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

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.

We challenge the view that the SPCN ERP component cannot track the memory maintenance of objects displayed along the vertical meridian. Owing to the receptive fields of posterior neurons straddling on the intersection of the two visual hemifields, bilateral N2pc (N2pcb) and SPCN (SPCNb) activity can be detected using a cued visual search design.

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.

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.