Intact Pairs Research Articles

Does Intact Pair Status Matter in the Study of African American Twins? The Carolina African American Twin Study of Aging

In twin research, typically both members of a pair must participate. Survivorship of members of intact pairs compared to surviving members of nonintact twin pairs may reflect differences in psychosocial and health factors, and represent a potential selection bias relative to the general population. The purpose of the present study is to examine health, cognition, and well-being among members of African American intact twin pairs compared to individuals from nonintact twin pairs. Data from the Carolina African American Twin Study of Aging (CAATSA) were used for analyses. Subjects ranged in age from 25 to 89 years of age (mean=59.78 years, SD=12.84 years). CAATSA implements a 3-h protocol to collect data on demographics, health, cognition, and well-being. Data from one randomly selected member of each twin pair (N=78) was compared to data from surviving members of nonintact twin pairs (N=52). The results indicated significant differences on 11 of the 39 measures (i.e., age, education, forced expiratory volume, mean standing and sitting systolic and diastolic blood pressures, cognitive impairment score, alpha span, digit symbol, and logical memory). In each case, members of intact twin pairs performed better than surviving members of nonintact twin pairs. After controlling demographic variables, only blood pressures differed between the groups. It appears that using only pairs in research on older African American twins may represent a selection bias in estimating origins of individual variability in cognitive functioning and health but not psychological well-being.

Prefrontal cortex contribution to associative recognition memory in humans: an event-related functional magnetic resonance imaging study

The present event-related fMRI study examined in ten healthy participants neural correlates of recognition memory for intact, rearranged, and new pairs of visual stimuli. The correct categorization of both intact and rearranged pairs relative to pairs of new, never presented before stimuli relies on the retrieval of associative information whereas the discrimination of rearranged pairs from intact pairs additionally involves a recall-to-reject process by which subjects retrieve additional information to reach a recognition decision. Relative to new pairs, both intact and rearranged pairs yielded activation in inferior prefrontal cortex bilaterally and left dorsolateral prefrontal cortex. Relative to intact pairs, rearranged pairs were associated with increased activity in left dorsolateral prefrontal cortex. The latter could reflect the neural correlates of a recall-to-reject process, a pivotal process of associative recognition memory.

Experimental manipulation of prior experience: Effects on item and associative recognition

Frequency of exposure to very low- and high-frequency words was manipulated in a three-phase (familiarisation, study, and test) design. During familiarisation, words were presented with their definition (once, four times, or not presented). One week (Experiment 1) or one day (Experiment 2) later, participants studied a list of homogeneous pairs (i.e., pair members were matched on background and familiarisation frequency). Item and associative recognition of high- and very low-frequency words presented in intact, rearranged, old-new, or new-new pairs were tested in Experiment 1. Associative recognition of very low-frequency words was tested in Experiment 2. Results showed that prior familiaris ation improved associative recognition of very low-frequency pairs, but had no effect on high-frequency pairs. The role of meaning in the formation of item-to-item and item-to-context associations and the implications for current models of memory are discussed.

The Minnesota Twin Registry: current status and future directions.

The Minnesota Twin Registry is a birth-record-based twin registry. Begun in 1983, it includes data for 4307 surviving intact pairs born in Minnesota between 1936 and 1955. In addition, the Registry includes 901 twin pairs born in Minnesota from 1904 to 1934, as well as 391 male pairs born in Minnesota from 1961 to 1964. The research focus is primarily on human individual differences assessed by self-report. Questionnaires completed by the participants include measures of personality, occupational interests, demographics, and leisure-time activities. We outline major contributions that have resulted from Registry research, as well as current and future research directions.

Different functions for different medial temporal lobe structures?

