Dynamic stability during running gait termination: Predictors for successful control of forward momentum in children and adults

Reported differences between children and adults with respect to COM horizontal and vertical position to maintain dynamic stability during running deceleration suggest that this relationship may not be as important in children. This study challenged the current dynamic stability paradigm by determining the features of whole body posture that predicted forward velocity and momentum of running gait termination in adults and children. Sixteen adults and 15 children ran as fast as possible and stopped at pre-determined location. Separate regression analyses determined whether COM posterior and vertical positions and functional limb length (distance between COM and stance foot) predicted velocity and momentum for adults and children. COM posterior position was the strongest predictor of forward velocity and momentum in both groups supporting the previously established relationship during slower tasks. COM vertical position also predicted momentum in children, not adults. Higher COM position in children was related to greater momentum; consistent with previously reported differences between children and adults in COM position across running deceleration. COM vertical position was related to momentum but not velocity in children suggesting that strategies used to terminate running may be driven by demands imposed not just by velocity, but also the mass being decelerated.     

Changes in dynamic balance control over time in children with and without Developmental Coordination Disorder

The aim of this study was to examine differences in underlying adaptations of dynamic balance in children with and without Developmental Coordination Disorder (DCD) during a Wii Fit game and to measure changes over time and after intervention.Twenty-eight children with DCD and 21 typically developing (TD) children participated in the study. Analyses of force plate variables showed that the TD group initially used a longer path length for the ski slope descent and tended toward more variation in Center of Pressure (CoP) displacement in lateral direction than the children with DCD. In contrast, the TD group showed a trend of fewer reversals per cm in both AP and lateral direction. After the nonintervention period, the TD group improved performance by decreasing the path length, while the DCD group improved by increasing the path length and by decreasing the number of reversals. After intervention, no changes were found in sway characteristics. Individual analyses within the DCD group showed that the path length per run fell more often within the 95% confidence Interval of the faultless runs. In conclusion both TD and DCD children modify the underlying kinetics of dynamic balance control, but in different ways and both lead to better performance.     

Use of public information when foraging: effects of time available to sample foods

It has been proposed that use of socially acquired information by animals should increase as the time available for individual resource sampling decreases. We gave Norway rat “observers” either 2 or 5 h day^–1 to sample four foods. Three of these foods were relatively palatable, but protein-poor; the fourth was relatively unpalatable, but protein-rich. We found that observer rats that for 2 h day^–1 both sampled foods and interacted with demonstrators eating only the protein-rich food ate more of the protein-rich food than did observers that sampled for 2 h day^–1 but had no opportunity to interact with demonstrators. On the other hand, observer rats that could sample foods for 5 h day^–1 ate equal amounts of protein-rich food whether they interacted with a demonstrator fed protein-rich food or not. Subsequent analyses showed that the time available to observers to sample foods, rather than the opportunity to interact with demonstrators determined whether such interaction influenced observers’ food choices. The results are consistent with the hypothesis that animals increase their use of public information in response to temporal constraints on opportunities for resource sampling. Received: 22 December 1998 / Accepted after revision: 27 February 1999     

Note on aversion learning to the shape of food by monkeys

Vervet and grivet monkeys were repeatedly tested eating bar- and circle-shaped cookies. One subject was always injected with lithium immediately after eating cookies with the circle shape and learned to avoid the circular cookies while continuing to eat the bar-shaped cookies. Another subject received similar treatment except that lithium injections were always delayed 30 minutes after access to the circle-shaped cookies. She also acquired a discriminative aversion. Aversion learning was not observed with 60-minute delayed toxicosis or with lithium injections administered unpaired with access to the cookies. The two types of cookies differed only in shape, and conditioning and test sessions were conducted in total darkness to preclude the use of visual cues. Therefore, the avoidance observed in subjects conditioned with immediate and 30-minute delayed toxicosis represents a conditioned aversion to the shape of the cookies as revealed by tactile cues. These findings illustrate that monkeys can learn to select food on the basis of tactile stimuli when such stimuli are conditioned with delayed aversive stimulation.     

Relationships among several methods of linearly constrained correspondence analysis

This paper shows essential equivalences among several methods of linearly constrained correspondence analysis. They include Fisher's method of additive scoring, Hayashi's second type of quantification method, ter Braak's canonical correspondence analysis, Nishisato's type of quantification method, ter Braak's canonical correspondence analysis, Nishisato's ANOVA of categorical data, correspondence analysis of manipulated contingency tables, Böckenholt and Böckenholt's least squares canonical analysis with linear constraints, and van der Heijden and Meijerink's zero average restrictions. These methods fall into one of two classes of methods corresponding to two alternative ways of imposing linear constraints, the reparametrization method and the null space method. A connection between the two is established through Khatri's lemma.The work reported in this paper has been supported by grant A6394 from the Natural Sciences and Engineering Research Council of Canada to the first author. We wish to thank Carolyn Anderson, Ulf Böckenholt, Henk Kiers, Shizuhiko Nishisato, Jim Ramsay, Tadashi Shibayama, Cajo ter Braak, and Peter van der Heijden for their helpful comments on earlier drafts of this paper.     

Component analysis with different sets of constraints on different dimensions

Many of the classical multivariate data analysis and multidimensional scaling techniques call for approximations by lower dimensional configurations. A model is proposed, in which different sets of linear constraints are imposed on different dimensions in component analysis and classical multidimensional scaling frameworks. A simple, efficient, and monotonically convergent algorithm is presented for fitting the model to the data by least squares. The basic algorithm is extended to cover across-dimension constraints imposed in addition to the dimensionwise constraints, and to the case of a symmetric data matrix. Examples are given to demonstrate the use of the method.The work reported in this paper has been supported by the Natural Sciences and Engineering Research Council of Canada, grant number A6394, and by the McGill-IBM Cooperative Grant, both granted to the first author. The research of H. A. L. Kiers has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences. We thank Michael Hunter for his helpful comments on earlier drafts of this paper.     

