Temporal dynamics of sequential motor activation in a dual-prime paradigm: Insights from conditional accuracy and hazard functions.

In response priming experiments, a participant has to respond as quickly and as accurately as possible to a target stimulus preceded by a prime. The prime and the target can either be mapped to the same response (consistent trial) or to different responses (inconsistent trial). Here, we investigate the effects of two sequential primes (each one either consistent or inconsistent) followed by one target in a response priming experiment. We employ discrete-time hazard functions of response occurrence and conditional accuracy functions to explore the temporal dynamics of sequential motor activation. In two experiments (small-N design, 12 participants, 100 trials per cell and subject), we find that (1) the earliest responses are controlled exclusively by the first prime if primes are presented in quick succession, (2) intermediate responses reflect competition between primes, with the second prime increasingly dominating the response as its time of onset is moved forward, and (3) only the slowest responses are clearly controlled by the target. The current study provides evidence that sequential primes meet strict criteria for sequential response activation. Moreover, it suggests that primes can influence responses out of a memory buffer when they are presented so early that participants are forced to delay their responses.

Studying the dynamics of visual search behavior using RT hazard and micro-level speed-accuracy tradeoff functions: A role for recurrent object recognition and cognitive control processes.

Thanks to the work of Anne Treisman and many others, the visual search paradigm has become one of the most popular paradigms in the study of visual attention. However, statistics like mean correct response time (RT) and percent error do not usually suffice to decide between the different search models that have been developed. Recently, to move beyond mean performance measures in visual search, RT histograms have been plotted, theoretical waiting time distributions have been fitted, and whole RT and error distributions have been simulated. Here we promote and illustrate the general application of discrete-time hazard analysis to response times, and of micro-level speed-accuracy tradeoff analysis to timed response accuracies. An exploratory analysis of published benchmark search data from feature, conjunction, and spatial configuration search tasks reveals new features of visual search behavior, such as a relatively flat hazard function in the right tail of the RT distributions for all tasks, a clear effect of set size on the shape of the RT distribution for the feature search task, and individual differences in the presence of a systematic pattern of early errors. Our findings suggest that the temporal dynamics of visual search behavior results from a decision process that is temporally modulated by concurrently active recurrent object recognition, learning, and cognitive control processes, next to attentional selection processes.

Analyzing Response Times and Other Types of Time-to-Event Data Using Event History Analysis: A Tool for Mental Chronometry and Cognitive Psychophysiology.

In this Methods article, we discuss and illustrate a unifying, principled way to analyze response time data from psychological experiments-and all other types of time-to-event data. We advocate the general application of discrete-time event history analysis (EHA) which is a well-established, intuitive longitudinal approach to statistically describe and model the shape of time-to-event distributions. After discussing the theoretical background behind the so-called hazard function of event occurrence in both continuous and discrete time units, we illustrate how to calculate and interpret the descriptive statistics provided by discrete-time EHA using two example data sets (masked priming, visual search). In case of discrimination data, the hazard analysis of response occurrence can be extended with a microlevel speed-accuracy trade-off analysis. We then discuss different approaches for obtaining inferential statistics. We consider the advantages and disadvantages of a principled use of discrete-time EHA for time-to-event data compared to (a) comparing means with analysis of variance, (b) other distributional methods available in the literature such as delta plots and continuous-time EHA methods, and (c) only fitting parametric distributions or computational models to empirical data. We conclude that statistically controlling for the passage of time during data analysis is equally important as experimental control during the design of an experiment, to understand human behavior in our experimental paradigms.

Neuropsychological evidence for the temporal dynamics of category-specific naming.

Multiple accounts have been proposed to explain category-specific recognition impairments. Some suggest that category-specific deficits may be caused by a deficit in recurrent processing between the levels of a hierarchically organized visual object recognition system. Here, we tested predictions of interactive processing theories on the emergence of category-selective naming deficits in neurologically intact observers and in patient GA, a single case showing a category-specific impairment for natural objects after a herpes simplex encephalitis infection. Fragmented object outlines were repeatedly presented until correct naming occurred (maximum 10 times), and the fragments increased in length with every repetition. We studied how shape complexity, object category, and fragment curvature influence the timing of correct object identification. The results of a survival analysis are consistent with the idea that deficits in recurrent processing between low- and high-level visual object representations can cause category-selective impairments.