Since the landmark study of the globally amnesic patient H.M. by Scoville and Milner (1957), the essential role of the medial temporal lobes (MTL) in long-term memory has been well established. Considerable success has been made dissociating declarative memory processes (explicit memory for events and facts), which rely on the integrity of the MTL, from nondeclarative memory processes, which rely on other cortical, subcortical, and cerebellar structures (Gabrieli 1998). There has also been progress in distinguishing frontal-lobe and MTL contributions to declarative memory. Determining the precise contributions of the structures within the MTL to declarative memory, however, has proved challenging. The MTL is comprised of multiple structures including the hippocampal formation (the dentate gyrus, CA fields, and the subiculum), the amygdala, entorhinal cortex, and surrounding perirhinal and parahippocampal cortices. The focal role of the amygdala in the emotional modulation of memory appears clear (Phelps and Anderson 1997), but there is little consensus about how to characterize the differential contributions of specific MTL structures to declarative memory. Isolating the function of each of these structures, however, is central to understanding declarative memory processing and how damage or disease disrupts such processing. Different MTL structures must be mediating different component processes underlying declarative memory, but the unanswered question is what are those different processes? One emerging idea about differential functions within the MTL is that the hippocampal formation is necessary for declarative memory tasks that require the processing of relations between multiple stimuli, whereas surrounding cortices mediate performance on tasks that rely on stimulus familiarity (or the converse, novelty). This hypothesis is consistent with the hierarchical connectivity of this region (Fig. 1A). Information from unimodal and polymodal association cortices enters the MTL through the perirhinal and parahippocampal cortices, which project to the entorhinal cortex, which in turn provides the major input to the hippocampal formation. This connectional anatomy suggests that the representational capacity of the perirhinal and parahippocampal cortices may be highly related to the incoming sensory information. These cortices may mediate familiarity-based memory processes linked to sensory aspects of items. In contrast, the hippocampus may generate a more abstract representation that integrates or binds multiple cortical sources of information, which is an important aspect of relational memory processes. According to this view, the degree to which a memory task recruits relational versus familiarity processes would determine its relative reliance on hippocampal versus nonhippocampal MTL structures. Tasks that rely greatly on relational processing, such as paired-associate learning, would be greatly impaired by damage to the hippocampal formation. In contrast, tasks that can be performed on the basis of stimulus familiarity, such as tests of item recognition, could be supported by the function of the surrounding MTL cortices in the absence of hippocampal function. Several convergent findings have supported this hypothesis, including evidence from lesion and electrophysiological studies in animals (for review, see Brown and Aggleton 2001; Eichenbaum 2001), and from neuropsychological (Vargha-Khadem et al. 1997), and neuroimaging (Eldridge et al. 2000; Yonelinas et al. 2001) studies in humans. A provocative challenge to this idea is set forth in a study by Stark et al. (2002) examining amnesic patients with bilateral MTL lesions thought to be limited to the hippocampal formation. By using patients with well-characterized lesions, the authors add an important contribution to the ongoing efforts to characterize MTL function in declarative memory. Examining patients with focal hippocampal damage allows the authors to test how damage to this structure affects memory tasks that differentially require relational and familiarity processing. In this study, hippocampal patients performed both single-item and associative recognition tasks. For single-item recognition, patients studied pictures of single faces or houses and after a delay, performed a yes/no recognition memory judgment. For associative recognition, patients studied face-house paired associates and after a delay, were presented with pairs of items. In this associative task, patients had to discriminate items that had been studied together (intact pairs) from items that had not been studied together (recombined pairs). These tasks differentially focus on the processing of relations between test items compared with the processing Corresponding author. E-MAIL gabrieli@psych.stanford.edu; FAX (650) 725-5699. Article and publication are at http://www.learnmem.org/cgi/doi/ 10.1101/lm.54702.

Parental division of labour and the shift from minimal to maximal role specializations: an examination using a biparental fish

In biparental species, parents often cooperate by emphasizing different parental roles. However, these parental sex differences often disappear when only one parent is present. For example, under natural conditions, the female convict cichlid fish, Archocentrus nigrofasciatus, typically remains with the offspring while the male spends most of his time patrolling the territory and chasing intruders. With the removal of the mate, either parent is capable of raising the offspring alone, and when doing so, they each perform all parental roles. We tested how the presence and absence of the mate and the presence and absence of an intruder influence parental sex-role specialization in A.nigrofasciatus. When presented with an intruder, widowed males left the offspring unattended more often and spent more time attacking the intruder compared with widowed females. For intact pairs, males showed a tendency to leave the offspring more than their mates but this sex difference was not significant. However, these paired individuals rarely left the offspring unattended. With an intruder present, paired males and females spent the least amount of time with the offspring (compared with widows and paired individuals without an intruder present), with males spending significantly more time with the intruder than females. Unlike pairs without an intruder, parents with an intruder changed roles only in support of the other parent. Thus, females rarely approached the intruder unless the male was also present and the male rarely approached the offspring without the female also being present. We speculated that the male's inability to remove the intruder caused females to support the male in attacking the intruder and the male returned to the offspring and joined the female during those periods when the intruder was least threatening.

