Do chimpanzees anticipate an object’s weight? A field experiment on the kinematics of hammer-lifting movements in the nut-cracking Taï chimpanzees

When humans are about to manipulate an object, our brains use visual cues to recall an internal representation to predict its weight and scale the lifting force accordingly. Such a long-term force profile, formed through repeated experiences with similar objects, has been proposed to improve manipulative performance. Skillful object manipulation is crucial for many animals, particularly those that rely on tools for foraging. However, despite enduring interest in tool use in non-human animals, there has been very little investigation of their ability to form an expectation about an object’s weight. In this study, we tested whether wild chimpanzees use long-term force profiles to anticipate the weight of a nut-cracking hammer from its size. To this end, we conducted a field experiment presenting chimpanzees with natural wooden hammers and artificially hollowed, lighter hammers of the same size and external appearance. We used calibrated videos from camera traps to extract kinematic parameters of lifting movements. We found that, when lacking previous experience, chimpanzees lifted hollowed hammers with a higher acceleration than natural hammers (overshoot effect). After using a hammer to crack open one nut, chimpanzees tuned down the lifting acceleration for the hollowed hammers, but continued lifting natural hammers with the same acceleration. Our results show that chimpanzees anticipate the weight of an object using long-term force profiles and suggest that, similarly to humans, they use internal representations of weight to plan their lifting movements.     

Prevalence and correlates of ICD-11 adjustment disorder: Findings from the Zurich Adjustment Disorder Study

Background/Objective: The 11th revision of the International Classification of Diseases (ICD-11) will provide a new definition of adjustment disorder (AjD). The aim of the present study is to report on prevalence and correlates of ICD-11 AjD in a high-risk sample. Method: Three hundred thirty persons who had lost their job involuntarily were sampled by local job centres. The Munich Composite International Diagnostic Interview was administered with a new AjD module. Associations between AjD and correlates were investigated with logistic regression analyses. Results: 27.3% of the participants reported the AjD core symptom pattern. 13.8% men and 17.2% women met diagnostic guidelines of ICD-11 AjD. Prevalence increased with age and exposure to multiple stressors. The AjD core symptom pattern was associated with various sociodemographic correlates (e. g., lower financial household budget), whereas the full ICD-11 diagnosis including the exclusion algorithm was not. Regarding work-related factors, AjD occurred with a lower probability if the last job position had higher responsibilities and more general confidence for the future. Conclusions: ICD-11 AjD has a high prevalence among persons who lost their jobs involuntarily. Healthcare professionals should be aware of this problem. Research to investigate the ICD-11 AjD concept in the general populations and other subpopulations is needed.     

Book reviews

Abstract

Oakhill, J. &; Garnham, A. (1988). Becoming a skilled reader, Oxford: Blackwell. Pp. 206. ISBN 0-631-15244X £27.50 (hardback), ISBN 0-631-15776X £8.95 (paperback).

Brewin, C. R. (1988). Cognitive foundations of clinical psychology. London: Lawrence Erlbaum Associates Ltd. Pp. 224. ISBN 0-86377-098-3 £24.50 (hardback), ISBN 0-86377-099-1 £8.95 (paperback).     

Gender bias in fame judgments: Implicit gender stereotyping or matching study phase fame?

Familiarized names are falsely judgedfamous more often than nonfamiliarized names. Banaji and Greenwald (1995) demonstrated a gender bias in thisfalse fame effect, with the effect being larger for male than for female names. This effect was interpreted as reflecting the operation of a gender stereotype. However, the famous male names were, in fact, better known than the famous female names. Thus, the presence of more famous male names during study may have contributed to the observed male-famous association. If so, there should be no gender bias if the studied famous male and female names are equally famous, and a reversed gender bias should emerge if the famous female names are more famous than the male names. In two experiments, these predictions were corroborated. A “classical” gender bias was found only when the famous males were more famous than the famous females. These findings are consistent with the hypothesis that the gender bias in fame judgments, rather than showing implicit gender stereotyping in the sense of a transsituational judgment bias, reflects the fact that, in test, participants select a proportion of fame judgments to male and female names so that it matches the relative degree of fame of male and female names encountered during study.     

Valence of distractor words increases the effects of irrelevant speech on serial recall

Participants memorized target words in silence or while ignoring neutral or valent (positive or negative) distractor words that could be either possessor-relevant or other-relevant. Distractor words impaired recall performance, but valent distractor words caused more disruption than neutral distractors, and negative distractors caused more disruption than positive distractors. The results are problematic for explanations of the irrelevant speech effect within working memory models that do not specify an explicit role of attention in the maintenance of information for immediate serial recall.     

Seeing or hearing? Perceptual independence,modality confusions,and crossmodal congruity effects with focused and divided attention

Observers were given brief presentations of pairs of simultaneous stimuli consisting of a visual and a spoken letter. In the visual focused-attention conditon, only the visual letter should be reported; in the auditory focused-attention condition, only the spoken letter should be reported; in the divided-attention condition, both letters, as well as their respective modalities, should be reported (forced choice). The proportions of correct reports were nearly the same in the three conditions (no significant divided-attention decrement), and in the divided-attention condition, the probability that the visual letter was correctly reported was independent of whether the auditory letter was correctly reported. However, with a probability much higher than chance, the observers reported hearing the visual stimulus letter or seeing the spoken stimulus letter (modality confusions). The strength of the effect was nearly the same with focused as with divided attention. We also discovered a crossmodal congruity effect: Performance was better when the two letters in a stimulus pair were the same than when they differed in type.     

Dissociating the effects of automatic activation and explicit expectancy on reaction times in a simple associative learning task

After repeated associations between two events, E1 and E2, responses to E2 can be facilitated either because participants consciously expect E2 to occur after E1 or because E1 automatically activates the response to E2, or because of both. In this article, the authors report on 4 experiments designed to pit the influence of these 2 factors against each other. The authors found that the fastest responses to a target in a reaction time paradigm occurred when automatic activation was highest and conscious expectancy lowest. These results, when considered together with previous findings indicating that, under most conditions, the relation between expectancy and reaction times is in the opposite direction, are indicative of a reversed association-an interaction pattern that J. C. Dunn and K. Kirsner (1988) demonstrated to be the only one that unambiguously points to the involvement of independent processes.