Generation and characterization of a delta F508 cystic fibrosis mouse model

We have generated mice carrying the most common mutation in cystic fibrosis (CF), delta F508, within the cystic fibrosis (Cftr) gene. Mutant animals show pathological and electrophysiological changes consistent with a CF phenotype. delta F508-/- mice die from peritonitis and show deficiencies in cAMP-activated electrogenic Cl- transport. These mice produce delta F508 transcripts and show the temperature-dependent trafficking defect first described for the human delta F508 CFTR protein. A functional CFTR Cl- channel not demonstrated by null CF mice or present at 37 degrees C was detected following incubation of epithelial cells at 27 degrees C. Thus, these mice are an accurate delta F508 model and will be valuable for testing drugs aimed at overcoming the delta F508 trafficking defect.

Bias in the priming of object decisions

Seven experiments examined priming effects for 3-dimensional line drawings in the object decision task. One of the most important previous findings about object decisions has been that the decision about a possible object is primed by previous presentation of the object, but the decision about an impossible object is not. Through the use of manipulations that can eliminate processes that retrieve episodic information (response time deadlines, memory load, forced choice, and similarity), equal size effects on impossible and possible objects were obtained. This is interpreted to mean that priming effects reflect a bias to respond "possible," which can be opposed for impossible objects by episodic information so as to yield the approximately null priming effect for impossible objects found in past experiments.

Calcium-activated chloride conductance is not increased in pancreatic duct cells of CF mice

Calcium-activated anion secretion is elevated in the pancreatic ductal epithelium of transgenic cf/cf mice which lack the cystic fibrosis transmembrane conductance regulator (CFTR). To elucidate whether this effect is due to increased activity of calcium-activated chloride channels, we have studied the relationship between CFTR and calcium-activated chloride currents in pancreatic duct cells isolated from Cambridge cf/cf mice. CFTR chloride currents activated by cAMP were detected in 59% (29/49) of wild-type cells and in 50% (20/40) of heterozygous cells. However, we could not detect any CFTR currents in the homozygous cf/cf cells (0/25). The maximum CFTR current density measured at a membrane potential of 60 mV was 23.5 +/- 2.8 pA/pF (n = 29) in wild-type cells, and about half that value, i.e. 12.4 +/- 1.6 pA/pF (n = 20) in heterozygotes (P = 0.004). Calcium-activated chloride currents were detected in 73% (24/33) of wild-type, 75% (21/28) of heterozygous and in 58% (7/12) of homozygous cf/cf cells. There was no significant difference between the steady-state calcium-activated current densities in the three genotypic groups; the current measured at 60 mV being 527 +/- 162 pA/pF (n = 24) from wild-type, 316 +/- 35 pA/pF (n = 21) from heterozygote and 419 +/- 83 pA/pF (n = 7) from homozygous cells. Our data suggest that lack of CFTR does not enhance the calcium-activated chloride conductance in murine pancreatic duct cells.

How should implicit memory phenomena be modeled?

In a reply to R. Ratcliff and G. McKoon's (1995a) article on bias in the object decision task, D. L. Schacter and L. A. Cooper (1995) critiqued their theoretical arguments and presented an updated view of priming in the object decision task. In the present article, the updated view is examined in detail, and it is questioned whether Schacter and Cooper's explanation of the data is sufficiently articulated to be falsifiable. It is also argued in the present article that evidence from other research domains is not directly supportive of the memory systems hypothesis and that the statistical power available in data from object decision experiments is not great enough to test some relevant hypotheses. Finally, the bias hypothesis (Ratcliff & McKoon, 1995a) is elaborated to show that it requires a particular pattern of experimental results and that it serves as a target phenomenon for modeling.

The genetic advantage hypothesis in cystic fibrosis heterozygotes: a murine study

1. The delta F508 mutation of the cystic fibrosis (CF) gene is of high frequency in man (1 in 25) and in homozygotes causes cystic fibrosis. It is suggested that cystic fibrosis heterozygotes withstand secretory diarrhoea better than normal individuals and so are genetically advantaged. This hypothesis has been examined by measuring electrogenic chloride secretion in gut epithelia of normal and heterozygous CF mice. 2. Chloride secretory responses of normal and heterozygous colonic epithelia to forskolin, vasoactive intestinal polypeptide (VIP), isoprenaline, cholera toxin, heat-stable enterotoxin (STa), guanylin, carbachol and lysylbradykinin were examined. No significant differences in responses of tissues of the two genotypes were found. 3. Responses of normal and heterozygous ileal epithelia to forskolin and glucose were investigated. Heterozygous tissues responded as well as normal tissues. 4. Frusemide (furosemide) caused virtually identical inhibition of the chloride secretory responses to forskolin in colonic epithelia of both genotypes. 5. No evidence to support the genetic advantage hypothesis in ileal or colonic epithelia of the null CF mouse has been found, at least for acute responses. If the hypothesis is true then either (a) other non-cystic fibrosis transmembrane conductance regulator (non-CFTR) transport processes are involved, (b) prolonged exposure to secretagogues is required, or (c) delta F508 CFTR is responsible for the protective effect.

