The resurrection of Tweedledum and Tweedledee: Bimodality cannot distinguish serial and parallel processes

Simultaneously presented signals may be processed in serial or in parallel. One potentially valuable indicator of a system’s characteristics may be the appearance of multimodality in the response time (RT) distributions. It is known that standard serial models can predict multimodal RT distributions, but it is unknown whether multimodality is diagnostic of serial systems, or whether alternative architectures, such as parallel ones, can also make such predictions. We demonstrate via simulations that a multimodal RT distribution is not sufficient by itself to rule out parallel self-terminating processing, even with limited trial numbers. These predictions are discussed within the context of recent data indicating the existence of multimodal distributions in visual search.     

Rasch-representable reaction time distributions

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.For helpful discussions, we are grateful to Hans Irtel, Christoph Micko, Hartmann Scheiblechner, and Hans-Henning Schultz.     

The temporal dynamics of visual search: evidence for parallel processing in feature and conjunction searches

Feature and conjunction searches have been argued to delineate parallel and serial operations in visual processing. The authors evaluated this claim by examining the temporal dynamics of the detection of features and conjunctions. The 1st experiment used a reaction time (RT) task to replicate standard mean RT patterns and to examine the shapes of the RT distributions. The 2nd experiment used the response-signal speed-accuracy trade-off (SAT) procedure to measure discrimination (asymptotic detection accuracy) and detection speed (processing dynamics). Set size affected discrimination in both feature and conjunction searches but affected detection speed only in the latter. Fits of models to the SAT data that included a serial component overpredicted the magnitude of the observed dynamics differences. The authors concluded that both features and conjunctions are detected in parallel. Implications for the role of attention in visual processing are discussed.     

Further evidence for a time-independent shift of the focus of attention

The separation between stimuli was manipulated in a same-different matching task. In Experiment 1, stimuli were upright Ts and/or Ls, whereas in Experiment 2, they were rotated Ts and/or Ls. In both experiments, mean reaction time (RT) for the same-different judgment did not increase as a function of interletter separation, suggesting either that the time needed to relocate attention was independent of distance, or that the stimuli were processed in parallel. These alternatives were tested in a third experiment, with a diagnostic for parallel processing proposed by Egeth and Dagenbach (1991). The diagnostic indicated that the rotated Ts and Ls in Experiment 2 were processed serially. If serial processing implies the utilization of attention, then the results of Experiment 2 suggest that relocation of attention is time-invariant with respect to distance.     

Response latencies in visual search involving redundant or irrelevant information

Three properties of models for comparison of multiattribute visual stimuli were considered: parallel vs serial processing, efficient vs exhaustive comparisons, and biased vs unbiased acceptance of attributes for processing. Ss performed two comparison tasks, matching-to-sample (M) and identification of odd stimuli (0), with color and form attributes presented singly and in redundant and nonredundant pairings. Analyses of means and of total distributions of response latencies supported the conclusion that parallel and efficient comparisons were the rule, along with a kind of partial selection of attribute to be processed. Ss differed in their relative speed of processing form and color attributes, and these differences accounted for most, but not all, of the differences among them in processing multiattribute stimuli.     

Attentional control in visual signal detection: effects of abrupt-onset and no-onset stimuli

The attention literature distinguishes two general mechanisms by which attention can benefit performance: gain (or resource) models and orienting (or switching) models. In gain models, processing efficiency is a function of a spatial distribution of capacity or resources; in orienting models, an attentional spotlight must be aligned with the stimulus location, and processing efficiency is a function of when this occurs. Although they involve different processing mechanisms, these models are difficult to distinguish empirically. We compared performance with abrupt-onset and no-onset Gabor patch stimuli in a cued detection task in which we obtained distributions of reaction time (RT) and accuracy as a function of stimulus contrast. In comparison to abrupt-onset stimuli, RTs to miscued no-onset stimuli were increased and accuracy was reduced. Modeling the data with the integrated system model of Philip L. Smith and Roger Ratcliff (2009) provided evidence for reallocation of processing resources during the course of a trial, consistent with an orienting account. Our results support a view of attention in which processing efficiency depends on a dynamic spatiotemporal distribution of resources that has both gain and orienting properties.     

