International Journal of Psychophysiology 32 Ž1999. 193]203

Integrated effect of stimulation at fixation points on EFRP ž eye-fixation related brain potentials / Koji KazaiU , Akihiro Yagi Department of Psychology, Kwansei Gakuin Uni¨ ersity, Uegahara, Nishinomiya, Hyogo, 662-8501 Japan Received 1 December 1998; received in revised form 21 February 1999; accepted 23 February 1999

Abstract The purpose of this study was to investigate the integrated effect of stimulation at the fixation points just before and just after saccadic eye-movement Žsaccade. on eye-fixation related brain potentials ŽEFRP: P75 and N105.. Checkerboard patterns were used as stimuli. In Experiment 1, changes in check sizes between two fixation points enhanced the amplitude of P75, while changes in the phases of patterns between the two points did not affect EFRP. This result showed that EFRP was affected by two fixation points, and that changes in the retinal image between the two points did not necessarily affect EFRP. In Experiment 2, the relationship between EFRP and check size was investigated in detail. A second order relationship between logarithm of check size and the latency of P75, and a linear relationship between logarithm of check size and the amplitude of N105 were found. The effect of check size on the amplitude of P75 which might explain the increased amplitude of P75 observed in Experiment 1 did not appear. These results suggest that EFRP might reflect relative higher processing than peripheral stimulation at one fixation point. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Saccadic eye-movement; Eye-fixation related brain potentials ŽEFRP.; Lambda response; Checkerboard patterns

1. Introduction In order to perceive the visual world, we make saccadic eye-movements Žsaccade. from one fixation point to another. When EEGs time-locked to the offset of saccades are averaged, eye-fixation related brain potentials ŽEFRP. can be obtained U

Corresponding author. Tel.: q81-798-54-6209; fax: q81798-52-7353. E-mail address: [email protected] ŽK. Kazai.

ŽYagi, 1995.. EFRP consist of several components. The most prominent component of EFRP is a large positive deflection with a peak latency of approximately 80 ms from the offset of the saccade. This is called the lambda response. Scott et al. Ž1981. examined the lambda response by averaging EEGs at onset of saccades. However, Yagi Ž1979, 1981a. found that the lambda response was linked to the offset of the saccade; i.e. the onset of eye-fixation. Other studies also support this finding ŽSzirtes et al., 1982; Billings,

0167-8760r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 7 6 0 Ž 9 9 . 0 0 0 1 0 - 0

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1989; Jagla and Zikmund, 1994.. Since suppression occurs during saccades Že.g. Zuber and Stark, 1966; Matin, 1974; Volkmann, 1986., the lambda response is thought to be evoked by the inflow of visual information after termination of the saccade. The lambda response like the so-called visual evoked potentials ŽVEP. is influenced by physical and psychological factors. For instance, the latency and amplitude of lambda responses vary with the size of the saccade ŽKurtzberg and Vaughan, 1977; Yagi, 1979., the arousalrattentional level of subjects ŽYagi, 1981b., and the physical properties of stimulus such as its luminance ŽGaarder et al., 1964. and contrast ŽYagi et al., 1992.. The latency and amplitude of lambda responses vary with stripe width in striped patterns ŽYagi et al., 1993.. Several authors ŽScott et al., 1981; Riemslag et al., 1987; Billings, 1989. have found similarities between the lambda response and pattern VEP in the same subject. These studies support the idea that lambda response can be a useful index of visual information processing as well as the VEP. When we view a picture or read sentences, for example, visual perception ordinarily consists of more than one fixation point. Visual information obtained at each eye fixation point is integrated into a perception. Therefore, the lambda response may be influenced by not only the stimulus at the fixation following the saccade, but also by the stimulus at the fixation prior to the saccade. For example, Yagi Ž1987. found a significant correlation between fixation duration and the amplitude of the lambda wave at the next fixation, in a case study with a subject who showed large lambda waves in raw EEGs. Kurtzberg and Vaughan Ž1977. postulated that the lambda response might represent a fusion of activity generated by stimulus at both the onset and offset points of the saccade. Yagi Ž1979. and Thickbroom et al. Ž1991. suggested that the lambda response might be the compound of an onset related component and an offset related component. These studies, however, did not directly investigate the relationship between stimuli at the onset and at the offset of the saccade. The purpose of the present study was to investigate the

