The experiment tested whether alcohol had any effect on reaction time.

Abstract

Objective: The experiment tested whether alcohol had any effect on reaction time.

Method: Subjects were required to identify the threshold at which a flickering light became constant (critical flicker fusion threshold) using a computerised flicker fusion system.  Frequency increased at a rate of 4 hertz per second.  Critical flicker fusion threshold is a well accepted and documented non-invasive measure of reaction time.  Ten female subjects were tested under control conditions and following ingestion of 2 units (80 mg) alcohol.  It was hypothesised that alcohol would cause an increase in reaction time, which would translate to a delay in recognising the critical flicker fusion threshold, thus higher frequency results.

Results:  Ingestion of 2 units (80mg) of alcohol was associated with an increase in mean critical flicker fusion threshold from 14.6 hertz to 15.4 hertz (p<0.0001).  This increase in mean critical flicker fusion threshold translated to an increase in reaction time equivalent to 0.2 seconds.

Conclusion:  2 units of alcohol had the effect of increasing reaction time by an average of 0.2 seconds, which has serious implications for the consumption of alcohol prior to tasks involving complex motor skills such as driving.

Introduction

Alcohol and its effects

Alcohol is believed to be the oldest drug used by humans, predating even the use of opium by 2000 years to around 8000 BC (Kerr, Hindmarch 1998).  Whilst legal age limits exist for the purchase of alcohol in the United Kingdom, it is widely regarded within the Western world as an acceptable drug.

In recent household studies in the UK it was found that 75% of men and 60% of women consumed at least one alcoholic drink per week.  In addition, 40% of men and 23% of women were found to have exceeded the national recommendations on alcohol consumption within the previous week (Office for National Statistics 2005).  The Institute For Alcohol Studies ranks the United Kingdom as 9th in per capita consumption of pure alcohol within European Nations, with 9.6 litres of pure alcohol being consumed per capita in 2002 (Institute for Alcohol Studies 2005).

Alcohol is known for its psychoactive effects, which include alterations in vision, motor tasks and skills such as car driving and flying.  In addition it is repeatedly shown, whether anecdotally or via scientific measurements, that a strong correlation exists between alcohol consumption and violence.

Alcohol is known to be a contributory factor in road accidents, with 9% of casualties showing evidence of alcohol consumption, this figure rising to 31% when considering pedestrians (The Scottish Office Central Research Unit 1998).  Research carried out in the 1980s by the Transport Research Laboratory indicated that alcohol was involved in 35% of fatal road traffic accidents, with the figure falling slightly to 31.5% in a similar study completed in 2000 (Tunbridge, Keigan & James 2001).   However neither of these reports explained why the association existed between alcohol and road traffic accidents, whether resulting in death or not. 

Of import for this report is the association between alcohol and reaction time.  The majority of alcohol consumers can identify a slowing down of their faculties following alcohol consumption, regardless of claims to the contrary.   Research has shown that alcohol impairs the ability of individuals to carry out complex motor tasks.

One example involved bus drivers being asked to drive a vehicle through a narrow space, or highlighting the fact that the gap was too narrow if necessary.  It was shown that alcohol consumption was correlated with a reduced ability to accurately guide the bus through the gap, coupled with an inability to accurately gauge the width of the gap.  Hence bus drivers who had consumed alcohol were more likely to judge a gap as to be wide enough when it was not, than those who had not consumed alcohol and whose spatial awareness remained intact (Rang, Dale & Ritter 1999a).

Recommended stopping distances at 30 miles per hour are 23 metres / 75 feet, of which 9 metres / 30 feet are the ‘thinking distance’.  This is based on an average reaction time of 0.7 seconds when the car is travelling at 44 feet / second.  Therefore if reaction times increase, stopping distances will do so also, with serious implications in an accident.

It has been indicated by some research that low levels of alcohol consumption have very little effect on reaction time if attention could be focussed on a single objective (Jaaskelainen et al. 1996).  Where attention needs to be divided between task objectives, even low blood alcohol levels were found to impair performance.  This suggests that alcohol is not going to greatly impair reaction time during simple tasks, but complex tasks which require several aspects to the performance would be much more likely to be impaired.  This was further supported by the research of Bartholow et al which found that response times per se were relatively unaffected by the presence of alcohol but the ability to respond appropriately to tasks that required complex attention were (Bartholow et al. 2003).  Indeed the authors implicate alcohol in impairments of cognitive processing, rather than the motor responses that result from these processes.  They cite data from studies that have shown that alcohol acts to reduce the ability to respond to stimuli as well as interpret and process the correct relevance of these stimuli.  This inability to respond fully to cues from the environment is described as the attention-allocation model, as the brain is selective in which cues are actually attended to and processing within the brain.  Further research has indicated that alcohol can sometimes actually improve the ability of subjects to resist distraction from a task (Erblich, Earleywine 1995) but this is not in keeping with the majority of research.

