Simultaneous Analysis of Early Components P1 and N1 and Phase and Non-Phase Alpha Activities Associated with Word Recall

Academic Editor: Kurt A. Jellinger Received: 12 February 2025 Revised: 28 April 2025 Accepted: 4 June 2025 Published: 9 June 2025 Citation: Vazquez-Marrufo, M.; Navarro-Martos, R.; NarbonaGonzalez, N.; Martin-Clemente, R. Simultaneous Analysis of Early Components P1 and N1 and Phase and Non-Phase Alpha Activities Associated with Word Recall. Sci 2025, 7, 84. https://doi.org/10.3390/ sci7020084 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Simultaneous Analysis of Early Components P1 and N1 and Phase and Non-Phase Alpha Activities Associated with Word Recall Manuel Vazquez-Marrufo 1,* , Remedios Navarro-Martos 1, Natividad Narbona-Gonzalez 2,3 and Ruben Martin-Clemente 4 1Experimental Psychology Department, Faculty of Psychology, University of Seville, 41018 Seville, Spain; [email protected] 2Faculty of Health Sciences, International University of La Rioja (UNIR), 26006 Logroño, Spain; [email protected] or [email protected] 3Research Unit, Neurointegra Clinic, 41960 Seville, Spain 4Signal Processing and Communications Department, Higher Technical School of Engineering, University of Seville, 41092 Seville, Spain; [email protected] *Correspondence: marr[email protected] Abstract: The study of non-phase modulation of different frequencies in the human electroencephalography (EEG) is revealing new mechanisms involved in information processing. In particular, it has been described that the alpha band, through a desynchronization of its non-phase component, could represent a mechanism for sensory gain in visual stimulus processing. One key question to address is whether this activity can be modulated (increased) by the recall of a previously memorized stimulus. The objective of this study is to answer this question by recording EEG activity with 58 electrodes and applying timefrequency analysis techniques (Temporal Spectral Evolution and the Hilbert Transform) in a sample of 27 human participants during a word recall task. The results of the study showed an increase in alpha phase modulation for recalled words compared to not recalled words, which included modulation of the P1 component. Additionally, alpha non-phase modulation also increased for recalled words, suggesting that the enhanced P1 component response could, in fact, be an indirect result of the attenuation of background neural noise, as proposed by the sensory gain hypothesis. Keywords: EEG; ERPs; Hilbert transformation; N1; non-phase alpha; P1; recalled word; Temporal Spectral Evolution 1. Introduction The analysis of various frequencies in the human EEG is providing valuable insights into information processing in the brain [ 1 – 3 ]. Undoubtedly, one of the most studied frequency bands is the alpha band (8–12 Hz) [ 4 , 5 ]. Initially considered an indicator of cortical idling [ 6 ], recent research has described alternative functions, suggesting an active role in information processing [ 7 , 8 ]. One of the latest proposals is that the alpha band may be involved in a sensory gain mechanism to enhance the processing of stimuli presented to a subject [9]. This recent proposal stems from advancements in technology applied to the timefrequency analysis of human EEG. Initially, spectral modulations were studied using the Fourier Transform (FFT) [ 10 ]; however, this type of analysis does not allow for the millisecond-resolution study of the temporal evolution of oscillations. Since the 1970s, Sci 2025,7, 84 https://doi.org/10.3390/sci7020084 Sci 2025,7, 84 2 of 13 various time-frequency techniques have been developed, such as event-related desynchronization/synchronization (ERD/ERS) [ 11 ], wavelets [ 12 ], and Temporal Spectral Evolution (TSE) [ 13 ]. The latter presents an advantage over other techniques as it can capture nonphase modulations related to stimulus presentation (non-phase locked, time-locked) [9]. The study of non-phase modulation in the alpha band (and other frequencies) is revealing new cognitive mechanisms that are not observable with traditional analyses focused solely on phase activity (i.e., event-related potentials) [ 14 , 15 ]. To date, various studies have shown that these non-phase modulations occur across different frequency bands (alpha, beta, gamma, etc.); sometimes manifesting as synchronies or desynchronies, with specific latencies and topographies, and demonstrating a high level of replicability in longitudinal