integrated effect of stimuli at both the onset and offset of the saccade on EFRP using checkerboard patterns as stimuli. 2. Experiment 1 The purpose of Experiment 1 was to examine the integrated effect of the stimuli at two fixation points Žonset point and offset point of the saccade. on EFRP. Two properties of the checkerboard patterns; i.e. the check sizes Ž309 and 1209. and the phase differences Žcongruent and reversal., were varied under six conditions. Under the congruent conditions, the two patterns presented at onset and offset of the saccade had the same alignment of black and white elements. Under the reversal conditions, two patterns presented at onset and offset of the saccade were counterphased. Thus, if the two patterns were presented at the same location successively, the viewer would perceive that a checkerboard pattern reverses white elements and black elements under the reversal conditions, while the viewer would perceive that a checkerboard pattern does not change under the congruent conditions. If the components of EFRP reflect either of the stimuli at the offset or onset points of the saccade, the difference in check sizes between the patterns at the two fixation points might not influence these components. The phase differences between the patterns at the two points, also, might not influence these components either. 2.1. Methods 2.1.1. Subjects Subjects were 16 students aged 18]24 Žnine males and seven females., who had normal or corrected-to-normal visual acuity. Informed consent was obtained after the experimental procedures were explained. 2.1.2. Stimulus and apparatus Stimuli were presented on a CRT display ŽSharp CU-21HD. which was placed at a distant of 62.4 cm from the eyes of the subjects. Each subject was seated in front of the CRT display in a sound proof and shielded room with the head moder-

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ately fixed by a head rest. The background color of the display was gray, and its size was 22 = 35.68 of visual angle. Two green fixation points of 129 in diameter were presented at positions 58 to the right and to the left of the display center. Therefore, the distance between the two fixation points was 108. Fig. 1 represents the schema of the stimulus presentation. A round black-and-white checkerboard pattern of 208 in diameter appeared around one of the two fixation points when eyes were directed to the fixation point, and remained there until the eyes left for the other fixation point. The element size of the pattern was 309 or 1209. The luminance level of the fixation points was 20 cdrm2 , and that of the background was 18 cdrm2 . The luminance levels of the black part and the white part of the checkerboard pattern were 2 cdrm2 and 35 cdrm2 , respectively. The presentation of stimuli was controlled with a personal computer ŽNEC PC-9801DS.. A horizontal electrooculographic ŽEOG. signal amplified with a differential amplifier ŽNihon Kohden AB-621G. at a time constant of 0.1 s was supplied to the input of a window-slicer ŽNihon Kohden EP-601J.. This window-slicer produced pulses when the EOG signal exceeded a given

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level. This level was set at 50% of the amplitude of the EOG when the subject made a horizontal saccade of 108. These pulses were used as the trigger to present two checkerboard patterns by turns. Therefore, one of the two checkerboard patterns disappeared and the other appeared during the saccade. One of the two checkerboard patterns was presented around the right fixation point when the subject’s eyes were directed to the right fixation point from the left fixation point. The other checkerboard pattern was presented around the left fixation point when the subject’s eyes were directed to the left fixation point from the right fixation point. The two green fixation points were always presented. 2.1.3. Procedure The task was to execute saccades between the two green fixation points on the display. Before the task, subjects practiced executing saccades at a rate of 70 times per minute with a metronome. In experimental trials, the subject was asked to make saccades at the same rate without the metronome. The subject made saccades for 16 s in one trial. They performed 10 trials under each condition. There were six conditions according to the combinations of the two check sizes of the

Fig. 1. Schematic representation of the stimulus presentation. One of the two stimulus patterns was presented around the right fixation point when the subject’s eyes were directed to the right fixation point from the left fixation point, and vice versa. Replacement of the patterns was triggered by a horizontal EOG. See the text for details.

