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The Negative Compatibility Effect (NCE) is a reversal in priming effects that can occur when a masked arrow prime is followed by an arrow target at a long stimulus-onset asynchrony (SOA). To test the explanation that the NCE is actually a positive priming effect elicited by mask features associated with the prime-opposed response, we devise masks that always point in the same direction as the prime, eliminating all antiprime features. We find large positive priming effects for arrow primes without masks and for arrow masks without primes. When a neutral mask is introduced, priming effects turn negative at long SOAs. In the critical case where the mask is an arrow in the same direction as the prime, the prime does not add to the positive priming effect from the mask shape, but instead strongly diminishes it and induces response errors even though all stimuli point in the same direction. No such feature-free inhibition is seen when arrows are replaced by color stimuli. We conclude that even though response activation by stimulus features plays a role in the NCE, there is a strong inhibitory component (though perhaps not in all feature domains) that is not based on visual features.

It is easier to indicate the ink color of a color-neutral noun when it is presented in the color in which it had been shown frequently before, relative to print colors in which it had been shown less often. This phenomenon is known as color-word-contingency learning. It remains unclear whether participants actually learn semantic (word-color) associations or/and response (word-button) associations. We here present a novel variant of the paradigm that can disentangle semantic and response learning, because word-color and word-button associations are manipulated independently. In four experiments, each involving four daily sessions, novel words (pseudowords such as enas, fatu or imot) were probabilistically associated either with a particular color, a particular response-button position, or with both. Neutral trials were also included, and participants’ awareness of the contingencies was manipulated. The data showed no impact of explicit contingency awareness, but clear evidence both for response learning and for semantic learning, with effects emerging swiftly. Deeper processing of color information, with color words presented in black instead of color patches to indicate response-button positions, resulted in stronger effects, both for semantic and response learning. Our data add a crucial piece of evidence lacking so far in color-word contingency learning studies: Semantic learning effectively takes place even when associations are learned in an incidental way.

[This corrects the article DOI: 10.1371/journal.pone.0212714.].

Ulrich and Vorberg (Attention, Perception, & Psychophysics 71: 1219–1227, 2009) introduced a novel approach for estimating discrimination performance in two-alternative forced choice (2AFC) tasks. This approach avoids pitfalls that are inherent when the order of the standard and the comparison is neglected in estimating the difference limen (DL), as in traditional approaches. The present article provides MATLAB and R routines that implement this novel procedure for estimating DLs. These routines also allow to account for processing failures such as lapses or finger errors and can be applied to experimental designs in which the standard and comparison differ only along the task-relevant dimension, as well as to designs in which the stimuli differ in more than one dimension. In addition, Monte Carlo simulations were conducted to check the quality of our routines.

In two experiments, we studied the temporal dynamics of the response time effects of masked visual prime stimuli, as a function of stimulus eccentricity and size. Experiment 1 factorially varied prime-target congruency, eccentricity, and mask-target stimulus onset asynchrony. Early facilitative and late inhibitory effects of congruency were observed at all eccentricities, with temporal dynamics modulated by eccentricity. To test whether this dependence on eccentricity is due to cortical magnification, Experiment 2 varied stimulus size as well. Response inhibition time courses were influenced by size and eccentricity jointly, with no discernible difference when stimuli were matched for cortical magnification. Analysis of the individual time course data revealed that the timescale of inhibition changes with the strength of the cortical representation of the prime stimulus. This imposes constraints on possible models.

Visual stimuli may remain invisible but nevertheless produce strong and reliable effects on subsequent actions. How well features of a masked prime are perceived depends crucially on its physical parameters and those of the mask. We manipulated the visibility of masked stimuli and contrasted it with their influence on the speed of motor actions, comparing the temporal dynamics of visual awareness in metacontrast masking with that of action priming under the same conditions. We observed priming with identical time course for reportable and invisible prime stimuli, despite qualitative changes in the masking time course. Our findings indicate that experimental variations that modify the subjective visual experience of masked stimuli have no effect on motor effects of those stimuli in early processing. We propose a model that provides a quantitative account of priming effects on response speed and accuracy.

