Sr. Steinhauer et al., Cognitive modulation of midbrain function: task-induced reduction of the pupillary light reflex, INT J PSYCP, 39(1), 2000, pp. 21-30
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.