Exp Brain Res (2003) 150:443–448 DOI 10.1007/s00221-003-1454-0

RESEARCH ARTICLE

T. S. Constantinidis · N. Smyrnis · I. Evdokimidis · N. C. Stefanis · D. Avramopoulos · I. Giouzelis · C. N. Stefanis

Effects of direction on saccadic performance in relation to lateral preferences Received: 27 September 2002 / Accepted: 26 February 2003 / Published online: 25 April 2003  Springer-Verlag 2003

Abstract A sample of 676 healthy young males performed visually guided saccades and antisaccades and completed the Porac-Coren questionnaire measuring lateral preferences. There was no difference in mean latency between rightward versus leftward saccades or for saccades executed in the left versus right hemispace. There was also no right/left asymmetry for individuals with left or right dominance as assessed by the lateral preferences questionnaire. The same results were observed for the latency of antisaccades and for the error rate in the antisaccade task. Finally, we did not confirm any substantial subpopulation of individuals with idiosyncratic left/right latency asymmetries that persisted both in the saccade and antisaccade task. These results suggest that neither latency nor antisaccade error rate are good indicators of lateral preferences in these tasks. Other oculomotor tasks might be more sensitive to hemifield differences, or cerebral hemispheric asymmetry is not present at the level of cortical organization of saccades and antisaccades. Keywords Saccades · Antisaccades · Latency · Handedness · Laterality · Hemifield · Hemispace

Introduction The latency of visually triggered saccades has been extensively studied and has been shown to vary according to the physical characteristics of the visual stimulus T. S. Constantinidis · N. Smyrnis · N. C. Stefanis · D. Avramopoulos · I. Giouzelis · C. N. Stefanis University Mental Health Research Institute, National University of Athens, Athens, Greece T. S. Constantinidis · N. Smyrnis ()) · I. Evdokimidis Cognition and Action Group, Neurology Department, National University of Athens, Aeginitio Hospital, 72–74 Vas. Sofias Av., 11528 Athens, Greece e-mail: [email protected] Tel.: +30-1-7289115 Fax: +30-1-7216474

(Doma and Hallett 1988a, 1988b; Becker 1989; Leigh and Zee 1999) as well as the experimental paradigm (Fuller 1996). Thus, the introduction of a time-gap between visual stimulus presentation and the onset of the response results in the reduction of latency (Fischer and Ramsperger 1984; Weber and Fischer 1995). On the other hand, the latency increases in the antisaccade task, i.e., when subjects have to saccade in the opposite direction from that of the visual stimulus (Everling and Fischer 1998). The question of hemispheric specialization in the programming and execution of saccadic eye movements has been studied both in terms of behavioral measures such as saccadic latency (Pirozzolo and Rayner 1980; Hutton and Palet 1986) and in terms of lesion studies in humans (Braun et al. 1992). Thus, it has been reported that saccades to the right visual hemifield have shorter latencies than saccades to the left hemifield and that this effect is observed only for right-handed subjects while this difference disappears in non-right-handed individuals (Pirozzolo and Rayner 1980; Hutton and Palet 1986). These findings are viewed as evidence supporting the hypothesis that in right-handed individuals, the left cerebral hemisphere seems to be a more efficient neural substrate for the generation of rightward saccades than the right hemisphere for the generation of leftward saccades (Pirozzolo and Rayner 1980). The hypothesis to explain this difference in processing efficiency was that it reflects an effect of learning directional scanning habits. Thus, English speakers have learned to read in a left-to-right direction and by consequence 90% of their saccades during reading are rightward (Pirozzolo and Rayner 1980). In a recent study (Honda 2002), left/right saccadic latency asymmetries were examined in healthy subjects and there was no latency difference even for right-handed individuals. There were, though, a few subjects with left/ right latency asymmetries in reflexive saccades, and these same individuals presented the same pattern of asymmetry in the execution of voluntary-saccades. These findings were interpreted in the context of a visuospatial atten-

