Abstract

In dynamically changing environments, spatial attention is not equally distributed across the visual field. For instance, when two streams of stimuli are presented left and right, the second target (T2) is better identified in the left visual field (LVF) than in the right visual field (RVF). Recently, it has been shown that this bias is related to weaker stimulus-driven orienting of attention toward the RVF: The RVF disadvantage was reduced with salient task-irrelevant valid cues and increased with invalid cues. Here we studied if also endogenous orienting of attention may compensate for this unequal distribution of stimulus-driven attention. Explicit information was provided about the location of T1 and T2. Effectiveness of the cue manipulation was confirmed by EEG measures: decreasing alpha power before stream onset with informative cues, earlier latencies of potentials evoked by T1-preceding distractors at the right than at the left hemisphere when T1 was cued left, and decreasing T1- and T2-evoked N2pc amplitudes with informative cues. Importantly, informative cues reduced (though did not completely abolish) the LVF advantage, indicated by improved identification of right T2, and reflected by earlier N2pc latency evoked by right T2 and larger decrease in alpha power after cues indicating right T2. Overall, these results suggest that endogenously driven attention facilitates stimulus-driven orienting of attention toward the RVF, thereby partially overcoming the basic LVF bias in spatial attention.

INTRODUCTION

In environments with multiple sources of information, there is competition between stimuli for attentional resources (Desimone & Duncan, 1995). By pushing attention to its limits, it has been shown that stimuli in the left visual field (LVF) win this competition over stimuli in the right visual field (RVF; Scalf, Banich, Kramer, Narechania, & Simon, 2007; Holländer, Corballis, & Hamm, 2005; Evert, McGlinchey-Berroth, Verfaellie, & Milberg, 2003). The dual-stream rapid serial visual presentation task (dual-stream RSVP) is a useful method to study such LVF bias. In this task, the participants are confronted with two streams of black letters (left and right from fixation) that are changing very rapidly (e.g., every 130 msec). Two targets embedded in these letters have to be identified, T1 and T2. The positions of both targets within one stream and between the two streams vary randomly; therefore, both streams have to be constantly monitored. In our version of the task, T1 is a letter of distinctive color and T2 is a black digit. Identification of T1 is usually good and equally distributed across visual fields. However, T2 is better identified when presented in the LVF than in the RVF (Matthews & Welch, 2015; Asanowicz, Śmigasiewicz, & Verleger, 2013; Verleger, Dittmer, & Śmigasiewicz, 2013; Verleger, Śmigasiewicz, & Möller, 2011; Śmigasiewicz et al., 2010; Verleger et al., 2009). This LVF advantage is especially large when T2 follows T1 with short lag in the opposite stream but remains constant across longer lags and the two spatial relations between T1 and T2 (i.e., same or opposite stream; Verleger et al., 2009, 2011).

There is agreement in the literature that the RH plays a dominant role during spatial attention processes, as it has been evidenced by numerous studies on the syndrome of hemineglect in clinical populations (Danckert & Ferber, 2006; Driver & Vuilleumier, 2001), pseudoneglect phenomena in nonclinical populations (e.g., Benwell, Harvey, & Thut, 2014), neuroimaging (Nobre et al., 1997; Corbetta, Miezin, Shulman, & Petersen, 1993), and behavioral experiments (e.g., Du & Abrams, 2010). It is probably this dominance of the RH that is expressed in dual-stream RSVP as the LVF advantage in the identification of T2. Indeed, the LVF advantage turned out to be independent from many factors such as eye movements (Verleger et al., 2009), overload of the left hemisphere (LH) with verbal input (Asanowicz et al., 2013), negative and positive priming (Verleger, Śmigasiewicz, Michael, & Niedeggen, 2012), and mutual inhibition between hemispheres (Śmigasiewicz, Weinrich, Reinhardt, & Verleger, 2014; Verleger et al., 2010) and only slightly depended on learned habits of reading direction (Śmigasiewicz et al., 2010; for a review of factors influencing the LVF advantage, see Verleger & Śmigasiewicz, 2015). In a recent study, it has been shown that weaker stimulus-driven orienting of attention toward the RVF than the LVF can account for this bias (Śmigasiewicz, Asanowicz, Westphal, & Verleger, 2015). In this study, a small red frame was presented either around the fixation cross (neutral control) or around the stimulus in one of the lateral streams 50 msec before T2 onset, serving as a salient cue attracting attention reflexively. T2 appeared in the same stream as the cue (valid cueing) or in the opposite stream (invalid cueing). If the LVF advantage is related to stimulus-driven orienting of attention then, by attracting attention reflexively, valid cues will improve and invalid cues will decrease the identification of right T2. This is because with valid cues T2 appears in the focus of attention and no further shifts of attention are required (therefore, there should be no difference between left and right T2s). However, with invalid cues, attention will be attracted to the wrong location and the appearance of T2 will trigger reorienting of attention, which will be weaker or/and slower toward the RVF. This hypothesis was confirmed: with valid cues left and right T2 were almost equally well identified and with invalid cues the identification of right T2 was particularly impaired leading to an increased LVF advantage.

