Developmental hypothyroidism disrupts visual signal detection performance in rats

Thyroid hormones (THs) are essential for proper brain development in mammals. TH insufficiency during early development causes structural and functional abnormalities in brain leading to cognitive dysfunction. The specific effects of developmental hypothyroidism on attention have not been well characterized in animal models. The present study was conducted to characterize the effects of developmental hypothyroidism on attention in rats, and tested the hypothesis that the hypothyroidism has adverse impacts on attention by means of a visual signal detection task. Pregnant rats were exposed to the anti-thyroid drug, methimazole (0.02% w/v) via drinking water from gestational day 15 through postnatal day (PND) 21 to induce maternal and neonatal hypothyroidism. Male offspring served as subjects for the task started on PND 90. A light stimulus (500 ms, 250 ms or 50 ms) was presented in signal trials and not in blank trials. The offspring were required to discriminate these signal events, and subsequently press the correct lever. The correct response for signal and non-signal events was considered as hit and correct rejection, respectively. The hypothyroid offspring exhibited a decreased hit response for short signals (250 ms and 50 ms) which requires the higher attentional demand. The total number of lever responses during inter-trial interval (ITI) was also increased in the hypothyroid group. The number of lever responses was negatively correlated with a hit response at 50 ms, not at 250 ms. These results suggest that developmental hypothyroidism disrupts signal detection performance via impairment of visual attention and the altered lever response behavior.