Nt Component Research Articles

Voluntary Control of Task Selection Does Not Eliminate the Impact of Selection History on Attention.

The human visual system can only process a fraction of the information present in a typical visual scene, and selection is historically framed as the outcome of bottom-up and top-down control processes. In this study, we evaluated how a third factor, an individual's selection history, interacts with top-down control mechanisms during visual search. Participants in our task were assigned to one of two groups in which they developed a history of either shape or color selection in one task, while searching for a shape singleton in a second task. A voluntary task selection procedure allowed participants to choose which task they would perform on each trial, thereby maximizing their top-down preparation. We recorded EEG throughout and extracted lateralized ERP components that index target selection (NT) and distractor suppression (PD). Our results showed that selection history continued to guide attention during visual search, even when top-down control mechanisms were maximized with voluntary task selection. For participants with a history of color selection, the NT component elicited by a shape target was attenuated when accompanied by a color distractor, and the distractor itself elicited a larger PD component. In addition, task-switching results revealed that participants in the color group had larger, asymmetric switch costs implying increased competition between task sets. Our results support the notion that selection history is a significant factor in attention guidance, orienting the visual system reflexively to objects that contradict an individual's current goals-even when these goals are intrinsically selected and prepared ahead of time.

Cueing distraction: electrophysiological evidence for anticipatory active suppression of distractor location.

It is well known that processing at upcoming target locations can be facilitated, but mixed results have been obtained regarding the inhibition of irrelevant locations when advance information about distractors is available on a trial-to-trial basis. Here, we provide electrophysiological evidence that distractor locations can be anticipatorily suppressed. In an additional singleton search task, distractor cues were presented before the search display, which were either fully predictive or non-predictive of the location of the upcoming salient colour distractor. The PD component of the event-related potential, a marker of active suppression, was elicited by lateral singletons and smaller following predictive than non-predictive cues, indicating that less suppression was required upon presentation of the distractor when its location was known in advance. Presumably, excitability of regions processing the predictively cued locations was anticipatorily reduced to prevent distraction. This idea was further supported by the finding that larger individual cueing benefits in reaction time were associated with stronger reductions of the PD. There was no behavioural benefit at the group level, however, and implications for the role of individual differences and for the measurement of inhibition in distractor cueing tasks are discussed. The enhancement of target locations, reflected by the NT component, was not modulated by the predictiveness of the cues. Overall, our findings add to a growing literature highlighting the importance of inhibitory mechanisms for the guidance of spatial attention by showing that irrelevant locations can be anticipatorily suppressed in a top-down fashion, reducing the impact of even salient stimuli.

Intertrial priming due to distractor repetition is eliminated in homogeneous contexts.

Targets are found more easily in a visual search task when their feature is repeatedly presented, an effect known as intertrial priming. Recent findings suggest that priming of distractors can also improve search performance by facilitated suppression of repeated distractor features. The efficacy of intertrial priming for targets can be potentiated by the expectancy of a specific target feature; systematic repetition shows larger intertrial priming than random repetition. For distractors, the underlying mechanism is less clear. We used the systematic lateralization approach to disentangle target- and distractor-related processing with subcomponents of the N2pc. We found no modulation of the NT component, which reflects prioritization of target processing. The ND component, which reflects attentional capture by irrelevant stimuli, however, showed intertrial priming: ND monotonically decreased with repetition of a distractor color, but only if a specific distractor feature was expected, and if the context induced a search that was vulnerable to attentional capture. These observations suggest that distractor priming only improves visual search if volitional control is relatively high. The results also suggest that intertrial priming for distractors is due to decreased attentional capture by repeatedly presented distractors, whereas target processing remains unaffected.

Context homogeneity facilitates both distractor inhibition and target enhancement

Homogeneous contexts were shown to result in prioritized processing of embedded targets compared to heterogeneous contexts (Duncan & Humphreys, 1989). The present experiment used behavioral and ERP measures to examine whether context homogeneity affects both enhancing relevant information and inhibiting irrelevant in contexts of varying homogeneity. Targets and distractors were presented laterally or on the vertical midline which allowed disentangling target- and distractor-related activity in the lateralized ERP (Hickey, diLollo, & McDonald, 2009). In homogeneous contexts, targets elicited an NT component from 150 ms on and a PD component from 200 ms on, showing early attention deployment at target locations and active suppression of distractors. In heterogeneous contexts, an NT component was also found from 150 ms on and PD was found from 250 ms on, suggesting delayed suppression of the distractor. Before 250 ms, distractors in heterogeneous contexts elicited a contralateral negativity, indicating attentional capture of the distractor prior to active suppression. In sum the present results suggest that top-down control of attention is more pronounced in homogeneous than in heterogeneous contexts.

Physcomitrella patens DCL3 Is Required for 22–24 nt siRNA Accumulation, Suppression of Retrotransposon-Derived Transcripts, and Normal Development