On the basis of these results, the question arises whether the LVF advantage might be reduced with attention endogenously oriented to T2's location. Endogenous orienting is usually studied in a paradigm where a centrally presented cue (e.g., an arrow) indicates the location of the next target with high probability (Posner, 1980). RTs are shorter and accuracy is higher for targets presented at the cued than at the uncued location (Posner, Snyder, & Davidson, 1980). This is because shifting attention to the cued location before target onset facilitates perception of the target (Eimer, 1993; Heinze, Luck, Mangun, & Hillyard, 1990). During the dual-RSVP task endogenous orienting might reduce the LVF advantage by facilitating stimulus-driven orienting toward the RVF. In line with this hypothesis are results of Experiment 2 reported by Verleger et al. (2009). In that experiment in one block of trials T2 followed T1 always in the same stream and in other block always in the other stream. T2 identification improved in the latter block, because participants knew that they had to shift attention to the other stream after T1 (such improvement was not observed in the block with T1 and T2 presented in the same stream, probably because accuracy in this condition was already relatively high). These results might suggest that the allocation of attention to the RVF initiated before target onset facilitated stimulus-driven orienting triggered by this target. There is no agreement in the literature whether endogenous attention does or does not facilitate exogenous attention. Usually independent influences of both orienting systems on target processing were obtained (Berger, Henik, & Rafal, 2005, Experiments 1–3; Chica, Botta, Lupiáñez, & Bartolomeo, 2012; Lupiáñez et al., 2004; Riggio & Kirsner, 1997). It is only sometimes that interactions have been reported. For instance, Berger et al. (2005, Experiment 4) demonstrated that during difficult task conditions exogenous cues had larger effects when preceding endogenous cues were valid rather than invalid. In a study measuring ERPs, Hopfinger and West (2006) observed enhanced processing of exogenous cues as well as of consecutive stimuli when presented at locations pointed to by preceding valid endogenous cues. This enhancement was reflected in increased positivity at 80–120 msec in the ERPs evoked by exogenous cues and at 150 msec in ERPs evoked by their following stimuli.

The aim of the present experiment was to study the influence of endogenous orienting of attention on the LVF advantage. To this end, cues were administered before trial onset, indicating with 100% certainty the location of T1 and T2. Importantly, if the LVF advantage can be at least partially overcome by endogenous orienting of attention, then with informative cues, the identification of right T2 will be particularly improved.

Insight into neural processes underlying the unequal distribution of spatial attention has been provided by EEG, in particular by three electrophysiological markers: the N2pc component, visual evoked potentials (VEPs), and power in alpha oscillations. The most reliable neural correlate of the LVF advantage is the N2pc component of ERPs evoked by T2 (Verleger, Dittmer, et al., 2013; Verleger, Talamo, Simmer, Śmigasiewicz, & Lencer, 2013; Verleger et al., 2009, 2011). N2pc is a negative deflection appearing at about 200 msec after target onset and is measured at lateral sites overlying temporo-occipital cortex as the difference between electrodes contralateral minus ipsilateral to the target (Eimer, 1996; Wascher & Wauschkuhn, 1996; Luck, Fan, & Hillyard, 1993). N2pc reflects attentional selection needed for identification of relevant stimuli (Tan & Wyble, 2015; Verleger et al., 2009, 2011; Eimer, 1996). In the dual-stream RSVP task, N2pc has shorter latencies when evoked by left T2 than by right T2, suggesting that left T2 is selected faster by the right hemisphere (RH) than right T2 by the LH (Śmigasiewicz et al., 2015; Verleger, Dittmer, et al., 2013; Verleger et al., 2009, 2011). As a neural correlate of the LVF advantage, the latency of N2pc should be modulated along with the size of the LVF bias. However, such modulation was not observed in the study with salient cues (Śmigasiewicz et al., 2015), possibly because the negativity evoked by T2 could not be successfully separated from the overlapping negativity evoked by cues. In the current experiment, cues are temporally separated from targets and thus no overlap with their activity should preclude N2pc measurement. Therefore, we expect that if the LVF advantage will be reduced with cues engaging endogenous orienting of attention, then also the latency difference between N2pc evoked by left and right T2 should be reduced. Informative cues, if effective, should also modulate N2pc evoked by T1 and T2 by decreasing its amplitude (Praamstra, 2006).

From the beginning of the trial onwards each pair of distractor stimuli evokes P1–N1 complexes. These VEPs recorded above lateral temporo-occipital cortex were slightly but significantly leading in the RH over the LH (Śmigasiewicz et al., 2014; Verleger, Dittmer, et al., 2013; Verleger et al., 2011). It has been hypothesized that this predominance of the RH in VEP latencies might be related to its better ability in constructing percepts at early stages of information processing (Verleger et al., 2011) or to the attentional bias toward the LVF already at the beginning of the trial (Śmigasiewicz et al., 2014). If reflecting attentional bias, these VEPs should be sensitive to the manipulation with endogenous orienting of attention. Thus, we expect to replicate the latency difference between RH and LH with uninformative cues and to obtain increased differences with cues directing attention to the left stream and decreased differences with cues directing attention to the right stream.

The oscillations of human EEG in the alpha range (8–12 Hz) have been associated with several cognitive processes like semantic processing and memory (Klimesch, 1999; Klimesch, Doppelmayr, Pachinger, & Russegger, 1997), attention (Yordanova, Kolev, & Polich, 2001; Klimesch, Doppelmayr, Russegger, Pachinger, & Schwaiger, 1998), and visual perception (Hanslmayr, Gross, Klimesch, & Shapiro, 2011; Kranczioch, Debener, Maye, & Engel, 2007). Power decreases or “event-related desynchronization” (ERD) in the alpha band in the prestimulus interval have been shown to predict successful target identification (Chaumon & Busch, 2014; Hanslmayr et al., 2007). Therefore, high amplitudes of alpha oscillations are interpreted as indicating an internally oriented brain state and low amplitudes an externally oriented brain state ready to process the external stimulation (Hanslmayr et al., 2011). ERD during the cue–target interval in attention cuing paradigms has been interpreted as a neural mechanism indicating the allocation of spatial attention: Alpha desynchronization in the hemisphere contralateral to targets was linked to facilitation in processing of attended stimuli (Van der Lubbe & Utzerath, 2013; Kelly, Gomez-Ramirez, & Foxe, 2009; Yamagishi, Callan, Anderson, & Kawato, 2008; Thut, Nietzel, Brandt, & Pascual-Leone, 2006), and alpha synchronization in the hemisphere ipsilateral to targets was linked to suppression of distracting stimuli (Rihs, Michel, & Thut, 2007; Kelly, Lalor, Reilly, & Foxe, 2006; Worden, Foxe, Wang, & Simpson, 2000). Such lateralization of ERD in the alpha range was replicated for endogenous orienting of attention (Gould, Rushworth, & Nobre, 2011) as well. Importantly, Janson, De Vos, Thorne, and Kranczioch (2014) interpreted the suppression of the amplitude of alpha oscillation before stimulus presentation in the RSVP task as reflecting preparatory processes. Thus, in the current experiment power of alpha oscillation in the interval between cue onset and onset of the stimulus streams will be analyzed. We expect to observe a decrease in alpha power after informative cues as compared with uninformative cues. If such decrease will be lateralized to the cued target location, it will be interpreted as reflecting allocation of spatial attention. If decreases in alpha power will be equally distributed across both hemispheres, they might be interpreted as reflecting preparatory processes, such as general effort invested or unspecific (not spatial) attention paid to the task.

