The effect of frequent and infrequent stimulation on Visually Evoked Potential (VEP)
Abstract
Visually evoked potential (VEP) also referred to as visually evoked response (VER) is a measurement of electrical signal recorded at the scalp over the occipital cortex as a response to light stimuli. The main aim of the experiment was to determine whether frequent and infrequent stimulation impact Visually Evoked Potential (VEP). It was hypothesized that the latency of N1 is a function of the size of the head as well as the distance. The study subject(s) were allowed to sit withing a range of 70 to 100 cm from the computer screen. Each data recording was 10 seconds and an average of the response was calculated for the best results. iWork data software was used for data acquisition, and EEG lead wires was connected to the scalp of the study subject. The results indicated that, N2 had the highest mean value of 135.65 against 104.78 and 65.81 for the P1 and N1 respectively. The results also indicated that, there was a correlation between N1 and P1 was 0.76. the value was close to 1 implying that the relationship was strong and positive.
Introduction
Visually evoked potential (VEP) also referred to as visually evoked response (VER) is a measurement of electrical signal recorded at the scalp over the occipital cortex as a response to light stimuli. Some types of VEP such as flash VEP are essential to patients suffering from extremely poor vision (Warren, Maraver, de Luca, and Kopp, 2020). In this context, their response to the pattern-reversal stimulus may either be absent or limited (Kardos, Kóbor, and Molnár, 2020). However, if measurable, the pattern response herein gives a more reliable and quantifiable waveform (Ladouce, Donaldson, Dudchenko, and Ietswaart, 2019). The waveform is represented by a series of amplitudes that reflect the damages in the visual pathway. The pattern formed may be studied either by number of cycles in every second or as the size of the checkboard pattern (Petro,v Kojouharova, Gaál, Nagy, Csizmadia, and Czigler, 2020). In this case, the smaller sizes allow the detection of smaller changes within the function.
As observed by Stróżak, Francuz, Augustynowicz, Ratomska, Fudali-Czyż, and Bałaj, (2016), the most widely studied type of VEP waveform is basically made up of an initial negative peak referred to as N1. The N1 peak is followed by a positive peak called P1 or P100 (since it is usually located at 100 msec). The P1 peak is followed by N2 (second negative peak), which is then followed by the second positive peak (P2).
Sulykos, Gaál, and Czigler, (2017), also noted that application of VEP in most clinical situation is limited due to its susceptibility to many factors that may result to abnormal waveforms even when there are no damages in the visual pathway. For instance, the reliability of VEP in clinical applications is affected by uncorrected refractive error, amblyopia, media opacity, inattention (intentional or unintentional), and fatigue. Also, the application of chemicals such as anaesthesia affect the VEP and the resultant waveform as indicated by Hayashi, and Kawaguchi (2017). Therefore, in most cases, VEP is not intended for diagnosis of optical neuropathy and is less accurate in case of quantification than basic perimetry (Mendes et al., 2009).
However, there are cases in which VEP can be clinically essential;
- In infants and inarticulate adults; to evaluate the visual pathway.
- In patients suspected to have nonorganic diseases; to confirm their intact visual pathways.
As discovered by (Saliasi, Geerligs, Lorist, and Maurits, (2013), working memory is regarded as a cognitive function that has a limitation in the capacity of individuals to store as well as actively manipulate information over a short time. The working memory is affected by age whereby an increase in age reduces the performance (Waller, Hazeltine, and Wessel, 2019). High performers have relatively larger P3 amplitude than low performers. Therefore, the higher P3 components indicate a more efficient utilization of cognitive resources. Therefore, the P3 component is used in active cognitive response; the higher the level of P3 component, the higher the cognitive efficiency of the visual pathway (Saliasi et al., 2013).
Hypothesis
The hypothesis for experiment 1 was stated as, “the latency of N1 is a function of the size of the head”. In this case, the study suggested that the size of the head determines the detection of N1 during the VEP analysis. For experiment 2, the hypothesis was stated as, “VEP components are modified by the selective attentional process”. In this case, the study proposed that the larger P3component would be observed for attended stimuli (squares) than that of the unattended stimuli (circles). Therefore, there would be an observable effect of attentional process on the initial VEP components of N1, P1, and N2. For experiment 3, the hypothesis was given as, “VEP changes are specific to the square stimuli from experiment 2 because the squares are visually differentiated from the circles but not due to selective attention”.
