Additive and multiplicative probabilistic models of infant looking times

Additive and multiplicative regression models of habituation were compared regarding the fit to looking times from a habituation experiment with infants aged between 3 and 11 months. In contrast to earlier studies, the current study considered multiple probability distributions, namely Weibull, gamma, lognormal and normal distribution. In the habituation experiment the type of contrast between the habituation and the test trial was varied (luminance, color or orientation contrast), crossed with the number of habituation trials (1, 3, 5, or 7 habituation trials) and crossed with three age cohorts (4, 7, 10 months). The initial mean LT to dark stimuli (around 3.7 s) was considerably shorter than the mean LT to green and gray stimuli (around 5 s). Infants showed the strongest dishabituation to changes from dark to bright (luminance contrast) and weak-to-no dishabituation to a 90-degrees rotation of the gray stimuli (orientation contrast). The dishabituation was stronger after five and seven habituation trials, but the result was not statistically robust. The gamma distribution showed the best fit in terms of log-likelihood and mean absolute error and the best predictive performance. Furthermore, the gamma distribution showed small correlations between parameters relative to other models. The normal additive model showed an inferior fit and medium correlations between the parameters. In particular, the positive correlation between the initial looking time (LT) and the habituation rate was likely responsible for a different interpretation relative to the multiplicative models of the main effect of age on the habituation rate. Otherwise, the additive and multiplicative models provided similar statistical conclusions. The performance of the model versions without pooling and with partial pooling across participants (also called random-effects, multi-level or hierarchical models) were compared. The latter type of models showed worse data fit but more precise predictions and reduced correlations between the parameters. The performance of model variants with auto-regressive time structures were explored but showed considerably worse fit. The performance of quadratic models that allowed non-monotonic changes in LTs were investigated as well. However, when fitted with LT data, these models did not produce non-monotonic change in LTs. The study underscores the utility of partial-pooling models in terms of providing more accurate predictions. Further, it agrees with previous research in that a multiplicative LT model is preferable. Nevertheless, the current results suggest that the impact of the choice of an additive model on the statistical inference is less dramatic then previously assumed.

Habituation in mosquito larvae Aedes aegypti is context-specific

Mosquito larvae live in water and perform a stereotyped escape response when a moving object projects its shadow on the surface, indicating potential risk of predation. Repeated presentations of the shadow induce a decrease in the response due to habituation, a form of non-associative learning defined as the progressive and reversible decrease in response to a specific reiterative innocuous stimulus. Nevertheless, habituation can be context-specific, which indicates an association between the context and the stimulus. The aim of this work was to study context-specificity in habituation in mosquito larvae Aedes aegypti. Larvae were individually placed in Petri dishes. Underneath, black, white or black-white striped cardboards were placed as backgrounds (visual context). Larvae were presented with a shadow produced by a cardboard square (training) over the course of 15 trials. After the fifteenth trial, the background was shifted and the stimulus was presented once again (test). To analyse habituation in different contexts, we developed a series of learning curve models. We performed a Bayesian model selection procedure using those models and the data from the experiments to find which model best described the results. The selected model was a Power-Law learning curve with six parameters (habituation rate, context-specific asymptotic habituation response -with one parameter per context, i.e. 3 parameters-, response-increase, and autocorrelation) describing the whole experimental setup with a generalised r2 of 0.96. According to the model, a single habituation rate would indicate that habituation was independent of the context, whilst asymptotic habituation would be context-specific. If the background was shifted after training, there was an increase in the response in the test, evincing context specificity in habituation.

Habituation and individual variation in the endocrine stress response in the Trinidadian guppy (Poecilia reticulata)

The vertebrate stress response enables individuals to react to and cope with environmental challenges. A crucial aspect of the stress response is the elevation of circulating glucocorticoids. However, continued activation of the stress response under repeated (or chronic) stress can be damaging to fitness. Under certain circumstances it may therefore be adaptive to habituate to repeated exposures to a particular stressor by reducing the magnitude of any associated release of glucocorticoids. Here, we investigate whether Trinidadian guppies (Poecilia reticulata) habituate to repeated exposure to a mild stressor, using a waterborne hormone sampling approach that has previously been shown to elicit a stress response in small fish. We also test for individual variation in the extent of habituation to this stressor. Concentrating on freely circulating cortisol, we found that the first exposure to the assay induced high cortisol release rates but that guppies tended to habituate quickly to subsequent exposures. There were consistent differences among individuals in their average cortisol release rate (after accounting for effects of variables such as body size) over repeated exposures. Our analyses did not find evidence of individual differences in habituation rate, although limitations in statistical power could account for this finding. We also present data on free 11-ketotestosterone, in addition to conjugated forms of both hormones. We discuss consistent individual differences around the general pattern of habituation in the flexible stress response, and highlight the potential for individual variation in habituation to facilitate selection against the deleterious effects of chronic stress.Summary statementTrinidadian guppies habituate quickly to repeated stress exposure, and exhibit consistent differences in their endocrine stress response. We provide a framework for analysing individual variation in habituation rate.

Auditory Startle Response in Blind Subjects

Blindness is often associated with hypervigilance and arousal. The auditory startle response has been suggested as a measure of arousal. This study evaluated the auditory startle response of 10 blind individuals and 10 sighted controls who were exposed to a series of 15 1000-Hz, 95-db, 0-rise-time pure-tone stimuli, administered to both ears. The subjects' heart-rate, skin-conductance, and orbicularis-oculi-electromyogram responses to the tones were measured. Blind subjects did not differ from controls in the magnitude and in the habituation-rate of their responses. Results were interpreted as showing that blind persons' capacity for discriminative learning and for regulation of arousal is unimpaired. Task-related anxiety of blind persons should be differentiated from stimulus-related anxiety.