Visible persistence refers to the continuation of visual perception after the physical termination of a stimulus. We studied an extreme case of visible persistence by presenting two matrices of randomly distributed black and white pixels in succession. On the transition from one matrix to the second, the luminance polarity of all pixels within a disk- or annulus-shaped area reversed, physically creating a single second-order transient signal. This transient signal produces the percept of a disk or an annulus with an abrupt onset and a gradual offset. To study the nature of this fading percept we varied spatial parameters, such as the inner and the outer diameter of annuli (Experiment I) and the radius and eccentricity of disks (Experiment III), and measured the duration of visible persistence by having subjects adjust the synchrony of the onset of a reference stimulus with the onset or the offset of the fading percept. We validated this method by comparing two modalities of the reference stimuli (Experiment I) and by comparing the judgments of fading percepts with the judgments of stimuli that actually fade in luminance contrast (Experiment II). The results show that (i) irrespective of the reference modality, participants are able to precisely judge the on- and the offsets of the fading percepts, (ii) auditory reference stimuli lead to higher visible persistence durations than visual ones, (iii) visible persistence duration increases with the thickness of annuli and the diameter of disks, but decreases with the diameter of annuli, irrespective of stimulus eccentricity. These effects cannot be explained by stimulus energy, which suggests that more complex processing mechanisms are involved. Seemingly contradictory effects of disk and annulus diameter can be unified by assuming an abstract filling-in mechanism that speeds up with the strength of the edge signal and takes more time the larger the stimulus area is.

A stochastic model for synchronization with a metronome is analyzed which generalizes Wing and Kristofferson's (1973a, 1973b) two-level model for tapping. The model includes a motor component, a timekeeper component, and a linear phase error correction mechanism. The model's predictions of the asymptotic dependence structure of the synchronization errors and interresponse intervals are derived. Problems of identifiability and estimation of parameters are discussed.

Linear phase correction models for synchronized tapping and their stochastic properties are presented. In the most general form they include a central timer, a motor execution, and a phase correction mechanism that acts on the physical or the perceived asynchrony. A central issue of the article is how to identify and estimate the model parameters from the data. Monte Carlo simulations show serious problems of parameter interdependency.

We typically fixate targets such that they are projected onto the fovea for best spatial resolution. Macular degeneration patients often develop fixation strategies such that targets are projected to an intact eccentric part of the retina, called pseudofovea. A longstanding debate concerns which pseudofovea-location is optimal for non-foveal vision. We examined how pseudofovea position and eccentricity affect performance in visual search, when vision is restricted to an off-foveal retinal region by a gaze-contingent display that dynamically blurs the stimulus except within a small viewing window (forced field location). Trained normally sighted participants were more accurate when forced field location was congruent with the required scan path direction; this contradicts the view that a single pseudofovea location is generally best. Rather, performance depends on the congruence between pseudofovea location and scan path direction.

Zusammenfassung. Nur wenige Kriterien müssen berücksichtigt werden, wenn man ein statistisches Verfahren für eine gegebene Fragestellung bestimmen will. Wir stellen einen Entscheidungsbaum vor, mit dessen Hilfe sich für viele einfache Versuchspläne angemessene Verfahren ermitteln lassen. Der Entscheidungsbaum modifiziert einen früheren ( Vorberg, 1981 ), und enthält weitere statistische Verfahren sowie aktualisierte Literaturverweise. An zwei Beispielen demonstrieren wir seine Benutzung.

Most studies of synchronization have focused on how an established phase relationship between self-produced events (e.g., finger taps) and the clicks of a metronome is maintained when the metronome is regular or subject to unpredictable perturbations. Here we study how synchronization is initially established, using an experimental paradigm in which the metronome is activated after the subject has executed a series of self-paced finger taps. In Exp. 1, the metronome period was constant and equal to the mean of the self-paced inter-response intervals, whereas the initial phase difference of the metronome from the taps varied across trials. The synchronization error patterns could be predicted by a linear phase correction model. Experiment 2 involved both period and phase correction. The initial phase difference was constant, whereas the metronome period varied across trials. The observed synchronization error patterns suggest that the subjects achieved synchronization either by reacting to the second metronome signal or by aiming at the third metronome signal. The pattern of the residual synchronization errors was consistent with the linear phase correction model. These results support the notion that period and phase correction mechanisms are called for by different task variables and contribute differently to sensorimotor synchronization.