Intact implicit memory for newly formed verbal associations in amnesic patients following single study trials.

This study examines the ability of amnesic patients to recover newly formed associations implicitly after a single study trial. Fifteen amnesic patients with various etiologies studied pairs by forming a sentence containing both words. At test, all participants saw 40 intact pairs, 40 rearranged pairs, and 40 new words. All pairs appeared side by side both at study and at test. For the implicit lexical-decision task, 40 nonwords were intermixed with the other pairs, and participants indicated whether both items were words. For the explicit speeded-recognition test, participants were asked to indicate whether both words had appeared at study. Despite being severely impaired on the explicit test, amnesic patients performed like healthy controls on the implicit test, with faster and more accurate responses to intact pairs than to recombined pairs. Contrary to existing theories, the results suggest that amnesic patients can form and retain new associations.

Priming of new associations in reading time: What is learned?

The priming of new associations has been a controversial topic, with some studies finding significant effects but others failing to replicate these results. Three studies investigated the priming of new associations in a reading time task, presenting lists of word pairs that were read aloud as quickly as possible. In Experiment 1, significant priming of new associations was found after two study presentations, replicating similar results by Moscovitch, Winocur, and MacLachan (1986). In Experiment 2, reading time was facilitated for intact pairs when word positions remained constant relative to when word positions were reversed. This suggested that the associative priming effect was related to specific lower level features of the word pairs rather than to abstract associations. In Experiment 3, the insertion of the wordand between test words eliminated the pairing-specific effect, placing the locus of new association priming at the transition between words within pairs. These findings demonstrate that the knowledge that supports priming of new associations in the reading time task involves perceptual or articulatory information about the transitions between words rather than abstract associative knowledge.

Description and primary results from an audiometric study of male twins.

The purpose of this study is to present descriptive results from an audiometric examination of a male twin sample, compare the values with normative databases, and explore whether there are age differences in genetic and environmental contributions to variation in hearing. Audiometric and questionnaire data were collected on a subsample of male twins who were identified through the population-based Swedish Twin Registry. Hearing examinations were completed for a total of 557 intact pairs, comprised of 250 identical (monozygotic [MZ] pairs and 307 fraternal (dizygotic [DZ] pairs aged 36 to 80 yr. A scale measuring hearing in the high-tone ranges was constructed by calculating the mean for the hearing threshold values obtained for both ears combined at 3000, 4000, 6000, and 8000 Hz for air conduction. To assess whether the twin values are representative of genera hearing function, the hearing thresholds were compared with values from two normative databases. Results suggest that these twin data provide a valid representation of hearing function in a cross-section of the Swedish male population, aged 35 and older. The mean values for hearing ability decreased across age, and variation increased. Twin similarity, estimated across four age groups using intraclass correlations, decreased from 0.716 to 0.516 for the MZ pairs and increased from 0.131 to 0.279 for the DZ pairs. These results suggest that variation in hearing ability in the high ranges is due to genetic and environmental factors and that environmental effects become more important with age. These data reflect typical age-related deterioration in hearing ability accompanied by greater individual differences in hearing function with age. Across all ages, genetic and environmental effects are important sources of variation in hearing. However, preliminary analyses suggest that the relative influence of environment increases with age. These environmental effects are of the nonshared type that are not associated with shared family environments but rather are explained by unique exposures.

