Filip Van Opstal

Post-error slowing: An orienting account

It is generally assumed that slowing after errors is a cognitive control effect reflecting more careful response strategies after errors. However, clinical data are not compatible with this explanation. We therefore consider two alternative explanations, one referring to the possibility of a persisting underlying problem and one on the basis of the low frequency of errors (orienting account). This latter hypothesis argues that infrequent events orient attention away from the task. Support for the orienting account was obtained in two experiments. Using a new experimental procedure, Experiment 1 demonstrated post-error slowing after infrequent errors and post-correct slowing after infrequent correct trials. In Experiment 2, slowing was observed following infrequent irrelevant tones replacing the feedback signals.

Dissecting the symbolic distance effect: Comparison and priming effects in numerical and nonnumerical orders

When participants are asked to compare two stimuli, responses are slower for stimuli close to each other on the relevant dimension than for stimuli further apart. Previously, it has been proposed that this comparison distance effect originates from overlap in the representation of the stimuli. This idea is generally accepted in numerical cognition, where it is assumed that representational overlap of numbers on a mental number line accounts for the effect (e.g., Cohen Kadosh et al., 2005). In contrast, others have emphasized the role of response-related processes to explain the comparison distance effect (e.g., Banks, 1977). In the present study, numbers and letters are used to show that the comparison distance effect can be dissociated from a more direct behavioral signature of representational overlap, the priming distance effect. The implication is that a comparison distance effect does not imply representational overlap. An interpretation is given in terms of a recently proposed model of quantity comparison (Verguts, Fias, & Stevens, 2005).

Hippocampal contribution to early and later stages of implicit motor sequence learning

Implicit motor sequence learning refers to an important human ability to acquire new motor skills through the repeated performance of a motor sequence. This learning process is characterized by slow, incremental gains of motor performance. The present fMRI study was developed to better delineate the areas supporting these temporal dynamics of learning. By using the serial color matching paradigm, our study focused on the motor level of sequence learning and tracked the time course of learning-related neural changes. Imaging results showed a significant contribution of the left anterior hippocampus in an early sequence acquisition stage (first scanning session) as well as during a later stage with stabilized learning effects (second scanning session). Hippocampal activation significantly correlated with the behavioral learning process and was affected by a change of the motor sequence. These results suggest a strong involvement of the hippocampus in implicit motor sequence learning. On the other hand, a very extensive and bilateral neural network of parietal, temporal and frontal cortical areas (including SMA, pre-SMA) together with parts of the cerebellum and striatum were found to play a role during random visuo-motor task performance.

The neural representation of extensively trained ordered sequences.

The role of the intraparietal sulcus (IPS) in number processing is largely agreed on. A current debate however concerns the specificity of the involvement of the IPS in representing numbers or ordinal sequences more generally. To test this specificity, we investigated whether the IPS would be activated by extensive training on an arbitrary ordered sequence. We found that the hippocampal-angular gyrus activation initially involved in learning the ordered sequences extends with extensive training to the left inferior frontal gyrus (left IFG), but not to the IPS. These results suggest that left IFG can be involved in processing ordinal information, and that there is no need for an IPS area specifically dedicated to the representation of all ordinal sequences. Instead, we propose that the locus of the representation might be determined by the nature of the stimuli rather than its ordinal nature per se.

Feeling the Conflict The Crucial Role of Conflict Experience in Adaptation

In the study reported here, we examined the role of conflict experience in cognitive adaptation to conflict. Although the experience of conflict is generally neglected in theoretical models of cognitive control, we demonstrated that it plays a critical role in cognitive adaptation. Using a masked-priming paradigm, we showed that conflict adaptation was present only after trials on which participants experienced response conflict. Furthermore, when subjective experience did not coincide with actual conflict, adaptation effects in the error rates were observed after the experience of conflict, not after response conflict. We conclude that the experience of conflict, and not response conflict per se, is the crucial factor underlying cognitive adaptation effects. The current findings provide a new perspective on the question of why the human cognitive system exerts cognitive control, and they suggest that a crucial role of subjective experience is to allow for top-down control of behavior.

