On the functional independence of numerical acuity and visual working memory.

Deciding where to direct our vehicle in a crowded parking area or where to line up at an airport gateway relies on our ability to appraise the numerosity of multitudes at a glimpse and react accordingly. Approximating numerosities without actually counting is an ontogenetically and phylogenetically primordial ability, given its presence in human infants shortly after birth, and in primate and non-primate animal species. Prior research in the field suggested that numerosity approximation is a ballistic automatism that has little to do with human cognition as commonly intended. Here, we measured visual working memory capacity using a state-of-the-art change detection task and numerosity approximation using a dot-comparison task, and found a null correlation between these two parametrical domains. By checking the evidential strength of the tested correlation using both classic and Bayesian analytical approaches, as well as the construct validity for working memory capacity and numerosity approximation estimates, we concluded that the present psychophysical evidence was sufficiently strong to support the view that visual working memory and numerosity approximation are likely to rely on functionally independent stages of processing of the human cognitive architecture.

Conscious perception of fear in faces: Insights from high-density EEG and perceptual awareness scale with threshold stimuli.

Contrary to the extensive research on processing subliminal and/or unattended emotional facial expressions, only a minority of studies have investigated the neural correlates of consciousness (NCCs) of emotions conveyed by faces. In the present high-density electroencephalography (EEG) study, we first employed a staircase procedure to identify each participant's perceptual threshold of the emotion expressed by the face and then compared the EEG signals elicited in trials where the participants were aware with the activity elicited in trials where participants were unaware of the emotions expressed by these, otherwise identical, faces. Drawing on existing knowledge of the neural mechanisms of face processing and NCCs, we hypothesized that activity in frontal electrodes would be modulated in relation to participants' awareness of facial emotional content. More specifically, we hypothesized that the NCC of fear seen on someone else's face could be detected as a modulation of a later and more anterior (i.e., at frontal sites) event-related potential (ERP) than the face-sensitive N170. By adopting a data-driven approach and cluster-based statistics to the analysis of EEG signals, the results were clear-cut in showing that visual awareness of fear was associated with the modulation of a frontal ERP component in a 150-300 msec interval. These insights are dissected and contextualized in relation to prevailing theories of visual consciousness and their proposed NCC benchmarks.

On target selection as reflected by posterior ERP components in feature-guided visual search.

The N2pc event-related potential is a widely studied ERP component that reflects the covert deployment of visuo-spatial attention to target stimuli displayed laterally relative to fixation. Recently, an analogous ERP component, named N2pcb, has been proposed as a marker of the deployment of visuo-spatial attention to targets displayed on the vertical midline. Two studies that investigated the N2pcb component found analogous results, using however two different algorithms to compute the amplitude of N2pcb. One study subtracted the ipsilateral activity elicited by a lateral target from the bilateral activity elicited by a target displayed on the vertical midline, whereas the other study subtracted the bilateral activity elicited by target-absent displays from the bilateral activity elicited by a target displayed on the vertical midline. Here we show both algorithms estimate properly the N2pc as well as the N2pcb components. In addition, we explored whether the singleton detection positivity (SDP) component, a posterior bilateral positivity temporally concomitant to N2pc recently reported in studies using singleton search, could be observed in the present study in which a target was defined by a combination of features. Given that such component was indeed found using feature search, we named this component posterior processing positivity (PPP), and showed that bilateral activity elicited by target-absent displays is an adequate baseline for its correct isolation.

A bilateral SPCN is elicited by to-be-memorized visual stimuli displayed along the vertical midline.

We recently showed that deploying attention to target stimuli displayed along the vertical meridian elicits a bilateral N2pc, that we labeled N2pcb (Psychophysiology). Here we investigated whether a different component, the sustained posterior contralateral negativity (SPCN), shows the same property when a varying number of visual stimuli are displayed either laterally or on the vertical meridian. We displayed one or two cues that designated candidate targets to be detected in a search array that was displayed after a retention interval. The cues were either on the horizontal meridian or on the vertical meridian. When the cues were on the horizontal meridian, we observed an N2pc followed by an SPCN in their classic form, as negativity increments contralateral to the cues. As expected, SPCN amplitude was greater when two cues had to be memorized than when only one cue had to be memorized. When the cues were on the vertical meridian, we observed an N2pcb followed by a bilateral SPCN (or SPCNb). Critically, like SPCN, SPCNb amplitude was greater when two cues had to be memorized than when only one cue had to be memorized. A series of additional parametrical and topographical comparisons between N2pcb and SPCNb revealed similarities but also some important differences between these two components that we interpreted as evidence for their distinct neural sources.