METHODS

Participants

Sixteen students from the University of Lübeck (eight men, mean age = 24 years, SD = 2.1 years) participated in the study. They provided informed written consent and reported normal or corrected-to-normal vision and no history of neurological disorders. In the end of the experiment, they were paid €7 per hour. All participants were right-handed, with an average score in the Edinburgh Handedness Inventory (Oldfield, 1971) of 80 (SD = 20). Four other participants were rejected from analysis: two because of systematic eye movements toward T1 and two because of poor identification of T1 (both below 2 SDs of the other participants).

Stimuli and Apparatus

Figure 1 illustrates the task. Stimuli were displayed on the computer 16-in. CRT screen, driven with 100 Hz. Participants were seated about 1.2 m from the monitor. With this distance 1 cm on the screen spanned 0.5°. A fixation point (a 3 × 3 mm small cross) was displayed at the center of the screen. Stimuli consisted of capital letters of the Latin alphabet and the digits 1–6 in Helvetica 35 font. They were displayed on the white background in the left and right stream with respect to the fixation point. Letter stimuli were 11 mm high and maximally 9 mm wide (their midpoints were 16 mm from fixation). In each trial, two targets had to be identified: The first target (T1) was a blue letter (D, F, G, J, K, L); the second target (T2) was a black digit (1, 2, 3, 4, 5, 6). The distractor set consisted of all letters presented in black. Cues consisted of words written in capital letters (10 mm high and 8 mm wide): “LINKS,” “RECHTS,” or “BEIDE” (“left,” “right,” and “both” in English), displayed above and below the fixation cross (1.25°). “Left” meant that the target would be presented in the left stream, “right” that it would be presented in the right stream, and “both” that it might appear in either stream. Cues above and below the fixation cross referred to T1 and T2, respectively. Presentation software, version 14.1 (Neurobehavioral Systems, Inc., Berkeley, CA) was used for experimental control.

Figure 1. 

Sequence of events in the experiment. Sequence of events is exemplified for a right T1–left T2 sequence in the two cue conditions: uninformative and informative. The arrow on the left informs about temporal distance between onsets of consecutive stimulus frames. All cues were presented 1800 msec before onset of the first stimulus pair. See Methods for details.

Figure 1. 

Sequence of events in the experiment. Sequence of events is exemplified for a right T1–left T2 sequence in the two cue conditions: uninformative and informative. The arrow on the left informs about temporal distance between onsets of consecutive stimulus frames. All cues were presented 1800 msec before onset of the first stimulus pair. See Methods for details.

Procedure

Participants were seated in a darkened room in front of the computer screen with the keyboard placed on a table. Each trial started with the presentation of the fixation cross in the screen center and of the cues displayed above and below. Participants were asked to fix their gaze on the cross during the entire trial. There were two different cues, in separate blocks: uninformative and informative. Uninformative cues consisted of the words “BEIDE” above and below the fixation cross. Informative cues consisted of the words “LINKS” or “RECHTS,” randomly selected both above (for T1) and below (for T2) the fixation cross. Informative cues indicated with 100% validity the target location. Cues were presented on the screen for 1.8 sec. Simultaneously with their offset, the first frame of distractor stimuli appeared for 130 msec, immediately followed by the next frame. T1 was presented randomly as sixth, eighth, or tenth frame and T2 randomly as first or fourth frame after T1 (Lag 1 or Lag 4, 130 or 520 msec after T1 onset). T2 was followed by five distractor frames. Thus, 12–19 frames of stimuli were displayed. Both targets could appear in the left or right stream (depending on cues) and were accompanied by a letter distractor in the opposite stream. Both targets were randomly selected from the target sets, distractors were randomly selected with replacement from the distractor set (with a restriction against immediate repetition and against equal characters simultaneously in the left and right stream). At the end of each trial, participants were asked to enter their responses on a standard keyboard, first the T1 letter on the middle row and then the T2 digit on the number pad, even if the answer was not known. The next trial started immediately after the T2 response. Eye position was measured by means of a remote infrared eye-tracker (600 series binocular, Eyegaze LC Technologies, Fairfax, VA) and online fed back by software (Interactive Minds, Dresden, Germany), which communicated with the Presentation program. At the beginning of each trial, fixation was checked by the program. In case of a deviation of more than 6 mm from vertical midline, a red exclamation mark was presented at midline, inducing shifts of gaze back to fixation.

The independent variables were Cue (informative, uninformative), Lag between T1 and T2 (1, 4), Other-target Side (same, different), and Target Side (left, right). These 16 conditions were replicated 45 times. The two cue conditions were presented in separate blocks counterbalanced across participants. Within each block, the sequence of the remaining conditions was random. Before the task proper, about 10 trials were presented in slow motion and 10 with original presentation rate for practice.

EEG Recording and Preprocessing

EEG was recorded with Ag/AgCl electrodes (Easycap, www.easycap.de) from 60 scalp sites, which were 8 midline positions from AFz to Oz and 26 pairs of symmetric left and right sites, and from the nose-tip. Online reference was placed at Fz, and after recording data were rereferenced to the nose-tip. Fpz was used as the ground. Four electrodes placed on the face were used to control for eye movements: two electrodes above and below the right eye for vertical EOG and two at the outer canthi for horizontal EOG. Data were amplified from DC to 250 Hz by a BrainAmp MR plus and stored at 500 Hz per channel. Data preprocessing and analysis was done with Brain Vision Analyzer software (version 2.02).

For stimulus-evoked potentials, the low-pass filter was set at 20 Hz. Afterwards segments were defined in each trial: For analysis of VEPs, segments started 100 msec before the first distractor pair and extended till 1600 msec. For analysis of T1- and T2-evoked potentials, segments started 100 msec before T1 onset and extended until 700 msec after T1 for T1-evoked potentials and until 500 msec after T2 for T2-evoked potentials. Bad segments were marked and removed: with zero lines, with overall minimum–maximum voltage differences of ≥200 μV for analysis of VEPs and ≥150 μV for analysis of target-evoked activity, with voltage steps between adjacent data points of ≥50 μV for VEP and ≥30 μV for target evoked potentials, and with absolute amplitudes of ≥100 μV uniquely for target evoked potentials. (Criteria had to be more liberal for VEPs because the VEPs evoked by the first frame were often large.) This artifact rejection procedure was chosen to be conservative enough to allow rejecting segments with blinks. Additionally, for analysis of VEPs, data were high-pass filtered at 3 Hz (IIR Butterworth Zero Phase Filter) to remove slow drifts from the signal. Distortions of the signal because of this filter were negligible and of low importance for the method used to evaluate hemispheric differences in VEPs. Finally, data were referred to the first 100 msec before the first distractor pair for VEPs or 100 msec before T1 onset for N2pc calculation as baseline. For analysis of VEPs, segments were averaged across artifact-free trials irrespective of identification accuracy of T1 or T2. For analysis of T1- and T2-evoked potentials, segments were averaged across artifact-free trials with accurate identifications of both T1 and T2. Averages of differences of contralateral–ipsilateral to T1 were formed for horizontal EOG. Any participant's data were rejected if these averages deviated from baseline by 10 μV within 700 msec after T1 onset, indicating eye movements of ≥0.7° toward T1, which led to excluding 2 of the 20 participants. Subsequently, averaged data were spatially filtered using the current source density (CSD) approach. CSD accentuates local effects while filtering out distant effects because of volume conduction (Kayser & Tenke, 2015). All averaged ERP waveforms at each electrode were transformed into reference-free CSD estimates (μV/m2) using the surface Laplacian algorithm with interpolation by spherical splines (Perrin, Pernier, Bertrand, & Echallier, 1989, 1990) implemented in Brain Vision Analyzer software with the following computation parameters: order of spline 4; maximal degree of Legendre polynomials 10; approximation parameter lambda 1.0e−005.

For cue-evoked EEG oscillations, segments were defined for each trial starting 100 msec before cue onset and extending for 2.1 sec (200 msec after the presentation of the first distractor pair). The data were low-pass filtered at 40 Hz, and ocular artifacts were corrected by linear regression using the method implemented in the Brain Vision Analyzer software. Data were referred to the first 100 msec of the segment as baseline and edited for artifacts by rejecting trials with zero lines and with overall minimum–maximum ≥150 μV.

Data Analysis

For calculation of behavioral parameters, percentages of trials with correct responses were computed in each condition for T1 as T1-correct relative to all trials and for T2 as T1-and-T2-correct relative to all T1-correct trials. ANOVA on behavioral parameters was performed with the repeated measurement factors Cue (uninformative, informative), Lag (1, 4), Other-target Side (same, different), and Target Side (left, right; with target being T1 or T2 depending on the analysis).

T1- and T2-related N2pc was measured separately for the LH and the RH. To avoid contamination caused by constant asymmetries between hemispheres, activity at either hemisphere in the condition with T1 or T2 on the ipsilateral side was subtracted from the activity at the same hemisphere in the condition with T1 or T2 on the contralateral side, for example, PO8 (left T1/T2) − PO8 (right T1/T2) for the RH and PO7 (right T1/T2) − PO7 (left T1/T2) for the LH (Verleger et al., 2011; Oostenveld, Stegeman, Praamstra, & van Oosterom, 2003). N2pc amplitudes and latencies evoked by T1 were measured at the most negative peak in the area 160–280 msec after T1 onset separately for each cue condition. ANOVA had two repeated measurement factors: Cue (informative, uninformative) and Hemisphere (PO8, PO7). N2pc evoked by T2 was measured at Lag 4 only, because N2pc evoked by T2 at Lag 1 could not be reliably separated from N2pc evoked by T1. To improve the signal/noise ratio, parameters of T2-evoked N2pc were measured in one-leave-out grand means (Miller, Ulrich, & Schwarz, 2009; Kiesel, Miller, Jolicœur, & Brisson, 2008). F values had to be corrected for their drastically diminished interindividual variance by division by (n − 1)2 (Ulrich & Miller, 2001). N2pc latency was determined by means of the “50% area” measure, that is, as the time point (relative to T2 onset) that divided the 150–350 msec Voltage × Time area-under-the-waveform into equal halves (Craston, Wyble, Chennu, & Bowman, 2009; Luck & Hillyard, 1990). To obtain reliable estimates of 50% area also in cases where the negative deflection did not span the entire 200 msec epoch, any positive values in this epoch were set to zero. N2pc amplitude was then measured as mean amplitude of the ±25 msec epoch around this 50% area latency. This measurement procedure was performed separately for each condition. ANOVA was performed with the two repeated measurement factors Cue (informative, uninformative), Other-target Side (same, different), and Hemisphere (PO8, PO7). Minimum numbers of trials included in the analysis were 8, with an average of 31 (SD = 9.2).

VEPs consisted of a series of P1 and N1 potentials evoked by the series of distractors from the beginning of the trial every 130 msec. For measurement of the lag between VEPs evoked in the LH and the RH, each participant's waveforms at PO8 and PO7, where VEPs were largest, were shifted against each other in 2-msec steps within ±50 msec, and the shift that rendered the largest cross-correlation was selected (Okon-Singer et al., 2011). Analysis included 800 msec of the wave duration, encompassing five P1 and N1 peaks. (The sixth N1 peak could already contain T1.) Latency shifts were tested against zero by t tests for the uninformative cue condition. Subsequently, an ANOVA was performed on differences between these latency shifts in informative cue conditions and the uninformative cue condition. Repeated measurement factors were Cued T1 Side (left, right) and Cued T2 Side (left, right).

Time–frequency analysis was performed with continuous wavelet transform applied to all trials irrespective of correctness of T1 and T2 identification. Morlet complex wavelets were computed on single trials in the 2100-msec interval between 100 msec before cue onset and until 200 msec after onset of the stimulus stream in the frequency range 1–40 Hz with linear 40 steps (Morlet parameter c = 7), and averaged power was calculated for the five cue conditions separately: cue uninformative, T1&T2 cued left, T1&T2 cued right, T1 cued left/T2 cued right, T1 cued right/T2 cued left. One participant had individual alpha power that deviated in the uninformative cue condition below 3 SDs of the other participants and therefore was rejected from this analysis (mean = 55 μV2, SD = 86 μV2). After inspection of time–frequency plots in each condition, the range of alpha frequency to be analyzed was defined as the average power between 10 and 12 Hz. Power was analyzed where values were at their maximum at 1300–1700 msec after cue onset separated into four epochs, each one encompassing 100 msec. ANOVA was performed on differences between informative and uninformative cue conditions with four repeated measurement factors: Recording Site (POz, PO8, PO7), Cued T1 Side (left, right), Cued T2 Side (left, right), and Epoch (1–4).

For all statistical analyses Greenhouse–Geisser-corrected p values will be reported, but ε values will not be indicated, for brevity. Likewise, partial eta-square will not be explicitly indicated, being easily derived from the reported F values by the formula ηp2 = (F/df)/(1 + F/df). IBM SPSS statistics version 20 was used. When interactions were significant, their sources were clarified by ANOVAs or t tests on subsets of the data.

RESULTS

Target Identification

Identification rates of T1 and T2 are presented in Figure 2A and compiled in Table 1.

Figure 2. 

Behavioral and N2pc results. (A) Identification rates for T1 (on the left) and for T2 (on the right). T1 accuracy was calculated as the percentage of correct T1 relative to all trials. T2 accuracy was calculated as the percentage of correct T2 in those trials where T1 was correctly identified. Dashed lines represent the uninformative-cue condition; solid lines represent the informative-cue condition. Black lines denote trials with T1 and T2 on the same side; blue lines denote trials with T1 and T2 on different sides. (B) CSD grand means at PO7 and PO8, including the N2pc components evoked by T1 (on the left) and by T2 (on the right; for T1–T2 lags of four frames only). Dashed lines represent the uninformative cue condition; solid lines represent the informative cue condition. Black lines represent recordings from the RH (PO8); blue lines represent recordings from the LH (PO7). The topographical maps show that N2pc was largest at lateral posterior-occipital sites centered at PO8 and PO7.

Figure 2. 

Behavioral and N2pc results. (A) Identification rates for T1 (on the left) and for T2 (on the right). T1 accuracy was calculated as the percentage of correct T1 relative to all trials. T2 accuracy was calculated as the percentage of correct T2 in those trials where T1 was correctly identified. Dashed lines represent the uninformative-cue condition; solid lines represent the informative-cue condition. Black lines denote trials with T1 and T2 on the same side; blue lines denote trials with T1 and T2 on different sides. (B) CSD grand means at PO7 and PO8, including the N2pc components evoked by T1 (on the left) and by T2 (on the right; for T1–T2 lags of four frames only). Dashed lines represent the uninformative cue condition; solid lines represent the informative cue condition. Black lines represent recordings from the RH (PO8); blue lines represent recordings from the LH (PO7). The topographical maps show that N2pc was largest at lateral posterior-occipital sites centered at PO8 and PO7.

Table 1. 

Percentages of Correct Identification of T1 (Relative to All Trials) and of T2 (Relative to All T1-correct Trials)

Lag14
T1T2 StreamSameDifferentSameDifferent
Target SideLeftRightLeftRightLeftRightLeftRight
Cue uninformative T1 87 (10) 89 (11) 86 (8) 85 (10) 89 (8) 88 (9) 86 (11) 88 (11) 
T2 97 (5) 96 (3) 77 (18) 53 (24) 89 (11) 80 (15) 93 (10) 77 (24) 
Cue informative T1 87 (16) 90 (9) 85 (14) 87 (12) 89 (12) 88 (12) 86 (15) 86 (8) 
T2 97 (4) 98 (3) 77 (19) 62 (24) 90 (12) 87 (18) 93 (7) 85 (17) 
Lag14
T1T2 StreamSameDifferentSameDifferent
Target SideLeftRightLeftRightLeftRightLeftRight
Cue uninformative T1 87 (10) 89 (11) 86 (8) 85 (10) 89 (8) 88 (9) 86 (11) 88 (11) 
T2 97 (5) 96 (3) 77 (18) 53 (24) 89 (11) 80 (15) 93 (10) 77 (24) 
Cue informative T1 87 (16) 90 (9) 85 (14) 87 (12) 89 (12) 88 (12) 86 (15) 86 (8) 
T2 97 (4) 98 (3) 77 (19) 62 (24) 90 (12) 87 (18) 93 (7) 85 (17) 

Values are presented as means across participants (SD).

T1 Identification

T1 was better identified when followed by T2 on the same side than on the other side, F(1, 15) = 18.7, p = .001. The main effect of Cue was not significant (F(1, 15) = 0.0, ns) nor were there any interactions of the Cue factor (F(1, 15) ≤ 2.5, p ≥ .1).

T2 Identification

The LVF advantage (T2 Side: F(1, 15) = 25.3, p < .001) was modulated by cues (Cue × T2 Side: F(1, 15) = 7.8, p = .01): It was larger with uninformative cues (F(1, 15) = 22.3, p < .01) than with informative cues (F(1, 15) = 19.3, p = .001) because of improvement of right T2 (Cue effect for right T2: F(1, 15) = 6.3, p = .02; for left T2: F(1, 15) = 0.2, ns).

Independent from cues, when T2 was presented after T1 on the same side with Lag 1, left and right T2 were almost perfectly identified, such that left and right T2s did not differ, and when T2 was presented after T1 on the different side with Lag 1, the LVF advantage was very large (Lag × Other-target Side × T2 Side: F(1, 15) = 13.1, p = .003; T2 Side effect in Lag 1 with T1 on the same side as T2: F(1, 15) = 0.2, ns; on the different side at Lag 1: F(1, 15) = 27.3, p < .001; for T1 on the same side at Lag 4: F(1, 15) = 13.8, p = .002; for T1 on the different side at Lag 4: F(1, 15) = 13.1, p = .003). This pattern of results was additionally reflected in the dual interactions of Other-target Side × T2 Side (F(1, 15) = 23.5, p < .001), Lag × Other-target Side (F(1, 15) = 37.6, p < .001), and in the main effect of Other-target Side (F(1, 15) = 40.3, p < .001).

EEG Results

T1-evoked Negativity

Figure 2B (left side) presents the grand average of T1-evoked CSD difference waveforms (contralateral T1 minus ipsilateral T1) recorded from PO8 and PO7 (for RH and LH, respectively) in the two cue conditions. As can be seen, T1-evoked N2pc was considerably smaller after informative cues than after uninformative cues, F(1, 15) = 22.7, p < .001. Besides, independent of cues, N2pc was evoked marginally earlier at the RH than at the LH, F(1, 15) = 3.9, p = .07.

T2-evoked Negativity

Figure 2B (right side) presents the grand average of T2-evoked CSD difference waveforms (contralateral T2 minus ipsilateral T2) recorded from PO8 and PO7 (for RH and LH, respectively) in the two cue conditions. As can be seen, there is negativity at around 150–350 msec after T2 onset, obviously earlier at PO8 than at PO7, F(1, 15) = 20.6, p < .001. Informative cues generally decreased the latency of N2pc (F(1, 15) = 6.1, p = .03), but this effect was significant for PO7 only (F(1, 15) = 5.2, p = .04; effect of Cue for PO8: F(1, 15) = 0.7, ns; cue effects were tested at either hemisphere even though the interaction between Hemisphere and Cue did not reach significance, F(1, 15) = 2.6, p = .1, because it was hypothesized a priori that informative cues will influence differently the processing of left and right T2). Furthermore, informative cues decreased the amplitude of N2pc, F(1, 15) = 4.9, p = .04. The difference between PO8 and PO7 did not reach significance, F(1, 15) = 2.8, ns. The differences between Same-target Side and Other-target Side were not significant for N2pc amplitude (main effect and all interactions: F(1, 15) < 3.0, ns) nor for N2pc latency (main effect and all interactions: F(1, 15) < 2.6, ns).

Distractor-evoked Potentials

Figure 3A presents the grand average of CSD waveforms of the first 1000 msec from the beginning of the distractor stream, recorded at PO8 and PO7 in the condition with uninformative cues. In this condition, VEPs were evoked earlier at PO8 than PO7 (by 6 msec on average), but the t test comparing this difference against null was not significant (t(1, 15) = 1.3, ns). This latency shift between PO8 and PO7 with uninformative cues was subtracted from the latency shifts in each cue condition (T1/T2 cued left/right), and these differences were analyzed (Figure 3B). Both factors, Cued T1 Side (F(1, 15) = 36.0, p < .001) and Cued T2 Side (F(1, 15) = 11.0, p = .01), were significant, indicating that when targets were cued to the left the latency shift between PO8 and PO7 was larger than when targets were cued right. Comparing each condition separately against uninformative cues, the latency shift between PO8 and PO7 increased by 6 msec when T1 was cued to the left (constant term of ANOVA: F(1, 15) = 4.7, p = .047) but did not change when T1 was cued to the right side (increase of 2 msec; constant term of ANOVA: F(1, 15) = 0.3, ns). For T2, both when cued to the left or to the right, PO8–PO7 differences, albeit differing from each other (cf. above) were not significantly larger than with uninformative cues (constant terms of ANOVA for T2 cued to the left side: F(1, 15) = 2.7, ns; for T2 cued to the right side: F(1, 15) = 1.6, ns).

Figure 3. 

EEG measurements before T1 onset: VEPs. (A) Visual potentials evoked by the stream of left and right background distractors during the first 1000 msec after the onset of the first distractor pair in the uninformative-cue condition. The black waveform was recorded at PO8 (RH), and the blue waveform was recorded at PO7 (LH). The topographical map shows the largest negativity at lateral posterior-occipital sites including PO8 and PO7. (B) The difference (in milliseconds) between conditions where T1/T2 were cued to the left/right side and the condition with uninformative cues. White bars present T1, black bars present T2, and vertical bars indicate confidence intervals.

Figure 3. 

EEG measurements before T1 onset: VEPs. (A) Visual potentials evoked by the stream of left and right background distractors during the first 1000 msec after the onset of the first distractor pair in the uninformative-cue condition. The black waveform was recorded at PO8 (RH), and the blue waveform was recorded at PO7 (LH). The topographical map shows the largest negativity at lateral posterior-occipital sites including PO8 and PO7. (B) The difference (in milliseconds) between conditions where T1/T2 were cued to the left/right side and the condition with uninformative cues. White bars present T1, black bars present T2, and vertical bars indicate confidence intervals.

Time–Frequency Analysis

Figure 4A presents the grand average of time–frequency plots (1–40 Hz) recorded from POz during the cue–trial interval (1.8 sec) in the uninformative cue condition. Evidently, there is large power in the upper alpha band (10–12 Hz) at 1000–1800 msec after cue onset with its focus over parieto-occipital cortex. Figure 4B presents the course of changes in alpha power over the interval between cue and first distractor pair averaged across 10–12 Hz for all cue conditions. Power in alpha band decreased with informative cues (constant term of ANOVA: F(1, 14) = 5.0, p = .04). This decrease did not differ between left and right Cued T1 Side (main effect and all interactions with T1 Side: F ≤ 1.8, ns) but was larger in Epochs 1–3 when T2 was cued to the right than to the left (effect of Cued T2 Side: F(1, 14) = 5.2, p = .04; T2 Side Cued × Epochs: F(1, 14) = 4.8; p = .03; effect of T2 Cued Side in Epochs 1–3: F(1, 14) ≥ 5.2; p = .04; in Epoch 4: F(1, 14) = 2.3, ns; constant term of ANOVA for T2 left in all epochs: F(1, 14) ≤ 4.5, p = .053; T2 right in Epochs 1–3: F(1, 14) ≥ 4.7, p = .048; in Epoch 4: F(1, 14) = 4.2, p = .06). Recording site had no effects (main effect and all interactions: F(2, 28) ≥ 1.8).

Figure 4. 

EEG measurements before T1 onset: Time–frequency analysis. (A) Time–frequency plot (1–40 Hz) recorded from POz during the interval between the onsets of cues and first distractor frames in the uninformative-cue condition. The topographical map in the bottom shows that the power of alpha was largest at posterior-occipital sites centered at POz. (B) Time course of alpha power averaged across frequencies 10–12 Hz over the interval between cue and first distractor pair averaged across POz, PO8, and PO7 recording sites. The black dashed line represents the uninformative-cue condition as depicted in A. Informative cue conditions are represented by solid lines: both T1 and T2 cued to the left by black color, T1 cued right and T2 cued left by gray color, both T1 and T2 cued right by dark blue color, and T1 cued left and T2 cued right by light blue color.

Figure 4. 

EEG measurements before T1 onset: Time–frequency analysis. (A) Time–frequency plot (1–40 Hz) recorded from POz during the interval between the onsets of cues and first distractor frames in the uninformative-cue condition. The topographical map in the bottom shows that the power of alpha was largest at posterior-occipital sites centered at POz. (B) Time course of alpha power averaged across frequencies 10–12 Hz over the interval between cue and first distractor pair averaged across POz, PO8, and PO7 recording sites. The black dashed line represents the uninformative-cue condition as depicted in A. Informative cue conditions are represented by solid lines: both T1 and T2 cued to the left by black color, T1 cued right and T2 cued left by gray color, both T1 and T2 cued right by dark blue color, and T1 cued left and T2 cued right by light blue color.

DISCUSSION

In the current experiment, we studied if endogenous cues can compensate for weak orienting of attention toward the RVF as reflected in the LVF advantage: We provided cues informing about location of T1 and T2 with 100% certainty, and we compared their effects with uninformative cues. First, target identification rates and their relation to the LVF advantage will be discussed. Then we will discuss the psychophysiological mechanisms by which cues led to improved T2 identification.

Target Identification

Results confirmed that endogenous cues can facilitate stimulus-driven orienting of attention because T2 identification improved with informative cues. Similar results were obtained by Berger et al. (2005) and Hopfinger and West (2006). The lack of such facilitation observed in many other studies (Chica et al., 2012; Lupiáñez et al., 2004; Riggio & Kirsner, 1997) may indicate that this occurs in difficult task conditions above all (Berger et al., 2005) and might be restricted to only one hemifield, like in the current study where T2 identification improved only in the RVF. Importantly, by improving the identification of only right T2, informative cues reduced the LVF advantage, thereby partially compensating for unequal exogenous orienting of attention induced by the targets. However, the LVF advantage was not eliminated with informative cues, which suggests that endogenous orienting cannot fully overcome this spatial bias. This need not be a property of endogenous orienting per se but rather might be due to the fact that the present endogenous cues, although reducing spatial uncertainty, left considerable temporal uncertainty with respect to target onset. This differs from exogenous cueing in this task as used by Śmigasiewicz et al. (2015), where cues appeared always 50 msec before T2 onset, thereby attracting attention to T2 at the right moment (if T2 indeed occurred in the cued stream).

N2pc Induced by T1 and T2

Further evidence for efficacy of the cue manipulations was provided by the analysis of T1- and T2-evoked N2pc. Its amplitudes were reduced after informative compared with uninformative cues. This result is in line with the study of Praamstra (2006), who also observed reduction of N2pc amplitude related to informative cues. However, in other studies, endogenous cues did not influence the N2pc component at all (Kiss, Van Velzen, & Eimer, 2008) or increased N2pc amplitude evoked by targets that were defined by combination of features (Seiss, Kiss, & Eimer, 2009). Thus, overall the relation between endogenous cues and N2pc amplitudes is not clear yet. Nevertheless, we interpret the reduction of N2pc amplitude as indicating that the selection of target was easier with informative cues. This decrease of N2pc amplitude in case of T1 indicates that, although its identification was already very high, it could be still facilitated. The identification rate is probably not precise enough to reflect this improvement in case of T1. The same applies to left T2, where decrease in N2pc amplitude was not accompanied by increase in target identification rate. In case of right T2, facilitation caused by cues was apparent at both behavioral (improved identification rate) and psychophysiological level (decreased N2pc amplitude). Overall, thanks to informative cues the processing of targets (Eimer, 1996) or inhibiting of distractors (Luck & Hillyard, 1994) might not need to be as intensive as with uninformative cues. Another possible interpretation of the discrepancy between the modulation of N2pc amplitude and target identification rates by informative cues as compared with uninformative cues may be derived from assuming that N2pc rather reflects attentional selection of the target whereas target identification that requires in-depth analysis follows later on and is reflected by contralateral delayed activity (Töllner, Conci, Rusch, & Müller, 2013; Mazza, Turatto, Umiltà, & Eimer, 2007; named also sustained posterior contralateral negativity). This would suggest that in the current study informative cues modulated target selection, but not target identification. This interpretation can account for T1 results. However, T2 is not a pop-out stimulus, and therefore, to be selected has to be at least partially identified as a digit. Thus, such clear separation into selection and identification processes probably does not apply to T2 processing.

