Spatial inhibition and the visual cortex: a magnetic resonance spectroscopy imaging study

Deficits in processing spatial information have been observed in clinical populations who have abnormalities within the dopamine (DA) system. As psychostimulants such as methamphetamine (MA) are particularly neurotoxic to the dopaminergic system it was of interest to examine the performance of MA-dependent individuals on a task of spatial attention.

51 MA-dependent subjects and 22 age-matched non-substance abusing control subjects were tested on a Spatial Stroop attention test. MR Spectroscopy (MRS) imaging data were analyzed from 32 MA abusers and 13 controls.

No group differences in response time or accuracy emerged on the behavioral task with both groups exhibiting equivalent slowing when the word meaning and the spatial location of the word were in conflict. MRS imaging data from the MA abusers revealed a strong inverse correlation between NAA/Cr ratios in the Primary Visual Cortex (PVC) and spatial interference (p=0.0001). Moderate inverse correlations were also seen in the Anterior Cingulate Cortex (ACC) (p=0.02). No significant correlations were observed in the controls, perhaps due to the small sample of imaging data available (n=13).

The strong correlation between spatial conflict suppression and NAA/Cr levels within the PVC in the MA-dependent individuals suggests that preserved neuronal integrity within the PVC of stimulant abusers may modulate cognitive mechanisms that process implicit spatial information.