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The speed of visual processing is central to our understanding of face perception.
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Yet the extent to which early visual processing influences later processing in distributed face processing networks, and the top-down modulation of such bottom-up effects, remains unclear. We used simultaneous EEG-fMRI to investigate cortical activity that showed unique covariation with ERP components of face processing C1, P1, N, P3while manipulating sustained attention and transient cognitive conflict employing an emotional face-word Stroop task.
Crucially, this covariation depended on sustained attentional focus and was absent for incongruent trials, suggesting flexible top-down gating of bottom-up processing. The speed of visual processing plays an important role in attempts to understand human visual perception in general, and face perception more specifically.
Processing speed can provide evidence about the number of computational steps that are proposed by theories of visual perception Thorpe, Investigating the time course of visual processing also allows for conclusions about the relative contributions of bottom-up and top-down influences on different stages of visual perception.
The latter findings were interpreted as lending support to theories that assume a strong role of bottom-up and feed-forward processing in visual recognition Liu et al.
A common means of investigating top-down modulation is the examination of task effects on visual perception. ERP research has shown that several top-down factors can influence activity in the visual cortex: Notably, recent research has demonstrated that even the primary visual cortex is susceptible to top-down modulation, including influences of attention, reward, and mood Bayer et al.
In contrast, the time course of task-related effects on object recognition remains a matter of debate, especially in the case of face perception for discussion, see Rousselet et al. A related question is the impact that early crkuzet visual cortex processing has on subsequent processing in more anterior crouze of the brain, including temporal, parietal and frontal cortical regions, and how such influences are affected by top-down processes. In the present study we investigated the influence of early information processing in the visual cortex on higher-order processing stages, and its top-down modulation by task-related attention, focusing specifically on the processing of faces.
Using simultaneous EEG-fMRI, trial-by-trial ERP amplitudes were used to identify brain regions that showed significant and unique covariation with different visual processing stages, from very early perceptual processing in the striate and extrastriate visual cortex as indicated by C1 and P1 components; Di Russo et al. Eimer,Polich, Furthermore, the influence of sustained attention and trial-by-trial cognitive conflict on such covariation was examined.
The emotional face-word Stroop task is well-suited for our research question: First, since face and word are presented simultaneously and in a superimposed manner, it allows for the investigation of sustained top-down attentional focus on one of the two object categories during identical perceptual crouzett. Second, contrasting congruent and incongruent face-word pairs provides insight into conflict processing, a mechanism at the intersection of bottom-up and top-down processing, which includes trial-by-trial conflict monitoring and resolution.
For the congruent versus incongruent contrast, we predicted changes to ERP-BOLD covariation, though made no specific too prediction. The study was reviewed and approved by the University of Reading research xrouzet committee. Participants provided informed consent prior to taking part in the study. The remaining sample of 15 participants 7 female participants had a mean age of All participants were right handed, had normal or corrected-to-normal vision and did not report any history of neurological or psychiatric disorders.
Stimuli consisted of portraits of unfamiliar females and males with superimposed affective words. The portraits displayed three different facial expressions fearful, happy, neutral; 71 stimuli per category. The superimposed words were positioned horizontally in the middle of the face, and presented in white colour. Faces and words were combined in two ways: In congruent trials, the word matched the facial expression e.
In incongruent trials, the word did not match the facial expression. Participants were instructed to perform an affective categorisation task on the stimuli by pressing the corresponding button on the button box.
We manipulated attentional focus by presenting stimuli in two types of blocks: In half of the blocks, the affective categorisation was performed on the face while ignoring the word Attend Face ; in the other half of blocks, categorisation was performed on the words Attend Word. Two comparisons were of particular interest. For the effect of top-down attention on face processing, we compared Attend Face vs. Attend Word in the congruent condition. For the effect of conflict processing on face processing, we compared congruent vs.
After providing informed consent, participants were given information concerning the task and the procedure.
After Tip preparation, participants performed 10 practice trials in order to practice the button assignments to affective categories. The experiment employed an event-related paradigm programmed in Matlab MathWorks utilizing the Psychophysics Toolbox extensions Brainard,Pelli, Optseq 2 Dale, was used to schedule the presentation of events by jittering the inter-trial interval in order to maximise statistical efficiency Post stimulus delay parameters: Participants were asked to limit eye blinks to the presentation of the fixation cross, and to avoid blinking during stimulus presentation.
An additional electrode was placed on the back of the participant, just left of the spinal column, for recording the electrocardiogram ECG. Data were referenced to electrode Cz; AFz was used as ground.
Gradient artefacts were identified using synchronized markers from the scanner and removed from continuous, baseline corrected data using the whole artefact for baseline correction. Further pre-processing and data analyses of segmented single-trial ERPs was performed using Matlab.
Reaction times RTs for correct trials and percentage of correct responses were calculated for each experimental condition and analysed with a mixed effects General Linear Model GLM with the fixed factors Attention face, word and Congruency congruent, incongruentand Subject as random factor.
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In addition, differences between congruent and incongruent trials would indicate effects of stimulus conflict. A first level GLM was modelled separately for each run of the experiment with separate regressors for congruent and incongruent trials. Regressors were created by convolving the temporal profile of each experimental condition with the double gamma haemodynamic response function in FSL. The contrast of interest compared activation in congruent vs. Single-level cc1 for each run were combined across all rop in a second level fixed effects analysis for each participant.
