Search PubMed⌕ Search

Biomedical subjects

E J O'Brien

Publications and source records attributed to E J O'Brien.

At least 19 recordsLinked to original sources
PubMed1998

Updating a situation model: a memory-based text processing view.

Previous research (J. E. Albrecht & E. J. O'Brien, 1993) demonstrated that readers were aware of an inconsistency between an earlier described characteristic of a protagonist and a subsequent target action carried out by the protagonist. In a series of 5 experiments, a qualification was added to the described characteristic that restricted the conditions under which the characteristic was operative. According to the here-and-now view of mapping, readers should use this qualification to maintain a fully updated model of the protagonist in active memory and should not experience comprehension difficulty when reading the target action. In contrast, according to the memory-based text processing view, the qualification would not be part of the active discourse model. Instead, it would be reactivated when the target action was read. Thus, readers should still experience comprehension difficulty. Results of all 5 experiments were consistent with the memory-based text processing view.

Humans↗
PubMed1995

The role of perspective in the accessibility of goals during reading.

An implicit assumption of several causal reasoning models is that readers adopt the goals of a narrative's protagonist during text comprehension. In apparent violation of this assumption, readers participating in Experiment 1 of the present study drew inferences relevant to a protagonist's goal even when that goal was already satisfied from the perspective of the protagonist. In Experiments 2 and 3, participants were explicitly asked to view the text situation from the point of view of the protagonist. In this case, the goals of the reader and the protagonist should be the same. In these experiments, participants focused on the goals of the protagonist only when those goals had not been satisfied from the perspective of the protagonist. These results are discussed in terms of reader- and character-based perspectives and in terms of text characteristics that cue perspective taking.

Adult↗
PubMed1992

Comprehension strategies in the development of a mental model.

It is generally assumed that the comprehension strategy used in the development of a mental model for narrative texts focuses on information that is relevant to the protagonist. Experiments 1a and 1b confirmed that readers remain sensitive to the location of the protagonist even when strategies based on text-base level representations predict this information should not be active. Experiments 2 and 3 tested the stronger claim that readers adopt the perspective of the protagonist. Ss did not notice information that was contradictory from the perspective of the protagonist unless explicitly instructed to adopt that perspective. It was concluded that, at the level of the mental model, readers focus on information relevant to the protagonist but that they do not adopt the perspective of the protagonist unless characteristics of the text induce such a strategy.

Adult↗
PubMed1991

Effects of centrality on retrieval of text-based concepts.

Four experiments examined the effect of centrality on retrieval time using a wide range of testing conditions. Subjects read passages containing concepts that were rated as central, moderately central, or peripheral to a passage, and then were asked to retrieve those concepts. Experiment 1 used a probe recognition task that contained foils that were unrelated to the passage, and it was found that central concepts were recognized more quickly than peripheral concepts. In Experiment 2, the foils were passage related, resulting in a much larger recognition advantage of central concepts over peripheral concepts. Experiment 3 used a speeded-recall task. Again central concepts were recalled more quickly than less central concepts. Experiment 4 ruled out a simple strength argument. The results of all four experiments are discussed in terms of text being represented as an integrated network. Within this network, central concepts are more interconnected, providing additional access routes that facilitate retrieval.

Analysis of Variance↗
PubMed1990

Elaborative inferencing as an active or passive process.

O'Brien, Shank, Myers, and Rayner (1988) reported that readers generated elaborative inferences only when a text contained characteristics that virtually eliminated the possibility of an inference being disconfirmed. We reanalyzed the data of O'Brien et al. (1988) and also conducted an experiment in which we varied (a) whether or not there was an anaphoric relation between a target word and its prior mention in the text and (b) the explicitness of the prior mention. Two refinements to O'Brien et al.'s conclusions are offered. First, the two text characteristics they manipulated (a strong biasing context or a demand sentence) may have produced different types of elaborative inferencing. We argue that a biasing context results in a passive form of elaborative inferencing, involving setting up a context of interpretation, whereas the presence of a demand sentence invites the reader to actively predict a subsequent expression. Second, clear evidence for either type of inference will be apparent only with truly anaphoric materials.

Attention↗
PubMed1990

Antecedent retrieval processes.