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checkerboard patterns Ž309 and 1209. and the phase differences Žcongruent or reversal. between the two checkerboard patterns. Table 1 indicates the combinations of the two factors in each condition. The conditions were: Ž1. LC, the element sizes of the two checkerboard patterns were large Ž1209., and the phases of the two patterns were congruent; Ž2. LR, the element sizes of the two patterns were large Ž1209., and the phases of the two patterns were reversed; Ž3. SC, the element sizes of the two patterns were small Ž309., and the phases of the two patterns were congruent; Ž4. SR, the element sizes of the two checkerboard patterns were small Ž309., and the phases of the two patterns were reversed; Ž5. SL, the check size of the pattern at the onset point of the saccade was small Ž309., and that at the offset point of the saccade was large Ž1209.; and Ž6. LS, the check size of the pattern at the onset of saccade was large Ž1209., and that at the offset of saccade was small Ž309.. 2.1.4. Recording EEGs were recorded from parietal site ŽPz., occipital site ŽOz. and the center of these two sites ŽPOz.. Linked earlobes were used as the reference in order to restrain the artifact from eyereye-lid movements subordinate to the saccade. The EEGs were amplified with differential amplifiers ŽNihon Kohden MME-3132. at a time constant of 1.0 s. The ground lead was attached to Table 1 Combination of ‘phase’ and ‘check size’ under each condition in Experiment 1 Condition

LC LR SC SR SL LS

Phase

Congruence Reverse Congruence Reverse ] ]

Check size Onset

Offset

1209 1209 309 309 309 1209

1209 1209 309 309 1209 309

Notes. ‘Phase’ indicates that phases of the patterns onset and offset points of the saccade were reversed congruent. ‘Onset’ indicates the check size of the pattern onset of the saccade, and ‘offset’ indicates the check size the pattern at offset of the saccade.

at or at of

the midline forehead. Eye movements were recorded by means of EOGs. A pair of Ag]AgCl electrodes was placed at the outer canthi of the two eyes for horizontal eye movements. Horizontal EOGs were amplified at two different time constants Ž0.1 and 5 s.. The horizontal EOGs with a time constant of 0.1 s were used to generate the trigger pulses for switching the stimulus patterns on the display on-line Žsee above., and the horizontal EOGs with a time constant of 5 s were used to detect the offset of the saccade off-line. Another pair of electrodes was placed above and below the left eye for vertical eye movements and eye blinking potentials. The vertical EOGs were also amplified at a time constant of 5 s. All of these signals were amplified at a high frequency cut off of 50 Hz, and digitized every 1.6 ms with a 14-bit A]D converter ŽTEAC PS9354.. 2.1.5. Data analysis All raw EEGs and EOGs were reviewed off-line on a computer terminal. The offset points of the saccades were determined by moving a graphic cursor manually on the horizontal EOG tracing on the computer terminal. The EEGs without artifacts between 200 ms before and 400 ms after the offset point of the saccade were averaged. The pattern appearing on each side of the display was identical during a specific trial. Thus, the brain potentials for SL and LS were obtained by selectively averaging the epochs corresponding to the conditions. The EFRP of SL consisted of two kinds of epochs as followings: One was an epoch when the subject made saccades to the right in the trials during which the check size of the left pattern was 309 and that of the right pattern was 1209. The other was an epoch when the subject made saccades to the left in the trials during which the check size of the right pattern was 309 and that of the left pattern was 1209. The EFRP of LS consisted of the counterparts in the same trials. Epochs Ž40]48. were averaged for each condition. The directions of saccades were counterbalanced. Repeated measure analyses of variance ŽANOVA. were conducted on data with Greenhouse]Geisser epsilon correction, but original degrees of freedom were reported to aid the

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interpretation of the statistical designs. In Experiment 1, the data from the four conditions ŽLC, LR, SC and SR. were subjected to a 2 Žphase. = 2 Žcheck size. ANOVA. Then the data from another combination of four conditions ŽLC, SC, SL and LS. were subjected to a single ANOVA. Tukey’s HSD test was used for post hoc comparisons. An alpha level of 0.05 was used for all statistical tests. 2.2. Results

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Table 2 Amplitudes of P75 Ž n s 16. and N105 Ž n s 14. in Experiment 1 Condition

LC LR SC SR SL LS

Amplitude ŽmV. P75

N105

5.96" 0.50 5.97" 0.77 5.78" 0.83 5.31" 0.70 8.28" 0.66 6.32" 0.74

4.44" 0.61 4.89" 0.82 8.45" 1.17 8.44" 1.18 7.43" 1.20 9.83" 1.16

Values are expressed as mean " 1 S.E.

