The groups did not differ significantly in age, education, ethnicity, or number of marijuana joints smoked per week; but did differ significantly on number of alcoholic drinks per week () (also see for age of initiating smoking, daily cigarette consumption, length of smoking history, lifetime exposure to smoking, mean dependence scores, and time from waking to smoking the first cigarette of the day). To determine whether age was associated with smoking behavior and/or exposure, bivariate correlation analyses between age and indices of cigarette dependence and behavior were performed. There were no significant correlations between the variables examined and age (HSI (r=0.01, p=0.47) and pack years (r=0.30, p=0.14)).
Immediately before scanning, cigarette craving was scored as 3.24 on average (range: 1.2–6.3) on a scale of 1–7 (UTS). Although 14 of the subjects had smoked within 2
h of scanning, 11 of them had not smoked for at least 2
h. Still, retrospective reports of nicotine withdrawal over the 7 days before testing (first nine items of the MNWS) indicated that the young smokers tested here generally did not suffer high levels of withdrawal. With a possible score of 30, the mean score was 10.1, with only 2 of the 25 participants reporting scores of 20 or above.
There were no significant group differences in task performance (). Inhibition success rate did not differ significantly from 50%, with few discrimination errors on Go trials; and values of the SSRT were in the range reported for healthy adults (Jenkinson et al, 2002
) in both groups. Median RT on Go trials was not correlated with SSRT, consistent with the assumptions underlying the Stop-signal race model (Logan, 1994
Successful response inhibition (Successful Stop>Baseline, Successful Stop>Go, and Successful Stop>Unsuccessful Stop contrasts) activated a broad neural network, consistent with previous reports (Aron and Poldrack, 2006
; Congdon et al, 2010
) with activation in bilateral IFG, cingulate cortex, supplementary motor area, striatum, thalamus, central opercular cortex, insular cortex, superior temporal gyrus, planum temporale, middle temporal gyrus, supramarginal gyrus, and occipital pole in both groups (; Supplementary Table 1). There were no significant group or gender differences.
Figure 1 Neural activation during successful inhibition. Direct group comparisons revealed no significant differences. For illustration only, conjunction group maps of all three contrasts for nonsmokers (blue) and smokers (red) are overlaid onto the averaged normalized (more ...) Go process
The Go process (Go>Baseline contrast) activated the supplementary motor cortex, precentral gyrus, and cingulate gyrus (data not shown). There were no significant group differences, and this contrast will not be discussed further.
Relationship between neural activation and response inhibition (SSRT)
Across all subjects, SSRT during successful inhibition was significantly negatively correlated with activation in the IFG, insula, cingulate, orbitofrontal cortex, and temporal gyrus. Individuals with greater stopping capacity (smaller SSRT) showed greater recruitment in these regions (; Supplementary Table 2). There were no positive correlations between neural activation and SSRT, and no group differences in correlation between activation and SSRT.
Figure 2 Correlation of stopping time (SSRT) with neural activation. Across all subjects, including nonsmokers and smokers, there was a significant negative correlation between stopping time (SSRT) and neural activation in the IFG, insula, and temporal gyrus, (more ...)
Smoking Behavior and Neural Activation During Response Inhibition
The HSI was negatively correlated with left and right MFG (x=−34, y=36, z=38; x=30, y=36, z=44), cingulate gyrus (x=0, y=14, z=28), supplementary motor cortex (x=0, y=−2, z=62), orbitofrontal cortex (x=−26, y=56, z=2), and right superior frontal gyrus (x=24, y=−2, z=66) in the Successful Stop>Baseline contrast (; Supplementary Table 3), and with left IFG (x=−52, y=14, z=14) in the Successful Stop>Go contrast (; Supplementary Table 3). There were no significant correlations with the Successful Stop>Unsuccessful Stop contrast. Cigarette craving and pack years were not significantly associated with neural activation during response inhibition.
Figure 3 Heaviness of Smoking Index and neural activation. In smokers, HSI was negatively correlated with left and right MFG (x=−34, y=36, z=38; x=30, y=36, z=44), cingulate gyrus (x=0, y=14, z=28), supplementary motor cortex (x=0, y=−2, z=62), (more ...)
Smoking Behavior and Neural Activation During Responding
To determine whether associations between smoking variables and neural activity were specific to response inhibition, the same smoking variables that were described above, including HSI and pack years, were subjected to correlation analyses with neural activation during the Go response (Go>Baseline and Go>Successful Stopping contrasts). There were no significant correlations between any of the smoking variables examined and neural activation during the Go response, except a negative correlation between HSI and activity in the supplementary motor cortex (x=2, y=−8, z=64) in the Go>Baseline contrast (data not shown).