A neural network predicting the amplitude of the N2pc in individual EEG datasets.

Objective.The N2pc is a small amplitude transient interhemispheric voltage asymmetry used in cognitive neuroscience to investigate subject's allocation of selective visuo-spatial attention. N2pc is typically estimated by averaging the sweeps of the electroencephalographic (EEG) signal but, in absence of explicit normative indications, the number of sweeps is often based on arbitrariness or personal experience. With the final aim of reducing duration and cost of experimental protocols, here we developed a new approach to reliably predict N2pc amplitude from a minimal EEG dataset.Approach.First, features predictive of N2pc amplitude were identified in the time-frequency domain. Then, an artificial neural network (NN) was trained to predict N2pc mean amplitude at the individual level. By resorting to simulated data, accuracy of the NN was assessed by computing the mean squared error (MSE) and the amplitude discretization error (ADE) and compared to the standard time averaging (TA) technique. The NN was then tested against two real datasets consisting of 14 and 12 subjects, respectively.Main result.In simulated scenarios entailing different number of sweeps (between 10 and 100), the MSE obtained with the proposed method resulted, on average, 1/5 of that obtained with the TA technique. Implementation on real EEG datasets showed that N2pc amplitude could be reliably predicted with as few as 40 EEG sweeps per cell of the experimental design.Significance.The developed approach allows to reduce duration and cost of experiments involving the N2pc, for instance in studies investigating attention deficits in pathological subjects.

A Time-Frequency Analysis for the Online Detection of the N2pc Event-Related Potential (ERP) Component in Individual EEG Datasets.

The N2pc event-related potential component measures direction and time course of selective visual attention and represents an important biomarker in cognitive neuroscience. While its subtractive origin strongly influences the amplitude, thus hindering its detection, other external factors, such as subject's inefficiency to allocate attention to the cued target, or the heterogeneity of the visual context, may strongly affect the elicitation of the component itself. It would therefore be extremely important to create a tool that, using as few sweeps as possible, could reliably establish whether an N2pc is present in an individual subject. In the present work, we propose an approach by resorting to a time-frequency analysis of N2pc individual signals; in particular, power at each frequency band (α/ß/δ/θ) was computed in the N2 time range and correlated to the estimated amplitude of the N2pc. Preliminary results on fourteen human volunteers of a visual search design showed a very high correlation coefficient (over 0.9) between the low frequency bands power and the mean absolute amplitude of the component, using only 40 sweeps. Results also seemed to suggest that N2pc amplitude values higher than 0.5 µV could be accurately classified according to time-frequency indices.Clinical Relevance - The online detection of the N2pc presence in individual EEG datasets would allow not only to study the factors responsible of N2pc variability across subjects and conditions, but also to investigate novel search variants on participants with a predisposition to show an N2pc, reducing time and costs and the possibility to obtain biased results.

Distilling the distinct contralateral and ipsilateral attentional responses to lateral stimuli and the bilateral response to midline stimuli for upper and lower visual hemifield locations.

