A number of findings supported our initial hypotheses that regions of decreased FA in users would be associated with lower performance on neurocognitive measures, whereas areas of increased FA would be linked to better performance, presumably due to compensatory processes. In temporal brain pathways comprising the right ILF and superior temporal gyrus, where FA was lower in the user group, performance on measures of speeded processing declined as FA decreased. Similarly, low FA in the ILF was associated with poorer scores on a measure of attention and working memory in users. Considering that this tract shows significant increases in FA from childhood to adolescence (Lebel, Walker, Leemans, Phillips, & Beaulieu, 2008; Schmithorst, Wilke, Dardzinski, & Holland, 2002), the current findings implicate weaknesses in cognitive functioning in users that may be linked to diminished fiber integrity. By comparison, in the right occipital region, where users demonstrated higher FA than controls, visuomotor switching performance and first trial verbal list learning improved with increasing FA, and suggests the possibility of a coincident compensatory process occurring in MJ+ALC users during this developmental period.