A hippocampal–parietal network for learning an ordered sequence

The dynamics of the neural network that underlies learning transitive structures of an ordered sequence remains poorly understood. To address this, in the present study we used fMRI to track the time course of transitive inference learning. The hippocampus and the angular gyrus were each shown to be closely related to the learning trajectory, but differentially so. Hippocampal activity was shown to consistently increase with learning but no correlation was found between performance and hippocampal activation, suggesting a general role for the hippocampus. Left angular gyrus activity was also found to consistently increase with training, but, in addition, correlated significantly with behavioral performance. This suggests an involvement of the angular gyrus in learning the ordinal associations between the stimuli.

Dissociation of the distance effect and size effect in one-digit numbers

Magnitude comparison of single digits is robustly characterized by a distance effect (close numbers are more difficult to compare than numbers further apart) and a size effect (for a given distance, comparison difficulty increases with increasing size). The distance effect indicates access to the mental number line (Dehaene, 1997), and the size effect is usually interpreted as indicating that the mental number line represents larger numbers more vaguely than smaller ones. In contrast, we have argued earlier (Verguts, Fias, & Stevens, 2005) that for symbolic numbers (Arabic or verbal notation), the size effect does not originate from the mental number line but, instead, originates from mappings to relevant output components that are specific for magnitude comparison. If the latter is true, it should be possible to dissociate the distance effect from the size effect in tasks other than magnitude comparison. In two experiments, we observed a robust distance effect insame/different judgments, which implies access to the mental number line. Yet the size effect was absent. Consistent with our prediction, this finding establishes a dissociation between the size effect and the distance effect.

The Neural Basis of Implicit Perceptual Sequence Learning

The present fMRI study investigated the neural areas involved in implicit perceptual sequence learning. To obtain more insight in the functional contributions of the brain areas, we tracked both the behavioral and neural time course of the learning process, using a perceptual serial color matching task. Next, to investigate whether the neural time course was specific for perceptual information, imaging results were compared to the results of implicit motor sequence learning, previously investigated using an identical serial color matching task (Gheysen et al., 2010). Results indicated that implicit sequences can be acquired by at least two neural systems: the caudate nucleus and the hippocampus, having different operating principles. The caudate nucleus contributed to the implicit sequence learning process for perceptual as well as motor information in a similar and gradual way. The hippocampus, on the other hand, was engaged in a much faster learning process which was more pronounced for the motor compared to the perceptual task. Interestingly, the perceptual and motor learning process occurred on a comparable implicit level, suggesting that consciousness is not the main determinant factor dissociating the hippocampal from the caudate learning system. This study is not only the first to successfully and unambiguously compare brain activation between perceptual and motor levels of implicit sequence learning, it also provides new insights into the specific hippocampal and caudate learning function.

The origins of the numerical distance effect: The same–different task

One of the most frequently used markers in research on numerical cognition is the distance effect. Recently, we have suggested that a distance effect can have different origins depending on the experimental task. By dissociating the comparison distance effect from the priming distance effect we revealed the need to study the origin of this effect before drawing any conclusions from it (van Opstal, Gevers, de Moor, & Verguts, 2008). Because a distance effect in a same–different task is also commonly used to study number representations (e.g., Dehaene & Akhavein, 1995), the present study aimed at uncovering the origin of the effect in this task. Computational and empirical results indicate clearly that the distance effect in the same–different task originates from number representations rather than a decision process.

Unconscious task application

The nature of unconscious information processing is a heavily debated issue in cognitive science (e.g., Kouider & Dehaene, 2007), and neuroscience (e.g., Crick & Koch, 1998). Traditionally, it has been thought that unconscious cognitive processing is restricted to knowledge that is strongly prepared by conscious processes (e.g., Dehaene et al., 1998). In three experiments, we show that the task that is performed consciously can also be applied unconsciously to items outside the current task set. We found that a same-different judgment of two target stimuli was also performed on two subliminally presented prime stimuli. This was true for target and prime stimuli from entirely different categories, as well as for prime and target stimuli at different levels of abstraction. These results reveal that unconscious processing can generalize more widely than previously accepted.