Reference EPFL-ARTICLE-147595doi:10.1007/s00426-006-0050-xView record in Web of Science Record created on 2010-03-23, modified on 2017-05-12

Control of relative timing is critical in ensemble music performance. We hypothesize that players respond to and correct asynchronies in tone onsets that arise from fluctuations in their individual tempos. We propose a first-order linear phase correction model and demonstrate that optimal performance that minimizes asynchrony variance predicts a specific value for the correction gain. In two separate case studies, two internationally recognized string quartets repeatedly performed a short excerpt from the fourth movement of Haydn's quartet Op. 74 no. 1, with intentional, but unrehearsed, expressive variations in timing. Time series analysis of successive tone onset asynchronies was used to estimate correction gains for all pairs of players. On average, both quartets exhibited near-optimal gain. However, individual gains revealed contrasting patterns of adjustment between some pairs of players. In one quartet, the first violinist exhibited less adjustment to the others compared with their adjustment to her. In the second quartet, the levels of correction by the first violinist matched those exhibited by the others. These correction patterns may be seen as reflecting contrasting strategies of first-violin-led autocracy versus democracy. The time series approach we propose affords a sensitive method for investigating subtle contrasts in music ensemble synchronization.

Inequalities on reaction time distribution functions for n -channel parallel models with an unlimited capacity assumption are presented, extending previous work on serial models. First, the first-terminating and the exhaustive cases are considered without any assumption on the distribution functions. Later, certain general stochastic dependence properties are introduced, and appropriate distribution inequalities are derived and illustrated by specific parametric examples. Moreover, the inequalities are extended to finite mixture distributions and to censoring.

Our ability to identify even complex scenes in rapid serial visual presentation (RSVP) is astounding, but memory for such items seems lacking. Rather than pictures, we used streams of more than 200 verbal stimuli, rushing by on the screen at a rate of more than 12 items per second while participants had to detect infrequent names (Experiments 1 and 2) or words written in capitals (Experiment 3). By direct and indirect tests, we investigated what is remembered of these masses of task-irrelevant distractor words and pseudowords embedded in an RSVP stream. Lexical decision, the indirect test applied either immediately after each stimulus train or with a delay, revealed strong long-term priming effects. Relative to stimuli not shown before, lexical decisions were faster and more accurate to words but slower to pseudowords. The size of these effects mirrored how often words and pseudowords had occurred in a stream, suggesting that memory traces are strengthened with successive presentations and survive for several minutes at least. Moreover, in a direct test (old-new categorization), words as well as pseudowords benefited from prior occurrence in an RSVP stream if they had occurred more than once. These findings parallel recent physiological and behavioral evidence for memory consolidation of distractor pictures in RSVP and highlight that, despite huge numbers of interfering stimuli, distractor words and pseudowords exhibit long-lasting memory effects. Consolidation seems to progress at higher cognitive levels at the same time that subsequent stimuli are perceptually processed.

Discrimination performance is often assessed by measuring the difference limen (DL; or just noticeable difference) in a two-alternative forced choice (2AFC) task. Here, we show that the DL estimated from 2AFC percentage-correct data is likely to systematically under- or overestimate true discrimination performance if order effects are present. We show how pitfalls with the 2AFC task may be avoided and suggest a novel approach for analyzing 2AFC data.

In their comment, Dell and O'Seaghdha (1991) adduced any effect on phonological probes for semantic alternatives to the activation of these probes in the lexical network. We argue that that interpretation is false and, in addition, that the model still cannot account for our data. Further­ more, and different from Dell and O'Seaghda, we adduce semantic rebound to the lemma level, where it is so substantial that it should have shown up in our data. Finally, we question the function of feedback in a lexical network (other than eliciting speech errors) and discuss Dell's (1988) notion of a unified production-comprehension system. Until recently, models of lexical access in speech production were almost exclusively based on speech error data. This is true both for the modular two-stage models and for the interactive connectionist models of lexical access. Both kinds of models were initially designed to account for the distributions of natu­ rally observed or experimentall y elicited speech errors. From the start, however, they were conceived as process models of normal speech production. Therefore, the ultimate test of such models cannot lie in their account of infrequent derailments of the process. Rather, the proof of their efficacy should be sought in the account of the normal process itself. An exclusively errorbased approach to lexical access in speech production is as ill-conceived as an exclusively illusion-based approach in vision research. One should, of course, hope that an ultimate theory of the normal process also has the potential of explaining ob­ served error distributions (or visual illusions, for that matter), but it should not be one's main concern.