In previous studies with dual RSVP, it was observed that the LVF advantage is mirrored in N2pc latency: N2pc is evoked earlier by left T2 than by right T2. Therefore, it was hypothesized that the LVF advantage resulted from faster selection of targets in the RH. This effect was here replicated with uninformative cues and was further modulated by informative cues: Informative cues shortened the latency of N2pc evoked by right T2, along with improved identification rate of right T2. Thus, the reorienting of attention toward the RVF was facilitated by prior knowledge about target location. With uninformative cues attention needs to be diffused across both streams of stimuli to identify T1 and T2. With informative cues attention is probably biased, at least partially, to one stream already before target onset (see below discussion about the influence of informative cues on VEPs). This bias is beneficial for all targets, as indicated by reduced N2pc amplitude. But it is particularly helpful for identifying right T2 because of the fragility of the stimulus-driven orienting system when attention must be suddenly directed to the RVF. Thus, informative cues caused bias of attention toward T2's location and thereby decreased the time necessary to orient to the RVF by the LH. In consequence, the asymmetry was reduced between both hemispheres in speed and efficacy of selecting T2. Nonetheless, this interpretation must be taken with caution. This is because the interaction Cue × T2 Side was not significant and the reduction of N2pc latency evoked by right T2 with informative cues was evidenced only in a follow-up test on this subset of data.

Alpha Oscillations between Onsets of Cues and Stimulus Streams

Further insight into mechanisms leading to the reduction of the LVF advantage with informative cues was provided by analyzing the power of EEG alpha oscillations between cue onset and the first distractor pair of the trial. After uninformative cues, alpha power increased gradually until beginning of the trial with its peak at about 1.7 sec after cue onset and with the topography over midline parieto-occipital cortex. Alpha power was reduced with informative cues, indicating that participants indeed processed the cues, possibly reflecting semantic processing and memorizing (Klimesch, 1999; Klimesch et al., 1997) or mobilization of nonspatial attention for the task (Janson et al., 2014). Interestingly, alpha reduction was particularly large after cues indicating T2 location in the RVF, which is the condition related to lowest T2 identification rates. Thus, desynchronization of alpha power in the current experiments was probably not only related to semantically processing and memorizing the cues (Klimesch, 1999) because then it should occur equally for all cues, as all cues had to be semantically processed and memorized, but was rather related to preparing the cognitive system to process the difficult T2 (Janson et al., 2014).

In the current experiment, uninformative and informative cue conditions were administered in separate blocks to ensure effective preparation to the trial and reduce the noise related to possible mistakes. However, this experimental strategy could have impact on memory processes: more demands on memory processes might be imposed in the informative than in the uninformative cue condition, introducing additional imbalance between those two conditions. The question arises whether similar alpha power reductions induced by informative compared with uninformative cues would be observed when the two cue conditions were intermixed within one block of trials. This issue needs further studies.

Distractor-evoked Potentials

In previous experiments with this task, distractor-evoked potentials were found to be leading at PO8 over PO7 (Verleger, Talamo, et al., 2013; Verleger et al., 2011). In the current experiment, this latency difference was not reliably replicated in the uninformative cue condition, although being numerically of the same size as in those preceding studies. However, VEPs were evoked earlier at the RH than at the LH when cues predicted T1 to occur on the left. Thus, cuing T1 to the left stream probably shifted attention slightly more to the LVF at the beginning of the trial already (see also Śmigasiewicz et al., 2014). Two questions arise. First, why did cuing T1 to the right stream not cause VEPs to be evoked earlier in the LH than in the RH? And second, why did T2 cuing not have such modulating effects on VEP latencies? Concerning the first question, earlier VEP latencies at the RH than at the LH may largely be caused by the RH's specialization in the processing of visually degraded stimuli at early visual stages (Grabowska & Nowicka, 1996; Hellige, 1980; Hellige & Webster, 1979). This argument is supported by our finding (Verleger, Talamo, et al., 2013) that VEPs were leading at the RH in the dual-RSVP task not only for background stimuli displayed at the horizontal but also at the vertical midline. It may be due to low signal-to-noise ratio that this effect is not always captured at a statistically significant level. With uninformative cues, the VEPs at the RH were leading over the LH by 6 msec. Therefore, cuing T1 to the right (and thereby shifting attention slightly to the right stream) could not so easily overcome this default preferential processing of background stimuli in the RH. Concerning the second question, it seems logical that participants directed their attention according to the location of T1, as it is always the first target to identify. It would be counterintuitive to direct attention to T2 location, because it was always known in advance where T1 will appear. Such priority given to T1 might result from experimental instruction given to participants: They were explicitly instructed to search for T1 first and then for T2.

Conclusions

Endogenous orienting plays a role in overcoming the LVF bias in the dual-RSVP task. Thus, knowing in advance that an important object, difficult to identify, will appear in the RVF, participants were enabled by using the cues to partially compensate for poor stimulus-driven orienting of attention toward the RVF. This compensation was accomplished by recruitment of more attentional resources when cues indicated T2 to be presented on the right (reflected by larger alpha power reduction when T2 was cued right) and by increasing the speed of the LH in selecting T2 (reduced latency of N2pc evoked by right T2 with informative cues). In consequence, informative cues improved the identification of right T2. The general effectiveness of the cue manipulation was confirmed by decreasing alpha power with informative cues, by changes in leftward bias of attention as reflected in VEP latency differences between RH and LH in response to T1 cues, and by reduced T1- and T2-evoked N2pc amplitudes with informative cues. Thus, advance knowledge about target locations resulted in more focused preparation to the task together with biasing attention according to future target location and thereby in easier identification of both targets.

Acknowledgments

This work was supported by grant Ve110/15-1/2 awarded from the Deutsche Forschungsgemeinschaft to R. V. as part of the network PAK270 “Neuro-cognitive mechanisms of conscious and unconscious visual perception.” We are grateful to two reviewers of a previous version of this manuscript for their useful suggestions.

Reprint requests should be sent to Kamila Śmigasiewicz, Klinik für Neurologie, Universität Lübeck, D 23538 Lübeck, Germany, or via e-mail: k.smigasiewicz@gmail.com.

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