Fig. 2 shows the grand averaged waves of EFRP at Oz over 16 subjects under the six conditions. The baseline was computed as the mean of all data points from 200 to 99.2 prior to the offset point of the saccade. Positive components Žlambda

response: P75. and negative components ŽN105. appeared under all six conditions, with a peak latency of approximately 75 ms and 105 ms, respectively, from the offset point of the saccades. Since N105 could not be recorded with two of the subjects, further analysis of N105 was conducted with the data of 14 subjects. Table 2 shows the mean amplitudes of P75 and N105. Table 3 shows their mean latencies. The peak amplitude of P75 was measured from the base line. The peak amplitude of N105 was calculated from the P75 peak to the N105 peak. The peak latencies of P75 and N105 were measured from the offset point of the saccade. 2.2.1. P75 (lambda response) As in Fig. 2, P75 was clearly observed in the grand averaged waves under all six conditions. The amplitude and latency of P75 under the four conditions ŽLC, LR, SC and SR. were subjected to phase Ž2. = check size Ž2. ANOVAs with reTable 3 Latencies of P75 Ž n s 16. and N105 Ž n s 14. in Experiment 1 Condition

Fig. 2. Grand averaged waves of EFRP over 16 subjects from the electrode site at Oz under the six conditions. The origin indicates offset of the saccade.

LC LR SC SR SL LS

Latency Žms. P75

N105

72.00" 2.37 69.50" 2.40 75.10" 1.68 74.70" 1.34 78.60" 2.03 69.80" 2.04

104.81" 2.37 103.54" 3.20 108.34" 3.41 107.43" 2.44 109.49" 2.58 104.57" 3.10

Values are expressed as mean " 1 S.E.

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peated measures, but there was no significant effect of the phase difference, the check size, nor their interaction. Then the amplitude and latency of P75 under the other combination of four conditions ŽLC, SC, SL and LS. were submitted to single ANOVAs with repeated measures. The single ANOVAs revealed significant main effects of the condition both on the amplitude Ž F3,45 s 5.86, P- 0.01. and on the latency Ž F3,45 s 5.77, P- 0.01.. Further analysis by Tukey’s HSD test revealed that the amplitude of SL was significantly larger than those of the other three conditions ŽLC, SC and LS., and that the latency of SL was significantly longer than those of LC and LS. 2.2.2. N105 Although N105 was clearly observed in the grand averaged waves under five conditions except SL, it was also observed under SL condition in the individual waveforms of the 14 subjects. The amplitude and latency of N105 under the four conditions ŽLC, LR, SC and SR. were subjected to phase Ž2. = check size Ž2. ANOVAs with repeated measures. Only the significant effect by the check size on the amplitude of N105 was revealed Ž F1,13 s 27.17, P- 0.01.. Then the amplitude and latency of N105 under the other combination of four conditions ŽLC, SC, SL and LS. were submitted to single ANOVAs with repeated measures. The single ANOVAs revealed significant main effects of the condition on the amplitude, but not on the latency. Further analysis by Tukey’s HSD test revealed that the amplitude of LC was significantly smaller than those of the other three conditions ŽSC, SL and LS.. 2.3. Discussion 2.3.1. Effects of the phase difference (congruent and re¨ ersal) of patterns between two fixation points There were no differences in the latencies and amplitudes of P75 Žlambda response. between LC and LR, nor between SC and SR. Furthermore, there were no differences in the latencies and amplitudes of N105 between LC and LR, nor between SC and SR. These results indicate that

the phase difference of the patterns did not affect P75 Žlambda response. or N105, regardless of the two different check sizes Ž309 and 1209. of the checkerboard patterns. Under LC and SC conditions, the two patterns which appeared around the left and the right fixation points were congruent. Retinal images at the two fixation points should be the same under these two conditions. In contrast, the two patterns which appeared around the left and the right fixation points were reversed under LR and SR conditions. Retinal images at the two fixation points should be quite different under these two conditions. Nevertheless, any differences were not found in P75 and N105 between the congruent conditions and the reversal conditions. An assumption that only one of the stimuli at the two fixation points affects P75 and N105 could explain this finding. However, this assumption does not explain the result that the P75 amplitude under SL Žthe check size at the onset point of the saccade was 309, and that at the offset point was 1209. was larger than those obtained under LC, SC and LS. Another assumption is presented in the next section, which could explain this result. 2.3.2. Effects of different check sizes of patterns between two fixation points The amplitude of P75 Žlambda response. obtained under SL was significantly larger than those obtained under LC, SC and LS. This result showed that P75 was affected by both the stimuli at the offset point and at the onset point of saccade. If P75 was affected by the stimulus only at the offset point of the saccade, there should be no difference in the P75 between SL and LC. The stimuli at the offset point of the saccade can be regarded to be the same between SL and LC. Furthermore, if P75 was affected by the stimulus only at the onset point of saccade, there should be no difference in the P75 between SL and SC, because the stimuli at the offset point of saccade could be regarded to be the same between SL and SC. However, there were statistically significant differences in the amplitudes between SL and LC, and between SL and SC. In addition, significant differences in latency and amplitude were found