Nijmegen University Nijmegen, The Netherlands Lexical access in object naming involves the activation of a set oflexical candidates, the selection of the appropriate (or target) item, and the phonological encoding of that item. Two views of lexical access in naming are compared. From one view, the 2-stage theory, phonological activation follows selection of the target item and is restricted to that item. From the other view, which is most explicit in activation-spreading theories, all activated lexical candidates are phonologically activated to some extent. A series of experiments is reported in which subjects performed acoustic lexical decision during object naming at different stimulus-onset asynchronies. The experiments show semantic activation of lexical candidates and phonological activation of the target item, but no phonological activation of other semantically activated items. This supports the 2-stage view. More- over, a mathematical model embodying the 2-stage view is fully compatible with the lexical deci- sion data obtained at different stimulus-onset asynchronies. One of a speaker's core skills is to lexicalize the concepts intended for expression. Lexicalization proceeds at a rate of two to three words per second in normal spontaneous speech, but doubling this rate is possible and not exceptional. The skill of lexicalizing a content word involves two components. The first one is to select the appropriate lexical item from among some tens of thousands of alternatives in the mental lexicon. The second one is to phonologically encode the selected item, that is, to retrieve its sound form, to create a phonological represen- tation for the item in its context, and to prepare its articulatory program. An extensive review of the literature on lexicalization can be found in Levelt (1989). This article addresses only one aspect of lexicalization, namely its time course. In particular, we examine whether the selection of an item and its phonologi- cal encoding can be considered to occur in two successive, non- overlapping stages. We acknowledge the invaluable contributions of John Nagengast and Johan Weustink, who programmed the computer-based experi- ments; ofGer Desserjer and Hans Fransen, who ran the experiments and assisted in data analysis; and of lnge Tarim, who provided graphi- cal assistance. We also acknowledge Gary Dell's and Picnic Zwitser- lood's detailed comments on an earlier version of this article, as well as the thorough comments of an anonymous reviewer. Herbert Schriefers is now at Freie Universit~it Berlin, Berlin, Federal Republic of Germany, and Thomas Pechmann is now at Universit~it des Saarlandes, Saarbriicken, Federal Republic of Germany. Correspondence concerning this article should be addressed to Wil- lem J. M. Levelt, Max-Planck-lnstitut for Psycholinguistik, Wundtlaan 1, NL-6525 XD Nijmegen, The Netherlands. 122 This is by no means a novel concept. One should rather say that it is the received view in the psycholinguistic literature (see especially Butterworth, 1980, 1989; Fromkin, 1971; Garrett, 1975, 1976, 1980; Kempen, 1977, 1978; Kempen & Huijbers, 1983; Levelt, 1983, 1989; Levelt & Maassen, 1981; Morton, 1969; Schriefers, Meyer, & Levelt, 1990). The first stage, lexical selection, makes available a semantically specified lexical item with its syntactic constraints. Kempen (1977, 1978) called this a lemma. Lemmas figure in grammatical encoding, specifically in the creation of syntactic frames. During the second stage, phonological encoding, phonological information is retrieved for each lemma. These phonological codes are used to create the articulatory plan for the utterance as a whole. Both Garrett (1976) and Kempen (1978), following Fry (1969), have stressed that the grammatical encoding and phonological encoding of an utterance normally run in parallel. Grammatical encoding, of which lexical selection is a proper part, is just slightly ahead of phonological encoding. The phonological encoding of a given item overlaps in time with the selection of a subsequent item. Only at the level of individual lexical items can one speak of successive stages. An item's semantic-syntactic makeup is accessed and used before its phonological makeup becomes available. Garrett (1975, 1976) argued for this separation of stages on the basis of speech error data. He distinguished between two classes of errors, word exchanges and sound exchanges, and could show that these classes differ in distributional properties. Word exchanges occur between phrases and involve words of the same syntactic category (as in this spring has a seat in it). Sound exchanges typically involve different category words in the same phrase (as in heft lemisphere). Word exchanges are

