Application of chaos theory to biology and medicine

The application of “chaos theory” to the physical and chemical sciences has resolved some long-standing problems, such as how to calculate a turbulent event in fluid dynamics or how to quantify the pathway of a molecule during Brownian motion. Biology and medicine also have unresolved problems, such as how to predict the occurrence of lethal arrhythmias or epileptic seizures. The quantification of a chaotic system, such as the nervous system, can occur by calculating the correlation dimension (D2) of a sample of the data that the system generates. For biological systems, the point correlation dimension (PD2) has an advantage in that it does not presume stationarity of the data, as the D2 algorithm must, and thus can track the transient non-stationarities that occur when the systems changes state. Such non-stationarities arise during normal functioning (e.g., during an event-related potential) or in pathology (e.g., in epilepsy or cardiac arrhythmogenesis). When stochastic analyses, such as the standard deviation or power spectrum, are performed on the same data they often have a reduced sensitivity and specifity compared to the dimensional measures. For example, a reduced standard deviation of heartbeat intervals can predict increased mortality in a group of cardiac subjects, each of which has a reduced standard deviation, but it cannot specify which individuals will or will not manifest lethal arrhythmogenesis; in contrast, the PD2 of the very same data can specify which patients will manifest sudden death. The explanation for the greater sensitivity and specificity of the dimensional measures is that they aredeterministic, and thus are moreaccurate in quantifying the time-series. This accuracy appears to be significant in detecting pathology in biological systems, and thus the use of deterministic measures may lead to breakthroughs in the diagnosis and treatment of some medical disorders.     

Low-dimensional chaos maps learning in a model neuropil (olfactory bulb)

Quantification of a chaotic system can be made by calculating the correlation dimension (D2) of the data that the system generates (Packard et al., 1980). The D2 algorithm, however, requires stationarity of the generator, a feature that biological data rarely reflect (Mayer-Kress et al., 1988). So we developed the “point correlation dimension” (PD2), an algorithm that accurately tracks D2 in linked data of different dimensions (Carpeggiani et al., 1991). We now present a mathematical argument that, for stationary data, individual PD2s converge to D2 and we demonstrate that the algorithm rejects contributions made by bursts of noise. Data were obtained from the surface of the olfactory bulb of the conscious rabbit (64 electrodes, 640 Hz each, 1.3 sec epochs) before and after presentation of a novel or habituated odor. D2 could be calculated in only 1 of 10 novel-odor trials, whereas PD2 could be calculated in all. Both algorithms indicated that a novel odor evokes a spatially uniform dimensional increase. The PD2 uniquely exhibited the dimensional decreases that occur during inspiration and the gradients of mean dimension present during the nonstimulated control state. These control gradients remained unchanged without odor experience, but showed spatially specific PD2 increases following odor habituation. It is interpreted that, 1) the PD2 issensitive, accurate, and appropriate for dimensional assessment of biological data, 2) that during analysis of unfamiliar information a singleglobal process is transiently evoked in the neuropil, and 3) after experience multiplespatially specific processes tonically map the sites of learning. Grant Support: National Institutes of Health, HL 31164 and NS27745     

Acquired dyslexia in three writing systems: study of a Portuguese-Japanese bilingual aphasic patient

The Japanese language is represented by two different codes: syllabic and logographic while Portuguese employs an alphabetic writing system. Studies on bilingual Portuguese-Japanese individuals with acquired dyslexia therefore allow an investigation of the interaction between reading strategies and characteristics of three different writing codes. The aim of this study was to examine the differential impact of an acquired brain lesion on the reading of the logographic, syllabic and alphabetic writing systems of a bilingual Portuguese-Japanese aphasic patient (PF). Results showed impaired reading in the logographic system and when reading irregularly spelled Portuguese words but no effects on reading regular words and nonwords in syllabic and alphabetic writing systems. These dissociations are interpreted according to a multi-route cognitive model of reading assuming selective damage in the lexical route can result in acquired dyslexia across at least three different writing codes.     

Timing during inhibitory conditioning

Analyses of second-by-second conditioned responding into a food receptacle by hungry rats (Rattus norvegicus) found that inhibition varies across the duration of a conditioned stimulus (CS) in a manner consistent with initial training. Variations in the arrival time of the unconditioned stimulus (US) supported temporally specific suppression of responding (Experiment 1). Summation and retardation tests (Experiments 2 and 3, respectively) revealed that points of greatest inhibition coincided with US omission at the time normally specified by the excitor. Our data provide a clear demonstration of fine-grained changes in the time course of inhibitory conditioning for the first time.     

