Cognitive modulation of midbrain function: task-induced reduction of the pupillary light reflex

The activation of processing resources has widespread effects in the nervou s system. A model of pupillary control systems (Steinhauer S.R., Hakerem G. , 1992. The pupillary response in cognitive psychophysiology and schizophre nia. Arm. N.Y. Acad. Sci. 658, 182-204) had predicted that ongoing cognitiv e activation should result in inhibition of the light reaction at the level of the oculomotor nucleus, n. III, in the midbrain. In this study, modific ation of parameters of the pupillary reaction to light were examined during varying task demands. The averaged light reaction was recorded from 33 mal e and female healthy volunteers during the performance of a serial 7 subtra ction task and compared to a 'no task' condition. For 13 subjects, an addit ional verbalization task with little processing demand (add 1) also was pre sented. Two types of effects were observed. Firstly, the tonic pupil diamet er increased from the no task to the easy (add 1) task, and increased furth er in the more demanding condition (subtract 7). Secondly, the extent of th e phasic light reaction was significantly reduced and the latency at the en d of the contraction was significantly decreased in the 'subtract 7' condit ion compared to both the no task and easy conditions (which did not differ from each other). The locus of interference with the light reaction was the Edinger-Westphal complex of the oculomotor nucleus, which is the motor cen ter for the pupillary sphincter muscles. Descending cortical influences inh ibited the activity of the Edinger-Westphal complex. Thus, increasing activ ation had a tonic inhibitory effect on this center, while higher levels of processing complexity produced a separate component of inhibition that inte racted with dynamic activation at this midbrain site. It was suggested that the variation in the light reaction is quantitatively responsive to varyin g processing loads, and may be utilized as a sensitive metric for a wide va riety of cognitive operations. (C) 2000 Elsevier Science B.V. All rights re served.