Thyroid hormones influence perceptual processing in a visual search paradigm.

The mechanisms that mediate the effects of thyroid hormones on higher cognitive processes are not well understood. In the present experiments, event-related potentials (ERPs) and behavioral measurements were recorded in a visual search paradigm where a target item had to be detected in a number of distractor items. 10 healthy subjects were given a daily dose 300 micrograms thyroxine (T4) p.o. and recordings were made before and after 3 weeks of treatment. All subjects developed hyperthyroidism with a completely suppressed TSH (1.6 +/- 0.5 mU/l vs. non detectable), a significant increase in T3 (1.5 +/- 0.2 vs. 1.9 +/- 0.2 ng/ml). T4 (6.6 +/- 1.6 micrograms/dl vs 11.9 +/- 2.7 micrograms/dl) and sex-hormone binding globulin (2.9 +/- 1.1 vs. 3.8 +/- 1.3 micrograms/ml; all p less than 0.0005). Compared to the control group (n = 15), thyroid hormones improved controlled serial visual processing, as indexed by changes of the late positive ERP component and target detection hit rate (in both cases p less than 0.01), whereas parallel detection of salient features remained unchanged. The data suggest a differential impact of thyroid hormones on visual information processing.