A contralateral posterior negativity elicited by lateral oddballs (N2pc) and a bilateral posterior negativity elicited by vertical midline oddballs (bilateral N2) are ERP components reflecting attentional deployment that have been rarely compared. In different tasks, we explored to what extent they reflect similar underlying mechanisms of attention. We used a multiple-frame procedure to present pop-out color oddballs among distractors. A homogeneous condition contained only distractors (0 oddballs) and served as a control condition that was subtracted from oddball-present conditions to isolate attention effects. The number of oddballs and the vertical hemifield containing them (upper vs. lower) were two critical factors. For the lower hemifield, the signal amplitude increased with the number of oddballs, otherwise had similar effects and scalp distributions, suggesting the bilateral N2 acted as a bilateral N2pc and likely reflected similar underlying generators. For the upper hemifield, component amplitude also increased with the number of oddballs, but the scalp distributions were positive and more centered, suggesting inverted generators across the two vertical hemifields. An ipsilateral positivity occurred about 50 ms after a contralateral positivity, similar in magnitude, producing a biphasic contra-minus-ipsi difference wave. Previously reported smaller negative N2pc components for upper hemifield oddballs likely reflected a negative lobe artificially created by the subtraction of a lagged positive ipsilateral response. The results compel us to argue for a systematic separation of data for upper versus lower hemifields in studies of visuo-spatial attention, and the use of an experimental design permitting the separate estimation of contralateral and ipsilateral responses.

Computer data simulator to assess the accuracy of estimates of visual N2/N2pc event-related potential components.

OBJECTIVE: Event-related potentials (ERPs) evoked by visual stimulations comprise several components, with different amplitudes and latencies. Among them, the N2 and N2pc components have been demonstrated to be a measure of subjects' allocation of visual attention to possible targets and to be involved in the suppression of irrelevant items. Unfortunately, the N2 and N2pc components have smaller amplitudes compared with those of the background electroencephalogram (EEG), and their measurement requires employing techniques such as conventional averaging, which in turn necessitates several sweeps to provide acceptable estimates. In visual search studies, the number of sweeps (Nswp) used to extrapolate reliable estimates of N2/N2pc components has always been somehow arbitrary, with studies using 50-500 sweeps. In-silico studies relying on synthetic data providing a close-to-realistic fit to the variability of the visual N2 component and background EEG signals are therefore needed to go beyond arbitrary choices in this context. APPROACH: In the present work, we sought to take a step in this direction by developing a simulator of ERP variations in the N2 time range based on real experimental data while monitoring variations in the estimation accuracy of N2/N2pc components as a function of two factors, i.e. signal-to-noise ratio (SNR) and number of averaged sweeps. MAIN RESULTS: The results revealed that both Nswp and SNR had a strong impact on the accuracy of N2/N2pc estimates. Critically, the present simulation showed that, for a given level of SNR, a non-arbitrary Nswp could be parametrically determined, after which no additional significant improvements in noise suppression and N2/N2pc accuracy estimation were observed. SIGNIFICANCE: The present simulator is thought to provide investigators with quantitative guidelines for designing experimental protocols aimed at improving the detection accuracy of N2/N2pc components. The parameters of the simulator can be tuned, adapted, or integrated to fit other ERP modulations.

A bilateral N2pc (N2pcb) component is elicited by search targets displayed on the vertical midline.

The study of visually elicited event-related potentials (ERPs) detected at posterior recording sites during visual search has enormously advanced our knowledge about how and when visuo-spatial attention locks onto one or more laterally presented target objects. The N2pc component to lateral targets has been pivotal to further our understanding of the mechanisms and time course of target selection in visual search. However, the N2pc cannot track visuo-spatial attention deployment to targets displayed along the vertical midline. Here, we introduce a new ERP marker (N2pcb component) that is elicited during the selection of such midline targets. In line with retinal and callosal projections from striate to ventral extrastriate cortex, this component reflects an enhanced negativity elicited by midline targets over both posterior hemispheres. By comparing the attentional selection of lateral and midline targets in a singleton search condition and a feature search condition, we show that the N2pcb is triggered at the same time as the N2pc to lateral targets, and shows the same onset latency difference between singleton and feature search. We conclude that the N2pcb and N2pc components reflect the same attentional target selection processes in visual search.

Functional dissociation of anterior cingulate cortex and intraparietal sulcus in visual working memory.