What Is Shaping RT and Accuracy Distributions? Active and Selective Response Inhibition Causes the Negative Compatibility Effect.

Inhibitory control such as active selective response inhibition is currently a major topic in cognitive neuroscience. Here we analyze the shape of behavioral RT and accuracy distributions in a visual masked priming paradigm. We employ discrete time hazard functions of response occurrence and conditional accuracy functions to study what causes the negative compatibility effect (NCE)-faster responses and less errors in inconsistent than in consistent prime target conditions-during the time course of a trial. Experiment 1 compares different mask types to find out whether response-relevant mask features are necessary for the NCE. After ruling out this explanation, Experiment 2 manipulates prime mask and mask target intervals to find out whether the NCE is time-locked to the prime or to the mask. We find that (a) response conflicts in inconsistent prime target conditions are locked to target onset, (b) positive priming effects are locked to prime onset whereas the NCE is locked to mask onset, (c) active response inhibition is selective for the primed response, and (d) the type of mask has only modulating effects. We conclude that the NCE is neither caused by automatic self-inhibition of the primed response due to backward masking nor by updating response-relevant features of the mask, but by active mask-triggered selective inhibition of the primed response. We discuss our results in light of a recent computational model of the role of the BG in response gating and executive control.

Impaired identification of impoverished animate but not inanimate objects in adults with high-functioning autism spectrum disorder.

The ability to identify animate and inanimate objects from impoverished images was investigated in adults with high-functioning autism spectrum disorder (HFA) and in matched typically developed (TD) adults, using a newly developed task. Consecutive frames were presented containing Gabor elements that slightly changed orientation from one frame to the next. For a subset of elements, the changes were such that these elements gradually formed the outline of an object. Elements enclosed within the object's outline gradually adopted one and the same orientation, outside elements adopted random orientations. The subjective experience was that of an object appearing out of a fog. The HFA group required significantly more frames to identify the impoverished objects than the TD group. Crucially, this difference depended on the nature of the objects: the HFA group required significantly more frames to identify animate objects, but with respect to the identification of inanimate objects the groups did not differ. The groups also did not differ with respect to the number and type of incorrect guesses they made. The results suggest a specific impairment in individuals with HFA in identifying animate objects. A number of possible explanations are discussed.

Time course of spatial contextual interference: event history analyses of simultaneous masking by nonoverlapping patterns.

Simultaneous masking refers to the impairment of performance on a visual target by simultaneously presented flankers. Whereas the spatial aspects of simultaneous masking have been studied extensively, the time course of these spatial influences is much less well understood. We here measure response latency and accuracy in a simultaneous masking paradigm and apply event history analysis to study the time course of target-flanker interactions. In our experiments, we presented a central target vernier flanked on both sides by 12 aligned distractor verniers that were either shorter, longer, or equal in length (Experiment 1), and that also were congruent or incongruent in their spatial offset with the target (Experiment 2). Response time distributions showed that there were more fast responses when the target was flanked by short flankers. Conditional accuracy functions showed that accuracy of responses dropped when the flankers had the same length as the target, but only for slow responses. These results are at odds with accounts based solely on lateral neural interactions or response competition, and instead suggest that top-down visual object-to-feature interference occurs when the target is not selected fast enough, congruent with object substitution theory.

Disturbed interplay between mid- and high-level vision in ASD? Evidence from a contour identification task with everyday objects.

Atypical visual processing in children with autism spectrum disorder (ASD) does not seem to reside in an isolated processing component, such as global or local processing. We therefore developed a paradigm that requires the interaction between different processes-an identification task with Gaborized object outlines-and applied this to two age groups of 6-to-10 and 10-to-14 year old children with and without ASD. Event history analyses demonstrated an identification disadvantage in the ASD group, which remained quite stable during the temporal unfolding of the outline. The typically developing group particularly outperformed the ASD group when more complex contours were shown. Together, our results suggest that the interplay between local and global processes and between bottom-up and top-down processes is disturbed in ASD.

