Alcohol-induced Impairment of Driving Performance in Adults with ADHD Jessica J. Weafer, B.A., Mark T. Fillmore, Ph.D., and Richard Milich, Ph.D. Department of Psychology, University of Kentucky INTRODUCTION  Recent research has shown that attention deficit/hyperactivity disorder (ADHD) persists into adulthood in 60-80% of cases (e.g., Barkley, Fischer, Smallish, & Fletcher, 2002), leading to impaired functioning in various domains.  One important problem associated with ADHD among adults concerns the possibility that the cognitive deficits might lead to impaired driving performance. Previous survey research has shown that adults with ADHD were more likely than controls to have been involved in accidents, receive traffic citations for speeding, and have their licenses suspended or revoked (Barkley, Murphy, DuPaul, & Bush, 2002)  To date, direct experimental evidence of impaired driving skills on a laboratory driving simulation task in individuals with ADHD has been scarce and inconclusive (Barkley, Murphy, et al., 2002; Barkley, Murphy, O’Connell, Anderson, & Connor, 2006).  An ecologically relevant benchmark upon which to compare driving impairment in ADHD is the performance of drivers who are considered to be legally intoxicated by alcohol at the blood alcohol concentration (BAC) of 80 mg/100 ml (0.08%) on a driving simulation task.  The possibility that sober drivers with ADHD display deficits in driving impairment characteristic of intoxicated drivers suggests that these drivers might be more sensitive to the effects of alcohol than drivers with no history of ADHD. STUDY OBJECTIVE: The present study involved two experiments that were designed to evaluate decrements of driving performance in adults with ADHD in terms of the types of driving decrements typically associated with alcohol intoxication. Experiment 1 compared driving simulation performance in drivers with ADHD to a community control sample whose driving performance was tested in a sober state and while legally intoxicated (80 mg/100 ml). Experiment 2 was a dose-response study of simulated driving in adults with ADHD and controls. EXPERIMENT 1 METHOD Participants: Fifteen participants with ADHD (5 men and 10 women; mean age = 21.5, SD = 1.5), and 23 control participants (13 men and 10 women; mean age = 22.0, SD = 1.7) with no history of ADHD participated in Experiment 1. Driving Simulation Task: Participants performed a simulated driving task that measured driving performance. They controlled the vehicle by moving a steering wheel and manipulating accelerator and brake pedals. The simulated driving test required 20 minutes to complete. Previous research has shown this task is a reliable measure of driving performance (Harrison & Fillmore, 2005). Procedure: The ADHD group attended a single test session in which their sober driving performance was assessed. Controls attended two test sessions. One session tested their driving performance while sober and the other session tested performance when intoxicated by a moderate dose (0.65 g/kg) of alcohol that produced a mean blood alcohol concentration (BAC) of 87.4 mg/100 ml (SD = 19.3). RESULTS Driving Deficits Associated with Alcohol Intoxication Alcohol impaired within-lane deviation, steering rate, and driving speed variation in controls, as demonstrated by 2 Session (sober vs. intoxicated) within-subjects ANOVAs (ps <.05; see Figure 1). Driving Deficits in Individuals with ADHD ADHD vs. Sober Controls Between-subjects ANOVAs comparing the ADHD group to sober controls confirmed that the ADHD group displayed greater within-lane deviation and steering rates (ps <.05), but no significant difference was found on the measure of driving speed variability (p =.21; see Figure 1). ADHD vs. Intoxicated Controls Between-subjects ANOVAs comparing the ADHD group to intoxicated controls confirmed that driving impairment in the ADHD group was comparable to that of the intoxicated controls in all measures (ps >.68; see Figure 1). DISCUSSION Experiment 1 showed that adults with ADHD exhibited decrements in driving performance that were comparable to those displayed by legally intoxicated drivers, both in terms of the aspects of driving performance that were impaired (e.g., driving precision, speed maintenance) and in terms of the magnitude of impairments observed. Experiment 2 replicated and extended these findings by showing that drivers with ADHD were more impaired than controls in all dose conditions. Moreover, those with ADHD were markedly impaired by the lower of the two doses (0.45 g/kg) which only yielded a peak BAC of 55.6 mg/100 ml, far below the legal limit BAC for prosecuting