Aim of the Experiment
The main aim of the experiment was to determine whether frequent and infrequent stimulation impact Visually Evoked Potential (VEP). Other minor aims of the study were to determine the cortical response of the VEP components N1, P1, and N2 towards a complex visual stimulus. The study aimed at discovering whether attention impacts the earlier VEP components.
Materials and Methods
The following apparatus were used in the experiment;
Computer
USB cable
IX-B3G USB recorder
Three EEG leads
EEG Electrodes (3)
Elastic headband
Alcohol swabs
The study subject(s) were allowed to sit withing a range of 70 to 100 cm from the computer screen. Each data recording was 10 seconds and an average of the response was calculated for the best results. iWork data software was used for data acquisition, and EEG lead wires was connected to the scalp of the study subject. The subjects (each at a time) were allowed to watch flashing lings while their eyes were being tested. The results collected were measured through the length of time take by the brain to respond to the stimuli. The USB cable was connected correctly to the laptop’s USB port and the IX-B3G USB port. The software was started by clicking on the Labscribe software. The start menu window was opened and Iworx was selected from the programs. The settings menu was obtained and the Load Group was opened from the settings folder. The menu was pulled down and VisualEvoked Potentials setting s was selected from the list of Human Psychophysiology. After configuring the settings, the experiment button was clicked. The EEG electrodes were connected correctly and alcohol swabs were used to clean the skin into which the electrodes were placed on the head. One electrode was placed on the top right side of the head. The other lead was placed at the edge of the eyebrow (in between the hairline and eyebrow). The third lead was attached to the occipital area about 2 In above the nape of the neck. An elastic headband was put on the head to hold the leads into position. The elastic headband was placed as high as possible above the ears. Again, the headband was made as tight as possible but not so tight to become uncomfortable to the study subject. The leads were attached to their respective inputs on the e iWire-B3G EEG cable. The subjects were instructed to avoid any movements that would distract their attention.
Results
Analysis for shows that N2 had the highest mean value of 135.65 against 104.78 and 65.81 for the P1 and N1 respectively. The ANOVA results produced a computed test statistic of F=34.39 with a probability value of 2.0599E-12. Since the probability value was less than 0.05, the study rejected the null hypothesis of equal means and concluded that the means were statistically different.
Exp 1. Checkerboard VEP
Table 1: Single ANOVA Analysis for Latency of VEP Components
| LATENCY | ||||||
| ANOVA: Single Factor | ||||||
| SUMMARY | ||||||
| Groups | Count | Sum | Average | Variance | ||
| N1 (N50) | 39 | 2566.4 | 65.8051282 | 447.601026 | ||
| P1 (P100) | 39 | 4086.3 | 104.776923 | 1146.2563 | ||
| N2 (N140) | 39 | 5290.46 | 135.652821 | 2574.33827 | ||
Table 2: Single ANOVA Analysis for Latency of VEP Components
|
ANOVA |
||||||
| Source of Variation | SS | df | MS | F | P-value | F crit |
| Between Groups | 95560.6855 | 2 | 47780.3427 | 34.3892279 | 2.0599E-12 | 3.07585264 |
| Within Groups | 158391.432 | 114 | 1389.39853 | |||
| Total | 253952.118 | 116 |
Table 3: Table 2: Single ANOVA Analysis for Amplitude of VEP Components
| AMPLITUDE | ||||||
| ANOVA: Single Factor | ||||||
| SUMMARY | ||||||
| Groups | Count | Sum | Average | Variance | ||
| N1 (N50) | 39 | 128 | 3.28205128 | 5.52414305 | ||
| P1 (P100) | 39 | 406.3 | 10.4179487 | 48.3273009 | ||
| N2 (N140) | 39 | 303.9 | 7.79230769 | 26.4333603 | ||
Table 4: P-Value Analysis
| ANOVA | ||||||
| Source of Variation | SS | df | MS | F | P-value | F crit |
| Between Groups | 1016.04667 | 2 | 508.023333 | 18.9832935 | 7.6548E-08 | 3.07585264 |
| Within Groups | 3050.82256 | 114 | 26.7616014 | |||
| Total | 4066.86923 | 116 |
The correlation between N1 and Nasion-Inion distance was observed in the results. In this case, with respect to latent data, the correlation between N1 and P1 was 0.76. the value was close to 1 implying that the relationship was strong and positive. The correlation between the latency of N1 and P1 was gauged as follows; (less than 0.25=weak correlation, between 2.5 and 7.5= moderate, above 7.5=strong).
