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PubMed2002

The electrophysiology of tactile extinction: ERP correlates of unconscious somatosensory processing.

We examined the electrophysiological correlates of left-sided tactile extinction in a patient with right-hemisphere damage. Computer-controlled punctate touch was presented to the left, right or both index fingers in an unpredictable sequence. The patient reported his conscious tactile percept ("left", "right" or "both"). He showed extinction on 75% of bilateral trials, reporting only right stimulation for these. Somatosensory evoked potentials for unilateral stimulation showed early components over contralateral somatosensory areas (P60 and N110) for either hand. In contrast to the results observed for age-matched controls, the patient's P60 was smaller in amplitude for left-hand touch over the right hemisphere than for right-hand touch over the intact hemisphere. Bilateral trials with extinction revealed residual P60 and N110 components over the right hemisphere in response to the extinguished left touch. These results demonstrate residual unconscious somatosensory processing of extinguished touch. They also suggest that tactile extinction can be caused by attenuation rather than elimination of somatosensory responses in the damaged hemisphere, with an underlying deficit even on unilateral trials.

Aged↗
PubMed2002

Spatial and temporal influences on extinction.

This study investigated the spatial and temporal characteristics of the attentional deficit in patients exhibiting extinction to determine the extent to which these characteristics can be explained by a theory of an underlying gradient resulting from the differential contribution of interacting cell populations. The paradigm required the identification of two letters whose spatial location was varied both within and across hemifields. Additionally, the interval between the appearances of the two stimuli was manipulated by changing the stimulus onset asynchrony (SOA). A final variable, that of expectancy, was introduced by making the stimulus location more or less predictable and examining the effect of this top-down contingency on performance. The findings were consistent across two patients and indicated the joint contribution of both spatial and temporal factors: the contralesional stimulus was maximally extinguished when it was preceded by the ipsilesional stimulus by 300-900 ms, but this extinction was reduced when the stimuli appeared further ipsilesionally. Interestingly, there was increased extinction of the contralesional stimulus when location was predictable. These findings support the hypothesis that the attentional deficit in extinction patients arises from a contralesional-to-ipsilesional gradient of cell populations that interact in a mutually inhibitory manner.

Aged↗
PubMed2002

Expectation-based attentional modulation of visual extinction in spatial neglect.

Visual extinction, the failure of patients with unilateral focal brain damage to report the contralesional of two simultaneously presented stimuli, may be modulated by characteristics of the display such as similarity, collinearity, or connectedness. Since these factors affect the perceptual configuration of stimuli, the modulation of extinction is believed to reflect low-level perceptual grouping. In the present study, patient AG did not show any modulation of contralesional detection when the ipsilesional and contralesional stimulus grouped by colour, by form, or both (Experiment 1). In contrast, identification of the contralesional stimulus was facilitated when the stimuli grouped (Experiment 2), suggesting a modulation of extinction by specific task demands. Experiment 3 used a cueing procedure to demonstrate modulation of extinction by expectation biases. Prior to stimulus presentation, AG was cued to attend to a particular feature (e.g. colour). After stimulus exposure he was prompted to identify the expected feature on valid trials and the unexpected feature on invalid trials. AG showed a significant validity effect for contralesional stimuli i.e. he identified the expected feature (e.g. colour) significantly better than the unexpected feature (e.g. form). These results suggest that competition for selection between visual stimuli may not only be influenced by perceptual characteristics of the display, but also by high-level factors such as the response criterion or expectation biases.

Attention↗
PubMed1997

Spatial extinction on double asynchronous stimulation.

Despite the fact that visual extinction is widely considered a space-based disturbance of selective attention, there has been little theoretical consensus about the nature of its pathogenic mechanism. A specific disruption in the ability to disengage attention from ipsilesional stimuli, or a loss of weight with which contralesional objects compete for visual selection, have been hypothesized to account for the disorder. We tested the merits of these two explanations in a right-hemisphere-lesioned patient, FB, who failed to recognize a contralesional target only when it was shown concurrently to an ipsilesional target (i.e. visual extinction). His task was to report two target letters presented in rapid succession to the left and right of the fixation point. The order of stimulus presentation (Left-First vs Right-First), and the intertarget interval (stimulus onset asynchrony) were varied systematically. We showed that contralesional extinction may occur for successively presented targets, not just for stimuli displayed at the same time. Of most importance, FB was seriously and equally impaired in dealing with a contralesional stimulus when this either preceded the ipsilesional stimulus or followed it by an interval less than about 600 msec. The data appear to contradict the disengagement hypothesis, which predicted a substantial reduction of extinction when a stimulus was displayed first into the lesioned side of space. We suggest that a competitive model of visual selective attention fits the data quite well.