drunk drivers. Thus, ADHD drivers exhibit a profile of impaired driving performance similar to a profile of alcohol-impairment that studies have reliably shown to be associated with substantially elevated traffic hazards and accident risk. Clear risks for hazardous driving are present in unmedicated drivers with ADHD, and the implications of these risks need to be addressed. REFERENCES Barkley, R.A., Fischer, M., Smallish, L., & Fletcher, K. (2002). The persistence of attention- deficit/hyperactivity disorder into young adulthood as a function of reporting source and definition of disorder. Journal of Abnormal Psychology, 111, Barkley, R.A., Murphy, K.R., Dupaul, G.J., & Bush, T. (2002). Driving in young adults with attention deficit hyperactivity disorder. Knowledge, performance, adverse outcomes, and the role of executive functioning. Journal of the International Neuropsychological Society, 8, Barkley, R.A., Murphy, K.R., O’Connell, T., Anderson, D., & Connor, D.F. (2006). Effects of two doses of alcohol on simulator driving performance in adults with attention-deficit/hyperactivity disorder. Neuropsychology, 20, Harrison, E.L.R., & Fillmore, M.T. (2005). Are bad drivers more impaired by alcohol? Sober driving precision predicts impairment from alcohol in a simulated driving task. Accident Analysis and Prevention, 37, Dependent Measures: Deviation of Lane Position. Degree of adjustment that the driver implements to maintain a desired position within the lane. Larger values indicate poorer driving precision. Steering Rate. Rate at which the driver turns the steering wheel to maintain position on the road. Impaired driving is characterized by abrupt, quick movements to the steering wheel, demonstrated by higher rates of steering movement. Speed Variation. Average speed was measured in mph, and speed variation was measured by the standard deviation of the average speed score during a test. The inability to maintain a constant speed is indicative of impaired driving. Figure 3. Mean steering rate under each dose (0.0 g/kg, 0.45 g/kg, and 0.65 g/kg) in the control and ADHD groups. Figure 1. Mean driving performance measures (deviation of lane position, steering rate, and speed variation) in controls while sober and intoxicated and in ADHD. Figure 4. Mean speed variation under each dose (0.0 g/kg, 0.45 g/kg, and 0.65 g/kg) in the control and ADHD groups. ACKNOWLEDGEMENT Supported by grant DA from the National Institute on Drug Abuse Experiment 2 Purpose: Given that impairment of driving performance in individuals with ADHD was found to be comparable to that of intoxicated controls, Experiment 2 tested the hypothesis that ADHD individuals may be more sensitive to the impairing effects of alcohol than controls in a dose-response study of driving simulation performance. METHOD Eight participants with ADHD (6 men and 2 women; mean age = 23.0, SD = 1.9) and 8 control participants (5 men and 3 women; mean age = 23.1, SD = 1.2) with no history of ADHD participated in Experiment 2. All participants attended three test sessions in which driving simulation performance was tested under three doses of alcohol: 0.65 g/kg, 0.45 g/kg, and 0.0 g/kg (placebo). Dose order was randomized and counter-balanced across participants. There were no group differences in BAC. The mean BACs during the driving test for the 0.45 g/kg and 0.65 g/kg doses were 55.6 mg/100 ml (SD = 9.5) and 78.9 mg/100 ml (SD = 18.2), respectively. Measures of driving performance were the same as in Experiment 1 and were analyzed by 2 Group (ADHD vs. controls) x 3 Dose (0.0, 0.45, 0.65 g/kg) ANOVAs. RESULTS Analysis of deviation of lane position revealed main effects of group (p =.02) and dose (p =.01). Figure 2 shows greater within-lane deviation in the ADHD group compared to controls and in response to active doses of alcohol. Analysis of steering rate revealed a main effect of group (p =.05). Figure 3 illustrates that this is due to increased steering rate in individuals with ADHD. Analysis of speed variation revealed a significant Group X Dose interaction (p =.04). Figure 4 illustrates that, compared to controls, those in the ADHD group displayed marked increase in speed variation in response to alcohol, and that the increase was most pronounced in the lower of the two doses (0.45 g/kg). Subjective effects of alcohol were also examined in each group. In general, those in the ADHD group reported greater self-perceived ability to drive regardless of dose and reported feeling less intoxicated by alcohol. Figure 2. Mean within-lane deviation under each dose (0.0 g/kg, 0.45 g/kg, and 0.65 g/kg) in the control and ADHD groups.