Aims

The aims of this project are to assess the long-term effects of adolescent cannabis use on cognitive functioning. Previous research has suggested heavy cannabis use in the teenage years may lead to persistent neuropsychological deficits; however much of the research to date has been cross-sectional and therefore unable to assess whether pre-exposure group differences are driving the association. A recent longitudinal study found chronic heavy cannabis use was associated with a decline in IQ in those with adolescent-onset cannabis use, but not in those with adult-onset use (Meier et al, 2012). However this study had a number of important limitations, including small group sizes and failure to account for a number of potential confounding variables adequately. The present study will address these issues with an independent, larger sample, and assess other factors that may contribute to the relationship.

Research Questions

1. Is heavy cannabis use in adolescence associated with a decline in IQ?

2. Does the neuropsychological functioning of different cannabis-exposure groups differ in childhood, i.e. before onset of drug use? Do the exposure groups differ in other ways (e.g. early environment, socioeconomic status) that might influence neuropsychological functioning?

3. Does adolescent cannabis use affect cognitive function once pre-exposure functioning and other potential confounders (e.g. mental illness or socioeconomic status) have been accounted for? If so what is the size of this effect, and is this also reflected in academic acheivement?

4. Are any deficits seen globally across all neuropsychological functioning, or are they specific to certain domains (e.g. attention or decision making)?

Exposure variables:

1. Cannabis use. This measure will be defined according to lifetime reported usage between ages 13-18 years.

2. Cannabis dependence. This measure will be defined according to criteria for cannabis dependence between ages 16-18 years.

Outcome variables:

1. IQ change from childhood to adolescence. This outcome will be defined based on performance on IQ assessments completed at ages 8 and 16.

2. Neuropsychological functioning. This outcome will be defined by performance on cognitive tasks completed between ages 16-18 years.

3. Academic acheivement. This outcome will be defined by performance on academic assessments completed at school year 11 (Key Stage 4).

Confounding variables:

1. Pre-exposure neuropsychological functioning. Pre-existing differences or deficits in cognitive and academic ability are likely to be related to post-exposure functioning

2. Pre-exposure academic performance. Problems at school in childhood may influence cognitive development and may also be related to the initiation of cannabis use

3. Childhood temperament and behaviour. Childhood traits may influence cognitive development and may also be related to initiation of cannabis use

4. Mental illness (including depression, psychosis and conduct disorder) will likely influence neuropsychological functioning and task performance

5. Sex, socioeconomic status (e.g. parental years of education) and early environment/ parental factors (e.g. parental IQ or drug use) will likely influence neuropsychological functioning

6. Other substance use and dependence (including alcohol and tobacco) may influence neuropsychological functioning

7. Recent cannabis exposure at testing will likely affect performance on neuropsychological assessment due to acute (use in the past 24 hours) and residual (use in the past month) effects of the drug

Reference

Meier, M.H., Caspi, A., Ambler, A., Harrington, H., Houts, R., Keefe, R. S. E., McDonald, K., Ward, A., Poulton, R., & Moffitt, T. E. (2012) Persistent cannabis users show neuropsychological decline from childhood to midlife. PNAS, 109, 1-8.