This article investigates properties of a representation based on the Rasch test model for reaction times (RT) that was proposed by Micko. Necessary and sufficient conditions for a set of RT distributions to be Rasch-representable are derived. It is shown that independent serial and independent parallel processing models cannot be reconciled with the representation. However, random extreme models compatible with the Reasch-representation exist that assume RT is determined by the longest or he shortest processing time of a random number of independent paraloel channels. Nonparametric properties of Rasch-representable distributions are derived that can be used for testing the model and for estimating its parameters. Conditions are presented for Rasch-representable distributions to form a scale family. Finally, Rasch-represent-able distributions are characterized interms of their hazard functions.

Forcing normal-sighted participants to use a distinct parafoveal retinal location for reading, we studied which part of the visual field is best suited to take over functions of the fovea during early stages of macular degeneration (MD). A region to the right of fixation lead to best reading performance and most natural gaze behavior, whereas reading performance was severely impaired when a region to the left or below fixation had to be used. An analysis of the underlying oculomotor behavior revealed that practice effects were accompanied by a larger number of saccades in text direction and decreased fixation durations, whereas no adjustment of saccade amplitudes was observed. We provide an explanation for the observed performance differences at different retinal locations based on the interplay of attention and eye movements. Our findings have important implications for the development of training methods for MD patients targeted at reading, suggesting that it would be beneficial for MD patients to use a region to the right of their central scotoma.

A frequent approach to study interactions of the auditory and the visual system is to measure event-related potentials (ERPs) to auditory, visual, and auditory-visual stimuli (A, V, AV). A nonzero result of the AV − (A + V) comparison indicates that the sensory systems interact at a specific processing stage. Two possible biases weaken the conclusions drawn by this approach: first, subtracting two ERPs from one requires that A, V, and AV do not share any common activity. We have shown before (Gondan and Röder in Brain Res 1073–1074:389–397, 2006) that the problem of common activity can be avoided using an additional tactile stimulus (T) and evaluating the ERP difference (T + TAV) − (TA + TV). A second possible confound is the modality shift effect (MSE): for example, the auditory N1 is increased if an auditory stimulus follows a visual stimulus, whereas it is smaller if the modality is unchanged (ipsimodal stimulus). Bimodal stimuli might be affected less by MSEs because at least one component always matches the preceding trial. Consequently, an apparent amplitude modulation of the N1 would be observed in AV. We tested the influence of MSEs on auditory-visual interactions by comparing the results of AV − (A + V) using (a) all stimuli and using (b) only ipsimodal stimuli. (a) and (b) differed around 150 ms, this indicates that AV − (A + V) is indeed affected by the MSE. We then formally and empirically demonstrate that (T + TAV) − (TA + TV) is robust against possible biases due to the MSE.

To demonstrate unconscious cognition, researchers commonly compare a direct measure (D) of awareness for a critical stimulus with an indirect measure (I) showing that the stimulus was cognitively processed at all. We discuss and empirically demonstrate three types of dissociation with distinct appearances in D-I plots, in which direct and indirect effects are plotted against each other in a shared effect size metric. Simple dissociations between D and I occur when I has some nonzero value and D is at chance level; the traditional requirement of zero awareness is necessary for this criterion only. Sensitivity dissociations only require that I be larger than D; double dissociations occur whensome experimental manipulation has opposite effects on I and D. We show that double dissociations require much weaker measurement assumptions than do other criteria. Several alternative approaches can be considered special cases of our framework.

