Causality and the allocation of attention during comprehension.

Recent research has suggested that each statement in a narrative text is understood by relating it to its causal antecedents and consequences and that the text as a whole is understood by finding a causal path linking its opening to its final outcome. Fletcher and Bloom (1988) have proposed that in order to accomplish this goal, while minimizing the number of times long-term memory has to be searched, readers focus their attention on the last clause of a narrative that has causal antecedents but no consequences in the preceding text. As a result, a statement that is followed by a causal antecedent should remain the focus of attention, while the same statement followed by a consequence should not. This prediction was tested and confirmed in three experiments which show that when a target statement is followed by a sentence that includes only causal antecedents, (a) continuation sentences related to it are read more quickly, (b) target words drawn from it are easier to recognize, and (c) subject-generated continuations are more likely to be causally related to it.

An on-line assessment of causal reasoning during comprehension.

Fletcher and Bloom (1988) have argued that as readers read narratives, clause by clause, they repeatedly focus their attention on the last preceding clause that contains antecedents but no consequences in the text. This strategy allows them to discover a causal path linking the text's opening to its final outcome while minimizing the number of times long-term memory must be searched for missing antecedents or consequences. In order to test this hypothesis, we examined the reading times of 25 subjects for each clause of eight simple narrative texts. The results show that: (1) causal links between clauses that co-occur in short-term memory (as predicted by the strategy) increase the time required to read the second clause; (2) potential causal links between clauses that never co-occur in short-term memory (again as predicted by the strategy) have no effect on reading time; and (3) reinstatement searches are initiated at the end of sentences that are causally unrelated to the contents of short-term memory or that contain clauses that satisfy goals no longer in short-term memory. These results support the claim that subjects engage in a form of causal reasoning when they read simple narrative texts.

Surface densities of Langerhans cells in relation to rodent epidermal sites with special immunologic properties.

Epidermal Langerhans cells bear surface receptors which implicate them as immunocompetent cells and they are now felt to play an important role both in delayed hypersensitivity and in skin allograft reactions. To determine the relationship between Langerhans cell availability and certain immunologic phenomena, surface densities were determined by ATP-ase and gold uptake in 3 rodent species: guinea pig, hamster, and mouse. Surface densities in epidermal specimens from the ear, back, foot pad, and buccal mucosa varied between 600 and 1500 cells/mm2. Significantly fewer cells were found in the hamster cheek pouch (130 cells/mm2) and in the mouse tail (110 cells/mm2 for C57BL/6J; 260 cells/mm2 for BALB/c nu/nu). Langerhans cells were absent from the central port;on of the cornea in all 3 species. Decreased Langerhans cell surface density may contribute to immunologic privilege as has been observed for the cornea and hamster cheek pouch and to the unusual allograft characteristics of mouse tail skin.

A new radiotracer technique involving 14C and 51Cr, for estimating the assimilation efficiencies of aquatic, primary consumers.

Gravimetric, radiotracer, and indicator methods currently available for estimating assimilation efficiencies, have been reviewed and their associated limitations have been discussed. It was concluded that the basic assumption implicit to gravimetric and indicator techniques, i.e. that all material contained within the faeces is derived from the food, does not generally hold. Radiotracer techniques are not based on this assumption but are time consuming. Consequently a new radiotracer technique analogous to indicator methods has been developed. In this technique the concentration of a non-absorbed indicator is expressed in terms of a radiotracer, 14C, which can be absorbed but which, at least initially, is only present in the food, rather than expressing it in terms of dry weight. 51Cr has been used as the nonabsorbed indicator.Use of these two isotopes in conjunction not only enables a distinction to be made between faecal material derived from food, and that derived from metabolic secretions but also facilitates estimation of assimilation efficiences fromsmall samples of faeces only. The new technique requires simply, measurement of the ratio 14C:51Cr in samples of both food and faeces.The applicability of conditions necessary for operation of the new technique has been tested on two species of freshwater gastropod, one feeding on epilithic algae, the other on bacteria, and its effectiveness has been tested by reference to results obtained from another, more conventional method involving 14C only.