INTRODUCTIONPrevious research indicates that a large cohort of veterans from the 1991 Gulf War report polysymptomatic conditions. These syndromes often involve neurocognitive complaints, fatigue, and musculoskeletal symptoms, thus overlapping with civilian illnesses from low levels of environmental chemicals, chronic fatigue syndrome, and fibromyalgia.METHODSTo test for time-dependent changes over repeated intermittent exposures, we evaluated objective performance on a computerized visual divided attention test in chronically unhealthy Gulf War veterans (n = 22 ill with low-level chemical intolerance (CI); n = 24 ill without CI), healthy Gulf War veterans (n = 23), and healthy Gulf War era veterans (n = 20). Testing was done before and after each of three weekly, double blind, low-level JP-8 jet fuel or clean air sham exposure laboratory sessions, including acoustic startle stimuli.RESULTSUnhealthy veterans receiving jet fuel had faster mean peripheral reaction times over sessions compared with unhealthy veterans receiving sham clean air exposures. Unhealthy Gulf veterans with CI exhibited faster post- vs. pre-session mean central reaction times compared with unhealthy Gulf veterans without CI. Findings were controlled for psychological distress variables.DISCUSSIONThese data on unhealthy Gulf veterans show an acceleration of divided attention task performance over the course of repeated low-level JP-8 exposures. The present faster reaction times are consistent with rat neurobehavioral studies on environmental toxicant cross-sensitization and nonlinear dose-response patterns with stimulant drugs, as well as some previous civilian studies using other exposure agents. Together with previous research findings, the data suggest involvement of central nervous system dopaminergic pathways in affected Gulf veterans.