Ion-transporting activity in the murine colonic epithelium of normal animals and animals with cystic fibrosis

Electrogenic ion transport in the isolated colonic epithelium from normal and transgenic mice with cystic fibrosis (CF mice) has been investigated under short-circuit current (Isc) conditions. Normal tissues showed chloride secretion in response to carbachol or forskolin, which was sensitive to the Na-K-2Cl cotransport inhibitor, frusemide. Responses to both agents were maintained for at least 12 h in vitro, but the responses to carbachol changed in format throughout this period. By contrast CF colons failed to show the normal secretory responses to carbachol and forskolin, most preparations showing a decrease in Isc that was immediately reversed by frusemide. In CF colons addition of Ba2+ ions or tetraethylammonium (TEA+) to the apical bathing solution antagonised the reduction in Isc caused by the secretagogues. It is concluded that the reduction in Isc in CF colons is due to electrogenic K+ secretion and this was confirmed by flux studies using rubidium-86. In normal colons exposed to TEA+ the responses to forskolin were greater, but not significantly so, presumably because the minor K(+)-secretory responses are dominated by major chloride-secretory responses. Again rubidium-86 fluxes showed an increase of K+ secretion in normal colons receiving forskolin. Since the amiloride-sensitive current was not different in CF and normal colons there was no evidence that the CF mice were stressed in a way that increased mineralocorticoid levels and hence K+ secretion. Knowledge of the phenotype of the colonic epithelium of the CF mouse sets the baseline from which attempts at gene therapy for the gut must be judged.

Empirical generality of data from recognition memory receiver-operating characteristic functions and implications for the global memory models

The experiments presented in this article examined the slope of the zeta-ROC (receiver-operating characteristic) function for recognition memory. The slope was examined as a function of strength and the variables study time, list length, word frequency, and category membership. For normal distributions of familiarity, the slope of the zeta-ROC is the ratio of the new-item to old-item standard deviations. R. Ratcliff, C.-F. Sheu, and S. D. Gronlund (1992) found that the slope was constant within standard error as a function of strength of encoding, which is inconsistent with the predictions of the global memory models. The results presented here extend this finding: The slope was constant as a function of strength of encoding, list length, and the number of related items from a category in the study list. Word frequency did affect the slope, but within a frequency class the slope was constant as a function of strength. The implications of these data for the global memory models, the attention likelihood model, and variants of these models are discussed.

Testing theories of language processing: an empirical investigation of the on-line lexical decision task

On-line lexical decision has been used to test major theoretical hypotheses about language comprehension. Contrary to several current models, A. Sharkey and N. Sharkey (1992) found that a word in a sentence did not give facilitation to an immediately following, highly associated test item. In this article it is shown that such facilitation can be obtained. Other theories have proposed that syntactic processes supply antecedents for implicit anaphors. In using a test item that was an associate of the antecedent of the anaphor, the authors were unable to replicate previous findings of facilitation at but not before the site of the anaphor. Across 9 experiments, obtaining facilitation depended on the choice of control condition. This dependency raises questions about previous on-line lexical decision results that have been used to support the immediacy of syntactic processing.

Sentential context and on-line lexical decision

J. L. Nicol and D. Swinney (1989) reported that lexical decision response times to a test word that was related to the implicit object of a verb were faster, when tested immediately after the verb, than response times to a control test word. They concluded from this result that the relation between the implicit object and the verb was understood during comprehension. In G. McKoon, R. Ratcliff, and G. Ward (1994), another interpretation was suggested, that the faster lexical decision response times were due to the better semantic-pragmatic fit of the related test words to the sentential contexts, and experimental data were presented to support this interpretation. In response, J.L. Nicol, J. D. Fodor, and D. Swinney (1994) pointed out some possible problems with these experiments. The experiment presented in this article undermines their arguments. By using sentences in which there was no implicit object, exactly the same pattern of results was found as originally reported by J. L. Nicol and D. Swinney, thereby implicating some factor other than syntactic processing of an implicit object as an explanation of their finding.