Previous electrophysiological studies of lateralized visual working memory (VWM) identified an ERP component, defined as contralateral delay activity (CDA), directly modulated by the number of items held in memory. One of the main candidate as the cortical source of this ERP component is the inferior intraparietal sulcus (IPS). Moreover, previous neuroimaging studies put forth evidence for the presence of a distributed VWM network involving also prefrontal areas and in particular the anterior cingulate cortex (ACC). Nonetheless, the understanding of the functional role of ACC is still debated. We recorded the high-density EEG in 20 healthy participants undergoing a VWM and a control task. Explorative cluster-based permutation statistics confirmed the posterior memory load dependent CDA modulation, but also identified an additional anterior cluster of electrodes whose amplitude was modulated by memory load. The source reconstruction revealed a memory load dependent activation in the IPS but also in the ACC, suggesting that these two areas might be nodes of a fronto-parietal circuit underlying VWM maintenance. Crucially, parietal and prefrontal areas showed a temporal dissociation, since IPS was more engaged in the early phase of visual information storage while the ACC was more active during the late phase. This pattern suggests a functional dissociation between the parietal cortex, which is involved in encoding and storage of information, and prefrontal areas, subserving cognitive control processes, including the boosting and protection of information from decay. Remarkably, the connection strength between IPS and ACC predicted the individual number of items held in memory. These findings are discussed within the theoretical account of a neural distributed model of VWM.

Reward motivation and neurostimulation interact to improve working memory performance in healthy older adults: A simultaneous tDCS-fNIRS study.

Several studies have evaluated the effect of anodal transcranial direct current stimulation (tDCS) over the prefrontal cortex (PFC) for the enhancement of working memory (WM) performance in healthy older adults. However, the mixed results obtained so far suggest the need for concurrent brain imaging, in order to more directly examine tDCS effects. The present study adopted a continuous multimodal approach utilizing functional near-infrared spectroscopy (fNIRS) to examine the interactive effects of tDCS combined with manipulations of reward motivation. Twenty-one older adults (mean age = 69.7 years; SD = 5.05) performed an experimental visuo-spatial WM task before, during and after the delivery of 1.5 mA anodal tDCS/sham over the left prefrontal cortex (PFC). During stimulation, participants received performance-contingent reward for every fast and correct response during the WM task. In both sessions, hemodynamic activity of the bilateral frontal, motor and parietal areas was recorded across the entire duration of the WM task. Cognitive functions and reward sensitivity were also assessed with standard measures. Results demonstrated a significant impact of tDCS on both WM performance and hemodynamic activity. Specifically, faster responses in the WM task were observed both during and after anodal tDCS, while no differences were found under sham control conditions. However, these effects emerged only when taking into account individual visuo-spatial WM capacity. Additionally, during and after the anodal tDCS, increased hemodynamic activity relative to sham was observed in the bilateral PFC, while no effects of tDCS were detected in the motor and parietal areas. These results provide the first evidence of tDCS-dependent functional changes in PFC activity in healthy older adults during the execution of a WM task. Moreover, they highlight the utility of combining reward motivation with prefrontal anodal tDCS, as a potential strategy to improve WM efficiency in low performing healthy older adults.

On pacing trials while scanning brain hemodynamics: The case of the SNARC effect.

Experimental designs used to describe psychological effects on overt human behavior are seldom suited to localize their corresponding neural substrates based on the analysis of stimulus-evoked brain hemodynamic responses. This is because stimuli in behavioral studies are usually separated by intertrial intervals (ITIs) in the order of 1 second or so following a behavioral response, which is notoriously too brief a time to detect a corresponding hemodynamic response. In fact, a solution commonly adopted in neuroimaging studies is to prolong the ITI up to several seconds. In doing so, the consequences of ITI variations between behavioral and neuroimaging design variants are either benignly neglected or explicitly assumed to be negligible. Here, we provide a systematic investigation of the consequence of manipulating ITI in a design optimized to study a well-established and highly replicable psychological phenomenon-the spatial numerical association of response codes (SNARC). The present exploration encompassed standard estimates of the SNARC effect (i.e., on reaction times and accuracy), estimates of ITI effects on the emotional state of participants before and after performing the SNARC task, as well as the degree of perceived task difficulty. The results showed that, in striking contrast to the common wisdom about the nil role of ITI, the substantial number of parametric differences observed between the two ITI conditions suggests that ITI plays a critical role in shaping the meaning of hemodynamic correlate of a psychological, at least the SNARC, effect.