Biochemical analyses on single amphidial glands, excretory-secretory gland cells, pharyngeal glands and their secretions from the avian nematode Syngamus trachea

The gape nematode, Syngamus trachea, has been used as a model to study nematode secretions. Individual and intact pairs of amphidial glands, pharyngeal glands and pairs of excretory-secretory gland cells have been dissected and their secretory products analysed. The protein profiles of each gland and the total nematode secretions were analysed on 12.5% homogeneous SDS-PAGE minigels. The protein analyses revealed that the structural protein profile of each gland is different. The amphidial gland secretes two major proteins of 36.0 and 41.5 kDa, the excretory-secretory gland cell secretes a protein of 28.2 kDa and a protein of 14.3 kDa, and the pharyngeal gland secretes proteins of 41.5 and of 14.6 kDa. Analysis of the total nematode secretions revealed all of the above major secretory proteins and an additional protein of 49.3 kDa. Syngamus trachea secretes acetylcholinesterases and its secretions contain multiple proteases.

Alzheimer's Disease in the National Academy of Sciences—National Research Council Registry of Aging Twin Veterans

To detect cases of Alzheimer's disease (AD) in a large population of twins living throughout the United States and to examine concordance for AD in twins as a function of age and genotype for apolipoprotein E (APOE). Nationwide survey. Multistage screening and field evaluation beginning with two telephone interviews and culminating with laboratory tests, longitudinal neuropsychological measures, physician examination, and diagnostic consensus among experts. Membership in 1990-1991 of intact pairs in the National Academy of Sciences--National Research Council Registry of veteran twins, then aged 62 to 73 years. Completeness of case detection was examined in collateral studies. Zygosity and APOE genotypes were determined by restriction mapping. Concordance was calculated by the proband method. Ninety subjects who screened positively for AD were studied in person, and 60 whose differential diagnoses included AD were followed up, as were their co-twins. Sensitivity of screening was estimated at greater than 99%, but 24% of subjects refused participation after initial screening. Seven of 38 diagnoses of AD have been confirmed at autopsy, and 31 other subjects eventually met criteria for probable or possible AD (prevalence estimate, 0.42%, 95% confidence interval, 0.29% to 0.56%), with good interrater reliability (intraclass r = .86). Excluding one discordant pair with unknown zygosity, concordance rates were 21.1% (4/19) for monozygotic and 11.1% (2/18) for dizygotic probands. Concordance was 50% for twins sharing the epsilon 4/epsilon 4 genotype at APOE, but there were no affected co-twins of 15 probands with onset before age 70 years, no epsilon 4 allele, and no family history of AD. The mean (SD) period of discordance in the latter pairs was 11.3 (3.3) years. The multistage case-detection approach achieved reliable and valid diagnoses of AD with high apparent sensitivity but substantial attrition after initial screening. Genetic influences in AD at this age are limited, except among homozygotes for allele epsilon 4 at APOE. Subjects with early-onset AD who lack the epsilon 4 allele are not rare, and their condition appears to have little genetic influence. They should be ideal for studies on environmental cause of AD.

Bacterial invasion into dentinal tubules of human vital and nonvital teeth

The difference in resistance to bacterial invasion into the dentinal tubules between vital and nonvital teeth has not been determined. This study was conducted to clarify the effect of vital pulp on bacterial invasion into the dentinal tubules. The specimens were 19 intact pairs of bilateral upper third molars of 19 healthy, young adult male volunteers. In each case, 30 or 150 days before extraction, pulpectomies and root canal fillings were carried out unilaterally and a class V cavity involving the dentin was made on the palatal surface of both the pulpectomized tooth and the nonpulpectomized opposite tooth. The cavities were left unprotected to expose them to oral flora until the extractions were done, and the extracted teeth were examined histologically. When extraction followed 150-day exposure to the oral flora, there was a statistically significant difference in the bacterial invasion rate between the vital and nonvital teeth. It was postulated that vital teeth were much more resistant to bacterial invasion into the dentinal tubules than were nonvital teeth, thereby suggesting that the vital pulp plays some important role in this process.

Sex-specific effects for body mass index in the new Norwegian twin panel.