Kinin-stimulated chloride secretion in mouse colon requires the participation of CFTR chloride channels

The effect of lysylbradykinin on electrogenic chloride secretion in the epithelium of the mouse colon has been investigated. The peptide was active only when applied to the basolateral surface and its effects were inhibited by the B2 receptor antagonist, Hoe 140, also applied to the same surface. The chloride channel blocker, niflumic acid, also inhibited the response to kinin when added apically. Cyclo-oxygenase inhibition with piroxicam attenuated the responses to kinin, indicating involvement of prostaglandins in the responses. It is concluded that lysylbradykinin increases chloride secretion by acting via B2 receptors and, as with other tissues, brings about secretion through the agency of multiple messengers. In colonic epithelia from cystic fibrosis (CF) mice lysylbradykinin was without effect, suggesting that the final effector process involves apically located cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels.

Empirical generality of data from recognition memory receiver-operating characteristic functions and implications for the global memory models

The experiments presented in this article examined the slope of the zeta-ROC (receiver-operating characteristic) function for recognition memory. The slope was examined as a function of strength and the variables study time, list length, word frequency, and category membership. For normal distributions of familiarity, the slope of the zeta-ROC is the ratio of the new-item to old-item standard deviations. R. Ratcliff, C.-F. Sheu, and S. D. Gronlund (1992) found that the slope was constant within standard error as a function of strength of encoding, which is inconsistent with the predictions of the global memory models. The results presented here extend this finding: The slope was constant as a function of strength of encoding, list length, and the number of related items from a category in the study list. Word frequency did affect the slope, but within a frequency class the slope was constant as a function of strength. The implications of these data for the global memory models, the attention likelihood model, and variants of these models are discussed.

Chloride secretion in response to guanylin in colonic epithelial from normal and transgenic cystic fibrosis mice

1. Guanylin, a 15 amino acid endogenous gut peptide, increased the short circuit current (SCC) in the epithelium of the mouse colon, but only when applied to the apical and not the basolateral surface. 2. By use of selective blockers of epithelial ion transport and modification of the bathing solution, it was concluded that guanylin increased electrogenic chloride secretion but also had a minor effect on electrogenic sodium absorption. In addition there were small residual currents which remained unresolved. 3. The threshold concentration of guanylin causing a SCC increase was less than 50 nM, but at concentrations 40 times greater no indication of a maximally effective concentration was found. 4. Two guanylin isomers with the same amino acid sequence but with the disulphide bridges joined in an alternate fashion showed no activity. Thus only guanylin with the greatest structural homology to heat stable enterotoxin (STa) showed biological activity. 5. The action of guanylin was virtually eliminated in colonic epithelia from transgenic cystic fibrosis (CF) mice. As these animals lack the chloride channel coded by the CF gene sequence, it is likely that the final effector process in murine colonic epithelia involves the CFTR (cystic fibrosis transmembrane conductance regulator) chloride channel. 6. Opportunistic infections of the gut generating STa lead to diarrhoeal conditions via an action of the toxin on apical guanylin receptors. Thus, as discussed, the CF heterozygote may have a genetic advantage in this circumstance.

Retrieving information from memory: spreading-activation theories versus compound-cue theories

McNamara (1992a) attacked compound-cue theories on a number of grounds. Using free association as a measure of distance between concepts in memory, he argued that compound-cue theories cannot explain mediated priming effects. The authors show that free-association production probabilities do not accurately predict priming effects, either directly or in the context of current spreading-activation models, and so remove the basis for McNamara's criticism. McNamara also claimed that compound-cue theories cannot account for the sequential effects of items that precede a target on responses to the target, but the authors show that sequential effects are consistent with compound-cue models if the target item is weighted more heavily than preceding items in the calculation of familiarity that determines response time and accuracy for the target. It is concluded that, although compound-cue and spreading-activation theories are both consistent with available data, the compound-cue theory, having less freedom, has passed more stringent tests.