studies [16–18]. The proposal that non-phase alpha activity may represent a sensory gain mechanism comes from a study that observed a key distinction: while phase modulation of the alpha band (linked to P1 and N1 components and thus to stimulus processing) exhibited a synchronous process, non-phase activity displayed a desynchronous process with a latency and topography very similar to phase activity [ 9 , 18 ]. In other words, the desynchronization of non-phase alpha would represent the attenuation of background noise in the brain at that frequency and in the same region responsible for processing the stimulus, ultimately enhancing the perception of the presented stimulus. This sensory gain mechanism appears to operate in the early stages of stimulus processing, based on its predominantly occipito-parietal topography for visual stimuli and its early latencies, around 120 ms [ 9 , 18 ]. However, it can also emerge at other moments, such as during the anticipation of stimulus onset [ 19 ] or in post-processing phases of the stimulus [17]. The objective of this study is to analyze simultaneously whether the recall of previously presented words can modulate both ERPs and phase and non-phase alpha band activities. In the case of ERPs, contradictory results have been reported. In particular, it seems that not always both components are modulated in visual memory tasks. In some cases, neither of the components is modulated by the recognition of the visual stimuli [ 20 ], and in other cases, modulation occurred only in one of the components [ 21 , 22 ]. Additionally, the simultaneous study of the P1 and N1 components along with the phase and non-phase modulations of the alpha band will help determine whether both domains (time and frequency) are interconnected, as suggested by the phase resetting hypothesis as a mechanism for the generation of ERPs [23]. Furthermore, to the best of our knowledge based on the existing scientific literature, there are no previous studies on the effect of word recall on non-phase-locked alpha activity. Since we consider non-phase-locked alpha activity to be a psychophysiological indicator of the sensory gain mechanism, the presence or absence of changes can provide highly informative insights into the events occurring during the early stages of information processing associated with word recall. In this particular study, we employed a combination of time-frequency techniques (TSE and Hilbert transform) to isolate the non-phase-locked component from the rest of the EEG signal. To investigate this, we will use a word memorization task followed by a recognition task, alongside electroencephalographic recordings with 58 scalp electrodes. 2. Materials and Methods 2.1. Ethical Statement This study was carried out in compliance with the Helsinki Declaration. The study protocol was approved previously by the ethics committee of the Junta de Andalucía (project Sci 2025,7, 84 3 of 13 code: PSI2010-16825). All participants enrolled in the present study signed informed consent before their inclusion. 2.2. Participants Twenty-seven healthy adults, who were university employees and students, were recruited from the University of Seville (16 women and 11 men). The participants’ ages ranged from 20 to 50 years (mean age: 37.7 ± 11.4). Only two participants were left-handed. 2.3. Cognitive Task Participants were seated in a sound-attenuated room in front of a Liquid Crystal Display (LCD) monitor. Stimuli were created by E-prime 2.0 (Psychology Software Tools, Inc., Pittsburgh, PA, USA). The experiment consisted of an initial stimulation routine in which participants were instructed to memorize the presented words. The number of words to remember was 20; all were bisyllabic, frequently used in the Spanish language, and were presented three times to each participant in a random order mixed with the other words to be remembered. Each word was displayed for 1 s in a central position, with a size of 3.6 × 1.2 degrees of visual angle, while participants’ eyes were positioned 70 cm from the screen. Next, to prevent continuous effort in recalling the words by the participant, a visual oddball task was presented in which the participant had to identify a target stimulus among other distractor stimuli (rectangles with a chessboard pattern in different colors) and indicate it by pressing a button. The task lasted 3 min. Finally, a stimulation routine was presented in which participants had to identify the words presented in