Reaction times for comparisons of successively presented visual patterns: Evidence for serial self-terminating search

Two-choice classification RTs were collected for eight conditions designed to vary the number of comparisons necessary between one or two visual patterns in perception and one or two in short-term memory (STM). Overall RT data supported both a serial self-terminating and parallel self-terminating model with distributed search times, while rejecting corresponding exhaustive models. Precise predictions for the parallel model proved difficult to derive; however, the serial model predicted the fine detail of the data surprisingly well. RTs suggested that Ss searched through all stimuli in memory first and that stimuli in both memory and perception were searched from right to left. Comparison times between identical stimuli were estimated to be longer than comparison times between different stimuli. Error rates increased with the number of hypothesized comparisons; predicted error rates, based on independence of rates within stages, also increased but failed to predict the empirical error rates very well.     

Interhemispheric categorization of pictures and words

Earlier studies suggest that interhemispheric processing increases the processing power of the brain in cognitively complex tasks as it allows the brain to divide the processing load between the hemispheres. We report two experiments suggesting that this finding does not generalize to word-picture pairs: they are processed at least as efficiently when processed by a single hemisphere as compared to processing occurring between the two hemispheres. We examined whether dividing the stimuli between the visual fields/hemispheres would be more advantageous than unilateral stimulus displays in the semantic categorization of simultaneously presented pictures, words, and word-picture pairs. The results revealed that within-domain stimuli (semantically related picture pairs or word pairs) were categorized faster in bilateral than in unilateral displays, whereas cross-domain stimuli (word-picture pairs) were not categorized faster in bilateral than in unilateral displays. It is suggested that interhemispheric sharing of word-picture stimuli is not advantageous as compared to unilateral processing conditions because words and pictures use different access routes, and therefore, it may be possible to process in parallel simultaneously displayed word-picture stimuli within a single hemisphere.     

Varieties of Binocular Interaction in Human Vision

Binocular processing was investigated using a quantitative, process-oriented metatheory of response times. The analyses are not confined to particular distributional assumptions or specific models. Upper and lower performance boundaries for probability summation in parallel processing are defined and compared with observed distributions of reaction times using a variety of dichoptic stimuli. Performance that exceeds the upper bound strongly suggests faciliatory convergence between the two eyes (binocular channel summation). Performance below the lower bound suggests that inputs to the two eyes are processed serially. The results indicate that binocular channel summation in subjects with normal stereo vision requires targets of the same luminance polarity (paired increments or decrements) in corresponding retinal locations. When corresponding retinal locations are stimulated with opposing luminance polarities (increment to one eye, decrement to the other), performance is consistent with probability summation, indicating that parallel ON and OFF pathways remain segregated at least to the level of binocular fusion. Further analyses of data from a stereo-blind observer suggests serial processing of binocular inputs.     

A stochastic theory of matching processes

A theory of matching processes is developed within which serial models and parallel models based on within-stage independent intercompletion times are defined. These models are then specialized to a class of processes possessing exponential intercompletion time densities and equivalence properties of parallel and serial models within this class are investigated. Nonequivalence theorems are proved that designate possible differences in “same” (+) and different (?) matching rates as important in testing parallel and serial models. An experimental paradigm is then derived with the property that if + rates are different from — rates, and if processing is self-terminating, then the parallel and serial models are distinguishable at the level of mean reaction times. The serial class of models that is tested by this paradigm includes a large number of stochastic distributions whose central assumption is additivity of element processing times. The corresponding parallel class of models is currently limited to those assuming exponential intercompletion times. A numerical example and an example of non-parametric relations predicted by the serial or parallel models are given. Some advantages and limitations of the present treatment are discussed.     

Parallel versus serial processing and individual differences in high-speed search in human memory

Many mental tasks that involve operations on a number of items take place within a few hundred milliseconds. In such tasks, whether the items are processed simultaneously (in parallel) or sequentially (serially) has long been of interest to psychologists. Although certain types of parallel and serial models have been ruled out, it has proven extremely difficult to entirely separate reasonable serial and limited-capacity parallel models on the basis of typical data. Recent advances in theory-driven methodology now permit strong tests of serial versus parallel processing in such tasks, in ways that bypass the capacity issue and that are distribution and parameter free. We employ new methodologies to assess serial versus parallel processing and find strong evidence for pure serial or pure parallel processing, with some striking apparent differences across individuals and interstimulus conditions.     