Inactivation of the murine cftr gene abolishes cAMP-mediated but not Ca(2+)-mediated secretagogue-induced volume decrease in small-intestinal crypts

The cellular volume of crypts isolated from 2- to 3-week-old mouse small intestine has been measured to assess the capacity of the epithelial cells to respond to secretagogues. Vasoactive intestinal polypeptide (VIP) or carbachol, respectively cAMP- and calcium-mediated secretagogues, produced a reduction crypt volume attributed to KCl loss through channels activated by the agonists. Consistent with the participation of separate chloride channels, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) blocked the carbachol- but not the VIP-induced volume decrease, whilst glibenclamide abolished the VIP effect without affecting the carbachol-induced volume decrease. Animals homozygous for a disrupted cftr gene, introduced by gene targeting, were also used as the source for crypt isolation. In these CFTR (-/-) crypts. VIP failed to elicit any reduction in cellular volume, while the response to carbachol was indistinguishable from that seen in crypts from age-matched control animals. These results are consistent with murine CFTR being a cAMP-activated chloride channel inhibited by glibenclamide and resistant to DIDS. A separate chloride conductance activated by calcium mobilization in small-intestinal crypts appears to be independent of CFTR.

Discourse models, pronoun resolution, and the implicit causality of verbs

Some interpersonal verbs, such as admire and amaze, describe an action or property of one person (the reactor) that is necessarily a response to an action or property of another (the initiator). We hypothesized that these verbs make the initiator relatively more accessible in a comprehender's discourse model and that this change in relative accessibility aids identification of the referent of a pronoun in a subsequent because clause. We predicted that, as a result, subjects would be faster to recognize a character's name after a because clause that uses a pronoun to refer to that character than after one that refers to some other character. Four experiments confirmed this prediction. Three further experiments demonstrated the importance of the verb's causal structure and of the presence of the connective because to this result.

Methods for dealing with reaction time outliers

The effect of outliers on reaction time analyses is evaluated. The first section assesses the power of different methods of minimizing the effect of outliers on analysis of variance (ANOVA) and makes recommendations about the use of transformations and cutoffs. The second section examines the effect of outliers and cutoffs on different measures of location, spread, and shape and concludes using quantitative examples that robust measures are much less affected by outliers and cutoffs than measures based on moments. The third section examines fitting explicit distribution functions as a way of recovering means and standard deviations and concludes that unless fitting the distribution function is used as a model of distribution shape, the method is probably not worth routine use.

TODAM and the list-strength and list-length effects: comment on Murdock and Kahana (1993a)

B. B. Murdock and M. J. Kahana (1993a) presented a continuous memory version of the theory of distributed associative memory (TODAM) model; they claimed that this model predicts list-strength and list-length findings, including those reported by R. Ratcliff, S. E. Clark, and R. M. Shiffrin (1990) and K. Murnane and R. M. Shiffrin (1991a). This model is quite similar to one discussed by R. M. Shiffrin, R. Ratcliff, and S. Clark (1990), who rejected the model on the basis of its inability to predict both an absent or negative list-strength effect (when strength is varied by repetitions) and a present list-length effect. In this comment we elaborate the earlier discussion and demonstrate that the version of TODAM proposed by B. B. Murdock and M. J. Kahana (1993a) indeed fails for this reason. We show this first for a somewhat simplified version of the model for which derivations are obvious and then in a simulation of the complete version using the parameter values suggested by B. B. Murdock and M. J. Kahana (1993a).

Discourse models, pronoun resolution, and the implicit causality of verbs

Some interpersonal verbs, such as admire and amaze, describe an action or property of one person (the reactor) that is necessarily a response to an action or property of another (the initiator). We hypothesized that these verbs make the initiator relatively more accessible in a comprehender's discourse model and that this change in relative accessibility aids identification of the referent of a pronoun in a subsequent because clause. We predicted that, as a result, subjects would be faster to recognize a character's name after a because clause that uses a pronoun to refer to that character than after one that refers to some other character. Four experiments confirmed this prediction. Three further experiments demonstrated the importance of the verb's causal structure and of the presence of the connective because to this result.

Production of a severe cystic fibrosis mutation in mice by gene targeting

We have used gene targeting in embryonic stem cells to introduce an HPRT mini-gene into the coding sequence of the murine cystic fibrosis gene (cftr). This insertion introduces a termination codon in frame with the cftr coding sequence to terminate prematurely the CFTR protein within the first nucleotide binding domain. Animals homozygous for the cftr disruption fail to thrive and display a range of symptoms including meconium ileus, distal intestinal obstructions, gastrointestinal mucus accumulation and blockage of pancreatic ducts. The animals also show lacrimal gland pathology. Tracheal and caecal transepithelial current measurements demonstrate the lack of a cAMP activatable Cl- channel. These animals will prove useful for the evaluation of new therapeutic drugs and gene therapy strategies.