The SNARC effect is not a unitary phenomenon.

Models of the spatial-numerical association of response codes (SNARC) effect-faster responses to small numbers using left effectors, and the converse for large numbers-diverge substantially in localizing the root cause of this effect along the numbers' processing chain. One class of models ascribes the cause of the SNARC effect to the inherently spatial nature of the semantic representation of numerical magnitude. A different class of models ascribes the effect's cause to the processing dynamics taking place during response selection. To disentangle these opposing views, we devised a paradigm combining magnitude comparison and stimulus-response switching in order to monitor modulations of the SNARC effect while concurrently tapping both semantic and response-related processing stages. We observed that the SNARC effect varied nonlinearly as a function of both manipulated factors, a result that can hardly be reconciled with a unitary cause of the SNARC effect.

Backward masking interrupts spatial attention, slows downstream processing, and limits conscious perception.

The attentional blink (AB) is a difficulty in correctly processing a target when it follows one or more other targets after a short delay. When no backward mask is presented after the last critical target, there is no or little behavioral AB deficit. The mask plays an important role in limiting conscious access to target information. In this electrophysiological study, we tested the impact of masking on the deployment and engagement of attention by measuring the N2pc and P3 components in an RSVP paradigm. We found that the presence of a mask in an AB paradigm reduced the amplitude of the N2pc, P3a, and P3b components. In addition to reducing encoding in memory, masking also reduced the effectiveness of the deployment and engagement of attention on the last target. We discuss the role of these findings in the context of current masking, consciousness, and AB models.

On the Role of the Inferior Intraparietal Sulcus in Visual Working Memory for Lateralized Single-feature Objects.

A consolidated practice in cognitive neuroscience is to explore the properties of human visual working memory through the analysis of electromagnetic signals using cued change detection tasks. Under these conditions, EEG/MEG activity increments in the posterior parietal cortex scaling with the number of memoranda are often reported in the hemisphere contralateral to the objects' position in the memory array. This highly replicable finding clashes with several reported failures to observe compatible hemodynamic activity modulations using fMRI or fNIRS in comparable tasks. Here, we reconcile this apparent discrepancy by acquiring fMRI data on healthy participants and employing a cluster analysis to group voxels in the posterior parietal cortex based on their functional response. The analysis identified two distinct subpopulations of voxels in the intraparietal sulcus (IPS) showing a consistent functional response among participants. One subpopulation, located in the superior IPS, showed a bilateral response to the number of objects coded in visual working memory. A different subpopulation, located in the inferior IPS, showed an increased unilateral response when the objects were displayed contralaterally. The results suggest that a cluster of neurons in the inferior IPS is a candidate source of electromagnetic contralateral responses to working memory load in cued change detection tasks.

Enhanced frontal activation underlies sparing from the attentional blink: Evidence from human electrophysiology.

Using the ERP method, we examined the processing operations elicited by stimuli that appear within the same temporal attention window. Forty subjects searched for letter targets among digit distractors displayed in rapid serial visual presentation (RSVP). ERPs were examined under conditions where a single target was embedded among distractors and compared to those recorded when two consecutive targets were embedded among distractors. Standard and independent component analyses revealed two temporally and topographically distinct ERP responses, a midfrontal P3a component peaking at about 300 ms followed by a midparietal P3b component peaking at about 450 ms. With minimal latency variations, the frontal P3a was amplified when elicited by two consecutive targets relative to a single target. The parietal P3b response was also amplified when elicited by two consecutive targets compared to a single target but, in contrast to P3a, it was also associated with a substantially longer time course. These results provide evidence for the involvement of frontal brain regions in the close-to-concurrent selection of two consecutive targets displayed in RSVP, and of posterior brain regions in the serial encoding of targets in visual working memory. The present findings are discussed in relation to current models of temporal gating of attention and the attentional blink effect.

Long-term continuous monitoring of the preterm brain with diffuse optical tomography and electroencephalography: a technical note on cap manufacturing.