Extending the simultaneous-sequential paradigm to measure perceptual capacity for features and words

In perception, divided attention refers to conditions in which multiple stimuli are relevant to an observer. To measure the effect of divided attention in terms of perceptual capacity, we introduce an extension of the simultaneous-sequential paradigm. The extension makes predictions for fixed-capacity models as well as for unlimited-capacity models. We apply this paradigm to two example tasks, contrast discrimination and word categorization, and find dramatically different effects of divided attention. Contrast discrimination has unlimited capacity, consistent with independent, parallel processing. Word categorization has a nearly fixed capacity, consistent with either serial processing or fixed-capacity, parallel processing. We argue that these measures of perceptual capacity rely on relatively few assumptions compared to most alternative measures.     

Selective attention to multidimensional auditory stimuli

Event-related brain potentials (ERPs) elicited by multidimensional auditory stimuli were recorded from the scalp in a selective-attention task. Subjects listened to tone pips varying orthogonally between two levels each of pitch, location, and duration and responded to longer duration target stimuli having specific values of pitch and location. The discriminability of the pitch and location attributes was varied between groups. By examining the ERPs to tones that shared pitch and/or locational cues with the designated target, we inferred interrelationships among the processing of these attributes. In all groups, stimuli that failed to match the target tone in an easily discriminable cue elicited only transitory ERP signs of selective processing. Tones sharing the "easy" but not the "hard" cue with the target elicited ERPs that indicated more extensive processing, but not as extensive as stimuli sharing both cues. In addition, reaction times and ERP latencies to the designated targets were not influenced by variations in the discriminability of pitch and location. This pattern of results is consistent with parallel, self-terminating models and holistic models of processing and contradicts models specifying either serial or exhaustive parallel processing of these dimensions. Both the parallel, self-terminating models and the holistic models provide a generalized mechanism for hierarchical stimulus selections that achieve an economy of processing, an underlying goal of classic, multiple-stage theories of selective attention.     

Display size effects in visual search: Analyses of reaction time distributions as mixtures

In a reanalysis of data from Cousineau and Shiffrin (2004) and two new visual search experiments, we used a likelihood ratio test to examine the full distributions of reaction time (RT) for evidence that the display size effect is a mixture-type effect that occurs on only a proportion of trials, leaving RT in the remaining trials unaffected, as is predicted by serial self-terminating search models. Experiment 1 was a reanalysis of Cousineau and Shiffrin's data, for which a mixture effect had previously been established by a bimodal distribution of RTs, and the results confirmed that the likelihood ratio test could also detect this mixture. Experiment 2 applied the likelihood ratio test within a more standard visual search task with a relatively easy target/distractor discrimination, and Experiment 3 applied it within a target identification search task within the same types of stimuli. Neither of these experiments provided any evidence for the mixture-type display size effect predicted by serial self-terminating search models. Overall, these results suggest that serial self-terminating search models may generally be applicable only with relatively difficult target/distractor discriminations, and then only for some participants. In addition, they further illustrate the utility of analysing full RT distributions in addition to mean RT.     

Variables influencing the mode of processing of complex stimuli

The present study attempts to specify some of the conditions under which parallel and serial processing may occur. The three variables studied were (1) type of task, (2) relative set for speed vs accuracy, and (3) practice. Pairs of multidimensional, geometric stimuli were presented either simultaneously or successively to S who was required to indicate whether they were the same or different. Each S participated in nine sessions. For half of the Ss speed was emphasized, and for the other half accuracy was emphasized. The results indicated that: (1) responses were faster with successive presentation than with simultaneous presentation; (2) with successive presentation, processing was serial; (3) in the simultaneous presentation condition, a gradual shift from serial to parallel processing occurred with practice; and (4) the speed and accuracy instructions used in this experiment produced no differential effects on latency or errors.     