Sex-specific effects for body mass index (BMI) were explored in a newly established, population-based Norwegian twin panel. The sample includes 5,864 individuals, aged 18-25 years, who responded to a questionnaire containing items for zygosity classification, height, weight, health, health-related behaviors, well-being, and demographic information. Among the 2,570 intact pairs who returned the questionnaire there were 416 identical (MZ) male pairs, 387 fraternal (DZ) male pairs, 528 MZ female pairs, 443 DZ female pairs, and 796 unlike-sexed pairs. Alternate sets of models testing for either sex-specific genetic or environmental parameters were evaluated using structural equation analysis. Results from the most parsimonious model indicated that the genes contributing to variation in BMI are not identical for men and women; rather, some genetic effects were shared by the sexes and some were unique to each sex. Total variation in BMI could be explained by sex-specific additive genetic effects, as well as genetic and non-shared environmental effects common to men and women. Estimates of heritability were .708 for men and .789 for women, and the male-female genetic correlation was 0.622. The series of models specifying sex-specific shared environment also fit the data and suggests that shared environmental factors may be important for males but not for females. The findings raise questions concerning the relationship between sex-specific effects for BMI and sex differences in health outcomes.

Sex-specific causal factors and effects of common environment for symptoms of anxiety and depression in twins.

Two thousand five hundred seventy intact pairs and 724 single responders from Norwegian twins aged 18-25 years completed questionnaires with information about anxiety and depression and perceived cotwin closeness. The aim of the study was the univariate estimation of sex-specific genetic and environmental effects on an index tapping symptoms of anxiety and depression. An index of social closeness between cotwins was significantly related to the cotwin correlation for anxiety/depression scores. MZ pairs were reported to be closer than DZ pairs, and like-sexed DZ pairs were closer than unlike-sexed pairs. The symptom data were adjusted for this apparent violation of the "equal-environment" assumption in twin studies, but the adjustment did not dramatically affect the parameter estimates of genetic and environmental effects on anxiety/depression. A model specifying male (aM) and female (aF) genetic additive effects, shared environment for males (cM), and individual environmental effects (eM and eF) fitted the adjusted data very well. An alternative model, specifying aM = aF, cM = cF, and eM = eF, and no correlation between those environmental factors shared by brothers and those shared by sisters, fitted equally well. Estimated proportions of total variance from the first model were aM2 = .30, aF2 = .52, and cM2 = .21. The estimates from the second model were aM2 = aF2 = .43 and cM2 = cF2 = .11.

Repetition priming for newly formed and preexisting associations: Perceptual and conceptual influences.

Three experiments demonstrate that association-specific repetition effects can be obtained for both newly formed and preexisting associations and that these effects are sensitive to modality of presentation. After studying a list of word pairs, participants were shown the original intact pairs and pairs formed by recombining the original pairs. In a lexical-decision task in which participants were asked to indicate whether both items were words, responses were faster to newly formed associations in the intact than in the recombined condition. This association-specific repetition priming effect was also observed for preexisting associations when a speeded relatedness judgment task was used. Both effects were found to be attenuated under cross-modal presentation. Finally, an explicit speeded recognition task revealed an associative effect that was not attenuated when modality was crossed for newly formed associations and was actually exaggerated for preexisting associations, suggesting that the repetition priming effects were not produced by conscious recollection. Results are discussed in terms of frameworks that are based either on perceptual representation systems or on a transfer-appropriate processing model.

Sensitivity analysis for models of population viability

A method of sensitivity analysis for population viability models is presented that uses logistic regression to evaluate the importance of model parameters that influence the risks of extinction. This approach is used to evaluate the importance of fecundity parameters and the initial number of non-breeding birds in a stochastic stage-structured model of helmeted honeyeater Lichenostomus melanops cassidix population dynamics. The regression analysis indicates which model parameters have the greatest impact on the risk of population decline. The results demonstrate that a simple expression containing the parameters of the model can encapsulate predictions of risk. This technique is proposed as an efficient alternative method of sensitivity analysis for population viability models. Of four fecundity parameters, the mean fecundity of intact pairs had the greatest influence on the risks faced by the helmeted honeyeater population. Mean fecundity of split pairs and the sex ratio of offspring were also important parameters. Over the range of parameters considered in this paper, environmental variation in fecundity and the initial number of non-breeding birds had little influence on the risks of decline. The importance of interactions between parameters was analysed.

