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The role of the solvent in the passive Q-switching of lasers by solutions of organic dyes has been examined in detail. It was found that several pure organic solvents could themselves partially Q-switch the laser, in the absence of the dye. This behaviour was observed for both ruby and neodymium: glass lasers. Pulse-widths as short as 20 nsec were occasionally recorded. Peak powers up to 2 MW have been observed using 1-chloronaphthalene to switch a ruby laser. It is proposed that Q-switching arises from an enhancement of reflectivity of the liquid, during the evolution of the laser pulse, through the formation of a periodic refractive index modulation in the liquid by the action of standing waves. [ABSTRACT FROM AUTHOR]

Objective: Full recovery is typical among individuals with a single, uncomplicated concussion/mild traumatic brain injury (MTBI). Less is known about the long-term effects of multiple concussion/MTBI. This study aims to compare psychomotor speed and cognitive flexibility in patients with a history of multiple MTBI to those with a history of a single MTBI. Method: Two groups of 7 males were included (age range 10 to 18 years). Previous MTBI's in the multiple concussion (MC) group ranged from 3 to 8 (M = 3.85, SD = 2.03). An age- and sex-matched control group with a history of a single concussion (SC) was used. All subjects participated in a standard clinical neuropsychological evaluation. Results: Mean FSIQ was within the average range for both groups (SC: M = 98.57, SD = 12.19; MC: M = 93.14, SD = 10.31; t(12) = 0.89, p = ns). Mean PSI was lower in the MC group (M = 88.43, SD = 14.47) compared to the SC group (M = 95.29, SD = 8.98), although not significantly different, t(12) = 1.02, p = ns. Differences were found between the two groups on Symbol Search, t(12) = 2.40, p = 0.03, with the MC group scoring lower. A trend was found between groups on DKEFS Letter-Number Switching, with the MC group scoring lower, t(12) = 1.88, p = 0.08. Conclusion(s): In this small group, differences were seen between adolescents with a single versus multiple MTBI. Evaluation of these findings in a larger sample is warranted to further clarify the results. [ABSTRACT FROM AUTHOR]