Given the existing data this experiment was designed to assess the ability of female subjects to respond to a change in a single form of stimulus.  There was no distraction, nor a divided attention focus required, in an effort to ensure that the effects of alcohol on reaction time, if any, were more obvious.

Flicker fusion threshold

The human eye is capable of distinguishing between intermittent stimuli such as flickering light, up to a threshold, which is usually around 16 Hertz.  The frequency at which the human eye is no longer able to distinguish individual stimuli is defined as the critical flicker fusion threshold.  It is at this frequency that the individual stimuli have fused to form a single continuous stimulus.  The flicker fusion threshold will vary between individuals depending on their eyesight, hence the use of a number os study participants.  It will also vary between an individual’s readings depending on their reaction time at each stage – ie the time at which they actually consciously register that the hitherto flickering stimulus has now become constant and are able to respond to this knowledge. 

The purpose of this experiment was to use the measurement of critical flicker fusion threshold as a correlate to reaction time. 

For this experiment the experimental hypothesis was that alcohol acts to increase the reaction time of female subjects.

The null hypothesis was that alcohol has no effect on the reaction time of female subjects. 

Thus it would be expected that an individual with a slower reaction time would give results indicating a higher critical flicker fusion threshold, measured in hertz. 

In other words it would be expected that the frequency at which subjects indicated that the flickering light (for full details of methodology please see below) had fused into a single light would be higher under alcohol conditions than control.  This would not be due to an enhanced ability to differentiate between flickering and constant light, rather a delay in the ability for this change to register and be processed by the brain, and the subject to press the button.

Method

Ten female subjects aged from 18-35 years, with a body mass index of 19-28 were selected as part of an open experiment into the effect of alcohol on reaction time.  All subjects were informed of the purpose of the experiment prior to taking part and were required to complete medical questionnaires to exclude medication that might affect the results of the experiment.   Known negative effects of alcohol consumption were also excluded and subjects all had a history of regular alcohol consumption of at least 2 units, once per week.

Subjects were required to refrain from eating or drinking for the 2 hours prior to each test, which took place on consecutive days, with the control (no alcohol) test taking place prior to the alcohol test.  The 2 hour nil by mouth regulation was put in place in an effort to standardise the absorption of the alcohol by reducing stomach contents to a more uniform amount, thus providing a similar surface area available for alcohol absorption in each study participant.

On arriving at the test room subjects were required to complete a health and safety questionnaire and were again reminded of the aims and purposes of the experiment.  Subjects were free to leave at any time, and signed consent forms to allow their results to be used. 

Following the initial briefing subjects were given a training briefing on the specialised equipment and allowed to take a small number of practise tests to familiarise themselves with the equipment requirements.  Following this training period a five-minute break was allowed.

For the test itself each subject was required to drink 250ml of pure orange juice, with a five-minute timespan being allowed for the drink to be consumed.  Forty minutes after the drink had been consumed subjects critical flicker fusion threshold was tested using the Model 12021 Flicker Fusion System (Lafayette Instruments).

This time scale was used as the 2 units of alcohol would have reached a peak blood alcohol concentration of approximately 80 mg/100 ml 45 minutes following ingestion (Wilson, Benjamin & Sreenivasan 2003).  Assuming absorption and metabolism at the accepted 4 mmol/l per hour (Rang, Dale & Ritter 1999b), the alcohol would be expected to have been removed completely from the body within 6 hours (Wilson, Benjamin & Sreenivasan 2003).

Subjects were requested to look in to the binocular eye piece at two white simultaneous lights.  The use of a separate light for each eye was used to prevent differences in eye focussing from causing conflicting critical flicker fusion thresholds.

The initial flash frequency of 4 hertz was set to ascending at a rate of 4 hertz / second.  The subject was provided with a push button connected to a 1 metre cable and was required to push the button when the flickering ceased and the lights became fused to a single light emission.  The point at which the button was pressed was taken as the critical flicker fusion threshold.  
Each subject was required to undertake ten reaction time recordings.

The experimental procedures on day 2 were identical to day 1, except that 2 units of alcohol (vodka), approximately 80mg of pure alcohol, had been added to the 250ml of pure orange juice that the subjects were required to drink.  A further ten reaction time recordings were made using the flicker fusion system.

Results

Each subject was able to provide 10 reaction time recordings, which ranged from a minimum of 11.5 Hertz (subject 9, recording 6, no alcohol) to a maximum of 19.4 Hertz (subject 3, recording 8, with alcohol). 

The mean for the control / no alcohol test was 14.6 + 3.6 Hertz.  The mean for the alcohol test was 15.4 + 4.0 Hertz.

Tables 1 and 2 below show the individual reaction times of each subject participant on the two tests.

Table 1. Reaction times of 5 female subjects with and without alcohol, as measured by critical flicker fusion threshold

Table 2. Reaction times of 5 female subjects with and without alcohol, as measured by critical flicker fusion threshold