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Department of Psychology, University of Western Ontario, London, Ontario, Canada, N6A 5C2
To gain a more complete picture of the influence of alcohol on visual performance we measured contrast sensitivity for a range of spatial and temporal frequencies in individuals with moderate blood alcohol levels.
Subjects consumed sufficient ethanol in fruit juice over a 20 minute period to raise BAL to approximately .08%. They were tested when BAL was approximately .06% in both the rising and falling phases. Patterns of stripes with a sinusoidal luminance profile were displayed briefly on a monitor. The subject's task was to detect the presence of these gratings. In the first study (4 males, 4 females) the gratings were stationary and were presented at six different spatial frequencies. In the second study (3 females) the same spatial frequencies were used but the gratings were also contrast reversed at different temporal frequencies (in different sessions), providing us with a spatiotemporal sensitivity profile.
Although there were individual differences, we found a small but significant reduction in contrast sensitivity at all spatial frequencies for stationary gratings. With counterphase flicker, performance loss was much greater at higher spatial and temporal frequencies.
The data suggest that alcohol does produce small, but real sensory deficits and that these cannot be attributed to impairments in pursuit eye-movement control.
For most purposes, the standard measure of visual performance is visual acuity, the ability to resolve fine detail. It may be assessed using a screening device, in which performance is defined with respect to the last line of letters on a chart that can be read or the smallest pattern in a display that can be identified. In most jurisdictions, an assessment of this type is part of a driver's license examination. Available evidence on the effect of alcohol on acuity is somewhat mixed, with some authors reporting a deficit and others not (cf. Newman & Fletcher, 1940; Nicholson et al., 1992).
Despite its popularity as a measure, acuity is limited in how much information it provides about visual capacity. In many situations involving driving, a person's ability to detect an object in his field of view is not acuity limited. With few exceptions, the failure to detect an object is not because it is too small to see. Although attention may play a major role, even with maximum attention there are circumstances when large objects may not be detected. Most commonly, this occurs when a driver attempts to discern relatively large, but faint, objects from their background. For example, when driving at dusk, or in foggy or snowy conditions, other vehicles that are quite large enough to be seen in bright sunlight may be barely visible. The most important variables, in addition to size, that determine visibility under these conditions are the contrast of the target against a background, and whether it is moving or flickering on and off. Contrast refers to the difference in luminance between adjacent light and dark regions in the visual field. Even a very large target becomes invisible if the contrast boundary is reduced sufficiently.
Given the almost infinite number of combinations of size and contrast of objects, it may seem an insurmountable task to provide a summary measure of sensitivity to different contrasts. However, there are several techniques that permit not only the evaluation of contrast sensitivity, but also allow researchers to obtain a more complete description of the overall functioning of the visual system (Campbell & Green, 1965). Contrast sensitivity may be determined by presenting the observer with "grating" patterns consisting of stripes whose luminance profile varies sinusoidally across the viewing screen. Thresholds are obtained by establishing the minimum detectable luminance difference between the dark and light bars. By measuring threshold at several different spatial frequencies (defined as the number of cycles of dark and light bars per degree of visual angle), a contrast sensitivity function (CSF) may be constructed.
Although the CSF itself is based on thresholds measured using sine-wave gratings, the mathematical properties of these stimuli permit predictions to be made about the potential visibility of a much wider range of stimuli (Campbell & Robson, 1968). The CSF therefore provides a much more complete description of visual sensitivity and more subtle deficits than those identified by acuity tests can be demonstrated (cf. Regan, Silver & Murray, 1977). It also provides a built-in measure of acuity, because a point is reached when a grating is so fine that it cannot be resolved, even at maximum contrast.
Two recent studies have examined contrast sensitivity following alcohol consumption. Zulauf, Flammer, and Signer (1988) reported a slight decrease in contrast sensitivity across the spatial frequency spectrum following an alcohol dose of .56 g/kg. They did not measure blood alcohol levels (BALs) directly, which is unfortunate, because they found differences in their results depending on when testing took place after alcohol administration. They also provided no information on the variability between subjects in their study. We examined the effects of alcohol on the CSF in our first experiment.
Andre, Tyrrell, Leibowitz, Nicholson, and Wang (1994) measured contrast sensitivity for sine-wave gratings that were either static or moved along a circular path to induce pursuit eye movements. They found a small reduction of sensitivity for static gratings and a much greater loss when the patterns were moved during testing. Although these data are consistent with the view that the a deficit in pursuit eye movements leads to the loss of sensitivity for the moving gratings, it is important to demonstrate that there is not a direct effect on temporal sensitivity independent of eye movements. It is known that alcohol can reduce the critical flicker fusion rate (Carpenter, 1962), but there are no data on contrast sensitivity for temporally modulated patterns. We investigated this question in a second experiment.
In the first experiment eight volunteers (4 males and females) with normal or corrected to normal vision, drawn from the University graduate student population participated. Their age range was 23 to 29 years and they were compensated for their time. They completed a drinking history questionnaire that indicated that all were moderate drinkers, in good health, with no family history of alcoholism. In the second experiment, because it required eight sessions, only four participants, drawn from the same population, were tested.