Two experiments investigated subliminal semantic and form priming in a word-completion task. Visual gap-words with a dominant and a subordinate solution were preceded by form-related or by semantically related words, which were briefly presented and sandwich-masked. Priming of the subordinate solution was assessed in Experiment 1, relative to a neutral condition. Both solutions were primed in Experiment 2. In the absence of conscious prime recognition, both semantic and form primes reliably increased the probability with which the primed solution was given. With our variant of fragment-completion, response priming can be ruled out as explanation. Moreover, effects were already present at first presentation, excluding an interpretation in terms of partial awareness due to massive repetition. The data demonstrate automatic activation at both form and semantic levels in the absence of conscious awareness.

Wing and Kristofferson (1973) have shown that temporal precision in self-paced tapping is limited by variability in a central timekeeper and by variability arising in the peripheral motor system. Here we test an extension of the Wing–Kristofferson model to synchronization with periodic external events that was proposed by Vorberg and Wing (1994). In addition to the timekeeper and motor components, a linear phase correction mechanism is assumed which is triggered by the last or the last two synchronization errors. The model is tested in an experiment that contrasts synchronized and self-paced trapping, with response periods ranging from 200–640 ms. The variances of timekeeper and motor delays and the error correction parameters were estimated from the auto-covariance functions of the inter-response intervals in continuation and the asynchronies in synchronization. Plausible estimates for all parameters were obtained when equal motor variance was assumed for synchronization and continuation. Timekeeper variance increased with metronome period, but more steeply during continuation than during synchronization, suggesting that internal timekeeping processes are stabilized by periodic external signals. First-order error correction became more important as the metronome period increased, whereas the contribution of second-order error correction decreased. It is concluded that the extended two-level model accounts well for both synchronization and continuation performance.

Publisher Summary This chapter presents a tutorial approach to the quantitative models of the origin and nature of temporal variability in movement sequences. Sometimes psychologists appear to approach behavior with the aim of explaining variability away. The approach to timing mechanisms differs in that the variability of timing is taken to be a feature of interest in its own right. The chapter focuses on departures from random variability evident in intervals defined by psychologically significant events or responses occurring in sequences of movement. The patterns of statistical dependence in the inter-response intervals can provide important clues to the functional arrangement of a number of underlying independent processes, each contributing a degree of variability. The chapter discusses the basic framework based on a two-level partitioning of variability in timing that distinguishes between central timing processes and mechanisms involved in the execution of movement. The chapter describes certain tools of probability and statistics that are needed in deriving the properties of the models considered and in relating their predictions to data. The statistical issues of estimation and testing the model is discussed in the chapter. The chapter illustrates the way a simple feedback scheme for phase correction may be developed within the model and generalizes the basic model to allow more complex forms of central timekeeping.

In normal vision, shifts of attention and gaze are tightly coupled. Here we ask if this coupling affects performance also when central vision is not available. To this aim, we trained normal-sighted participants to perform a visual search task while vision was restricted to a gaze-contingent viewing window ("forced field location") either in the left, right, upper, or lower visual field. Gaze direction was manipulated within a continuous visual search task that required leftward, rightward, upward, or downward eye movements. We found no general performance advantage for a particular part of the visual field or for a specific gaze direction. Rather, performance depended on the coordination of visual attention and eye movements, with impaired performance when sustained attention and gaze have to be moved in opposite directions. Our results suggest that during early stages of central visual field loss, the optimal location for the substitution of foveal vision does not depend on the particular retinal location alone, as has previously been thought, but also on the gaze direction required by the task the patient wishes to perform.

Was zeichnet Experten vor Anfangern aus, was macht sie zu erfolgreicheren Problemlosern? Welches Wissen liegt ihrem Expertentum zugrunde und wie kann es zielfuhrend beim Losen neuer Probleme eingesetzt werden? Weshalb ist die Uberlegenheit von Experten relativ eng auf den Gegenstandsbereich ihres Expertentums eingeschrankt?

A generalization of W. J. McGill's (1963, Stochastic latency mechanisms. In R. D. Luce, R. B. Bush, & E. Galanter (Eds.), Handbook of mathematical psychology (Vol. 1). New York: Wiley) random search model is examined in which the items in the search area are allowed to vary with respect to accessability. In order to analyze the model, it is equivalently described as a serial and as a parallel model. Predictions are derived for the means and variances of the interresponse times and of the number of recalls by time t. The predictions are compared to those made by the individual differences generalization of McGill's model proposed by D. G. Morrison (1979) , Journal of Mathematical Psychology, 19, 307–315). In a final section, maximum likelihood estimation of parameters is discussed.