In three experiments we examined the nature of the search for antecedents during reading. Subjects read passages that contained two possible antecedents: one appearing early in the passages and the other appearing late. The degree of elaboration was varied, with one antecedent receiving additional elaboration for half the passages and the other antecedent receiving additional elaboration for the remaining half. Reading time differences from the first two experiments demonstrated that late antecedents are reinstated more quickly than early antecedents and elaborated antecedents are reinstated more quickly than nonelaborated antecedents. Experiment 1 showed that concepts that fall in the path of an antecedent search can be activated by the search. Experiments 2 and 3 showed that this activation is restricted to concepts that are from the same general category as the target antecedent.

Attention↗
PubMed1988

Structure of tropomyosin-troponin T cocrystals.

Crystals formed from a mixture of tropomyosin and troponin T have an open double-stranded lattice structure with a diamond-shaped repeat. In some regions the appearance in electron micrographs of negatively stained specimens changes from this double-diamond lattice to a more condensed banded crystal form. The double-diamond lattice has plane group symmetry cmm with unit cell 76.3 by 21.7 nm. The molecules form continuous chains along the diagonal of the unit cell and the diagonal length (79.4 nm) is that expected for two tropomyosin molecules joined end-to-end. Computer filtering of the micrographs shows that the strands of the lattice are thicker from the acute vertex of the large diamond to a point about half-way along the side of the diamond, where there is a small blob of density. At the acute vertex of the diamond is a large blob of density which is accentuated, however, by being at the lattice node where strands cross each other, and which is much weaker in regions of the micrographs where the crystals have condensed laterally. The results indicate that troponin T is a long thin molecule running in contact with the tropomyosin strands over 40-50% of the tropomyosin molecular length. The small globular region may represent the end-to-end overlap of tropomyosin but is more likely to be a globular region at the C-terminal region of troponin T.

Animals↗
PubMed1988

Elaborative inferences during reading: do they occur on-line?

Four experiments were conducted to examine the extent to which readers construct elaborative inferences on-line during reading. In Experiment 1, gaze durations were measured while subjects read anaphors to target antecedents that referenced a particular category member either explicitly or implicitly. When the context strongly suggested a particular category member, gaze durations on an anaphor were the same following either an implicit or an explicit antecedent. When the context did not suggest any particular category member, gaze durations were significantly longer following an implicit antecedent. The results confirmed that, with sufficient context, readers will generate a simple elaborative inference on-line. These results were replicated in Experiment 2 in which the materials did not strongly signal the inference but a sentence designed to encourage subjects to infer was included. In Experiment 3, this "demand sentence" was not included, and readers did not appear to construct the targeted inference. The results of Experiment 4 confirmed that once generated, elaborative inferences are stored as part of the long-term-memory representation of a passage.

Adult↗
PubMed1987

Antecedent search processes and the structure of text.

Kintsch and van Dijk (1978) assumed that when a reader encounters a reference to a concept no longer available in short-term memory, a search through long-term memory for the original concept is necessary. In the present study, four experiments addressed the nature of this search process. In the first two, subjects read passages that contained two possible antecedents: one appearing early in the passages and the other appearing relatively late. Reading time differences demonstrated that late antecedents are reinstated more quickly than early antecedents, which is in disagreement with predictions from several search models within the Kintsch and van Dijk framework. The best account of the reinstatement time differences assumes that text is represented as an integrated network accessed by a backward parallel search. Experiment 3 demonstrated that naming time, as used in the current studies, measures only current activation and is not sensitive to differences in long-term memory strength. Experiment 4 provided further support for the assumptions of a backward parallel-search model by showing that concepts appearing between an antecedent and the end of a passage are often accessed during the search for the antecedent.

Cognition↗
PubMed1986

Anaphoric inference during reading.

Three experiments provide evidence that an anaphoric noun phrase reinstates its antecedent in the course of comprehension. Subjects read a series of texts each containing a target item. Immediately after the last line of each text, the item was probed using a recognition task in Experiment 1 and a naming task in Experiment 2. Subjects were faster to respond to the item when the last line contained an anaphoric reference to it than when the last line referred to a different item from the text. Additional control conditions ensured that the effect was not due to semantic priming and that the probed item was not in working memory when the last line was encountered. A third experiment suggested that previous evidence for reinstatement reflected interference from a change of topic in the last line rather than facilitation due to reinstatement of the probed item.

Cognition↗
PubMed1985

Structure of myosin decorated actin filaments and natural thin filaments.

