title: Worth the wait: effects of age of onset of marijuana use on white matter and impulsivity.
abstract: BACKGROUND
  Marijuana (MJ) use continues to rise, and as the perceived risk of using MJ approaches an all-time historic low, initiation of MJ use is occurring at even younger ages. As adolescence is a critical period of neuromaturation, teens and emerging adults are at greater risk for experiencing the negative effects of MJ on the brain. In particular, MJ use has been shown to be associated with alterations in frontal white matter microstructure, which may be related to reports of increased levels of impulsivity in this population.
  
  
  OBJECTIVE
  The aim of this study was to examine the relationship between age of onset of MJ use, white matter microstructure, and reported impulsivity in chronic, heavy MJ smokers.
  
  
  METHODS
  Twenty-five MJ smokers and 18 healthy controls underwent diffusion tensor imaging and completed the Barratt Impulsiveness Scale. MJ smokers were also divided into early onset (regular use prior to age 16) and late onset (age 16 or later) groups in order to clarify the impact of age of onset of MJ use on these variables.
  
  
  RESULTS
  MJ smokers exhibited significantly reduced fractional anisotropy (FA) relative to controls, as well as higher levels of impulsivity. Earlier MJ onset was also associated with lower levels of FA. Interestingly, within the early onset group, higher impulsivity scores were correlated with lower FA, a relationship that was not observed in the late onset smokers.
  
  
  CONCLUSIONS
  MJ use is associated with white matter development and reported impulsivity, particularly in early onset smokers.
authors.affiliation: Cognitive and Clinical Neuroimaging Core, McLean Imaging Center, McLean Hospital, 115 Mill St, Belmont, MA, 02478, USA, gruber@mclean.harvard.edu.
authors.forename: Staci A
authors.initials: SA
authors.lastname: Gruber
authors.forename: Mary Kathryn
authors.initials: MK
authors.lastname: Dahlgren
authors.forename: Kelly A
authors.initials: KA
authors.lastname: Sagar
authors.forename: Atilla
authors.initials: A
authors.lastname: Gönenç
authors.forename: Scott E
authors.initials: SE
authors.lastname: Lukas
mesh_terms: D017668:Age of Onset; D001921:Brain / Q000187:drug effects / Q000254:growth & development / Q000473:pathology*; D003337:Corpus Callosum / Q000187:drug effects / Q000254:growth & development / Q000473:pathology; D038524:Diffusion Magnetic Resonance Imaging; D056324:Diffusion Tensor Imaging; D005260:Female; D005454:Fluorescence Polarization; D007839:Functional Laterality; D006801:Humans; D007175:Impulsive Behavior; D008297:Male; D008385:Marijuana Smoking / Q000473:pathology* / Q000523:psychology*; D011569:Psychiatric Status Rating Scales; D066127:White Matter / Q000187:drug effects / Q000254:growth & development / Q000473:pathology*; D055815:Young Adult