Method
Over five years, 384 subjects were tested; 129 male and 255 female. All were Psychology undergraduate students aged 18-21. Subjects used the 11th version of the Barratt Impulsivity Scale questionnaire2. This is made up of 30 statements, of which the subjects were asked to rank the accuracy on a scale of 1-4 (1 being “rarely/never”, 2 being “occasionally”, 3 being “often”, and 4 being “almost always/always”). They were instructed not to hesitate for too long over individual answers, and to complete the questionnaire honestly. Of the 30 statements, 11 are related to non-planning impulsivity, 11 to motor impulsivity, and 8 to attentional impulsivity. 19 are positively loaded, and 11 are negatively loaded. This gives a total possible score of 120.
The affective “go – no go” task was executed on computers, using PsyScope. Each subject was asked to choose heads or tails in order to determine whether to start with positive or negative words as the target stimuli. This was to counterbalance any effect of starting with a particular type of word. Participants were instructed to respond as quickly and accurately as possible (by pressing the space bar) to target words, while making no response to distracter words. 10 blocks of 18 words each were presented, alternating between positive and negative blocks. In positive blocks, positive words were the target stimuli, and negative words were the distracters. The reverse was true for the negative blocks.
Subjects wore headphones, through which a loud noise was played if the subject made an incorrect response to a distracter word (commission error). No sound was played for a correct response, or for failure to respond to a target word.
Results
The mean, variance, and standard deviation of the key measures in the BIS and the go – no go task are shown in table 1 below. They were calculated using the following equations:
Mean:
Variance:
Standard deviation:
Table 1 – key measures on BIS and go – no go task
In order to confirm a normal distribution of BIS scores, the results were plotted in a histogram (see figure 1 below) using Microsoft Excel’s Analysis Toolpak. As the distribution is approximately normal, parametric tests may be used.
Figure 1 – distribution of BIS total scores for 384 subjects
Table 2 below shows the BIS total scores sorted according to gender. The mean, variance, and standard deviation were calculated using the equations described above.
Table 2 – gender sorted BIS total scores
Using the F test with α=0.05, the variances are found to be statistically equal (F = 1.247, critical value of F=1.3). Therefore a normal unpaired t-test can be used. The null hypothesis is that the mean of the results for men is equal to the mean of the results for women.
Pooled variance = 96.840 (from table 1 above)
The critical value of t for 382 degrees of freedom (two-tailed, with p= 0.05) is 1.966. Therefore there is no significant difference between the total BIS scores for men and women.
Scores in the BIS were then correlated (using the Excel correlation function) with key variables from the affective go – no go task to give r and t values seen in table 3 below. The null hypothesis is that there is no correlation between BIS score and errors or bias on go – no go task.
Table 3 – correlation BIS total scores and results of affective go – no go task
The critical value for t (382 degrees of freedom, two-tailed, p=0.05) is 1.966. Therefore all three variables in the go – no go task correlate with the BIS total score. The probability of seeing these results if the null hypothesis were true is less than 0.05.
In order to further investigate the correlation between impulsivity and go – no go performance, the individual sub-factor scores were correlated with go – no go variables, as seen in table 4 below.
Table 4 – correlation between BIS sub-factor scores and results of affective go – no go task
Again, the critical value of t is 1.966. It can be seen that motor impulsivity shows significant correlation with all three go – no go variables, while attentional impulsivity shows no significant correlation with any. Non-planning impulsivity correlates with affective bias, but not with omission or commission errors.
Discussion
The results of the gender comparison test are similar to those shown in previous BIS experiments2. The gender difference is insignificant.
As expected, there was a significant correlation between BIS total score and number of commission errors in the affective go – no go task. Motor impulsivity was the only sub-factor to correlate with commission errors. This is related to acting without thinking4, and so it may be that highly impulsive people are unable to inhibit the impulse to act and respond.
Both BIS total score and motor impulsivity correlated with omission errors, contrary to the predictions. This could be due to the distracting effect of the noise played on commission errors. However, if this were the case, attentional impulsivity would be expected to show a significant correlation, as the distraction would act to inhibit focus and concentration.
Affective bias correlated strongly with BIS total score, motor impulsivity, and non-planning impulsivity. This is not what was expected. Healthy subjects tend to show a small positive bias3. A positive bias means a slower response to positive words than to negative words. These results show that a greater impulsivity correlates with a larger positive bias, while a lower impulsivity is linked to a larger negative bias. It may be that those with a greater impulsivity find it more difficult to inhibit responses to negative words than to positive words, and so when switching between positive and negative blocks, they are faster to respond to negative words. People with a lower non-planning impulsivity may be able to plan for future blocks more readily, and so inhibition is more easily reversed. However, if this were true, we would expect no bias for those with low impulsivity, as they could switch between inhibition.
An additional factor that may have influenced go – no go task performance is that subjects were already informed that the Barratt Scale questionnaire was to investigate their impulsivity, and they had already completed it and calculated their scores. Their expectations of results may have influenced their performance on the task.
References
1. Edman et al. (1983). Impulsivity and speed and errors in a reaction time task. Acta Psychologica. 53: 1-8.
2. Patton et al. (1995), Factor structure of the Barratt Impulsiveness Scale. Journal of Clinical Psychology. 51: 768–774.
3. Murphy et al, (1999). Emotional bias and inhibitory control processes in mania and depression. Psychological Medicine. 29: 1307-1321.
4. International Society for Research on Impulsivity - http://impulsivity.org/BIS-11/history-and-development-of-the-bis
