Description of data in 'alcohol.dat' variables... Alcohol Level mg/dl Decrease % decrease in smooth-pursuit velocity Gender 1 = Female 2 = Male data obtained by personal communication between jh and author BLOOD ALCOHOL AND EYE MOVEMENTS Letter by P. Bittencourt to The Lancet, Nov 1, 1980: p.981. SIR, - Alcohol is known to affect eye movements [ref] Smooth pursuit and saccades are impaired when the blood ethanol concentration is in the region 60 - 100 mg/dl [ref] but the magnitude of the effect in relation to the blood concentration has not beenexamined. The maximum velocity reached by the eyes during smooth pursuit is between 40 and 100û/s.[ref Above this threshold, eye tracking is accomplished largely by saccades. Since vision is not perfect during saccades [ref] a decrease in the velocity capacity of the smooth-pursuit system will impair vision and even lead to nystagmus.[ref] The smooth-pursuit system may be involved in producing the slow component of optokinetic nystagmus [ref] and in suppression of vestibular induced nystagmus, both of which are important for maintaining foveal (optimum) vision.[ref] To study the relations between blood alcohol concentration and smooth- pursuit velocity, six male and six female healthy volunteers, aged 21-42 (mean 30) years, had baseline eye-movement tests done between 1530 and 1630 h. The volunteers then drank whisky and water or gin and tonic until they considered themselves unfit to drive. Eye-movement tests were then repeated and blood was taken for determination of ethanol concentration by gas-liquid chromatography.7 A standardised electro-oculographic procedure [ref] was used to record smooth- pursuit eye movements on magnetic tape. The velocity of smooth-pursuit eye movements was measured by off-line computer analysis. This method measure the maximum velocity that the eyes can reach in smooth pursuit, before breaking down into saccades. The smooth-pursuit threshold velocity was 58.5 +/- 11 (SD) û/s before and 43 +/- 13.6 (SD) û/s after alcohol ingestion (p<0.01) when alcohol blood concentrations were 83 +/- 34 (SD) mg/dl. There was a direct linear relation (r=0.67, p<0.05) between the decrease in smooth-pursuit velocity and the alcohol concentration (figure) [figure shows plot of Smooth-pursuit velocity % decrease on y axis versus Blood alcohol (mg/dl) on x axis*, with legend "direct linear correlation between blood alcohol concentrations and % decrease in smooth-pursuit velocity" and footnote: "n=12, r=0.67, p<0.05"]. This result provides reliable evidence that at a blood ethanol of 80 mg/dl (the legal limit in the U.K. for driving purposes) smooth-pursuit eye movements -- i.e., the capacity of the eyes to track objects moving slowly across the visual fields -- is impaired by about 25%. *To reconstruct Figure, the data (slightly rounded) are given below: Subject 1 2 3 4 5 6 7 8 9 10 11 12 Sex F F F F F F M M M M M M Alcohol(x) 84 67 71 105 109 74 148 45 67 20 84 120 % Decrease(y) 31 33 2 38 12 6 58 13 48 2 20 51 SUM x = 994 SUM y = 314 SUM xx = 95242 SUM yy =12520 SUM xy = 30924 QUESTIONS a What test do you think the authors used to show that the decrease of 58.5 - 43.0 = 15.5 degrees per second is statistically significant? Do you have enough data to verify their calculations? b Verify that the p value is indeed less than 0.05 for the "direct linear relation" c In fitting a regression line of the % decrease (y) on the blood alcohol (x), which would be a more suitable candidate (i) a straight line through the origin? (ii) a straight line with nonzero intercept? Why? [Answer as if you had been asked the question before seeing the data.] d The authors claim that the direct linear relation (r=0.67, p<0.05) "provides reliable evidence that at a blood ethanol of 80 mg/dl the capacity of the eyes to track objects moving slowly across the visual fields is impaired by about 25%?" Why might you disagree with this claim? For example, if a judge quoted you this statement, how would you defend yourself? [Your defense must be statistical but intelligible to the judge, who has never taken a course in statistics; also, do not go beyond the data at hand... ie don't get into a discussion of whether eye movements are the most relevant parameter of impairment] e List 3 suggestions (1 sentence each) for improving on this study: