A Bilateral SPCN Is Elicited By To-be-memorized Visual Stimuli Displayed Along The Vertical Midline Part 1

Abstract

We recently showed that deploying attention to target stimuli displayed along the verticalmeridianelicits abilateralN2pc,thatwe labeledN2pcb(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. 

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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.

KEYWORDS

Cued visual search, ERPs, SPCN, visual working memory.

1 | INTRODUCTION

In order to identify visual stimuli of interest, we are required to scan our complex environment. In most cases, finding such objects does not seem to pose any insurmountable obstacle to our daily living. At the neural level, however, visual search involves a complex set of processes required to maintain a stable representation of the visual environment in spite of the massive changes of the retinal images caused by head and/or eye movements(e.g., Henderson, 2008; Hollingworth et al.,  2008). 

Visuospatial attention and visual working memory are said to play a crucial role in these processes,with visuo-spatial attention often described as a filter set to individuate target stimuli, and visual working memory as a system optimized to maintain target information in a representationalstate amenable to further,higher-level processing.

Studying visual attention and visual working memory in the lab using event-related potentials (ERPs) has advanced our understanding of both these key aspects of human cognition, especially after the discovery that each of them is associated with a distinctive ERP signature. The ERP signature of the deployment of visuo-spatial attention to candidate targets is the N2pc component (Eimer, 1996; Luck & Hillyard, 1994). 

N2pc is often studied in the context of visual search tasks. When a target is displayed laterally relative to fixation,N2pc manifestsitself as a transient negativity enhancement usually unfolding in a 200–300  ms timewindow at parieto-occipital sites (i.e., PO7/PO8) contralateral to the visual hemifield in which the target is displayed. The ERP signature of the active maintenance of a laterally displayed stimulus in visual working memory is the sustained posterior contralateral negativity component (SPCN; Jolicœur et al.,  2008; alternatively named contralateral delay activity, or CDA, byVogel & Machizawa, 2004; contralateral negative slow wave, or CNSW, by Klaver et al., 1999; contralateralsearch activity, or CSA, by Emrich et al., 2009). 

SPCN was initially explored using cued change detection tasks, in which subjects are typically cued to memorize objects displayed in either visual hemifield for later comparison with objects that can unpredictably remain the same or one of which can be changed. SPCN is often detected at the same recording sites as those used to observe N2pc (i.e., PO7/PO8) and,similarly to N2pc, manifestsitself as a larger negativity contralateral to the visual hemifield in which target information is displayed. This surface similarity aside, SPCN onsets later (at about 400 ms1 ) and lasts substantially longer than N2pc, namely, as long as objects are retained in visual working memory (see Luria et al., 2016, for a comprehensive review). 

Furthermore, unlike N2pc,2 a distinctive feature of SPCN is that its amplitude increases as the number of objects to be retained in memory is increased, as long as this number does not exceed an individual's visual working memory capacity (Vogel & Machizawa,  2004), which averages to about 3 objects across individuals (Balaban et al., 2019; Cowan, 2001).

Source localization analyses of MEG recordings have localized the neural generators of the N2pc in the extrastriate visual cortex, in the infero-temporal cortex, with a possible early parietal contribution (Hopf et al.,  2000, 2002, 2006; Jolicœur et al.,  2011). MEG and fMRI recordings concur that the neural generators of SPCN are located in the parietal cortex, in the intra-parietal sulcus in particular, and in more lateral/ventral regions also involved in the generation of N2pc activity (Becke et al.,  2015; Brigadoi et al.,  2017; Duma et al.,  2019; Jolicœur et al.,  2011; Naughtin et al.,  2016; Robitaille et al.,  2010; Todd & Marois,  2004; Xu & Chun,  2006). 

Although some uncertainty remains as to whether N2pc and SPCN have exactly the same or slightly different neural sources, it is important for the present purposes to note that the receptive fields of neurons located in the aforementioned regions and receiving inputs from foveal retinal receptors extend into the ipsilateral hemifield, a subset of them for as much as 2° of visual angle (Hubel & Wiesel,  1967; Nakamura et al.,  2007; Papaioannou & Luck, 2020; Wandell et al., 2007; Zeki, 1993). As a result, visual input displayed along (or close to) the vertical meridian activates homologous neurons located in posterior regions of both hemispheres, and is therefore bilaterally represented in the posterior cortex.

Doro et al.  (2020) have recently explored whether N2pc reflects this neuroanatomical organization of the receptive fields of neurons underpinning the selection and encoding phases of target information. Using a visual search task in which singleton or feature targets could be displayed laterally or aligned to the vertical meridian, we observed N2pc activity in its classical form, namely, as a larger negativity for contralateral relative to ipsilateral PO7/PO8 recording sites when targets were displayed laterally relative to the vertical meridian. 

Targets displayed along the vertical meridian elicited a bilateral negativity, that we quantified as the average activity detected at PO7 and PO8, that was undistinguishable from the contralateral negativity elicited by lateral targets. This pattern suggested that “midline” targets elicit a bilateral N2pc (or N2pcb; Doro et al., 2020; Monnier et al., 2020) that, like N2pc (e.g., Feldmann-Wüstefeld & Schubö, 2015; Mazza et al.,  2009), onsets earlier in singleton search than in feature search. Evidence for the supposed similarity between N2pc and N2pcb has also been reported by Monnier et al. (2020), who showed that N2pc and N2pcb share an additional property. It is now well established that the amplitude of N2pc is substantially reduced, sometimes even reversed in polarity, for lateral targets displayed above the horizontal meridian, that is, in the upper visual hemifield, compared to those displayed below the horizontal meridian, that is, in the lower visual hemifield (e.g., Bacigalupo & Luck, 2019; Luck et al., 1997). 

A likely explanation of this N2pc asymmetry refers to the neuroanatomical organization of the retinotopic topography in the posterior cortex. Stimuli in the lower visual field project to more dorsal regions of the posterior cortex, whereas stimuli in the upper visual field project to more ventral regions of the posterior cortex. Relative to ventral regions, dorsal regions are closer to the scalp, and this explains why N2pc can be more easily detected for stimuli in the lower visual field compared to stimuli in the upper visual field. 

In fact, using a singleton search design, Monnier et al. (2020) observed a fully-fledged N2pc for lateral targets in the lower visual hemifield, and an N2pc polarity reversal for lateral targets in the upper visual hemifield (i.e., a contralateral positivity). Critically, an identical pattern was observed for N2pcb for midline targets when these targets were presented above versus below fixation, a result that was interpreted as suggesting a similarity of the neural sources of N2pc and N2pcb.

The issue at stake in the present context is the lack of a test for SPCN conceptually analogous to those provided by Doro et al. (2020) and Monnier et al. (2020) for N2pc. Would a midline stimulus that must be retained in visual working memory elicit a bilateral SPCN (or SPCNb) of equal amplitude compared to the contralateral portion of the SPCN elicited by a lateral stimulus? Moreover, would SPCNb share with SPCN the peculiar property to scale in amplitude with the number of midline visual stimuli? Of course, given the overlap, or close proximity, of the neural generators of N2pc and SPCN activity, the expected answers to both these questions are in the positive. 

Perhaps, an issue that warrants close inspection in relation to the possible distinction of the neural sources of N2pc and SPCN would be to observe a different modulation of N2pc and SPCN asfar asthe vertical elevation of the visualstimuli is concerned. Would the amplitude of SPCN/SPCNb — similarly to the amplitude of N2pc/N2pcb — be reduced to nil, or even reversed in polarity, for stimuli displayed in the upper visual hemifield compared to SPCN/SPCNb elicited by stimuli displayed in the lower visual hemifield? To answer all these questions, we employed a cued visual search task akin to that of Carlisle et al. (2011), that is illustrated in Figure 1.

One or two colored squares (cues) with a gap on one side were displayed either on the horizontal meridian (left or right of fixation) or on the vertical meridian (above or below fixation) at the beginning of each trial. The cues of given color (e.g., green) indicated the candidate target(s),and subjects were instructed to memorize the position of the gap(s) for later search in an array composed of uniformly white distractor gapped squares, accompanied by a differently colored (blue) distractor in the opposite hemifield so as to avoid sensory imbalance. 

The task required first to select the candidate target(s) based on color, to keep the information about the gap position(s) in memory for a short interval (1 s), and finally to inspect a square of the same color as the cue(s) for a correspondence in gap position. The information needed to answer all the above questions were extracted from ERP activity time-locked to the cue array onset. We estimated SPCN activity in the typical form, as the difference between ERP activity contralateral and ipsilateral to lateral cues recorded at PO7/PO8 electrodes. 

Similarly to how Doro et al.  (2020) estimated N2pcb activity to midline search targets in their design, we estimated SPCNb activity asthe difference between the bilateral ERP activity recorded at PO7/PO8 electrodes elicited by cues displayed on the vertical meridian and the ipsilateral ERP activity elicited by lateral cues. We expected to find clear SPCN activity during the retention of lateral cues that should be larger for two cues than for one cue, as reported by Carlisle et al. (2011). The new question asked here was whether we would find SPCNb activity of similar amplitude when the cues were presented aligned to the vertical midline. As argued in the foregoing introduction, this is what we expected, and in fact what we found.

2 | METHOD

2.1 | Participants

Twenty-one students at the Guangzhou University (4 males; mean age = 23 years, SD = 2.4) took part in the present experiment after providing written informed consent. All participants had normal or corrected-to-normal visual acuity, and all reported normal color vision and no history of neurological disorders. The experiment was vetted by the local ethics committee.

2.2 | Stimuli and procedure

An example of the stimuli and an illustration of the sequence of events on four trials in the experiment are shown in Figure 1. The stimuli were displayed on the black background (CIE: 0.312/0.329, 1.0 cd/m2 ) of a 17” CRT computer monitor with a refresh rate of 60 Hz, at a viewing distance of about 60 cm. The stimuli in the cue array (marked by the cyan bar on the timeline in Figure 1) were 2 or 4 equiluminant outlined squares (1.2° ×  1.2°, 0.2° line thickness), colored in green (CIE: 0.278/0.393,20 cd/m2 ) or in blue (CIE: 0.213/0.272, 20 cd/m2 ) with a gap (0.3°) on the left, right, top, or bottom side.
When the cue array was composed of 2 gapped squares, each gapped square was displayed 3.5° to the left/right or above/below the center of the monitor. When the cue array was composed of 4 gapped squares, the 2 more eccentric gapped squares were presented 3.5° to the left/right or above/ below the center of the monitor and the 2 less eccentric gapped squares were presented 1.8° to the left/right or above/below the center of the monitor. The stimuli in the search array (marked by the orange bar on the timeline in Figure 1) were 12 gapped squares identical in dimension to those composing the cue array, 10 of which were displayed in white (CIE: 0.313/0.329, 90 cd/m2 ), with the addition of two gapped squares, one blue and one green (same colors as the cues) always displayed laterally (i.e., left/right) on opposite sides relative to the center of the monitor. 

The stimuli in the search array were arranged along a notional circle of 5.8° in diameter and positioned in correspondence to the number locations on a clock face. With the exception of the positions aligned to the vertical meridian (i.e., the positions at 12 and 6 o'clock), all other positions on opposite sides relative to the center of the screen were equally likely to be occupied by the blue and green gapped squares.

Prior to the beginning of the experiment, each participant was informed about the task-relevant color (i.e., either blue or green, counterbalanced across participants) designating cues and targets in the cue and search arrays, respectively. For each participant, the task-relevant color was kept constant for the entire experiment. Each trial began with the presentation of a white fixation dot (0.4° × 0.4°) at the center of the monitor. Participants were instructed to maintain gaze on the fixation dot, avoiding head and/or eye movements until the end of the trial. 

Participants started each trial by pressing the spacebar using the thumb of the left orrighthand. Afterthe spacebar press, an interval of 500–800 ms (randomly jittered using a rectangular distribution) elapsed before the onset of the cue array, which was exposed for 200 ms. Participants had to memorize the position of the gap(s) of the cue(s) in the task relevant color. Participants had therefore to memorize the gap position of 1 cue (1C trials in Figure 1) or the gap positions of 2 cues (2C trials in Figure 1). The cues in the cue array could unpredictably and with equal probability be presented on the horizontal meridian (i.e., to the left/ right of fixation) or on the vertical meridian (i.e., above/ below fixation). 

The gap position(s) of the cue(s) in the cue array had to be memorized regardless of their spatial arrangement. The cue array was followed by an interval of 1000 ms, followed by the onset of the search array that was exposed for 2000 ms. On half of the trials, the search array contained a target, that is, a gapped square identical to the cue in 1C trials, or to either cue in 2C trials. On the other half of the trials, the target was absent. In the search array, the gap position of the (e.g., blue) cue never matched that of the (green) distractor. 

Participants were instructed to use the ‘L’ or ‘A’ of the computer keyboard (counterbalanced across participants) to indicate whether a target was present or absent, with equal emphasis on response speed and accuracy. Following the detection of the participant's response, the fixation dot disappeared and an inter-trial interval of 1000 ms elapsed before the presentation of the fixation dot indicating the beginning of the next trial. Participants were informed that, during the intertrial interval, they were allowed to make eye blinks.

Participants performed a total of 10 blocks of 96 experimental trials each. Half of the participants started with 5 blocks of 1C trials, followed by 5 blocks of 2C trials. This order wasreversed forthe other half of the participants. Each series of 5 blocks was preceded by 18 to 24 1C or 2C practice trials, depending on which trials participants had to perform in the following blocks. Participants were informed they could take a short break between one block and the next.

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