Left: Negative correlation (2-way Spearman) of glutamate (Glu) levels in thalamus (average of left and right) with cigarettes per day over last 30 days. Note: There are data overlapping from two participants at the points (15,7.2) and (20,6.6), each marked by “2”, for a total of 17 observations and 15 degrees-of-freedom. Right: Correlation with pack-years (2-way Spearman, participant age partialled out). Individual smokers represented by green triangles. Metabolite levels in Institutional Units (IU) corrected for voxel CSF content. Higher levels of tobacco use are associated with lower thalamic Glu. Similar results were observed for Glx (Glx = Glu + glutamine).