the first routine from another set of 20 new words that served as a control for the number of intrusions. The words were of the same size and position as those used in the first routine but displayed during two seconds. Each word—both those to be identified and the new ones to be ignored—was presented three times in random order and intermixed with the others. Participants were instructed to press the right mouse button with their index finger whenever they recognized the presented word as one that had appeared during the memorization task. At the end of the recording, the number of correct responses in the recognition o-f memorized words was calculated, as well as the percentage of intrusions made by the participant. 2.4. EEG Recording and Analysis EEG data were recorded from 58 electrodes (Ag/AgCl) in standard locations of a 10-10 system (American Electroencephalographic Society, 1994) [ 24 ] (Figure 1), then amplified with BrainAmp amplifiers (BrainProducts GmbH, Gilching, Germany) and digitized at a rate of 500 Hz using Recorder software v.1.26 (BrainProducts GmbH, Germany) [ 25 ]. The electrodes used in the present study were limited to a 3 × 7 matrix located in posterior regions where the maximum activity of all analyzed activities (ERPs, evoked and induced alpha modulations) was observed: CP5, CP3, CP1, CPz, CP2, CP4, CP6, P5, P3, P1, Pz, P2, P4, P6, PO5, PO3, PO1, POz, PO2, PO4, PO6 (see Figure 1). An online reference was placed at the ear lobes, while an offline reference was set to the common average. Eye movements were monitored by placing electrodes on the outer lateral eye orbits for horizontal (HEOG) and on the superior and inferior orbits of the left eye for vertical (VEOG) movements. A bandpass filter was applied within the 0.01–100 Hz range. Impedance levels were kept below 5 kΩthroughout the recording. Sci 2025,7, 84 4 of 13 Figure 1. Locations of electrodes on the scalp. EEG data were collected using all electrodes displayed in the figure. However, only the 3 × 7 electrode array highlighted in red was used for the subsequent analysis. Abbreviations: F (frontal), FC (fronto-central), C (central), CP (central-parietal), P (parietal), PO (parieto-occipital), L (Lines 1–6; “0” refers to the midline). The following protocol was used: eye movement correction to eliminate artifacts, based on the algorithm developed by Gratton et al. [ 26 ]; segmentation of the data in a − 200 to 1000 ms interval; baseline correction from − 200 to 0 ms; and rejection of any artifacts exceeding ± 75 µ V at electrodes Pz, and HEOG. After applying the artifact rejection protocol, the average number of trials across subjects was 47.5 for the recalled words condition and 47.7 for the not recalled words condition. From this point, the analysis of phase and non-phase activities proceeded differently. Phase activity was analyzed by applying the following steps: averaging (ERP modulations were analyzed), bandpass filtering in the 8–12 Hz range (48 dB/octave Butterworth zero phase), rectification, low-pass filtering at 5 Hz (48 dB/octave Butterworth zero phase), and baseline correction from − 200 to 0 ms. In contrast, non-phase activity was calculated using this protocol: bandpass filtering in the 8–12 Hz range (48 dB/octave Butterworth zero phase), rectifying the signal, averaging all trials for each experimental condition, subtracting phase activity from this result, applying 5 Hz low-pass filtering (48 dB/octave Butterworth zero phase), and baseline correction from −200 to 0 ms (Figure 2) [18]. It is important to highlight that both procedures include the same steps up to the point of obtaining phase or non-phase modulations. The main difference lies in the order of these steps. The critical step is the rectification of the signal. In the case of the non-phase section study, rectification is applied to the EEG recording segments before they are averaged. This approach allows us to observe non-phase modulations, which are usually canceled out by the averaging process. A detailed description of the procedure can be found in [27]. Latency values for ERPs and phase and non-phase modulations were calculated at the peak/valley amplitude for each participant to evaluate whether ERPs and phase/non-phase activities occur at similar latencies for recalled words compared to not recalled words [ 28 ]. The electrode showing the highest ERP amplitude and phase/non-phase activities was positioned at PO5. To calculate the possible effects on