Dynamic emotional messages differentially affect 6-month-old infants’ attention to eyes and gaze cues

Infants often experience interactions in which caregivers use dynamic messages to convey their affective and communicative intent. These dynamic emotional messages may shape the development of emotion discrimination skills and shared attention by influencing infants’ attention to internal facial features and their responses to eye gaze cues. However, past research examining infants’ responses to emotional faces has predominantly focused on classic, stereotyped expressions (e.g., happy, sad, angry) that may not reflect the variability that infants experience in their daily interactions. The present study therefore examined forty-two 6-month-old infants’ attention to eyes vs. mouth and gaze cueing responses across multiple dynamic emotional messages that are common to infant-directed interactions. Overall, infants looked more to the eyes during messages with negative affect, but this increased attention to the eyes during these message conditions did not directly facilitate gaze cueing. Infants instead showed reliable gaze cueing only after messages with positive and neutral affect. We additionally observed gender differences in infants’ attention to internal face features and subsequent gaze cueing responses. Female infants spent more time looking at the eyes during the dynamic emotional messages and showed increased initial orienting and longer looking to gaze-cued objects following positive messages, whereas male infants showed these gaze cueing effects following neutral messages. These results suggest that variability in caregivers' communication can shape infants’ attention to and processing of emotion and gaze information.     

Motivated Reasoning in an Explore-Exploit Task

The current research investigates how prior preferences affect causal learning. Participants were tasked with repeatedly choosing policies (e.g., increase vs. decrease border security funding) in order to maximize the economic output of an imaginary country and inferred the influence of the policies on the economy. The task was challenging and ambiguous, allowing participants to interpret the relations between the policies and the economy in multiple ways. In three studies, we found evidence of motivated reasoning despite financial incentives for accuracy. For example, participants who believed that border security funding should be increased were more likely to conclude that increasing border security funding actually caused a better economy in the task. In Study 2, we hypothesized that having neutral preferences (e.g., preferring neither increased nor decreased spending on border security) would lead to more accurate assessments overall, compared to having a strong initial preference; however, we did not find evidence for such an effect. In Study 3, we tested whether providing participants with possible functional forms of the policies (e.g., the policy takes some time to work or initially has a negative influence but eventually a positive influence) would lead to a smaller influence of motivated reasoning but found little evidence for this effect. This research advances the field of causal learning by studying the role of prior preferences, and in doing so, integrates the fields of causal learning and motivated reasoning using a novel explore-exploit task.     

The effect of exercise intensity on joint power and dynamics in ergometer double-poling performed by cross-country skiers

The purpose of this study was to examine the effect of increasing exercise intensity on the role of joint powers in ergometer double poling (DP), while taking specific dynamic constraints into account. One main question was whether lower-body power contribution increased or decreased with increasing intensity. Nine male Norwegian national-level cross-country skiers performed ergometer DP at low, moderate, high and maximal intensity. Kinematics, and ground (GRF) and poling (F_poling) reaction forces were recorded and used in link segment modeling to obtain joint and whole-body dynamics. Joint powers were averaged over the cycle, the poling (PP) and recovery (RP) phases. The contribution of these average powers was their ratios to cycle average poling power. At all intensities, the shoulder (in PP) and hip (mostly in RP) generated most power. Averaged over the cycle, lower-body contribution (sum of ankle, knee and hip power) increased from ∼37% at low to ∼54% at maximal intensity (p < .001), originating mostly from increased hip contribution within PP, not RP. The generation of larger F_poling at higher intensities demanded a reversal of hip and knee moment. This was necessary to appropriately direct the GRF vector as required to balance the moment about center of mass generated by F_poling (control of angular momentum). This was reflected in that the hip changed from mostly absorbing to generating power in PP at lower and higher intensities, respectively. Our data indicate that power-transfer rather than stretch-shortening mechanisms may occur in/between the shoulder and elbow during PP. For the lower extremities, stretch-shortening mechanisms may occur in hip, knee and trunk extensors, ensuring energy conservation or force potentiation during the countermovement-like transition from body lowering to heightening. In DP locomotion, increasing intensity and power output is achieved by increased lower-body contribution. This is, at least in ergometer DP, partly due to changes in joint dynamics in how to handle dynamic constraints at different intensities.     

Simple games as dynamic, coupled systems: randomness and other emergent properties

From a game theory perspective the ability to generate random behaviors is critical. However, psychological studies have consistently found that individuals are poor at behaving randomly. In this paper we investigated the possibility that the randomness mechanism lies not within the individual players but in the interaction between the players. Provided that players are influenced by their opponent’s past behavior, their relationship may constitute a state of reciprocal causation [Cognitive Science 21 (1998) 461], in which each player simultaneously affects and is affected by the other player. The result of this would be a dynamic, coupled system. Using neural networks to represent the individual players in a game of paper, rock, and scissors, a model of this process was developed and shown to be capable of generating chaos-like behaviors as an emergent property. In addition, it was found that by manipulating the control parameters of the model, corresponding to the amount of working memory and the perceived values of different outcomes, that the game could be biased in favor of one player over the other, an outcome not predicted by game theory. Human data was collected and the results show that the model accurately describes human behavior. The results and the model are discussed in light of recent theoretical advances in dynamic systems theory and cognition.