Correction of the ion transport defect in cystic fibrosis transgenic mice by gene therapy

Cystic fibrosis (CF) is a lethal inherited disorder affecting about 1 in 2,000 Caucasians. The major cause of morbidity is permanent lung damage resulting from ion transport abnormalities in airway epithelia that lead to mucus accumulation and bacterial colonization. CF is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that encodes a cyclic-AMP-regulated chloride channel. Cyclic-AMP-regulated chloride conductances are altered in airway epithelia from CF patients, suggesting that the functional expression of CFTR in the airways of CF patients may be a strategy for treatment. Transgenic mice with a disrupted cftr gene are appropriate for testing gene therapy protocols. Here we report the use of liposomes to deliver a CFTR expression plasmid to epithelia of the airway and to alveoli deep in the lung, leading to the correction of the ion conductance defects found in the trachea of transgenic (cf/cf) mice. These studies illustrate the feasibility of gene therapy for the pulmonary aspects of CF in humans.

Design and application of NMR-compatible bioreactor circuits for extended perfusion of high-density mammalian cell cultures

MR spectroscopy of cultured cells allows non-invasive analyses of the metabolism of cells with specific phenotypes under defined conditions. This technique can be used to investigate the intracellular metabolism of cells or extended to critically evaluate phenomena observed by in vivo MRS. In this paper, a cell maintenance system is described which allows MR analyses with unparalleled spectral resolution, S/N and stability. This system consists of a 25 mm diameter hollow fiber bioreactor and a supporting circuit. The hollow fiber reactor was chosen because it yields a large filling factor which can be perfused through defined volumes. The fibers were 300 microns diameter microporous (0.2 micron) cellulose acetate/cellulose nitrate membranes with high porosity, which allow bulk convective flow throughout the extracapillary space. This flow (Starling flow) is necessary to disrupt steady-state gradients in substrates and waste products. In many respects, the design of the supporting circuit is more important than the bioreactor itself, since it provides the reactor with the proper chemical and physical environment. Hence, this circuit can be applied to a variety of bioreactor configurations. The circuit consists of a hollow fiber oxygenator and a bleed-and-feed system housed in a temperature-controlled cabinet. Culture of mammalian cells in this reactor yields 31P spectra which have excellent spectral and temporal resolution. At confluence, endogenous 31P line widths were typically < 10 Hz (at 162 MHz) and well resolved spectra were obtained in < 30 s.

Spreading activation versus compound cue accounts of priming: mediated priming revisited

Spreading activation theories and compound cue theories have both been proposed as accounts of priming phenomena. According to spreading activation theories, the amount of activation that spreads between a prime and a target should be a function of the number of mediating links between the prime and target in a semantic network and the strengths of those links. The amount of activation should determine the amount of facilitation given by a prime to a target in lexical decision. To predict the amount of facilitation, it is necessary to measure the associative links between prime and target in memory. Free-association production probability has been the variable chosen in previous research for this measurement. However, in 3 experiments, the authors show priming effects that free-association production probabilities cannot easily predict. Instead, they argue that amount of priming depends on the familiarity of the prime and target as a compound, where the compound is formed by the simultaneous presence of the prime and target in short-term memory as a test item.

Inference during reading

Most current theories of text processing assume a constructionist view of inference processing. In this article, an alternative view is proposed, labeled the minimalist hypothesis. According to this hypothesis, the only inferences that are encoded automatically during reading are those that are based on easily available information, either from explicit statements in the text or from general knowledge, and those that are required to make statements in the text locally coherent. The minimalist hypothesis is shown to be supported by previous research and by the results of several new experiments. It is also argued that automatically encoded minimalist inferences provide the basic representation of textual information from which more goal-directed, purposeful inferences are constructed.

Testing global memory models using ROC curves

Global memory models are evaluated by using data from recognition memory experiments. For recognition, each of the models gives a value of familiarity as the output from matching a test item against memory. The experiments provide ROC (receiver operating characteristic) curves that give information about the standard deviations of familiarity values for old and new test items in the models. The experimental results are consistent with normal distributions of familiarity (a prediction of the models). However, the results also show that the new-item familiarity standard deviation is about 0.8 that of the old-item familiarity standard deviation and independent of the strength of the old items (under the assumption of normality). The models are inconsistent with these results because they predict either nearly equal old and new standard deviations or increasing values of old standard deviation with strength. Thus, the data provide the basis for revision of current models or development of new models.