amplitude, the mean voltage value was exported for each electrode in the 3 × 7 matrix (see Figure 1) across two time intervals: (1) within the latency range of the P1 component (90–130 ms) and (2) within the latency range of the N1 component (140–180 ms). Sci 2025,7, 84 5 of 13 Figure 2. ERPs and phase and non-phase alpha modulations. The waveforms correspond to the PO5 electrode. To examine the potential voltage difference between the two conditions (recalled vs. not recalled words) for ERPs and spectral modulations (phase and non-phase), the intervals were defined based on the grand average peaks or valleys (90–130 and 140–180 ms, corresponding to the P1 and N1 windows). To ensure comparability for the topographic amplitude analyses between the two types of activity (phase and non-phase), the non-phase activity was converted to absolute values. To check the possibility of cross-contribution between phase and non-phase modulations, a phase angle analysis was performed for both modulations. For this, after subtracting the average trial from each individual trial, the Hilbert transform was applied to represent the alpha band-filtered EEG signals as complex waveforms of the form xn(t) = Cn(t) exp (i w n (t)), where x n (t) denotes any of the average-subtracted trials, such as the nth, w n (t) is its instantaneous phase or angle (measured in radians), and iis the imaginary unit [ 29 , 30 ]. Figures 3and 4display polar plots showing the distribution of these angles, grouped by subject. 2.5. Statistical Analysis Statistical analyses were performed using SPSS v29 (IBM) [ 31 ]. The potential effect of the “RECALL” factor on ERP latencies (P1 and N1 components) was analyzed separately using paired t-tests. In the case of spectral modulation analysis, a repeated-measures ANOVA (RM-ANOVA) was conducted with the following factors and levels: (1) “RECALL” (levels: Recalled/Not recalled word); and (2) “TYPE OF ACTIVITY” (levels: Phase (P)/Nonphase (NP)). For topographical differences in ERP amplitude (P1 and N1), an RM-ANOVA was applied with the following factors and levels: (1) “RECALL” (levels: Recalled/Not recalled word); (2) “ANTERO-POSTERIOR LOCATION” (levels: Central-parietal, Parietal, and Parietal-posterior); and (3) “LATERAL-MEDIAL LOCATION” (L1, L2, L3, L4, L5, L6, L7) (see Figure 1). For the analysis of phase and non-phase alpha amplitudes, the same factors and levels used in the ERP amplitude analysis were applied, with the addition of an extra factor: (4) “TYPE OF ACTIVITY” (levels: Phase (P)/Non-phase (NP)). In all cases, a Bonferroni correction was applied in the multiple comparison post hoc analysis. Topographic map amplitude correlation analyses were conducted using Pearson’s r. In accordance with the recommendations proposed by Kileny and Kripal [ 32 ] and referenced in Vázquez-Marrufo et al. [ 33 ], the significance threshold of 0.05 was adjusted by dividing Sci 2025,7, 84 6 of 13 it by the total number of contrasts performed across all correlation analyses (n = 6). As a result, a revised significance level of p< 0.008 was established. Figure 3. Polar plot of phase values results for phase-locked and non-phase-locked activities in the alpha band (8–12 Hz) for each of the twenty-seven participants and for the recalled words. Green dots correspond to phase-locked activity, and red crosses correspond to non-phase-locked activity. Sci 2025,7, 84 7 of 13 Figure 4. Polar plot of phase values results for phase-locked and non-phase-locked activities in the alpha band (8–12 Hz) for each of the twenty-seven participants and for the not recalled words. Green dots correspond to phase-locked activity, and red crosses correspond to non-phase-locked activity. Sci 2025,7, 84 8 of 13 3. Results 3.1. Behavior The analysis of the participants’ behavioral responses showed an average accuracy rate of 79.3 ± 15.9% and a percentage of 20.5 ± 13.5% for intrusions. These results suggest that the task was suitable for measuring the participants’ recall ability, without being overly difficult or too easy in terms of recalling the word list. 3.2. ERPs The study of the P1 and N1 components showed that there were no latency differences due to the “RECALL” factor (t 26 = − 0.65, p= 0.518 and t 26 = − 0.46, p= 0.644, for P1 and N1, respectively). The mean latency values are shown in Table 1, and the ERP traces for recalled and non-recalled words are presented in Figure 2. Table 1. Mean latency and amplitude values at the electrode with maximum voltage (PO5) for ERPs (P1 and N1) and phase and non-phase alpha activities. Physiological Parameter Latency (ms) 1 Recalled Word Not Recalled Word P1 component 110 ±11.6 112 ±14.1 N1 component 158 ±14.9 160 ±18.8 Alpha phase-locked 128 ±23.3 130 ±28.4 Alpha non-phase locked 132 ±40.2 144 ±50.5 Amplitude (µV) 1 Recalled word Not recalled Word P1 component (90–130 ms) 7.19 ±4.86 6.49 ±4.34 N1 component (140–180 ms) −2.85 ±4.69 −2.47 ±4.7 Alpha phase-locked (90–130 ms) 2.28 ±1.36 0.91 ±0.35 Alpha non-phase locked (90–130 ms) 1.48 ±0.99 0.55 ±0.36 Alpha phase-locked (140–180 ms) 2.14 ±1.34 0.88 ±0.37 Alpha non-phase locked (140–180 ms) 1.43 ±0.99 0.62 ±0.35 1Latency values are expressed in milliseconds and amplitude values in microvolts. Regarding amplitude, the export of the mean value between 90 and 130 ms after stimulus onset (corresponding to the P1 component) showed an increase in the amplitude of this component for recalled words compared to new words (F(2,52) = 6.82, p= 0.002, N2: 0.208 ). The mean value for each condition at the electrode with the maximum amplitude is presented in Table 1. A post-hoc comparison revealed that the increase was observed across all electrodes included in the analysis (3 × 7 matrix) (see Figure 1). No other interaction involving the “Recall” factor was found to be significant. On the other hand, the analysis performed in the 140–180 ms interval for the study of the N1 component did not show, unlike what was observed with the P1 component, a statistically significant increase in amplitude (Figure 1) for recalled words compared to new words (F(1,26) = 0.79, p= 0.380, N2: 0.030). 3.3. Alpha Phase and Non-Phase Activities Concerning the latency values of the phase and non-phase alpha responses, the mean values for the different experimental conditions can be found in Table 1. The repeated measures ANOVA conducted to study the latency of these modulations revealed that there were no statistically significant differences due to the main factors “RECALL” (Recalled word/Not recalled word) F(1,26) = 2.65, p= 0.115, N2 : 0.093); “TYPE OF ACTIVITY” Sci 2025,7, 84 9 of 13 (Phase/Non-phase) F(1,26) = 1.96, p= 0.172, N2 : 0.070); or the interaction of both factors (F(1,26) = 1.63, p= 0.212, N2: 0.059) (Figure 2). In the case of the amplitude, ANOVA analysis of the “TYPE OF ACTIVITY” factor revealed that, in the first interval (90–130 ms), phase activity exhibited greater amplitude than non-phase activity across all electrodes analyzed in the 3 × 7 matrix (F(1,26) = 31.47, p< 0.001, N2 : 0.548) (see Table 1for values at the electrode with the highest amplitude) (Figure 2). Regarding the “RECALL” factor, the statistical analysis revealed that the four-way interaction “RECALL” דTYPE OF ACTIVITY” דANTERO-POSTERIOR LOCATION” × “LATERAL-MEDIAL LOCATION” was significant (F(12,312) = 1.81, p= 0.045, N2 : 0.065). Post hoc analysis indicated that all electrodes in the 3 × 7 matrix exhibited greater amplitude for both phase and non-phase activities for recalled words compared to new words (Figure 2). When the same analyses were conducted for the 140–180 ms window, an initial result showed that phase activity had greater amplitude than non-phase activity (factor “TYPE OF ACTIVITY”: F(1,26) = 22.98, p< 0.001, N2 : 0.469) (see Table 1for mean values) (Figure 2). With respect to the “RECALL” factor, the four-way interaction involving this factor along with “TYPE OF ACTIVITY”, “ANTERO-POSTERIOR LOCATION”, and “LATERALMEDIAL LOCATION” was also significant (F(12,312) = 3.30, p< 0.001, N2 : 0.113). Subsequent post hoc analysis showed that all electrodes, for phase and non-phase activities, exhibited higher amplitude when the word was recalled compared to new words. Regarding the analysis of phase values, the polar plots (Figures 3and 4) shows that the values observed for non-phase activity were randomly distributed and not concentrated around those associated with phase modulation. 3.4. Topographic Correlation Analysis The correlation analyses first showed that the map associated with the ERPs produced when the word was recalled compared to when the word was new to the subject was virtually identical (90–130 ms interval: r = 0.999, p< 0.001; 140–180 ms interval: r = 0.974, p< 0.001). That is, the P1 component was almost identical for both types of words, and somewhat less so for the N1 component (see Figure 5). Figure 5. Topographic maps for each condition (recalled/not recalled word) and type of activity (ERPs or alpha activity).