Aged↗
PubMedDate not supplied

Analysis of exhausts emitted by i.c. engines and stationary burners, by means of u.v. extinction and fluorescence spectroscopy.

Optical investigations of the exhausts emitted by internal combustion (i.c.) engines and a stationary burner were performed, in order to assess their relative role as sources of organic matter to the atmosphere. Extinction spectra of air-diluted exhausts in the 200-400 nm u.v. band reveal the expected existence of gaseous trace-species (NO, NO2 and SO2) and carbonaceous particulate matter (soot). In addition, after subtracting the absorption contribution from known species, a strong residual absorption band remains below 250 nm, which is attributed to organic aromatic matter, involving no more than two aromatic rings. A set of ex situ extinction and laser induced fluorescence (LIF) experiments were carried out on condensed combustion-water samples. Extinction measurements from the water samples show absorption spectra similar to those observed from air-diluted samples, which are attributed to low volatility organic compounds, as they are trapped in the condensed phase. Combining the indications of extinction data for both air-diluted and condensed samples, it is suggested that the absorbing species might be molecular clusters of one/two aromatic rings. LIF spectra from condensed samples evidence two fluorescence bands, centered above 300 and 400 nm, respectively, whose intensities correlate with the combustion regimes. Analogous optical analysis on rain samples, collected in an urban area, showed that rain absorption and fluorescence spectra are similar to those found in condensed exhaust samples, which is consistent with the prevailing contribution of i.c. engines to the urban air pollution. The combined experimental data suggest that the absorbing and fluorescent species trapped in the condensed samples are organic (aromatic) compounds, involving mostly one two aromatic rings structural units, since they do not absorb above 250 nm. The overall molecular weight of the trapped material is likely heavy as they show low volatility.

Journal Article↗
PubMed2000

Early-session increases in responding during extinction.

After training under a variable-interval 60-s schedule of reinforcement, four rats were exposed to 30-min extinction tests, which occurred either at the start or at the end of the session (each session being 50-min long). Response rate in extinction decreased when the extinction test occurred at the end of the session, but first increased and then decreased when the extinction test occurred at the start of the session. Consistent with other recent results, this finding suggests that some variable, other than reinforcement, contributes to early-session increases in responding.

Journal Article↗
PubMed2003

Protein synthesis subserves reconsolidation or extinction depending on reminder duration.

When learned associations are recalled from long-term memory stores by presentation of an unreinforced conditioned stimulus (CS), two processes are initiated. One, termed reconsolidation, re-activates the association between the conditioned and unconditioned stimuli and transfers it from a stable protein synthesis-independent form of storage to a more labile protein-dependent state. The other is an extinction process in which presentation of the CS alone degrades the association between CS and US. To address the mechanistic relationship between reconsolidation and extinction, we have used an invertebrate model of contextual memory, which involves an association between the learning context and a visual danger stimulus. Here, we show that re-exposure duration to the learning context acts as a switch guiding the memory course toward reconsolidation or extinction, each depending on protein synthesis. Manipulation of this variable allows findings of impaired extinction to be discriminated from those of disrupted reconsolidation.

Animals↗
PubMed1988

Patterns of generic extinction in the fossil record.

Analysis of the stratigraphic records of 19,897 fossil genera indicates that most classes and orders show largely congruent rises and falls in extinction intensity throughout the Phanerozoic. Even an ecologically homogeneous sample of reef genera shows the same basic extinction profile. The most likely explanation for the congruence is that extinction is physically rather than biologically driven and that it is dominated by the effects of geographically widespread environmental perturbations influencing most habitats. Significant departures from the congruence are uncommon but important because they indicate physiological or habitat selectivity. The similarity of the extinction records of reef organisms and the marine biota as a whole confirms that reefs and other faunas are responding to the same history of environmental stress.

Animals↗
PubMedDate not supplied

A land-bridge island perspective on mammalian extinctions in western North American parks.

In recent years, a number of authors have suggested several geometric principles for the design of nature reserves based upon the hypothesis that nature reserves are analogous to land-bridge islands. Land-bridge islands are islands that were formerly connected to the mainland and were created by a rise in the level of the ocean. Land-bridge islands are considered supersaturated with species in that the ratio of island to mainland species numbers is higher than expected from the area of the island. As a result, the rate of extinction should exceed the rate of colonization on a land-bridge island, resulting in a loss of species that is suggested to be related to the size and degree of isolation of the island. If nature reserves are considered to be similar to land-bridge islands, because most are slowly becoming isolated from their surroundings by habitat disturbance outside the reserves, several predictions follow. First, the total number of extinctions should exceed the total number of colonizations within a reverse; second, the number of extinctions should be inversely related to reserve size; and third, the number of extinctions should be directly related to reserve age. I report here that the natural post-establishment loss of mammalian species in 14 western North American national parks is consistent with these predictions of the land-bridge island hypothesis and that all but the largest western North American national parks are too small to retain an intact mammalian fauna.

