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INTRODUCTION
Sports-related concussion occurs with some frequency. Among high school athletes, for instance, 5.5% of all injuries are concussions with an estimated 62,816 new cases of concussion annually (Powell & Barber-Foss, 1999). Football accounts for 63% of these injuries. The rate of concussion is similarly high in professional sports with an estimated .41 concussions per National Football League game (Pellman et al., 2004).
Sports-related concussion has gained increasing attention in the neuropsychology literature. Early work by Barth and colleagues (Barth et al., 1983; Rimel et al., 1981) in the 1980s set the stage for a plethora of empirical investigation into the neuropsychological impact of concussion in sports and the resolution of cognitive sequelae over time. In addition, other researchers have suggested the possibility that repeated exposure to sports-related activities such as heading a soccer ball may cause a more subtle concussion (e.g., headaches, dizziness, feeling "dazed," etc.) with an associated dose-response effect (Webbe & Ochs, 2003; Witol & Webbe, 2003).
Although it is clear that most patients suffer at least some acute cognitive difficulties associated with concussion or mild traumatic brain injury (MTBI) more generally, the nature and course of postacute cognitive recovery remains an area of intense controversy. In non-sports-related MTBI, most cases recover completely within the first 3 months (Dikmen et al., 1986, 1995; Gentilini et al., 1985; Gronwall & Wrightson, 1974; Levin et al., 1987), however, a significant minority continue to manifest cognitive deficits beyond that point, with prevalence estimates varying across study from 7-8% (Binder et al., 1997) to 33% (Rimel et al., 1981). In addition, a number of individuals continue to report distressing symptoms for months (Alves et al., 1993; Dikmen et al., 1986; Hartlage et al., 2001; Powell et al., 1996) or years postinjury (Alexander, 1992; Deb et al., 1999; Hartlage et al., 2001). Frequently these complaints involve a constellation of physical, emotional, and cognitive symptoms collectively known as postconcussion syndrome (PCS).
Four meta-analytic reviews have been conducted on neuropsychological outcomes in MTBI (Belanger et al., 2005; Binder et al., 1997; Schretlen & Shapiro, 2003; Zakzanis et al., 1999) which together suggest (1) small overall effects, (2) somewhat larger effects in certain domains (i.e., attention largest), (3) a decrease in effect size with time since injury, and (4) effect...