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between SL and LS. These results indicate that P75 was influenced by differences in the check sizes between the stimulus patterns at the onset and at the offset point of the saccade. In other words, the P75 reflected stimuli at both onset and offset points of the saccade. N105 also showed a similar tendency to P75, since there were significant differences in the amplitudes between SL and LC, and between LS and LC. It will be useful to discuss again the result that the phase difference did not affect P75 and N105. This result implies that EFRP did not change morphologically in spite of the change in the retinal image between the two fixation points, as long as the check sizes were equal. In contrast to the phase difference, changes in check sizes between two fixation points had influences on EFRP. Therefore, in the present experiment, the property of the stimulus that affected EFRP was the check size of the checkerboard patterns. It should be noted that the effect of SL and the effect of LS were not the same despite the same difference in check sizes Ž30]1209, or 120]309. between the two fixation points. That is, the amplitude of P75 under SL was larger than that under LS, while the amplitude of N105 under LS was larger than that under SL. This finding suggests that the stimulus at onset point of the saccade and the stimulus at offset point of the saccade do not equally affect EFRP. In the present experiment, the amplitudes of N105, under the conditions where the check sizes of patterns were small ŽSC and SR., increased compared with those under conditions where the check sizes of patterns were large ŽLC and LR.. This result suggests an effect of check size on EFRP. However, to date no known studies have examined the effect of check size on EFRP. The relationship between check size and EFRP was examined in the next experiment. 3. Experiment 2 Little attention has been paid to the effect of check size on lambda response or EFRP. The purpose of Experiment 2 was to examine the effect of check size on P75 Žlambda response. and N105. Check sizes were 309, 459, 609, 909 and 1209.

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This examination might clarify the effect on EFRP by the different check sizes between two fixation points which was observed in Experiment 1. 3.1. Methods 3.1.1. Subjects Subjects were 14 students aged 20]26 Žseven males and seven females., who had normal or corrected-to-normal visual acuity. One of them had participated in Experiment 1. 3.1.2. Stimulus In Experiment 2, only the check size of the checkerboard pattern was varied Ž309, 459, 609, 909 and 1209.. The effect of the phase difference between the stimuli at the two fixation points was ignored in Experiment 2, because the phase difference did not affect EFRP in Experiment 1. Other properties of the stimulus were precisely the same as those in Experiment 1. 3.2. Results Fig. 3 shows the grand averaged waves of EFRP at Oz over 14 subjects in response to the five check sizes. The baseline was computed as the mean of all data points from 200 to 99.2 ms prior to the offset point of the saccade. Positive ŽP75. and negative ŽN105. components appeared under all five conditions as in Experiment 1. Fig. 4 shows the modifications of latency and amplitude of the two components by check size. 3.2.1. Effects of check size on P75 (lambda response) In Fig. 3, P75 was clearly observed in the grand averaged waves under all five conditions. Fig. 4a indicates the relationship between check size and P75 amplitude. The amplitude of P75 showed very small changes to the different check sizes. There was no significant effect of check size on the amplitude of P75. Thus, the results obtained in Experiment 1 on the effect of check size on P75 was replicated. Fig. 4b indicates the relationship between check size and P75 latency. A single ANOVA with repeated measures revealed a significant main effect of check size on latency Ž F4,52 s 3.03, P-

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Fig. 3. Grand averaged waves of EFRP over 14 subjects from the electrode site at Oz under the five conditions. The origin indicates offset of the saccade.