Endogenous 24 nt short interfering RNAs (siRNAs), derived mostly from intergenic and repetitive genomic regions, constitute a major class of endogenous small RNAs in flowering plants. Accumulation of Arabidopsis thaliana 24 nt siRNAs requires the Dicer family member DCL3, and clear homologs of DCL3 exist in both flowering and non-flowering plants. However, the absence of a conspicuous 24 nt peak in the total RNA populations of several non-flowering plants has raised the question of whether this class of siRNAs might, in contrast to the ancient 21 nt microRNAs (miRNAs) and 21–22 nt trans-acting siRNAs (tasiRNAs), be an angiosperm-specific innovation. Analysis of non-miRNA, non-tasiRNA hotspots of small RNA production within the genome of the moss Physcomitrella patens revealed multiple loci that consistently produced a mixture of 21–24 nt siRNAs with a peak at 23 nt. These Pp23SR loci were significantly enriched in transposon content, depleted in overlap with annotated genes, and typified by dense concentrations of the 5-methyl cytosine (5 mC) DNA modification. Deep sequencing of small RNAs from two independent Ppdcl3 mutants showed that the P. patens DCL3 homolog is required for the accumulation of 22–24 nt siRNAs, but not 21 nt siRNAs, at Pp23SR loci. The 21 nt component of Pp23SR-derived siRNAs was also unaffected by a mutation in the RNA-dependent RNA polymerase mutant Pprdr6. Transcriptome-wide, Ppdcl3 mutants failed to accumulate 22–24 nt small RNAs from repetitive regions while transcripts from two abundant families of long terminal repeat (LTR) retrotransposon-associated reverse transcriptases were up-regulated. Ppdcl3 mutants also displayed an acceleration of leafy gametophore production, suggesting that repetitive siRNAs may play a role in the development of P. patens. We conclude that intergenic/repeat-derived siRNAs are indeed a broadly conserved, distinct class of small regulatory RNAs within land plants.

THE EVAPOTRANSPIRATIVE DEMAND AND WATER USE EFFICIENCY OF MAIZE CROP (ZEA MAYS L.) IN THE SEMI-ARID TRACT OF INDIA

Mai ze is an Impo rta nt component o f th e d ry lau d fanning system o f India. In th e present study.d ata fur five years' period for three 51ation s. viz.. New Delhi. Rajendran agar an d Bellary in th e dry farm ing tr act ofInd ia have been iuili zed to und erstand evapotra nspira tive deman d of the crop in relat ion to pan-evaporat ion , andsolar rad iation. Biomet ric cha racte ristic. e.g.. heil:ht ofthe pla nt has also been analysed \'U-U-I';S meteorological facton.' V,lIer use efficie ncy of maul," has also been workedout nnd discu ssed .
 Th e analysis indica ted th ai th e maize plant uses maximum water during the flowering stage. Th e ETJEP rati ogradu ally in creases fmm germination aua ining a maximum value 8 to 10we eks after sowing.Th is ratio is found 10 besignilican tl)' correlated with the plan t height . The ETIR, curve was found 10 follow a sigm oid patt ern reac hingmaximum during flowering stage. Th e water use efficiency of maize crop vari es ove r wide margins in limeand space.

Discontinuous transcription inLeptomonas seymouri: presence of intact and interrupted mini-exon gene families

Mature mRNAs of trypanosomatid protozoa result from the joining of at least two exons, which are initially transcribed as separate RNAs. In all trypanosomatids examined to date, the first exon (mini-exon) is encoded by approximately 200 tandemly reiterated genes. In characterizing the mini-exon genes of Leptomonas seymouri, we identified two predominant size classes of repetitive sequences that hybridized strongly to the L. seymouri mini-exon sequence. These two sequences are arranged as interspersed clusters. DNA sequence analysis of a clone representing the smaller size class demonstrated that these sequences have the capacity to encode a mini-exon donor (med)RNA corresponding to the 86 nt component seen in Northern blots of L. seymouri RNA. The larger size class comprises a family of related sequences, some of which contain DNA inserted into the mini-exon portion of the medRNA gene. The specific insert identified here (LINS 1) is exclusively associated with medRNA sequences, and is present in approximately 20% of the larger size class of L. seymouri medRNA genes. Disregarding the insertion, the sequences of the smaller bona fide mini-exon genes and the gene copy containing the insert were almost identical. The insert sequence is transcribed in the same direction as medRNA to yield at least four small non-polyadenylated RNAs, which appeared not to be linked to medRNA sequences.

Triumph of the Nomads, A History of Aboriginal Australia

This is a review of Geoffrey Blainey's Triumph of the Nomads, a History of Aboriginal Australia (1976). Disciplines Anthropology | Social and Behavioral Sciences This review is available at ScholarlyCommons: http://repository.upenn.edu/anthro_papers/64 218 ANTHROPOLOGICAL QUARTERLY tion in a Bolivian town, despite truly radical changes in social structure. Using simple diagrams to illustrate his analyses, Heath describes the changes in patronclient networks for curing, land, justice, public works, and marketing. Robert Shirley, in the context of greater economic development, traces the process and patterning of patronage in the large system of cooperatives in Sao Paulo State. Esther Hermitte, an Argentine investigator, breaks new relating a network of women in patron-client relationship. They are the weavers and local marketers of vicuiTa and Ilama ponchos, the only cash products in an.otherwise subsistence farming valley on the eastern slopes of the Andes. Only women weave, and they guard the privileges connected with this occupation, supporting the definition of male weavers as homosexual. Males without land or other occupation, then, generally do nothing or take migratory work outside the region, reinforcing the womens' hold on the only local industry. Breaking even newer ground, Nancie Gonzalez enters the realm of patron-client relations at the international level. She summarizes the economic and political interdependence of the Dominican Republic (cl ent) and the United States (patron) over the past hundred years. Basically the relati n involves a powerful oligarchy, the elite in many areas of national life in the Republic, and its position in the disbursement of valuable resources-notably, foreign aid. Gonzalez' paper should be read-it may inspire more research into the workings of personalized-formal relations among those doing business at the international level. To date, the expertise of the heads of even the largest U.S. corporations does not extend to competently handling the grey areas, the interstices in world markets. Personalized dealings, often with a patroncli nt component, are frequently identified in the popular press with graft, corruption, and bribery. However, it seems that dishonesty, as such, is possible in any human system, rationally formalized or not. Perhaps for the forseeable future Americans will need to learn how to work ethically within patron-client-like systems. Anthropologists can surely make a key contribution, toward this end, for which Structure and Process in Latin America can serve as a