This article provides a preliminary theoretical formulation of a threshold theory for acquired brain injury. The theory addresses the concept of brain reserve capacity as a major factor in explaining threshold differences in the onset of clinical symptoms or the expression of impaired test performance after acquired brain injury. The article provides the first critical review of the literature relevant to this theory.In the past decade the concept of a threshold factor has attracted sporadic interest in medicine to account for instances of protection from or vulnerability to clinical symptoms when the central nervous system is diseased. This interest has largely been focused on aging and dementia, although a heuristic example of the threshold concept is seen in Parkinson's disease (PD;Quinn, Rossor, & Marsden, 1986). According to Roth (1986, p. 45),

These data suggest that the risk of subtle cognitive deficits may be increased in asymptomatic stages of HIV-1 infection. However, these deficits are not associated with neurologic changes and do not seem to affect subjects' social functioning.

We administered a battery of standardized neuropsychological measures to assess cognitive functions in a group of 769 HIV-1 seronegative, 727 asymptomatic HIV-1 seropositive (CDC Groups 2 and 3), and 84 symptomatic HIV-1 seropositive (CDC Group 4) homosexual/bisexual men enrolled in the Multicenter AIDS Cohort Study (MACS). Measures included tests of attention, memory, and psychomotor speed. Comparison of group means revealed significant differences in performance between HIV-1 seronegative and symptomatic HIV-1 seropositive subjects on measures of memory and on measures with strong motor and psychomotor timed components. These findings support the sensitivity of these neuropsychological instruments for detecting cognitive changes that may be related to HIV-1, and are consistent with other reports of neuropsychological abnormalities in symptomatic HIV-1-infected individuals. Asymptomatic seropositive men, on the other hand, did not differ significantly from seronegative subjects on any of the neuropsychological measures. Only 5.5% of the asymptomatic HIV-1 seropositive men showed abnormal performance on individual tests. This proportion did not differ significantly from that of seronegative controls. Further, among asymptomatic seropositive subjects, we found no statistically significant differences as a function of duration of HIV infection or level of immune system functioning. Thus, results from this study support the hypothesis that the frequency of neuropsychological abnormalities in asymptomatic HIV-1-infected homosexual men is low, and not statistically different from that of seronegative controls.

1960, 16, 421. 4.-WOLFSON, W. and BACHELIS, L. An abbreviated form of the WAIS verbal scale. J . elin. Psychol., T h i s latter finding is significant in that schizophrenics re resented roughly 9270 of this group. The authors are presently investigating this problem. T i e hypothesis of performance change as a function of change in the continuity of item difficulty has been advanced.

In this article, the authors provide a comprehensive review of the research of mild head injury in children and adolescents from 1970 to 1995. Because of marked variability in methodologies across studies, a preliminary box-score tally was computed, without regard to studies' scientific or methodological merit. These results revealed 13 adverse, 18 null, and 9 indeterminate findings related to neuropsychological, academic, or psychosocial outcome. When studies were classified based on methodological merit, the stronger studies were generally associated with null outcomes across domains. However, a few of the less stronger neuropsychological studies (5 of 40) reported subthreshold and transitory alterations during the early postinjury period. At the present time, cautious acceptance of the null hypothesis is recommended until more definitive studies are conducted that address the problems raised in this review.