only 10.8 percent performed at or below the 5% level of the large sample of nonpatient controls. The task has nevertheless shown sensitivity to TBI and to frontal lobe damage (Henry & Crawford, 2004a; Henry & Crawford, 2004b), in particular to left-sided frontal damage (Delis et al., 2001). There is also some evidence from the Burgess et al. (1998) study to support the test’s ecological validity relative to carer-reported everyday executive problems on the Dysexecutive Questionnaire (DEX; Wilson, Alderman, Burgess, Emslie, & Evans, 1996), as well as relative to the difference in DEX scores based on other- versus self-ratings. The task has in addition shown predictive validity in terms of the level of functional independence after mild to severe TBI (median/range GCS score = 8/3-15) following discharge from acute rehabilitation (Hanks, Rapport, Millis, & Deshpande, 1999).

2.6.6 Information processing speed: Choice-reaction task. A two-choice reaction