Redundancy phenomena are affected by response requirements

Results are reported for two go/no-go reaction time (RT) experiments, in which the redundant targets advantage was investigated. These experiments were replications of two earlier choice reaction time (CRT) experiments, in which letter stimuli were used. Important differences between the go/no-go RT experiments and the CRT experiments were obtained. Equal and significant redundancy advantages were obtained whether redundant targets were compared with a single target presented with a noise letter or without noise. In the CRT experiments, the advantage was not obtained in the comparison with a single target presented alone. Noise letters did not slow the RTs to single targets with which they were presented as was the case with CRT. Since the differing results of the two procedures depend on the response requirements, explanation of differing CRT data in terms of perceptual or attentional concepts is probably inappropriate. The presence and absence of response competition in the two situations may be the best interpretation. The results tend to support a conclusion of the parallel processing of two letter stimuli separated spatially by as much as 3 degrees.     

Obsessive-compulsive tendencies are associated with a focused information processing strategy

The study examined the hypothesis that obsessive–compulsive (OC) tendencies are related to a reliance on focused and serial rather than a parallel, speed-oriented information processing style. Ten students with high OC tendencies and 10 students with low OC tendencies performed the flanker task, in which they were required to quickly classify a briefly presented target letter (S or H) that was flanked by compatible (e.g., SSSSS) or incompatible (e.g., HHSHH) noise letters. Participants received 4 blocks of 100 trials each, two with 50% compatible trials and two with 80% compatible trials and were informed of the probability of compatible trials before the beginning of each block. As predicted, high OC participants, as compared to low OC participants, had slower overall reaction time (RT) and lower tendency for parallel processing (defined as incompatible trials RT minus compatible trials RT). Low, more than high OC participants tended to adjust their focused/parallel processing including a shift towards parallel processing in blocks with 80% compatible trials and in trials following compatible trials. Implications of these results to the cognitive theory and therapy of OCD are discussed.     

Parallel processing and the psychological refractory period

The serial processor theory proposed by Welford is the traditional explanation of the delay in responding to the second stimulus presented in psychological refractory period paradigm experiments. This paper reviews results which do not accord either with that theory or with later modifications of it by Smith and Tolkmitt. To explain these results a model is proposed which contains three propositions: (a) parallel processing of concurrent stimuli, (b) fixed total processing capacity, (c) capacity allocation either dependent on the relative difficulty of the stimuli or set by the subject according to experimental instructions. The predictions of this model for the effects of stimulus difficulty and interstimulus interval on reaction time and interresponse interval are computed. These predictions are compared with those of other parallel processing models of the psychological refractory period which have been proposed by Herman and Kantowitz, by Kahneman and by Keele.     

Visual search time based on stochastic serial and parallel processings

Two stochastic latency models were developed in order to account for random fluctuations of the search time that the S consumed when he was asked to search for a single target number or one of two (or three) target numbers through the sets of small two-digit numbers randomly distributed on a white square. It was shown that the search time distribution for a single target number can be adequately described in terms of McGill’s two-step serial processing model. The parallel processing model whose structure is exponential was found to fit approximately the search time distributions for the first target number that the S detects among the two (or three) target numbers predesignated. This model, however, does not satisfactorily handle the long search times.     

Intrusion patterns in rapid serial visual presentation tasks with two response dimensions.

A rapid serial visual presentation (RSVP) paradigm using both one and two response dimensions was used to test parallel processing models of stimulus dimensions. Fifty subjects were asked to report the identity and/or color of a target uppercase word inserted in a series of lowercase words. The results produced a predominance of posttarget intrusions for color responses and a predominance of pretarget intrusions for identity responses. The requirement of a response to a second dimension impaired hit rates but did not change the pattern of intrusions. An examination of the distributions of intrusions in each response dimension as a function of the response given to the other dimension showed an unexpectedly high percentage of simultaneous hits, a moderate covariation between both responses, and the same patterns of intrusions when compared with the general distributions. While these results seem to be compatible with parallel models of processing for stimulus dimensions, two modifications to this model are suggested. First, the processing of response dimension(s) needs some attentional resources. Second, provision for a mixed model is indicated, which would include trials where no illusory conjunctions are formed.