Animals↗
PubMed1994

Purging inbreeding depression and the probability of extinction: full-sib mating.

Inbreeding depression has been a topic of interest in recent years from a number of perspectives, particularly in the captive breeding of endangered species. Generally, the goal of captive breeding is to avoid the detrimental effects of inbreeding depression and to retain genetic variation for future adaptation. However, an important component of another suggested approach to captive breeding is to purge rapidly the population of its genetic load so that its long-term fitness is not compromised. I have examined the effectiveness of purging the genetic load by documenting both the reduction in inbreeding depression and the increase of the probability of extinction when there is continuous full-sib mating. When the genetic load is the result of lethals, the inbreeding depression is quickly purged without a high probability of extinction, except when the total genetic load is high. On the other hand, if the load is due to detrimentals of relatively small effect, the genetic load becomes fixed, the mean fitness is reduced, and the probability of extinction may be greatly increased. In other words, the success of such a programme to purge genetic load without an increase in the probability of extinction is highly dependent upon the genetic basis of inbreeding depression, information that is not readily available for most species.

Adaptation, Physiological↗
PubMed2005

Two episodes of microbial change coupled with Permo/Triassic faunal mass extinction.

Microbial expansion following faunal mass extinctions in Earth history can be studied by petrographic examination of microbialites (microbial crusts) or well-preserved organic-walled microbes. However, where preservation is poor, quantification of microbial communities can be problematic. We have circumvented this problem by adopting a lipid biomarker-based approach to evaluate microbial community changes across the Permo/Triassic (P/Tr) boundary at Meishan in South China. We present here a biomarker stratigraphic record showing episodic microbial changes coupled with a high-resolution record of invertebrate mass extinction. Variation in the microbial community structure is characterized by the 2-methylhopane (2-MHP) index (a ratio of the abundance of cyanobacterial biomarkers to more general bacterial biomarkers). Two episodes of faunal mass extinction were each preceded by minima in the 2-MHP index, followed by strong maxima, likely reflecting microbial responses to the catastrophic events that caused the extinction and initiated ecosystem changes. Hence, both cyanobacterial biomarker and invertebrate fossil records provide evidence for two episodes of biotic crisis across the P/Tr boundary.

Animals↗
PubMed2005

Predicting the risk of extinction from shared ecological characteristics.

Understanding the ultimate causes of population declines and extinction is vital in our quest to stop the currently rampant biodiversity loss. Comparison of ecological characteristics between threatened and nonthreatened species may reveal these ultimate causes. Here, we report an analysis of ecological characteristics of 23 threatened and 72 nonthreatened butterfly species. Our analysis reveals that threatened butterflies are characterized by narrow niche breadth, restricted resource distribution, poor dispersal ability, and short flight period. Based on the characteristics, we constructed an ecological extinction risk rank and predicted which of the currently nonthreatened species are at the highest risk of extinction. Our analysis reveals that two species currently classified as nonthreatened are, in fact, at high risk of extinction, and that the status of a further five species should be reconsidered.

Animals↗
PubMed2005

Asynchronous extinction of late Quaternary sloths on continents and islands.

Whatever the cause, it is extraordinary that dozens of genera of large mammals became extinct during the late Quaternary throughout the Western Hemisphere, including 90% of the genera of the xenarthran suborder Phyllophaga (sloths). Radiocarbon dates directly on dung, bones, or other tissue of extinct sloths place their "last appearance" datum at approximately 11,000 radiocarbon years before present (yr BP) or slightly less in North America, approximately 10,500 yr BP in South America, and approximately 4,400 yr BP on West Indian islands. This asynchronous situation is not compatible with glacial-interglacial climate change forcing these extinctions, especially given the great elevational, latitudinal, and longitudinal variation of the sloth-bearing continental sites. Instead, the chronology of last appearance of extinct sloths, whether on continents or islands, more closely tracks the first arrival of people.

Animals↗
PubMed2006

Latent extinction risk and the future battlegrounds of mammal conservation.