Negatively stained paracrystals of reconstituted thin filaments decorated with myosin subfragment 1 (S1), at high calcium concentrations (greater than or equal to 10(-5) M), exhibit pgg plane group symmetry with component filaments having 28 subunits in 13 turns of the actin genetic helix. Isolated S1 decorated F-actin filaments trapped in a stain film were also observed to form spontaneously paracrystals with pgg plane group symmetry. We conclude that a favourable S1-S1 interaction must exist in order to stabilize these structures. Three-dimensional helical reconstructions, calculated from these paracrystals show S1 to be curved, 12 to 14 nm long and tilted with respect to the helical axis, in broad agreement with previous reconstructions calculated from isolated particles. Reconstructions of S1 and HMM decorated filaments that resolve actin show a principal myosin binding site located on the side of the actin subunit reported by Taylor & Amos [J. molec. Biol. 147, 297-324 (1981)] and a possible small interaction on the opposite side. The appearance, symmetry and helical reconstructions of isolated F-actin filaments decorated with heavy meromyosin (HMM) were similar to those of S1 decorated filaments, except at high radii where extra mass was observed. This probably arose from the connection between the two heads of HMM bound to the same long-pitch strand of actin. In contrast to most studies on thin filaments, which use reconstituted filaments, we present data on natural I-segments of muscle homogenates. Individual filaments exhibited actin helical symmetry which on reconstruction gave a two-domain motif oriented consistently with its long axis approximately perpendicular to the helical axis, but inclined towards the 5.9 nm genetic helix. Our original interpretation of these maps [Seymour & O'Brien, Nature, Lond. 283, 680-2 (1980)] depended upon reconstructions from F-actin paracrystals, which suggested actin was rather symmetrical in shape. New data from two- and three-dimensional crystal studies and reconstructions of actin-tropomyosin filaments show that actin is rather elongated and consists of two domains. These results indicate that actin contributes towards both domains of our I-segment motif and are consistent with the monomer long axis lying approximately perpendicular to the helical axis. Although tropomyosin is not resolved, comparison of the actin-tropomyosin and I-segment reconstructions suggests that tropomyosin is strongly merged with the actin domain at a lower radius from the helical axis and that the domain at higher radius arises solely from actin.

Actins↗
PubMed1981

Structure of filamin and the F-actin-heavy merofilamin complex.

Rotary-shadowed filamin molecules appear as long, highly flexible rods curved into a variety of configurations. The particles observed were 2.7 nm wide but had contour lengths of either 98 nm or 193 nm. The longer particles are probably end-to-end dimers of the shorter but it is not clear how many polypeptide chains these particles contain. Heavy merofilamin, obtained by digestion of filamin with a calcium-activated protease from muscle, has been used to investigate where filamin binds on the actin filaments. Negatively stained filaments of actin plus heavy merofilamin resemble those of pure actin; occasionally rod-shaped material sticking out from the filament is observed suggesting that the elongated shape of filamin is maintained after digestion. Optical diffraction patterns of electron micrographs of paracrystals of actin plus heavy merofilamin indicate that the helical symmetry of the actin filament is unchanged, but the observed interfilament spacing is larger than in F-actin paracrystals. Increased intensity of the second layer-line reflection is observed, suggesting that additional material is lying along the grooves of the actin helix. The elongated shape of filamin and its ability to bind to F-actin in a way similar to tropomyosin suggest a possible role for this protein in regulating the organization and aggregation of actin filaments.

Actins↗
PubMed1981

Changes of thick filament structure during contraction of frog striated muscle.

The strongest myosin-related features in the low-angle axial x-ray diffraction pattern of resting frog sartorius muscle are the meridional reflections corresponding to axial spacings of 21.4 and 14.3 nm, and the first layer line, at a spacing 42.9 nm. During tetanus the intensities of the first layer line and the 21.4-nm meridional decrease by 62 and 80% respectively, but, when the muscle is fresh, the 14.3-nm meridional intensity rises by 13%, although it shows a decrease when the muscle is fatigued. The large change in the intensity of the 21.4-nm meridional reflection suggests that the projected myosin cross-bridge density onto the thick filament axis changes during contraction. The model proposed by Bennett (Ph.D. Thesis, University of London, 1977) in which successive cross-bridge levels are at 0,3/8, and 5/8 of the 42.9-nm axial repeat in the resting muscle, passing to 0, 1/3, and 2/3 in the contracting state, can explain why the 21.4-nm reflection decreases in intensity while the 14.3-nm increases when the muscle is activated. The model predicts a rather larger increase of the 14.3-nm reflection intensity during contraction than that observed, but the discrepancy may be removed if a small change of shape or tilt of the cross-bridges relative to the thick filament axis is introduced. The decrease of the intensity of the first layer line indicates that the cross-bridges become disordered in the plane perpendicular to the filament axis.

Animals↗