Dynamic norm-based encoding for unfamiliar shapes in human visual cortex.

Previous studies have argued that faces and other objects are encoded in terms of their deviation from a class prototype or norm. This prototype is associated with a smaller neural population response compared with nonprototype objects. However, it is still unclear (1) whether a norm-based representation can emerge for unfamiliar or novel object classes through visual experience at the time scale of an experiment and (2) whether the results from previous studies are caused by the prototypicality of a stimulus, by the physical properties of individual stimuli independent from the stimulus distribution, and/or by the trial-to-trial adaptation. Here we show with a combined behavioral and event-related fMRI study in humans that a short amount of visual experience with exemplars from novel object classes determines which stimulus is represented as the norm. Prototypicality effects were observed at the behavioral level by behavioral asymmetries during a stimulus comparison task. The fMRI data revealed that class exemplars closest to the prototypes--the perceived average of each class--were associated with a smaller response in the anterior part of the visual object-selective cortex compared with other class exemplars. By dissociating between the physical characteristics and the prototypicality status of the stimuli and by controlling for trial-to-trial adaptation, we can firmly conclude for the first time that high-level visual areas represent the identity of exemplars using a dynamic, norm-based encoding principle.

Identification of everyday objects on the basis of Gaborized outline versions.

Using outlines derived from a widely used set of line drawings, we created stimuli geared towards the investigation of contour integration and texture segmentation using shapes of everyday objects. Each stimulus consisted of Gabor elements positioned and oriented curvilinearly along the outline of an object, embedded within a larger Gabor array of homogeneous density. We created six versions of the resulting Gaborized outline stimuli by varying the orientations of elements inside and outside the outline. Data from two experiments, in which participants attempted to identify the objects in the stimuli, provide norms for identifiability and name agreement, and show differences in identifiability between stimulus versions. While there was substantial variability between the individual objects in our stimulus set, further analyses suggest a number of stimulus properties which are generally predictive of identification performance. The stimuli and the accompanying normative data, both available on our website (http://www.gestaltrevision.be/sources/gaboroutlines), provide a useful tool to further investigate contour integration and texture segmentation in both normal and clinical populations, especially when top-down influences on these processes, such as the role of prior knowledge of familiar objects, are of main interest.

Time-course contingencies in perceptual organization and identification of fragmented object outlines.

To study the dynamic interplay between different component processes involved in the identification of fragmented object outlines, the authors used a discrete-identification paradigm in which the masked presentation duration of fragmented object outlines was repeatedly increased until correct naming occurred. Survival analysis was used to investigate whether and when different types of information-such as contour integration cues (proximity, collinearity, and fragment density), fragment properties (low vs. high curvature), stimulus complexity (global symmetry, number and saliency of the parts), and memory factors (natural vs. artifactual)-influenced the timing of identification. The results show that the importance of these different types of information can change over the time course of object identification, indicating so-called time-course contingencies. Most important, the straight segments of a contour played a larger role for complex outlines with high part saliency during early (bottom-up) grouping processes, whereas the curved segments of object outlines were more important during later (top-down) matching processes for simpler outlines with lower part saliency. This new insight can explain why different studies on shape-based object identification have produced seemingly contradictory results.

Subordinate categorization enhances the neural selectivity in human object-selective cortex for fine shape differences.

There is substantial evidence that object representations in adults are dynamically updated by learning. However, it is not clear to what extent these effects are induced by active processing of visual objects in a particular task context on top of the effects of mere exposure to the same objects. Here we show that the task does matter. We performed an event-related fMRI adaptation study in which we derived neural selectivity from a release of adaptation. We had two training conditions: "categorized objects" were categorized at a subordinate level based on fine shape differences (Which type of fish is this?), whereas "control objects" were seen equally often in a task context requiring no subordinate categorization (Is this a vase or not?). After training, the object-selective cortex was more selective for differences among categorized objects than for differences among control objects. This result indicates that the task context during training modulates the extent to which object selectivity is enhanced as a result of training.

Identification of everyday objects on the basis of fragmented outline versions.