0.05.. Further analysis by Tukey’s HSD test revealed that the latency to the check size of 309 was significantly longer than those to 609 and 909. A second order polynomial correlation was determined between the mean latency of P75 and logarithm of check size, expressed by the equation approximated by least square regression analysis: P75 latency Žms. s 215.96y 148.13= log Žcheck size. q 39.60= wlog Žcheck size.x 2 , R 2 s 0.99, F2,2 s 105.60, P- 0.01. 3.2.2. Effects of check size on N105 N105 was clearly observed in the grand averaged waves under all five conditions. Fig. 4c indicates the relationship between check size and N105 amplitude. A single ANOVA with repeated measures revealed significant main effects of check size on the amplitude Ž F4,52 s 15.79,

Fig. 4. The relationships between check size and amplitudes Ža, P75; c, N105., and the relationships between check size and latencies Žb, P75; d, N105.. These figures depict the means of all subjects with bars of "1 S.E. Significant regression equations are as follow: Žb. P75 latency Žms. s 215.96y 148.13= log Žcheck size. q 39.60= wlog Žcheck size.x2 , R 2 s 0.99; Žc. N105 amplitude ŽmV. s 22.29y 7.64= log Žcheck size., R 2 s 0.83.

P- 0.01.. Further analysis by Tukey’s HSD test indicated that amplitudes to the check sizes of 1209 and 909 were significantly less than those to the other three check sizes. There was a linear correlation between the mean amplitude of N105 and logarithm of check size, expressed by the equation approximated by least square regression analysis: N105 latency Žms. s 22.29y 7.64= log Žcheck size., R 2 s 0.83, F1,3 s 14.80, P- 0.05. Fig. 4d indicates the relationship between check size and N105 latency. A single ANOVA with repeated measures revealed a significant main

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effect of check size on latency Ž F4,52 s 4.73, P0.01.. Further analysis by Tukey’s HSD test revealed that latency to the check size of 909 was significantly longer than those to the check sizes of 459 and 309. Thus, the results obtained in Experiment 1 on the effect of check size on N105 were also replicated. 3.3. Discussion The results of Experiment 2 revealed a second order polynomial correlation between the latencies of P75 and logarithm of check sizes, and showed a linear correlation between the amplitudes of N105 and logarithm of check sizes. Therefore, the increased amplitude of P75 observed in Experiment 1 may not be explained by the effect of check size itself, because the amplitude of P75 showed only a small change to check sizes in Experiment 2. Several studies have reported the relationships between check size and the components of pattern reversal VEP Že.g. Siegfried, 1975; Ristanovic and Hajdukovic, 1981; Kurita-Tashima et al., 1991.. Though the components of VEP might not necessarily correspond to that of EFRP, the present study indicates that VEP and EFRP have the same property of varying with check size. Yagi et al. Ž1993. reported that the amplitude and latency of lambda response varied with stripe width, although no studies have yet examined the relationship between P75 Žlambda response. and check sizes. The results of the present experiment could be compared to those of Yagi et al. Ž1993. in terms of the fundamental spatial frequency of the stimulus. Because both stripe width and check size can be described by the fundamental spatial frequency of the pattern ŽBodis-Wollner et al., 1990.. Thus, it might be concluded that the lambda response Žor EFRP. varies with the spatial frequency of the stimulus, although the results in the present study did not always coincide with the previous findings by Yagi et al. Ž1993.. This discrepancy could be attributed to the difference in orientations of the power spectra between the striped pattern and the checkerboard pattern ŽBodis-Wollner et al., 1990., or to the difference

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in the total size of the stimulus pattern ŽYiannikas and Walsh, 1983.. 4. General discussion The purpose of the present study was to examine whether EFRP ŽP75 and N105. reflect visual information processing at only one or more than one fixation point, by using checkerboard patterns as stimuli. In Experiment 1, the check sizes and the phases of the stimuli were manipulated. The change in the check sizes between the two points enhanced the amplitude of P75. This result proved that EFRP reflects visual information processing at more than one fixation point. The change in phases between the two points did not affect EFRP. This result suggests that changes in the retinal image between the two points did not necessarily affect EFRP. In Experiment 2, the relationship between EFRP and check size was investigated in detail, under the conditions of five different check sizes from 309 to 1209. As the result, two significant relationships were obtained: a second order relationship between logarithm of check size and the latency of P75, and a linear relationship between logarithm of check size and the amplitude of N105. Therefore, the amplitude of N105 reflected the check size of the stimulus more clearly than did the amplitude of P75. The effect of check size on the amplitude of P75 which might explain the increased amplitude of P75 observed in Experiment 1 did not appear. Experiment 2 supported the finding in Experiment 1 that the change in check sizes between the onset point and the offset point of the saccade was necessary to enhance the amplitude of P75. Yagi Ž1979. and Thickbroom et al. Ž1991. found that the lambda response included the onset related component and the offset related component, and that these two components were merged into a single component when subjects made relatively small saccades. Thickbroom et al. Ž1991. also suggested that these components were identical to pattern movement VEP. These studies and the results of the present research support the hypothesis that lambda response reflects sti-

