Repetition priming in the stop signal task: the electrophysiology of sequential effects of stopping.

Inhibition of a response affects the processing of subsequent stimuli. When a response has to be made to a stimulus to which a response was previously inhibited, response time increases. In this study, we investigated the neurophysiological underpinnings of this repetition priming phenomenon. We aimed at distinguishing between two possible mechanisms. Firstly, it could be that slowing after a successful inhibition trial originates at the response execution level and is due to the reactivation of the system responsible for motor inhibition interfering with execution of the go response. The second possibility is that interference occurs at the more abstract level of conflicting action goals or plans (i.e. "stop" and "go") that are activated prior to response execution. We analyzed activity over primary motor cortices and the parietal cortex in a stop signal task. Stimulus repetition led to a decrease in activity over primary motor cortices but irrespective of history of stopping. Stopping on the previous trial did affect the stimulus-locked parietal P300 only on repetition of the stimulus, mimicking the behavioral pattern. Furthermore, the P300 was lateralized and affected by both stimulus onset and response time, suggesting that the interference caused by inhibition priming is situated between stimulus perception and response execution. Taken together, these findings show that the prolonged response times to a stimulus that was previously successfully inhibited to, do not originate from reactivated suppression of motor output, but are caused by interference between a stop and a go goal in parietal cortex that hampers translation from stimulus to response.

Effect of the static magnetic field of the MR-scanner on ERPs: evaluation of visual, cognitive and motor potentials.

OBJECTIVE: This work investigates the influence of the static magnetic field of the MR-scanner on ERPs extracted from simultaneous EEG-fMRI recordings. The quality of the ERPs after BallistoCardioGraphic (BCG) artifact removal, as well as the reproducibility of the waveforms in different environments is investigated. METHODS: We consider a Detection, a Go-Nogo and a Motor task, eliciting peaks that differ in amplitude, latency and scalp topography, repeated in two situations: outside the scanner room (0T) and inside the MR-scanner but without gradients (3T). The BCG artifact is removed by means of three techniques: the Average Artifact Subtraction (AAS) method, the Optimal Basis Set (OBS) method and the Canonical Correlation Analysis (CCA) approach. RESULTS: The performance of the three methods depends on the amount of averaged trials. Moreover, differences are found on both amplitude and latency of ERP components recorded in two environments (0T vs 3T). CONCLUSIONS: We showed that, while ERPs can be extracted from simultaneous EEG-fMRI data at 3T, the static magnetic field might affect the physiological processes under investigation. SIGNIFICANCE: The reproducibility of the ERPs in different recording environments (0T vs 3T) is a relevant issue that deserves further investigation to clarify the equivalence of cognitive processes in both behavioral and imaging studies.

Irrelevant digits affect feature-based attention depending on the overlap of neural circuits.

Feature-based attention was investigated by examining the effect of irrelevant information on the processing of relevant information. In all experiments, irrelevant information consisted of digits whose semantic information is known to be processed in parietal areas. Between experiments we varied the degree of parietal involvement in the processing of the relevant feature. The influence of the irrelevant digit on the binary manual response task on the relevant feature was measured by the SNARC effect, a spatial numerical association of response codes demonstrating faster left than right hand responses for small numbers and faster right than left hand responses for large numbers. When processing of the relevant feature depended on parietal cortex, as is the case for orientation processing (exps. 1 and 4), there was an effect of the digit's semantic value on response times. Conversely, there was no effect of the irrelevant digit on the processing of color (exps. 2 and 3) or shape (exp. 5), which rely only minimally on parietal resources. After ruling out alternative explanations we conclude that the efficiency of feature-based attention is determined by the degree of neural overlap of structures dedicated to process relevant and irrelevant information.

Are Arabic numerals processed as pictures in a Stroop interference task?

In a picture-word interference task, picture naming is interfered by an incongruent word, but word naming is hardly hindered by the presence of an incongruent picture. In this study, we investigated whether Arabic digits are processed more like pictures or like words. We report two experiments in which Arabic digits and verbal numerals were confronted in a Stroop task. Arabic digit naming is interfered by the presence of an incongruent verbal numeral, while naming the verbal numeral is not influenced by the presence of an incongruent Arabic digit. In a second experiment, we excluded the hypothesis that the results are due to ignoring the Arabic digits: interferences from an incongruent distracter were similar for both notations in a semantic classification task. It seems that an asemantic conversion for Arabic digits is too slow to influence naming times, and that Arabic digit naming, like picture naming, is semantically mediated.