Diffuse optical tomography (DOT) has recently proved useful for detecting whole-brain oxygenation changes in preterm and term newborns' brains. The data recording phase in prior explorations was limited up to a maximum of a couple of hours, a time dictated by the need to minimize skin damage caused by the protracted contact with optode holders and interference with concomitant clinical/nursing procedures. In an attempt to extend the data recording phase, we developed a new custom-made cap for multimodal DOT and electroencephalography acquisitions for the neonatal population. The cap was tested on a preterm neonate (28 weeks gestation) for a 7-day continuous monitoring period. The cap was well tolerated by the neonate, who did not suffer any evident discomfort and/or skin damage. Montage and data acquisition using our cap was operated by an attending nurse with no difficulty. DOT data quality was remarkable, with an average of 92% of reliable channels, characterized by the clear presence of the heartbeat in most of them.

Lack of visual field asymmetries for spatial cueing in reading parafoveal Chinese characters.

In two experiments, we investigated whether visual field (VF) asymmetries of spatial cueing are involved in reading parafoveal Chinese characters. These characters are different from linearly arranged alphabetic words in that they are logograms that are confined to a constant, square-shaped area and are composed of only a few radicals. We observed a cueing effect, but it did not vary with the VF in which the Chinese character was presented, regardless of whether the cue validity (the ratio of validly to invalidly cued targets) was 1:1 or 7:3. These results suggest that VF asymmetries of spatial cueing do not affect the reading of parafoveal Chinese characters, contrary to the reading of alphabetic words. The mechanisms of spatial attention in reading parafoveal English-like words and Chinese characters are discussed.

The attentional blink impairs detection and delays encoding of visual information: evidence from human electrophysiology.

This article explores the time course of the functional interplay between detection and encoding stages of information processing in the brain and the role they play in conscious visual perception. We employed a multitarget rapid serial visual presentation (RSVP) approach and examined the electrophysiological P3 component elicited by a target terminating an RSVP sequence. Target-locked P3 activity was detected both at frontal and parietal recording sites and an independent component analysis confirmed the presence of two distinct P3 components. The posterior P3b varied with intertarget lag, with diminished amplitude and postponed latency at short relative to long lags-an electroencephalographic signature of the attentional blink (AB). Under analogous conditions, the anterior P3a was also reduced in amplitude but did not vary in latency. Collectively, the results provide an electrophysiological record of the interaction between frontal and posterior components linked to detection (P3a) and encoding (P3b) of visual information. Our findings suggest that, although the AB delays target encoding into working memory, it does not slow down detection of a target but instead reduces the efficacy of this process. A functional characterization of P3a in attentive tasks is discussed with reference to current models of the AB phenomenon.

The attentional blink freezes spatial attention allocation to targets, not distractors: evidence from human electrophysiology.

Previous work found a significant reduction of the amplitude of the N2pc ERP component during the attentional blink in response to lateral visual targets, suggesting that the allocation of attention to visual targets is impaired during the attentional blink. Recent theorizing on the processes reflected by the N2pc suggests the possibility of distinct sets of neural mechanisms underlying its generation, one responsible for target activation, and one for distractor inhibition. To disentangle whether either or both of these mechanisms are impaired during the attentional blink, an RSVP sequence of circles, equidistant from fixation was used. The first target frame (T1) contained the same repeated target colour circle and target whereas the second target frame (T2) contained a distractor colour singleton as well as a target colour singleton. Only the target or only the distractor was presented at a lateral position; the other singleton was presented on the vertical midline so as not to elicit any event-related lateralization. Impaired T2 report accuracy at a short stimulus-onset asynchrony (SOA) was accompanied by a significant delay of the N2pc to lateral T2 targets when compared to a long SOA condition. No such delay was found when the lateralized stimulus was a distractor, suggesting that the attentional blink impacts attention allocation to targets, not distractors. We also observed a lateralized component earlier than the N2pc, a posterior contralateral positivity (Ppc) that did not depend on T1-T2 SOA and that was elicited by both lateral targets and distractors. We conclude that, contrary to N2pc, the Ppc likely reflects activity of bottom-up mechanisms responding unselectively to asymmetrical visual displays.