Sensitivity analysis for models of population viability

A method of sensitivity analysis for population viability models is presented that uses logistic regression to evaluate the importance of model parameters that influence the risks of extinction. This approach is used to evaluate the importance of fecundity parameters and the initial number of non-breeding birds in a stochastic stage-structured model of helmeted honeyeater Lichenostomus melanops cassidix population dynamics. The regression analysis indicates which model parameters have the greatest impact on the risk of population decline. The results demonstrate that a simple expression containing the parameters of the model can encapsulate predictions of risk. This technique is proposed as an efficient alternative method of sensitivity analysis for population viability models. Of four fecundity parameters, the mean fecundity of intact pairs had the greatest influence on the risks faced by the helmeted honeyeater population. Mean fecundity of split pairs and the sex ratio of offspring were also important parameters. Over the range of parameters considered in this paper, environmental variation in fecundity and the initial number of non-breeding birds had little influence on the risks of decline. The importance of interactions between parameters was analysed.

The survival of a cooperative tradition in the intergroup discontinuity context

Interindividual-intergroup discontinuity is the tendency, in mixed-motive situations, for groups to interact more competitively, or less cooperatively, than individuals, even though mutual competition yields worse results that mutual cooperation. The present laboratory experiment attempted to assess whether the discontinuity effect could be reduced by the establishment of an intergroup cooperative tradition. Cooperation between intact pairs of groups was induced by placing in each group confederates who successfully convinced their fellow group members of the long-term advantage of trust and cooperation. On subsequent blocks of trails, the confederate was replaced by an actual subject, and then each of the original subjects was replaced so as to simulate generational turnover. By the time all initial members were replaced, their level of cooperation had eroded to the level of groups never experiencing mutual cooperation. The results are discussed in the context of other means of reducing discontinuity.

Word frequency effects in associative and item recognition.

The word frequency effect (WFE) has been taken as evidence that recall and recognition are in some way fundamentally different. Consequently, most models assume that recall and recognition operate via very different retrieval mechanisms. Experiment 1 showed that the WFE reverses for associative recognition, which requires discrimination between intact test pairs and recombinations of study list words from different study pairs. Experiment 2, in which word triples were used, revealed an interaction between word frequency and test type: for item recognition, performance was better for low-frequency words; however, for associative recognition and free recall, performance was better for high-frequency words. In Experiment 3, item recognition was tested: although overall performance was better for low-frequency words, the recognition advantage for items in intact pairs was larger for high-frequency words, suggesting two components in recognition memory. These results imply common mechanisms in recall and recognition. Theoretical implications are discussed within the framework of the SAM model.

The role of the cortex in play fighting by rats: developmental and evolutionary implications.

Play is a distinctive behavior of young mammals, especially mammals with a well-developed forebrain. For this reason it is thought that there may be a relation between forebrain evolution and highly elaborated play behavior. This study investigated the contribution of the cortex to play behavior by comparing play in control and neonatally decorticated rats (Rattus norvegicus). Play fighting in rats involves the combination of attack by one rat and defense by the recipient, with pinning arising when specific patterns of defense are used. Whether paired with another decorticate or with an intact pairmate, decorticates attacked pairmates as frequently as did intacts, and they were just as likely to defend against playful attacks as were intacts. Where decorticates differed from intacts was on a measure of pinning, in which one rat stands over a supine partner, decorticate rats displayed a reduction of 50% relative to control rats during the juvenile stage in which play is most pronounced (days 25 to 40). Juvenile decorticate rats adopted types of defensive responses which were less likely to result in the pinning configuration. Thus, a reduced pinning frequency reflects an altered pattern of defense, not a reduced level of play fighting. Rather, the decorticate patterns of defense were typical of those defensive responses displayed by adult rats. That is, decorticate juveniles exhibit a precociously mature pattern of playful defense. As intact controls mature, they come to resemble the decorticates in their defensive responses, and hence the difference in pinning frequency between decorticate and intact pairs diminishes. This suggests that the cortex may inhibit the escalation of defense in juveniles and thus promote prolonged ventral-ventral contact during play fighting. The results further suggest that the cortex is involved in the development of adult behavioral skills by facilitating juvenile play.