Antecedentes de Investigación sobre el tema

Briefly, simple information processing speed refers to the speed at which respondents react to straightforward stimuli (e.g. present/absent) and is thus determined by basic perceptual, motor and attentional processes. One type of simple processing speed is choice-reaction time on a visuospatial task that involves presentation of target stimuli to which a fast finger-press response is required, according to the spatial location (left/right) of the target stimuli (see Chapter 2 for a description of the task used here). Such choice-reaction tasks are sufficiently simple to reasonably attribute individual differences in response times to the efficiency of basic mental operations. At the same time, these tasks are also sufficiently demanding in the sense of tapping cognitive efficiency instead of being based on purely sensorimotor operations (Rypma et al., 2006). Complex information processing speed, which refers to the speed of responses to stimuli that require more complex judgements to be made, interacts with high-order cognitive processes including executive control (Chiaravalloti, Christodoulou, Demaree, & DeLuca, 2003). From here on these are referred to as ‘simple processing speed’ and ‘complex processing speed’. ‘Cognitive processing speed’ (e.g. Kochunov et al., 2010; Turken et al., 2008) can then be deduced from the subtraction of simple processing speed from complex. For example, the Trail Making Test (TMT; Reitan, 1958; Reitan & Wolfson, 1985; see Chapter 2) is a widely used measure of executive function, where performance on Trail A (TMT-A) depends on simple processing speed, and performance on the more demanding Trails B (TMT-B) on complex

processing speed, thereby allowing such deduction.

Impairments of processing speed are common following moderate to severe TBI, and are also observed in mild TBI in early stages post-injury (Frencham et al., 2005). In general, when brain-injured individuals are compared with healthy controls, they are substantially slower on a range of processing speed measures, including simple and more complex tasks (Fong, Chan, Ng, & Ng, 2009). Where no particular distinction has been made between types of processing speed, post-TBI deficits have been shown to increase with increasing severity of injury as well as with increasing task difficulty (Tombaugh, Rees, Stormer, Harrison, & Smith, 2007).

Efficient inter-regional communication within the fronto-parietal attentional network is believed to act as the neural basis of processing speed across a broad range of cognitive tasks (Rypma et al., 2006). For example, Perry et al. (2009) who studied white matter tract structure and complex processing speed on the TMT Trails B in normal cognitive aging found that higher anisotropy within the inferior fronto-occipital fasciculi, especially on the right, correlated with faster performance, independently of age. Speed of information processing may thus partly depend on the integrity of this and other white matter tracts connecting the frontal with more posterior regions (see Figure 5-1).

White matter pathways that interconnect the distinct regions of the motor network are involved in the integration of sensory input and motor output functions, and may thus play an important role in perceptual and motor processing speed. The motor network is a well-defined neural system that predominantly consists of the primary motor cortex, supplementary motor area, and the cerebellum (Kasahara et al., 2010). The corpus callosum is the principal connective structure between the brain’s two hemispheres and also connects the left with the right primary motor cortex (Wahl et al., 2007). The corpus callosum contains large numbers of myelinated fibres running in parallel (Shimony et al., 1999), a feature of white matter that in general supports its integrity (Beaulieu, 2002). Based on a relationship between white matter integrity and nerve conduction velocity (Jack, Noble, & Tsien, 1983), overall processing speed on a given task could be mediated by the structure of tracts that support the various perceptual, motor and cognitive processes involved. Several DTI studies have shown that the structural coherence of the corpus callosum is reduced after TBI, which could compromise the interhemispheric functional interactions within the motor network. In healthy individuals the

integrity of the corpus callosum has been suggested to predict individual differences in visuomotor processing speed (Schulte, Sullivan, Muller-Oehring, Adalsteinsson, & Pfefferbaum, 2005), as well as the decline in processing speed associated with normal cognitive ageing on a range of simple tasks (e.g. Bucur et al., 2008; Kennedy & Raz, 2009; Kochunov et al., 2010; Sullivan et al., 2001; Sullivan, Rohlfing, & Pfefferbaum, 2010).

Niogi, Mukherjee, Ghajar, Johnson, Kolster, Sarkar et al. (2008) explored the relationship between white matter structure and processing speed on a visuospatial choice- reaction task (indexed by the average reaction time for the congruent and incongruent conditions) in a sample of 34 patients with mild TBI, compared with 26 healthy controls (the same task was used in Niogi, Mukherjee, Ghajar, Johnson, Kolster, Lee et al., 2008 to index conflict processing ability). In patients, as the number of white matter structures showing reduced integrity increased, average choice-reaction times also increased. The predominant regions showing reduced anisotropy as compared with the healthy controls were the anterior corona radiata, the uncinate fasciculi, the genu of the corpus callosum, the inferior longitudinal fasciculi, and the cingulum bundles. Given the involvement of other frontal connections, in addition to the motor fibres of the anterior corona radiata, these results may reflect the executive components of the task used. All in all, these studies are consistent in suggesting a role for the structure of fronto-parietal, motor, and interhemispheric connections in information processing speed that may also underpin the deficits observed following TBI.

The present study will investigate the relationship between white matter structure and simple processing speed only. This is both because of the involvement of executive processes in complex processing speed, which would undesirably complicate exploring the relationship between the structure of specific white matter tracts and processing speed itself, and because post-TBI impairments have been demonstrated across a range of measures, including the simple ones.

5.1.5 Investigating the relationship between white matter tract structure and