Abstract
Filial imprinting is associated with induction of predisposed preference to animations that bear visual features of Johansson's biological motion (BM), and the induction is limited to a few days after hatching. As thyroid hormone (3,5,3′-triiodothyronine, T3) plays a critical role in determining the sensitive period of imprinting, we examined if exogenously applied T3 (or iopanoic acid, IOP; a selective inhibitor for converting enzymes) could also sensitize (or desensitize) the BM induction. Chicks were trained by using a non-BM stimulus (rotating red toy) according to a conventional imprinting procedure. Trained chicks were tested for preference to a point-light BM animation (walking chick) over a non-BM animation (linear motion), and for the preference for the familiarized stimulus (red toy) over an unfamiliar one (yellow toy). In 1-day chicks, those injected with IOP showed significantly lower scores than controls on both BM and imprinting tests. In 4-days chicks, those injected with T3 showed higher scores than control, but the difference in BM score was not significant. Imprinting and the accompanying T3 surge may be necessary for the predisposed BM preference to appear in 1-day chicks. Even after the conventional sensitive period is over, exogenous T3 can partly re-sensitize the BM preference as it does imprinting.
Highlights
Filial imprinting constitutes the earliest step of the social attachment formation in precocial birds such as chickens and ducks (Lorenz, 1935; Hess, 1958; Matsushima et al, 2003; Horn, 2004)
To examine if biological motion (BM) preference is associated with imprinting depending on the T3 level, we plotted the individual BM preference score against the imprinting score after merging data obtained from these four groups (Figure 2C)
We conclude that thyroid hormone sensitizes the induction of BM preference in a manner associated with the degree of imprinting
Summary
Filial imprinting constitutes the earliest step of the social attachment formation in precocial birds such as chickens and ducks (Lorenz, 1935; Hess, 1958; Matsushima et al, 2003; Horn, 2004). Imprinting upregulates gene expression of the converting enzyme (type 2 iodothyronine deiodinase, Dio2) in newly-hatched (1-day old) chicks. Infusion of exogenous T3 (intra-venous injection) augments the imprinting score in 1-day chicks, and it reopens the sensitive period even in 4-days old chicks. Chicks can be re-imprinted to novel object for a substantially longer period than the sensitive period assumed so far. Thyroid hormone would reorganize the neural mechanisms for visual perception. The chicks could develop durable social cohesion selectively to live animals such as their mother hen, even after they were initially imprinted to non-animate artifact
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