Sensitive Period For Imprinting Research Articles

Differential expression of olfactory genes in Atlantic salmon (Salmo salar) during the parr-smolt transformation.

The anadromous salmon life cycle includes two migratory events, downstream smolt migration and adult homing migration, during which they must navigate with high precision. During homing migration, olfactory cues are used for navigation in coastal and freshwater areas, and studies have suggested that the parr–smolt transformation has a sensitive period for imprinting. Accordingly, we hypothesized that there would be significant changes in gene expression in the olfactory epithelium specifically related to smoltification and sampled olfactory rosettes from hatchery‐reared upper growth modal juvenile Atlantic salmon at 3‐week intervals from January to June, using lower growth modal nonsmolting siblings as controls. A suite of olfactory receptors and receptor‐specific proteins involved in functional aspects of olfaction and peripheral odor memorization was analyzed by qPCR. Gene expression in juveniles was compared with mature adult salmon of the same genetic strain caught in the river Gudenaa. All mRNAs displayed significant variation over time in both modal groups. Furthermore, five receptor genes (olfc13.1, olfc15.1, sorb, ora2, and asor1) and four olfactory‐specific genes (soig, ependymin, gst, and omp2) were differentially regulated between modal groups, suggesting altered olfactory function during smoltification. Several genes were differentially regulated in mature salmon compared with juveniles, suggesting that homing and odor recollection involve a different set of genes than during imprinting. Thyroid hormone receptors thrα and thrβ mRNAs were elevated during smolting, suggesting increased sensitivity to thyroid hormones. Treatment of presmolts with triiodothyronine in vivo and ex vivo had, however, only subtle effects on the investigated olfactory targets, questioning the hypothesis that thyroid hormones directly regulate gene expression in the olfactory epithelium.

Metabolic activity in the hyperstriatum of 2-day-old chicks during optomotor and contrasting visual stimulation

Our earlier report of differences in metabolic activity within the visual regions of the hyperstriatum and ectostriatum, in 2-day-old chicks compared with 23-day-old chicks, suggested that two visual pathways within the visual system develop at different rates. Here we have investigated whether the demands of varying visual environments will increase the activity of the hyperstriatum accessorium (HA) in 2-day-olds. Metabolic activity in the HA was monitored in 2-day-old chicks by the radioactive 2-deoxyglucose technique during monocular stimulation with three different visual environments: moving stripes in a rotating drum, which induced eye and head movements, a featureless white environment, and the complex visual environment of the home cage with other chicks. Although a small but significant level of activity was found in HA in the hemisphere opposite the open eye, the activity did not vary with the visual treatment. On the other hand, a raised level of activity in the hyperstriatum dorsale (HD) appeared in chicks viewing the rotating stripes, indicating that at this age the thalamo-hyperstriatal pathway may be involved in processing whole-field visual movement. The optomoter environment also produced high activity in the medial hyperstriatum ventrale (MHV), a region that has been implicated in memory formation of imprinting. We suggest that during the sensitive period for imprinting, HA may either have not developed its fully functional capacity, or that following or during imprinting it is actively shut down to protect itself and associated regions from interfering visual input. In contrast to the 2-day-olds, 17-day-old chicks in a visually rich cage environment, had high levels of activity in HA, demonstrating that the functional maturation of the HA, related to performance in the cage environment, is complete at least 6 days earlier than previously observed.

Development of filial attachment to static visual features of an imprinting object

Three experiments investigated the fact that the static visual features of an imprinting object (its color, size, and shape characteristics, as represented by the object remaining stationary during its presentation) can gradually acquire control over filial behavior by virtue of their repeated spatial-temporal pairing with visual motion—an effect that has been interpreted as a process of classical conditioning. Experiment 1 found that the static features can acquire control if they are made conspicuous in some other, nonmovement way (i.e., by manipulating the relative illumination of the imprinting object). Further experiments then failed to find any qualitative difference between the behavioral control developed by conspicuous but static aspects of a duckling’s environment and the control developed by the static features of a moving object, either in terms of persistence of the acquired control (Experiment 2) or in terms of possible restrictions imposed by the sensitive period for imprinting (Experiment 3). Taken together, these results support the plausibility of a perceptual learning hypothesis, although the classical conditioning view was not itself contradicted. The present findings are also relevant to the broader issue of whether any valid distinctions exist between the behavioral control exerted by static aspects of a precocial bird’s environment and the control exerted by animate, presumably social stimuli.

Imprinting Behavior: Pituitary-Adrenocortical Modulation of the Approach Response

Plasma corticosterone concentrations in newly hatched ducklings exposed to an imprinting model are inversely related to the strength of approach behavior. Injections of corticosterone before imprinting reduces following, whereas alpha1-10-adrenocorticotropin or antiserum to corticosterone augments following behavior. The sensitive period for imprinting may be regulated by changes in the pituitary-adrenocortical axis.

Posthatch age and developmental age as a baseline for determination of the sensitive period for imprinting.

Two experiments comparing developmental age and posthatch age were performed. In the first experiment 143 Peking ducklings of different posthatch ages were imprinted, and their following responses examined in a simultaneous choice test. A sensitive period was evident on the basis of posthatch age. In the second experiment 99 ducklings were separated in three clusters of different posthatch ages, which did not vary within the clusters relative to their probability to be imprinted. The ducklings of each cluster were imprinted at various developmental ages, and the results show the involvement of developmental age. Thus, both experiments considered together favor the concept of an interaction of developmental and posthatch age. Imprinting and the determination of the sensitive period are dependent on prehatch and posthatch learning, so both developmental age and posthatch age are involved in setting the limit of the sensitive period.

Changes in chicks' responses to novel moving objects over the sensitive period for imprinting

The responses of isolated domestic chicks to a standard moving object on each of the first 5 days after hatching are described and the relations between some of the behavioural measures are analysed. Except on the first day after hatching when nearly all the chicks approached and nestled against the moving model, the majority of the chicks avoided for at least part of the test. The time spent avoiding in each minute steadily declined during the test and many chicks stopped avoiding altogether. The majority of the chicks which ceased to avoid the model then approached, nestled against it and subsequently followed. There was a strong positive correlation between the point at which avoidance ceased and the point at which positive responses were first given. After following had begun the time spent following in each minute tended to increase. The persistence of avoidance and the higher values of the strength of avoidance were negatively correlated with the strength of following. The positive correlation between the lower values of the strength of avoidance and the strength of following was attributed to their both being measures of locomotive ability. The proportion of birds which gave positive responses declined from the first to the second day and the proportion avoiding increased over the same period. The persistence of avoidance, the strength of avoidance, the latency of positive responses and the strength of following all increased up to the third or fourth day but thereafter declined. The increase in the strength of following is attributed to developing locomotive ability. The other initial changes are regarded as consequences of the process by which the isolated chicks learn the characteristics of the static environment in which they are reared. The negative relations between the measures of avoidance and the strength of following support the view that they are all measures of the birds' ability to discriminate between familiar and unfamiliar objects.