Although Attneave (1954 Psychological Review 61 183 193) and Biederman (1987 Psychological Review 94 115-147) have argued that curved contour segments are most important in shape perception, Kennedy and Domander (1985 Perception 14 367-370) showed that fragmented object contours are better identifiable when straight segments are shown. We used the set of line drawings published by Snodgrass and Vanderwart (1980 Journal of Experimental Psychology: Human Learning and Memory 6 174-215), to make outline versions that could be used to investigate this issue with a larger and more heterogeneous stimulus set. Fragments were placed either around the 'salient' points or around the midpoints (points midway between two salient points), creating curved versus relatively straight fragments when the original outline was fragmented (experiment 1), or angular and straight fragments when straight-line versions were fragmented (experiment 2). We manipulated fragment length in each experiment except the last one, in which we presented only selected points (experiment 3). While fragmented versions were on average more identifiable when straight fragments were shown, certain objects were more identifiable when the curved segments or the angles were shown. A tentative explanation of these results is presented in terms of an advantage for straight segments during grouping processes for outlines with high part salience, and an advantage for curved segments during matching processes for outlines with low part salience.

Similarity, typicality, and category-level matching of morphed outlines of everyday objects.

During visual object categorisation, a match must be found between the input image and stored information about basic-level categories. Graf [2002 Form, Space and Object (Berlin: Wissenschaftlicher Verlag Berlin)] suggested the involvement of analogue transformational, shape-changing processes in aligning the memory representation of the category with the perceptual representation of the current stimulus. Here we compare the predictions of alignment models with those of exemplar-based models, using morphing between four exemplar outlines within each of eleven categories. Overall, with increasing transformational distance between two exemplars of the same category, reaction times to decide whether they belong to the same category in a sequential matching paradigm increased, while rated similarity between the two exemplars decreased. However, in contrast to alignment accounts, exemplar-based accounts can correctly predict the observed dissociation between typicality and categorisation time, and allow the observed deviations from sequential additivity and nonlinear relations between transformational distance and rated similarity. Discussion of integrations of exemplar-based theories with neglected processes, such as information accumulation, response competition, response priming, and gain-modulation leads to a view of the recognition process from input to response, which increases the validity and scope of modern exemplar-based categorisation and recognition models.

The representation of subordinate shape similarity in human occipitotemporal cortex.

We investigated the coding of subordinate shape similarity in human object-selective cortex in two event-related functional magnetic resonance adaptation (fMR-A) experiments. Previous studies using faces have concluded that there is a narrow tuning of neuronal populations selective to each face, and that tuning is relative to the expected "average" face (norm-based encoding). Here we investigated these issues using outlines of animals and tools occupying a particular position on different morphing sequences per category. In a first experiment, we inferred the width of neural tuning to exemplars by examining whether the release from adaptation with increasing shape changes between two stimuli asymptotes. In a second experiment, we compared the response to central and extreme positions in shape space while controlling for the number of presentations of each unique stimulus to study whether the expected "average" category exemplar plays a role. The current fMR-A results show that a small change in exemplar shape produces a large release of adaptation, but only for outline shape changes of animals and not for man-made tools. Furthermore, our results suggested that central and extreme positions were not treated differently. Together, these results suggest a narrow tuning in object-selective cortex for individual exemplars from natural object categories, consistent with an exemplar-based encoding principle.

The concavity effect is a compound of local and global effects.

Using a change detection paradigm, Barenholtz, Cohen, Feldman, and Singh (2003) found that changes in concave regions of a contour are more easily detected than changes in convex regions. In a series of three experiments, we investigated this concavity effect using the same paradigm. We observed the effect in wire-like stimuli as well as in silhouettes (Experiment 1) and in complex, smoothed images as opposed to angular polygons (Experiment 2). We also observed a systematic effect of the magnitude of the change (Experiment 1). Furthermore, we find that the effect cannot be attributed to either local or global processing effects, but rather to a combination of both "mere" concaveness and an effect due to changes in the perceived part structure of the stimulus object (Experiment 3). For our data analysis, we used a nonparametric bootstrap method, which greatly increases sensitivity (compared to more traditional analyses like ANOVA).