Disruption of the cystic fibrosis transmembrane conductance regulator gene in embryonic stem cells by gene targeting

We have successfully disrupted the cftr (cystic fibrosis transmembrane conductance regulator) gene at its endogenous locus in embryonic stem cells by gene targeting. We are using a double replacement strategy to introduce subtle mutations into exon 10. We report here the first step of creating a null mutation by insertion of a functional hprt (hypoxanthine phosphoribosyl transferase) mini-gene into exon 10 of the cftr gene. Targeted embryonic stem cell clones were identified by PCR screening and confirmed by Southern blot analysis. One of the cftr targeted clones has been injected into recipient blastocysts and shown to contribute to chimaeras. The targeted clones will now be used as the starting point for a second gene targeting step to remove the hprt gene in exon 10 with the concomitant introduction of the delta F508 mutation or other mutations.

Using promotion to increase dental practices

Providers of professional services are beginning to experiment with promotional activities as a means to increase business and to remain competitive in the 1990s. The authors report the results of a nationwide survey of dentists that was conducted to identify the incidence and impact of promotional tool use among dental professionals. Of particular interest is the effect of different promotional tools on dentists' patient contact activity. Interestingly, dentists employed a variety of promotional tools in their practices and generally viewed promotion as acceptable. Yet, only dentists who used publicity as their sole promotional mechanism reported significantly improved patient contact activity.

Pronoun resolution and discourse models

Psychological investigations of pronoun resolution have implicitly assumed that the processes involved automatically provide a unique referent for every pronoun. We challenge this assumption and propose a new framework for studying pronoun resolution. Drawing on advances in discourse representation and global memory modeling, this framework suggests that automatic processes may not always identify a unique referent for a pronoun. In 9 experiments, we demonstrate that, unlike noun anaphors, pronouns sometimes do not produce relative facilitation of their referents in comparison with nonreferents. We argue that research on pronoun resolution must consider the discourse contexts in which pronouns are likely to occur.

Pronoun resolution and discourse models

Psychological investigations of pronoun resolution have implicitly assumed that the processes involved automatically provide a unique referent for every pronoun. We challenge this assumption and propose a new framework for studying pronoun resolution. Drawing on advances in discourse representation and global memory modeling, this framework suggests that automatic processes may not always identify a unique referent for a pronoun. In 9 experiments, we demonstrate that, unlike noun anaphors, pronouns sometimes do not produce relative facilitation of their referents in comparison with nonreferents. We argue that research on pronoun resolution must consider the discourse contexts in which pronouns are likely to occur.

Spreading activation versus compound cue accounts of priming: Mediated priming revisited

Spreading activation theories and compound cue theories have both been proposed as accounts of priming phenomena. According to spreading activation theories, the amount of activation that spreads between a prime and a target should be a function of the number of mediating links between the prime and target in a semantic network and the strengths of those links. The amount of activation should determine the amount of facilitation given by a prime to a target in lexical decision. To predict the amount of facilitation, it is necessary to measure the associative links between prime and target in memory. Free-association production probability has been the variable chosen in previous research for this measurement. However, in 3 experiments, the authors show priming effects that free-association production probabilities cannot easily predict. Instead, they argue that amount of priming depends on the familiarity of the prime and target as a compound, where the compound is formed by the simultaneous presence of the prime and target in short-term memory as a test item.

Connectionist models of recognition memory: constraints imposed by learning and forgetting functions

Multilayer connectionist models of memory based on the encoder model using the backpropagation learning rule are evaluated. The models are applied to standard recognition memory procedures in which items are studied sequentially and then tested for retention. Sequential learning in these models leads to 2 major problems. First, well-learned information is forgotten rapidly as new information is learned. Second, discrimination between studied items and new items either decreases or is nonmonotonic as a function of learning. To address these problems, manipulations of the network within the multilayer model and several variants of the multilayer model were examined, including a model with prelearned memory and a context model, but none solved the problems. The problems discussed provide limitations on connectionist models applied to human memory and in tasks where information to be learned is not all available during learning.

List-strength effect: I. Data and discussion

Extra items added to a list cause memory for the other items to decrease (the list-length effect). In one of the present studies we show that strengthening (or weakening) some items on a list harms (helps) free recall of the remaining list items. This is termed the list-strength effect. However, in seven recognition studies the list-strength effect was either absent or negative. This held whether strengthening was accomplished by extra study time or extra repetitions. The seven studies used various means to control rehearsal strategies, thereby providing evidence against the possibility that the findings were due to redistribution of rehearsal or effort from stronger to weaker items within a list. Current models appear unable to predict these results. We suggest that different retrieval operations underlie recall and recognition, as in the SAM model of Gillund and Shiffrin (1984), which can be made to fit the results with certain relatively minor modifications.