The stimuli consisted of sinusoidally modulated luminance gratings whose spatial and temporal frequency and contrast could be varied under computer control. They were displayed on the face of a Tektronix 608 display monitor with a white P4 phosphor. The luminance of the screen was 21 cd m-2. A large white screen (45° x 39°) with a 6 cm circular aperture was placed in front of the monitor. At the viewing distance of 114 cm the aperture subtended 3° of visual angle. The background screen was illuminated to provide an approximate colour and luminance match to the stimulus display. The participants viewed the display binocularly with their heads resting in a chin rest to prevent excessive head movements and maintain a constant viewing distance. A hand-held response key allowed them to initiate trials and to respond whether they had detected a target on each trial.
Participants were tested in counterbalanced alcohol and placebo sessions on separate days. The alcohol (40% neutral grain spirits) was mixed with the subject's juice of choice in a 1:4 ratio and consumed over 20 min. Blood alcohol concentrations were estimated every 15 min throughout the experiment from breath samples obtained using an Alcometer 7410 (Drager, Inc.). Participants reached an average peak BAL of .077%, although sensitivity was measured when their BAL reached 0.06% on both the rising and the falling portion of the blood alcohol curve or, in the placebo condition, 15 and 45 min after consumption of the juice.
Contrast sensitivity for stationary sine-wave gratings was measured for spatial frequencies of 0.5, 0.75, 1, 2, 4, 6, 8, 12 and 14 cyc deg-1, using a transformed random dual staircase procedure (Cornsweet, 1962; Levitt, 1971). In this procedure, the stimulus was presented for 500 ms and its contrast varied from trial to trial according to the detection rule: two "yes" responses to decrease the contrast by 2dB and one "no" to increase it by the same amount. Two randomly interleaved staircases were run for each spatial frequency, with the final six reversals on each being used to calculate threshold. All spatial frequencies were tested in a single experimental run. Participants could leave the laboratory once their BAL had dropped below 0.05%.
The same nine spatial frequencies were tested, but in each of four separate sessions the gratings were counterphase modulated at one of four different temporal frequencies (1, 3, 6 and 12 Hz). In this context, counterphase modulation refers to the fact that the gratings were reversed in contrast at the specified rate; thus there was temporal change without movement of the stimulus pattern. To accommodate the slowest reversal rate, the gratings were presented for 750 ms on each trial. Control non-alcohol sessions were run for each of the temporal conditions.
The averaged CSFs for the eight subjects in the alcohol and the placebo conditions are shown Figure 1. Because there were no significant differences between the rising and falling phases of BAL, these data were also pooled. It may be seen from the placebo/alcohol ratio that, with the exception of the highest frequency, average sensitivity was reduced by approximately 20%. We note also that there were marked individual differences, with some subjects showing small effects and others quite large deficits.
Average contrast sensitivity (+/- 1 SE) as a function of spatial frequency. Sensitivity is calculated as the reciprocal of the contrast threshold. Inset: the sensitivity ratio between the placebo/alcohol conditions. A value greater than 1 corresponds to an alcohol-induced deficit.
As in the first experiment, there was no significant difference between the rising and falling blood alcohol curves and the data presented in Figure 2 represent the average of the two post-alcohol runs. This Figure shows the effects of introducing temporal modulation. For the lower modulation rates the data are similar to those for stationary gratings, that is, a small decline in sensitivity following alcohol. However, at the highest temporal frequency, 12 Hz, the loss was much greater, averaging a reduction of sensitivity to 52% of the non-alcohol values.
Average contrast sensitivity (+/- 1 SE) as a function of spatial frequency for four temporal modulation rates. Each panel shows a different rate as indicated
The results of the first experiment are consistent with those of both Zulauf et al. (1988) and Andre et al. (1994), in showing that, on the average, alcohol produces a small reduction in contrast sensitivity. This small reduction is maintained when the gratings are temporally modulated at low rates. However, at the highest rate we tested, performance declined considerably. It is difficult to make a direct comparison between these data and those of Andre et al. because there is no simple way to translate the rotary motion used in that experiment with the contrast modulation used here. However, the current data suggest that rapid temporal processing, either for moving the eyes or at a more fundamental sensory level, are quite strongly affected by alcohol.
What is the implication of these results for the driver who has consumed a moderate amount of alcohol? It seems likely that under normal daylight driving conditions the loss of sensitivity would be insufficient to prevent a driver from detecting obstacles in his field of view. Although it is possible that objects might appear somewhat fainter than if he had not consumed alcohol, they would still remain quite visible. This would apply both to stationary and slowly flashing targets. Under less favourable conditions, such as heavy fog, where objects may be very faint, the loss of sensitivity could have the effect of reducing the visibility of an object below the threshold of detectability. This deficit is likely to be enhanced under conditions where an object is moving rapidly with respect to the driver, or flickering quickly. It is important to stress also that, even though all participants show some loss, a number of them showed much greater deficits. These individuals probably would have greater difficulties in detecting fainter targets.
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