In this step, Attention Attend Face, Attend Wordwhich had been manipulated between runs, was included as an additional contrast. For frouzet analyses across participants, contrasts of interest congruent vs. For all ERP components, artefact-free single-trial amplitudes were z-normalised across experimental conditions; on trials with artefacts, amplitude values were replaced with the respective mean i.
ERP waveforms and topographies. For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article. For the analyses of ERPs, component amplitudes of artefact-free trials were averaged per subject and experimental condition Congruency 2 X Attention 2 and analysed using a mixed-effects GLM, with the fixed factors Congruency and Attention and Subject as a random factor.
Normalized z-scored single-trial ERP values for each ERP component and Congruency condition were convolved with the FSL canonical hemodynamic response function and included as parametric regressors in the fMRI analyses 8 additional regressors: C1, P1, N and P3 for congruent and incongruent trials.
GLM analysis of fMRI data was identical to that described above, except for the addition of these parametric regressors.
We then identified brain regions showing significant covariation of ERP amplitudes with BOLD response in the Attend Face vrouzet, separately for each congruency condition.
However, since this analysis would not account for any variance that is shared between components, we also included crouzdt contrasts for pairs of components e. RT analyses showed longer reaction times for trials where participants attended to faces compared to words, F 1, Furthermore, RTs were longer for incongruent than for congruent trials, F 1, A significant interaction between Attention and Congruency reflected a larger effect of cruozet when participants attended to faces compared to words, F 1, Accuracy rates were higher in the word condition than in the face condition, F 1, A significant interaction between Attention and Congruency was based on a larger effect of Congruency in the face condition compared to the word condition, F 1, Together, RTs and accuracy indicate greater interference of conflicting information for the Attend Face condition than for Attend Word.
Importantly for the attentional focus manipulation, there was a significant crrouzet in reaction times between congruent trials in the Attend Face vs. Responses were slower for faces than for words in the congruent condition i.
For RTs and accuracy rates, see and Fig. When participants performed the categorisation task on the facial expressions Attend Facea number of rop regions showed significantly higher activation for croyzet than for congruent trials see Fig. These regions included large parts of the visual cortex, superior and middle frontal gyrus, and bilateral middle temporal cortex.
For a complete list of significant activations see Table 1. In the Attend Word condition, there were no significant activation differences between congruent and incongruent trials. Significant activations were limited to the Attend Face condition. Analyses revealed no significant effects of Attention or Congruency for the amplitudes of C1, P1, and N P3 amplitudes showed a main effect of Attention, F 1, N amplitudes in the congruent attend face condition showed ceouzet covariation with a region at the intersection of the lateral occipital cortex and angular gyrus Fig.
No significant covariations were found for incongruent trials. In order to test whether this area showed selective coupling to faces in the congruent condition, we extracted covariation parameter estimates within this region for all four Attention x Congruency conditions and performed post-tests on covariation parameter estimates using a mixed effects GLM with Attention and Congruency as fixed factors and Subject as a random factor.
Covariation parameter estimates for Attention x Congruency conditions within significant clusters of covariation for the A N component and B C1 component. When participants attended to the face in congruent c11, C1 amplitudes showed tkp covariations with a number of brain regions, including the precuneous cortex and the posterior cingulate gyrus, as well as left-lateralized activations in the lateral parietal-occipital cluster crouuzet above, middle temporal cortex and temporal pole see Fig.
In incongruent trials, analyses revealed no voxelwise covariations of C1 amplitudes in the Attend Face condition. We performed a mixed effects GLM with the fixed factors Attention, Congruency and cluster, and Subject as random factor. For the N cluster, there were no significant differences in BOLD activation between experimental conditions.
The EEG-fMRI analyses included additional contrasts for pairs of components in order to account for possible shared variance between components.
N and P3 showed shared variance in crouet number of brain regions, including significant bilateral covariations in the lateral parietal-occipital cluster. Further regions were located in the posterior cingulate cortex and the middle frontal gyrus see Fig. Notably, all significant shared covariations were again limited to the congruent condition. Covariations with amplitudes of N and P3 shared variance in the Attend Face condition in the lateral parietal-occipital cluster bilaterallyposterior cingulate gyrus and middle frontal gyrus.
Using simultaneous EEG-fMRI during a face-word emotional Stroop task, the present study investigated the influence of visual cortex activity on higher-order face processing stages, and its top-down modulation by task-related attention.
We predicted that, in the Attend Face condition, early ERP fop would covary from trial to trial with early and later stage face processing regions, and that this covariation would be reduced criuzet absent during the Attend Word condition.
Importantly, these covariations were crluzet to congruent trials in the Attend Face condition, showing their dependence on sustained attentional focus and modulation by trial-by-trial conflict processing. The lateral parietal-occipital cluster was located at the intersection of the superior lateral occipital cortex and v1 gyrus, extending into posterior middle temporal gyrus.
This region has previously been associated with a number of functions including face processing Leveroni et al. The more anterior brain regions are associated with emotion processing Adolphs,Rolls and Grabenhorst,Sabatinelli et al.