Global conservation prioritization usually emphasizes areas with highest species richness or where many species are thought to be at imminent risk of extinction. However, these strategies may overlook areas where many species have biological traits that make them particularly sensitive to future human impact but are not yet threatened because such impact is currently low. In this article, we identify such areas for the world's mammals using latent extinction risk, the discrepancy between a species' current extinction risk and that predicted from models on the basis of biological traits. Species with positive latent risk are currently less threatened than their biology would suggest, usually because they inhabit regions or habitats still comparatively unmodified by human activity. Using large new geographic, biological, and phylogenetic databases for nearly 4,000 mammal species, we map the global geographic distribution of latent risk to reveal areas where the mammal fauna is still relatively unthreatened but has high inherent sensitivity to disturbance. These hotspots include large areas such as the Nearctic boreal forests and tundra that are unrepresented in most current prioritization schemes, as well as high-biodiversity areas such as the island arc from Indonesia to the south Pacific. Incorporating latent extinction risk patterns into conservation planning could help guard against future biodiversity loss by anticipating and preventing species declines before they begin.

Animals↗
PubMed2003

Abundance not linked to survival across the end-Cretaceous mass extinction: patterns in North American bivalves.

Ecological studies suggest that rare taxa are more likely to go extinct than abundant ones, but the influence of abundance on survivorship in the fossil record has received little attention. An analysis of Late Maastrichtian bivalve subgenera from the North American Coastal Plain found no evidence that survivorship is tied to abundance across the end-Cretaceous mass extinction (65 million years ago), regardless of abundance metric or spatial scale examined. The fact that abundance does not promote survivorship in end-Cretaceous bivalves suggests that the factors influencing survivorship during mass extinctions in the fossil record may differ from those operating during intervals of background extinction.

Animals↗
PubMed2002

Rapid prehistoric extinction of iguanas and birds in Polynesia.

The Tongoleleka archaeological site on Lifuka Island, Kingdom of Tonga, is a rich accumulation of pottery, marine mollusks, and nonhuman bones that represents first human contact on a small island in Remote Oceania approximately 2,850 years ago. The lower strata contain decorated Lapita-style pottery and bones of an extinct iguana (Brachylophus undescribed sp.) and numerous species of extinct birds. The upper strata instead feature Polynesian Plainware pottery and bones of extant species of vertebrates. A stratigraphic series of 20 accelerator-mass spectrometer radiocarbon dates on individual bones of the iguana, an extinct megapode (Megapodius alimentum), and the non-native chicken (Gallus gallus) suggests that anthropogenic loss of the first two species and introduction of the latter occurred on Lifuka within a time interval too short (a century or less) to be resolved by radiometric dating. The geologically instantaneous prehistoric collapse of Lifuka's vertebrate community contrasts with the much longer periods of faunal depletion on some other islands, thus showing that the elapse time between human arrival and major extinction events was highly variable on oceanic islands as well as on continents.

Animals↗
PubMed1976

Calculation of immigration and extinction curves from the species-area-distance relation.

Quantitative models of the species-area-distance relation, based on equilibria between immigration and extinction rates, have been tested against data for birds on 52 Solomon islands. Biologically reasonable models account for 98% of the variance in species number. The data are adequate to permit determination of immigration and extinction curves and the values of seven associated parameters. The resulting curves are very concave. Extinction rates vary almost exactly as the reciprocal of area, but the effect of area on immigration rates is slight. Recognition of major differences among species in immigration and extinction rates and in dispersal distances proves essential to a successful model.

Journal Article↗
PubMed1978

Mathematical theory of group selection: Structure of group selection in founder populations determined from convexity of the extinction operator.

Genetic analysis for group selection is developed for the case of a biallelic locus (A, a) undergoing group selection of founder populations only. By contrast to R. Levins"E = E(x) models, extinction now depends on genetics at the propagule stage but acts uniformly on larger populations. Biological evidence supports this hypothesis, which also allows mathematical treatment at once simpler and biologically more general than the Fokker-Planck partial differential equation formalism adopted by Levins. It is presently possible to handle cytogenetics of both diploid and haplodiploid type. The model is set up as a quasideterministic recursion in the 5-simplex Sigma(5), collapsing both drift and mendelian selection effects into a single parameter u, which is a Fisher-Kimura-Ohta fixation probability. In the analysis, it is shown that the stability of the fixed points is determined by the convexity of the extinction operator acting on propagules, assumed to be of size 2.THUS, [FORMULA: see text] in which E(1) and E(3) are extinction probabilities for phenotypically uniform founder populations and E(2) is the corresponding probability for founder populations of mixed phenotype. Further parameter regions are defined where fixation of the group-selected gene is globally stable, and this is still possible even when extinction pressure acting on carrying capacity populations becomes weak relative to a fixed mendelian selection strength.

Journal Article↗