Two routes for the processing of verbal numbers: evidence from the SNARC effect.

The functional locus of the semantic system is an important issue in number processing. In the present article, the necessity of addressing a central semantic magnitude system in the processing of printed verbal number words is evaluated by looking at the presence of a spatial-numerical association of response codes or SNARC effect. This effect consists of an association of number magnitude and response-preference (preferred responses to small numbers with the left hand and to large numbers with the right hand) and reflects semantic access. Two experiments were run. In Experiment 1, participants performed a parity judgment task which requires access to number semantics. A SNARC effect was observed. In Experiment 2 a phoneme monitoring task was used, which can, in principle, be performed through direct asemantic transcoding. No SNARC effect occurred. Apparently, written number words access the semantic system only if this is necessary for correct task completion. Hence, a semantic and an asemantic route can be postulated for the processing of word numerals. These observations contrast with the processing of Arabic numerals for which semantic effects are omnipresent. Implications of this explicit demonstration of a dissimilarity between the processing of digits and of number words are discussed.

The Whorfian hypothesis and numerical cognition: is 'twenty-four' processed in the same way as 'four-and-twenty'?

Recent theoretical developments have redefined a Whorfian effect as a processing difference due to the language of the individual, and no longer as a marker for or against linguistic determinism. Within this framework. Whorfian effects can be used to investigate whether a particular part of the cognitive system is penetrable by language processes or forms an encapsulated module, provided the experimenter ensures that the target language difference is not caused by peripheral input or output processes. In this article, we examine the possibility of a Whorfian effect in numerical cognition by making use of the fact that in the Dutch number naming system the order of tens and units is reversed (i.e. 24 is read 'four-and-twenty'). In a first experiment, we asked native French- and Dutch-speaking students to name the solution of addition problems with a two-digit and a single-digit operand (e.g. 20 + 4 =?, 24 + 1 =?). The order of the operands was manipulated (20 + 4 vs. 4 + 20) as well as the presentation modality (Arabic vs. verbal). Three language differences emerged from this study. Experiment 2, however, showed that these differences were all due to input or output processes rather than differences in the addition operation (i.e. the differences between Dutch and French disappeared when subjects were asked to type the answer rather than pronounce it). On the basis of these findings, we question the idea that mathematical operations are based on verbal processes.

About the influence of the presentation format on arithmetical-fact retrieval processes.

This article presents the results of two experiments. In Experiment 1, French-speaking participants were asked first to retrieve the product of two numbers presented in Arabic or verbal code, and then to perform a number-matching task on the same material to assess the encoding time difference between numerals in the two formats. Experiment 2 involved the same multiplication task with Dutch-speaking participants who name two-digit numbers in reverse order. The format effects obtained by Campbell and Clark (1992); Campbell (1994) for multiplication were replicated. However, several observations suggest that some of these effects may be due to encoding time differences between word and digit numerals. The same size-by-format interaction was found for the number-matching task as for the multiplication task, and the effect disappeared with practice in the multiplication task. Finally, despite the fact that the linguistic structure of number names differs between French and Dutch, the types of error produced in both groups were identical. The last result does not match with the hypothesis that operand intrusion errors are due to interference between reading processes and arithmetical-fact retrieval processes. Implications of these findings for the debate about the nature of arithmetical-fact retrieval are discussed.

Comparing color-word and picture-word Stroop-like effects: a test of the Glaser and Glaser (1989) model.

Glaser and Glaser (1989) assume that the processing of colors and pictures is highly similar in that, compared to words, both kinds of stimulis have privileged access to semantic information. This assumption was tested in the present research. In Experiment 1, the season corresponding to the color or to the word of color-word Stroop stimuli had to be named (e.g., green for spring). In Experiment 2, subjects had to name the season corresponding to the picture or the word of a picture-word stimulus (e.g., flower for spring). According to Glaser and Glaser (1989), privileged semantic processing of colors and pictures should be evidenced by a larger interfering power of color and picture distractors than of word distractors. However, the asymmetric pattern of interference was observed only with picture-word stimuli (Experiment 2), but not with color-word stimuli (Experiment 1), suggesting that, unlike pictures, colors do not have privileged access to semantic information. It was also found that word distractors interfered with the semantic processing of pictures, a result that is incompatible with the dominance rule postulated by Glaser and Glaser (1989). From these results, an adapted version of the Glaser and Glaser model is proposed: colors are assumed to have privileged access to a separate color-processing system and the pattern of interference depends upon the relative activation strength of the response alternatives activated by the target and the distractor.