The female advantage in object location memory is robust to verbalizability and mode of presentation of test stimuli

This study extends Duff and Hampson's [Duff, S., & Hampson, E. (2001). A sex difference on a novel spatial working memory task in humans. Brain and Cognition,47, 470-493] finding of a sex-related difference in favor of females for an object location memory task. Twenty female and 20 male undergraduate students performed both manual and computer-generated versions of the task using stimuli that varied in degree of verbalizability. A 2x2x3 ANOVA with Sex as a between-subjects factor, and Presentation (manual or computer) and Stimuli (common objects, common shapes, and novel shapes) as within-subjects repeated measures revealed a significant main effect for Sex. Females made fewer errors than males regardless of presentation and across the three levels of verbalizability (i.e., stimulus types); moreover, the effect size was considered "large" [Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). Berlin: Springer]. These findings are interpreted within the context of the current literature that demonstrates a female advantage for object location memory (e.g., [Voyer, D., Postma, A., Brake, B., & Imperato-McGinley, J. (2007). Gender differences in object location memory: A meta-analysis. Psychonomic Bulletin and Review, 14, 23-38]).     

A male advantage for spatial and object but not verbal working memory using the n-back task

Sex-related differences have been reported for performance and neural substrates on some working memory measures that carry a high cognitive load, including the popular n-back neuroimaging paradigm. Despite some evidence of a sex effect on the task, the influence of sex on performance represents a potential confound in neuroimaging research. The present study investigated sex-related differences in verbal, spatial, and common object versions of the high cognitive load "n-back" working memory task. Eighteen male and 18 female undergraduates completed all 3 versions of the task. A mixed ANOVA, with Sex (male and female) as the between-subjects factor and Condition (verbal, spatial, and object) as the within-subjects repeated measure revealed that males were significantly more accurate than females on the spatial and object versions of the n-back task and performed equivalently to females on the verbal version of the task. Although the expected female advantage for verbal working memory was not found using this effortful n-back task, these results support a male advantage for high cognitive load spatial and object working memory. Future research should take into account the influence of sex on performance of the n-back task, and examine sex-related differences in working memory using other paradigms.     

Violence and Latin‐American preadolescents: A study of social brain function and cortisol levels

The present study investigated exposure to violence and its association with brain function and hair cortisol concentrations in Latin‐American preadolescents. Self‐reported victimization scores (JVQ‐R2), brain imaging (fMRI) indices for a social cognition task (the ‘eyes test’), and hair cortisol concentrations were investigated, for the first time, in this population. The eyes test is based on two conditions: attributing mental state or sex to pictures of pairs of eyes (Baron‐Cohen, Wheelwright, Hill, Raste, & Plumb, 2001). The results showed an association among higher victimization scores and (a) less activation of posterior temporoparietal right‐hemisphere areas, in the mental state condition only (including right temporal sulcus and fusiform gyrus); (b) higher functional connectivity indices for the Amygdala and Right Fusiform Gyrus (RFFG) pair of brain regions, also in the mental state condition only; (c) higher hair cortisol concentrations. The results suggest more exposure to violence is associated with significant differences in brain function and connectivity. A putative mechanism of less activation in posterior right‐hemisphere regions and of synchronized Amygdala: RFFG time series was identified in the mental state condition only. The results also suggest measurable effects of exposure to violence in hair cortisol concentrations, which contribute to the reliability of self‐reported scores by young adolescents. The findings are discussed in light of the effects of exposure to violence on brain function and on social‐cognitive development in the adolescent brain. A video abstract of this article can be viewed at https://www.youtube.com/watch?v=qHcXq7Y9PBk     

Living in History: How War, Terrorism, and Natural Disaster Affect the Organization of Autobiographical Memory

ABSTRACT— Memories of war, terrorism, and natural disaster play a critical role in the construction of group identity and the persistence of group conflict. Here, we argue that personal memory and knowledge of the collective past become entwined only when public events have a direct, forceful, and prolonged impact on a population. Support for this position comes from a cross-national study in which participants thought aloud as they dated mundane autobiographical events. We found that Bosnians often mentioned their civil war and that Izmit Turks made frequent reference to the 1999 earthquake in their country. In contrast, public events were rarely mentioned by Serbs, Montenegrins, Ankara Turks, Canadians, Danes, or Israelis. Surprisingly, historical references were absent from (post–September 11) protocols collected in New York City and elsewhere in the United States. Taken together, these findings indicate that it is personal significance, not historical importance, that determines whether public events play a role in organizing autobiographical memory.