Table 5: Summary of statistics for Latency of VEP Components
| LATENCY | ||||
| N1 (N50) | P1 (P100) | N2 (N140) | P3 (P300) | |
| N1 (N50) | 1 | |||
| P1 (P100) | 0.76240559 | 1 | ||
| N2 (N140) | 0.49494141 | 0.87328777 | 1 | |
| P3 (P300) | 0.19821258 | 0.4238074 | 0.33182506 | 1 |
Table 6: Table 5: Summary of statistics for Amplitude of VEP Components
| AMPLITUDE | ||||
| N1 (N50) | P1 (P100) | N2 (N140) | P3 (P300) | |
| N1 (N50) | 1 | |||
| P1 (P100) | 0.24996001 | 1 | ||
| N2 (N140) | 0.16792043 | 0.78001629 | 1 | |
| P3 (P300) | -0.0964055 | 0.00917118 | 0.09723945 | 1 |
Expt. 2: Circle vs Square: effect of visual attention
Table 7: Single ANOVA Mean Data for Circle and Square VEP Components
| Circle | Square | |||
| LATENCY | AMPLITUDE | LATENCY | AMPLITUDE | |
| N1 (N50) | 58.40371429 | 1.537142857 | 56.91891892 | 2.6405405 |
| P1 (P100) | 100.3257143 | 6.84 | 95.45945946 | 8.072973 |
| N2 (N140) | 138.6614286 | 5.425714286 | 131.5932432 | 6.5918919 |
| P3 (P300) | 289.1818182 | 3.763636364 | 294.5472222 | 6.8611111 |
Figure 1: Graphical representation of the statistical analysis Circle vs Square: effect of visual attention
Expt 3: Circle vs Square: effect of interfering WM task.
Table 8: Single ANOVA Mean Analysis for Circle vs. Square WM VEP
| Single ANOVA Mean Analysis for Circle vs. Square WM VEP | ||||
| Circle | Square | |||
| Latency | Amplitude | Latency | Amplitude | |
| N1 (N50) | 56.853125 | 1.77190625 | 52.434375 | 3.118212121 |
| P1 (P100) | 91.23125 | 5.1784375 | 85.5503125 | 5.951636364 |
| N2 (N140) | 123.7640625 | 4.45953125 | 115.9109375 | 5.272818182 |
| P3 (P300) | 283.3875 | 3.757357143 | 295.178125 | 4.645454545 |
Figure 2: Graphical representation of Single ANOVA Mean Analysis for Circle vs. Square WM VEP
Discussion
The results indicated that means were statistically different. The results of the experiment were in line with the study hypothesis. It was seen that the N1 latency was correlated with Nasion-Inion distance. In other words, the results indicated the presence of the strong and positive correlation between distance and latency for N1. Therefore, the latency for N1 was significantly influenced by the distance. In this case, an increase in the distance lead to a decrease in the latency for N1 and vice versa. The results also indicated a variation in the VEP components for simple complex with comparison to the complex stimuli patterns. Furthermore, the study results indicated that VEP components were influenced by visual attention rather than due to selective attention. Therefore, the results of the study indicated an accomplishment of the major aims of the study. The results from the experiment were congruent with previous studies such as by waller et al. (2019), that indicated a correlation between distance and latency of N1 VEP component. However, there were differences in some aspects of the results with some of the previous studies. For instance, the study results differed from the results obtained by Warren et al. (2020). According to Warren et al. (2020) the latency of P3 and its amplitude are not influenced intentional response. However, according to the results from this experiment, there was no significant influence of distance on the amplitude. The results of this experiment were susceptible to errors as a result of intentional or unintentional distractions that could have created the discrepancy with the previous studies.
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