List-strength effect: II. Theoretical mechanisms

Ratcliff, Clark, and Shiffrin (1990) examined the list-strength effect: the effect of strengthening (or weakening) some list items upon memory for other list items. The list-strength effect was missing or negative in recognition, missing or positive in cued recall, and large and positive in free recall. We show that a large number of current models fail to predict these findings. A variant of the SAM model of Gillund and Shiffrin (1984), involving a differentiation hypothesis, can handle the data. A variant of MINERVA 2 (Hintzman, 1986, 1988) comes close but has some problems. Successful variants of a variety of composite and network models were not found (e.g., Ackley, Hinton, & Sejnowski, 1985; Anderson, 1972, 1973; Metcalfe Eich, 1982; Murdock, 1982; Pike, 1984). The results suggest constraints on the future development of such models.

List-strength effect: II. Theoretical mechanisms

Ratcliff, Clark, and Shiffrin (1990) examined the list-strength effect: the effect of strengthening (or weakening) some list items upon memory for other list items. The list-strength effect was missing or negative in recognition, missing or positive in cued recall, and large and positive in free recall. We show that a large number of current models fail to predict these findings. A variant of the SAM model of Gillund and Shiffrin (1984), involving a differentiation hypothesis, can handle the data. A variant of MINERVA 2 (Hintzman, 1986, 1988) comes close but has some problems. Successful variants of a variety of composite and network models were not found (e.g., Ackley, Hinton, & Sejnowski, 1985; Anderson, 1972, 1973; Metcalfe Eich, 1982; Murdock, 1982; Pike, 1984). The results suggest constraints on the future development of such models.

A bias interpretation of facilitation in perceptual identification

In a typical perceptual identification task, a word is presented for a few milliseconds and masked; then subjects are asked to report the word. It has been found that an earlier presentation of the test word will improve identification of the test word by as much as 30%. In addition, this facilitation has been shown to be preserved under amnesia. In this article we examine a fundamental question: Is the facilitation the result of bias toward the earlier presented item, an improvement in perceptual sensitivity, or both? The experiments presented here use a forced choice procedure to show that prior presentation of an item biases the subject to choose that item but does not improve discriminability. This result is obtained when the distractor items are visually similar to the target items. When distractors are dissimilar, earlier presentation affects neither bias nor discriminability. Two models of word identification are examined in light of the bias effects, and implications for understanding savings in amnesia are also examined.

Similarity information versus relational information: differences in the time course of retrieval

Two experiments are reported that examine the time course of retrieval in a sentence matching procedure. Subjects learned lists of active and passive sentences and were tested with sentences in active or passive, correct or incorrect versions; for example, if "John hit Bill" was a studied sentence, "Bill hit John" would be an incorrect active test sentence. A response signal procedure was used so that accuracy could be measured as a function of time. The data show that sentences containing words from studied sentences are discriminable early in processing from sentences containing all new words, but discrimination of correct from incorrect versions of studied sentences occurs only later in processing (after 600-700 ms). These results demonstrate that different kinds of information are available at different points during the time course of retrieval and so suggest that modifications are required of models that provide only a unitary value for the amount of match between a test probe and information in memory. Early in processing, the growth of accuracy can be explained by a simple model that assumes independent contributions to total amount of match for each of the content words of a sentence, but this independent processing model cannot account for discrimination later in processing. Several, more general, memory models are examined with respect to their abilities to produce independent item information early in processing and relational information later in processing.

Semantic associations and elaborative inference

In this article, a theoretical framework is proposed for the inference processes that occur during reading. According to the framework, inferences can vary in the degree to which they are encoded. This notion is supported by three experiments in this article that show that degree of encoding can depend on the amount of semantic-associative information available to support the inference processes. In the experiments, test words that express possible inferences from texts are presented for recognition. When testing is delayed, with other texts and test items intervening between a text and its test word, performance depends on the amount of semantic-associative information in the text. If the inferences represented by the test words are not supported by semantic associates in the text, they appear to be only minimally encoded (replicating McKoon & Ratcliff, 1986), but if they are supported by semantic associates, they are strongly encoded. With immediate testing, only 250 ms after the text, performance is shown to depend on semantic-associative information, not on textual information. This suggests that it is the fast availability of semantic information that allows it to support inference processes.

Semantic associations and elaborative inference

In this article, a theoretical framework is proposed for the inference processes that occur during reading. According to the framework, inferences can vary in the degree to which they are encoded. This notion is supported by three experiments in this article that show that degree of encoding can depend on the amount of semantic-associative information available to support the inference processes. In the experiments, test words that express possible inferences from texts are presented for recognition. When testing is delayed, with other texts and test items intervening between a text and its test word, performance depends on the amount of semantic-associative information in the text. If the inferences represented by the test words are not supported by semantic associates in the text, they appear to be only minimally encoded (replicating McKoon & Ratcliff, 1986), but if they are supported by semantic associates, they are strongly encoded. With immediate testing, only 250 ms after the text, performance is shown to depend on semantic-associative information, not on textual information. This suggests that it is the fast availability of semantic information that allows it to support inference processes.

