Neural basis of endogenous and exogenous spatial orienting: A functional MRI study

Whole-brain functional magnetic resonance imaging (MRI) was used to examine the neural substrates of internally (endogenous) and externally (exogenous ) induced covert shifts of attention. Thirteen normal subjects performed th ree orienting conditions: endogenous (location of peripheral target predict ed by a central arrow 80% of the time), exogenous (peripheral target preced ed by a noninformative peripheral cue), and control (peripheral target prec eded by noninformative central cue). Behavioral results indicated faster re action times (RTs) for valid than for invalid trials for the endogenous con dition but slower RTs for valid than for invalid trials for the exogenous c ondition (inhibition of return). The spatial ex-tent and intensity of activ ation was greatest for the endogenous condition, consistent with the hypoth esis that endogenous orienting is more effortful (less automatic) than exog enous orienting. Overall, we did not observe distinctly separable neural sy stems associated with the endogenous and exogenous orienting conditions. Bo th exogenous and endogenous orienting, but not the control condition, activ ated bilateral parietal and dorsal premotor regions, including the frontal eye fields. These results suggest a specific role for these regions in prep aratory responding to peripheral stimuli. The right dorsolateral prefrontal cortex (BA 46) was activated selectively by the endogenous condition. This finding suggests that voluntary, but not reflexive, shifts of attention en gage working memory systems.