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mulus at the onset as well as at the offset of the saccade. However, the studies of Yagi Ž1979. and Thickbroom et al. Ž1991. do not fully explain the present results. Thickbroom et al. Ž1991. reported that the latency of the saccade onset related component was 100 ms from onset, while the latency of the saccade offset related component was 74 ms from offset of the saccade. In the present study, the size of the saccade was 108 with a duration of approximately 60 ms. The mean latency of P75 was 76 ms from offset of the saccade. This interval between onset of the saccade and the latency of P75 is great enough to separate the onset related component and the offset related component, although the onset related component was not observed in the current study. Thus, the changes in EFRP ŽP75. in Experiment 1 could not be attributed to a simple summation of the onset component and the offset component that correspond to pattern movement VEPs. The results of Experiment 2 confirmed the effect of the check size on the amplitude of N105: the smaller check size enhanced the amplitude of N105. Furthermore, Experiment 1 and Experiment 2 indicated that the stimulus at the offset point of the saccade had a greater influence on N105 than the stimulus at the onset point of the saccade. For example, the result in Experiment 1 that N105 under LS was larger than that under LC was probably due to the smaller check size of the pattern at the offset point of the saccade under LS. On the other hand, N105 under SL was larger than that under LC in spite of the same check sizes at the offset point of the saccade. This result, however, could be attributed to the effect of changes in check sizes between the two fixation points under SL. An assumption that the effect of the check size at the offset point and the effect of changes in check sizes were equal could explain the result that the amplitudes of N105 did not differ between SC and SL. It is obvious that EFRP did not reflect the very retinal image of the stimulus at the fixation point, since EFRP reflected the stimuli at both onset and offset points of the saccade. Furthermore, EFRP did not reflect superimposition of the retinal images at the two points, as EFRP under the

SL condition was different from that under the LS condition in Experiment 1. The superimposition of retinal images at the two fixation points under SL was identical to that under LS. This finding seems to agree with our casual experience that we ordinarily do not perceive superimposition of the afterimage of the previous fixation point on the current retinal image. It would be a plausible explanation for the present results that representation of the stimulus at the onset point of the saccade affects the processing of stimulus at the offset point of the saccade. In other words, EFRP may reflect the relative higher processing of integrating visual information across saccades rather than the summation of activities caused by peripheral stimulations at each fixation point. Acknowledgements The present study is supported by MITI ŽMinistry of International Trade and Industry. and NEDO Žthe New Energy and Industrial Technology Development Organization.’s Project on ‘Human Sensory Measurement Application Technology’. References Billings, R.J., 1989. The origin of the occipital lambda wave in man. Electroencephalogr. Clin. Neurophysiol. 72, 95]113. Bodis-Wollner, I., Brannan, J.R., Ghilardi, M.F., Mylin, L.H., 1990. The importance of physiology to visual evoked potentials. In: Desmedt, J.E. ŽEd.., Visual Evoked Potentials. Clinical Neurophysiology Updates, vol. 3. Elsevier Science Publishers B.V., Amsterdam, pp. 1]24. Gaarder, K., Krauskopf, J., Graf, V., Kropfl, W., Armington, J.C., 1964. Averaged brain activity following saccadic eye movement. Science 146, 1481]1483. Jagla, F., Zikmund, V., 1994. Differences in eye movement related potentials with visually triggered horizontal and vertical saccades. In: d’Ydewalle, G., van Resbergen, J. ŽEds.., Visual and Oculomotor Functions. Elsevier Science B.V., Amsterdam, pp. 19]30. Kurita-Tashima, S., Tobimatsu, S., Nakayama-Hiromatsu, M., Kato, M., 1991. Effect of check size on the pattern reversal visual evoked potential. Electroencephalogr. Clin. Neurophysiol. 80, 161]166. Kurtzberg, D., Vaughan, H.G., 1977. Electrophysiological observations on the visuomotor system and neurosensorium. In: Desmedt, J.E. ŽEd.., Visual evoked potentials in man: new developments. Clarendon Press, Oxford, pp. 314]331.

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