A bias interpretation of facilitation in perceptual identification

In a typical perceptual identification task, a word is presented for a few milliseconds and masked; then subjects are asked to report the word. It has been found that an earlier presentation of the test word will improve identification of the test word by as much as 30%. In addition, this facilitation has been shown to be preserved under amnesia. In this article we examine a fundamental question: Is the facilitation the result of bias toward the earlier presented item, an improvement in perceptual sensitivity, or both? The experiments presented here use a forced choice procedure to show that prior presentation of an item biases the subject to choose that item but does not improve discriminability. This result is obtained when the distractor items are visually similar to the target items. When distractors are dissimilar, earlier presentation affects neither bias nor discriminability. Two models of word identification are examined in light of the bias effects, and implications for understanding savings in amnesia are also examined.

Time course of item and associative information: Implications for global memory models

The time course of availability of associative and item information was examined by using a response signal procedure. Associative information discriminates between a studied pair of words and a pair with words from two different studied pairs. Item information is sufficient to discriminate between a studied pair and a pair not studied. In two experiments, discriminations that require associative information are delayed relative to those based on item information. Two additional experiments discount alternative explanations in terms of the time to encode the test items or task strategies. Examination of the global memory models of Gillund and Shiffrin (1984), Hintzman (1988), and Murdock (1982) shows that the models treat item and associative information inseparably. Modifications to these models which can produce separate contributions for item and associative information do not predict any difference in their availability. Two possible mechanisms for the delayed availability of associative information are considered: the involvement of recall in recognition and the time required to form a compound cue.

Contextually relevant aspects of meaning

A series of six experiments investigated whether inferences about contextually relevant aspects of meaning were encoded into memory during reading. In all the experiments, subjects studied short paragraphs. Then, test sentences were presented that expressed relevant aspects of meaning that had not been explicitly stated in the paragraphs. For example, for a paragraph about searching for the correct color to paint a picture of a tomato, a relevant aspect of meaning would be that tomatoes are red. The test sentences were presented either immediately following the relevant paragraph or after a delay. With immediate testing, it was argued that the facilitation obtained in verification latency could result from processes occurring either when the context was read or when the test sentence was verified. With delayed testing, evidence was found to support the hypothesis that contextually relevant aspects of meaning are incorporated into the memory representation of the paragraph, but such evidence was obtained only when the retrieval environment encouraged the use of newly learned information in the decision process on the test sentence.

More on the distinction between episodic and semantic memories

In reply to a critique of the episodic/semantic distinction (McKoon, Ratcliff, & Dell, 1986), Tulving (1986) argues that the usual rules for testing theories do not apply to the classification of phenomena with respect to memory systems. But we respond that the classification methods proposed by Tulving (1986) may be detrimental to the advancement of theory and that whole domains of inquiry should not be excluded from the usual criteria of experimental psychology.

Memory connections between thematically similar episodes

Recent theories about the representation of thematic information in memory propose that two episodes that share a theme are connected together through a thematic structure. We investigated the use of such cross-episode connections in comprehension and memory in six experiments. Experiments 1 and 2 used a priming technique; it was found that verification time for a test sentence from one story was speeded by an immediately preceding test sentence from a thematically similar story but only when subjects were given instructions to rate the similarities of the stories. In the remaining experiments, a single test sentence was presented immediately after a story was read, with timing controlled by presenting the story one word at a time. Response time for a test sentence from a previously read story was facilitated if the immediately preceding story was thematically similar, but only if the previously read story was extensively prestudied. We conclude that, during reading of an episode, thematic information may be encoded so as to lead to activation of similar episodes and formation of connections in memory between episodes, but such encoding is not automatic and depends on subjects' strategies and task difficulty.

A critical evaluation of the semantic-episodic distinction

Tulving (1983, 1984) has recently claimed that a wide range of evidence supports the distinction between episodic and semantic memory systems. He has provided a list of features to describe the differences between the two systems and a set of experimental results to demonstrate the distinction. In this article, we present opposing evidence that invalidates many of the distinguishing features and contradicts interpretations of the supporting experiments. In addition, we argue that the question of whether there are two separate memory systems cannot be answered without a specific theory about the differences between the systems.