Several multicomponent models of memory (Bower, 1967) are presented and applied to the retention of three-digit numbers. Their application is based on the notion that both the digits making up a number stimulus and the position in which they appear can be interpreted as the components of a ‘memory vector’. The models are fitted to the data obtained in a Peterson-type experiment in which a single three-digit number was shown per trial. During the retention interval the subject engaged in an arithmetical task for either 13.6, 19.6, or 25.6 s. Following this, the subject attempted recall. Confidence ratings were also obtained. Twenty-four different types of correctly and incorrectly recalled responses were scored. Their frequency distributions were best predicted at each retention interval by a ‘dual encoding’ model which relies on the assumption that stimuli are stored both component-wise and by means of a single ‘unit code’ encoding the entire stimulus number. It is also shown that the confidence ratings may be successfully predicted from estimates of the expected number of components retained. About 80% of the rating variance is predicted by a parameter-free procedure.

In this paper we present a simple stochastic model for the study of timing of skilled performance. The model is based on the notion that temporal variability and serial dependence observed in behavior sequences depend on the precision of a hypothetical central timing system as well as on the temporal jitter generated by the executing motor system. Within the theoretical framework of such a model it is possible to determine, for example, how much of the temporal variability in skilled performance is generated centrally, and how much is due to the precision lost during the execution of the motor program by the motor system. This general approach has been used first by Wing and Kristofferson’ in their study of timing in simple tapping situations. We have proposed extensions of the basic model to repetitive rhythmic tapping; our goal was to determine whether rhythmic performance is controlled by hierarchically organized timing mechanisms, as contrasted to simple serial mechanisms.* Analyses of actual musical performance in terms of such a model have been reported by Shaffer.’ A major problem with these models is that they rest on rather strong assumptions which are not easily accessible to empirical test. Of course, the conclusions reached from applications of the models remain dubious if the validity of the underlying assumptions can be questioned. In the following, we show how to overcome some of these problems by extending the model to synchronous two-handed performance. The model we propose rests on weaker and testable assumptions; moreover, it permits rather strong inferences about the structure of the postulated timing system given that the model’s validity can be established. It should be noted that several independent investigators have discovered recently how powerful the analysis of two-handed performance is in the study of timing and motor c o n t r ~ l . ~ . ~ In this paper we focus on the properties of the central timing structures underlying synchronous rhythm production; in a companion paper’ we investigate the statistical aspects of the motor system in more detail. The outline of the paper is as follows. First, we present the model and derive a testable prediction which is checked on data from an experiment involving two-handed rhythmic tapping. The model is then used as a

Direct tests of One-Stage and Two-Stage theories of discrimination learning and concept identification by means of shift experiments yield ambiguous results due to interpretational problems. These difficulties can be avoided by analysing the original acquisition data. A test of the theories is suggested for concept identification experiments which is based on an analysis of the dependence between appropriately constructed response subsequences. Predictions for the theoretical alternatives are derived from a generalized all-or-none model. Results from a concept identification study with adult subjects reject One-Stage theories and support Two-Stage theories.

Reece's (1976) generalization of the Wing-Kristofferson model for the timing of interresponse intervals is critically examined. A correct analysis of the model is presented. The crucial parameter of the model which determines the degree of dependence between the time-keeper and motor-delay processes is shown to be non-identifiable. A statistical problem concerning the estimation of the autocovariance function is pointed out, and a solution is proposed.

Four-category conjunctive concept problems were learned under nine different conditions with the number of irrelevant dimensions and the percentage of correct feedback as independent variables. Concept identification performance could be adequately predicted by assuming that the two relevant dimensions defining the conjunctive problems were learned concurrently and independently. A generalized version of the Bower-Trabasso theory of concept identification, encompassing a distinct response learning phase, was applied to the data. The theory adequately described performance under conditions with consistently correct feedback. Several alternative models for learning under misinformative feedback conditions were developed from the theory. Using parameter estimates from data under correct feedback conditions, these models were found to predict performance under misinformative feedback fairly well.