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tional bias that is specific for some individuals, a form of idiosyncratic left/right asymmetry of saccade latencies. The question of left/right asymmetries of antisaccade latencies and errors in relation to handedness has not been addressed in detail. In one study it was reported that in an antisaccade task with a gap condition subjects performed more errors when the stimulus was presented in the right hemifield (Fischer et al. 1997). The data presented in this study stem from the ASPIS project (Athens Study on Psychosis Proneness and Incidence of Schizophrenia) and originate from the analysis of oculomotor behavior in 2,075 apparently normal young male subjects (Evdokimidis et al. 2002; Smyrnis et al. 2002). In the present study, we have examined the latencies of visually triggered saccades and antisaccades and calculated the distractibility factor, in a subpopulation of 676 subjects for whom lateral preferences (hand, foot, eye and ear left/right preference) were also measured. The aim was to investigate the possible relations of lateral preferences with the above-mentioned eye movement parameters.

Methods Population The sample of the present study consisted of 676 Hellenic Air Force conscripts (age 21€3 years, range 18–25 years) who completed the Porac-Coren questionnaire of lateral preferences (Porac and Coren 1981), and performed visually guided saccades and antisaccades. This sample was a subgroup of a larger sample of 2,075 young males who participated in the above-mentioned study. Visually guided saccade task The task was preceded by a calibration procedure, with saccades at 5 and 10 to the left and to the right of a central fixation point (FP). During the task, the subject initially fixated a target that could appear on an imaginary horizontal line either at the center of the screen or at a peripheral position located at 8, 6, 4, or 2 to the right or to the left of the screen center. After a variable period of 1– 2 s, a second target appeared either to the left or to the right of the first target. The second target could appear at one of five distances 2, 4, 6, 8 and 10 from the first target to the right or to the left, if possible (e.g., if the first target was at 8 to the left of the center then only one location was available to the left, the one at 10). The subject had 1.5 s to respond and then the second target disappeared and the screen was blank for a period of 1 s before a new target appeared. Each subject performed 90 trials (18 trials for each first target position).

antisaccade trials (randomly chosen from the set of nine target positions). Data acquisition and preprocessing The data acquisition and preprocessing have being previously described in detail (Evdokimidis et al. 2002). Briefly we used infrared oculography for eye movement recordings (IRIS; Skalar, Delft, The Netherlands) with a sampling rate of 600 Hz. The analysis was performed offline, using an interactive program for marking the saccade initiation. The first derivative of the position record was used to calculate the instantaneous velocity curve. This curve in turn was used to detect the onset of saccadic eye movements using the criterion that five consecutive velocity values (8 ms duration at 600 Hz) were above a predefined noise level (the noise level was determined by taking the root mean square of the signal in 15 windows of 20 values each, covering the first 500 ms of the 1-to 2-s period of central fixation, and then taking the median of these 15 values). The end of the saccade was the return of the instantaneous velocity to the noise level. A criterion of change of velocity sign was used to exclude blinks (that occur as increases of velocity in one direction followed immediately by increases in the opposite direction). Finally, the program marked saccadic eye movements and blinks on the position record for inspection by a human observer. All trials with artifacts (blinks, eye movements just prior to the stimulus, etc.) were excluded from further analysis. We included only saccades with a latency that was within the window of 80–400 ms and antisaccades within 80–600 ms. Subjects that performed less than 40 valid antisaccade or saccade trials based on these criteria have been excluded (3.3% of the population in the antisaccade task and 1% in the saccade task), and are not included in the sample of 676 individuals that are studied in this report. Data analysis Visually guided saccades We grouped each subject’s saccades according to the direction of the eye movement into left-directed saccades (LDs) and rightdirected saccades (RDs). We also grouped saccades according to the hemispace in which the saccade was performed (both the starting and end position falling in the same hemispace) into left hemispace saccades (LHs) and right hemispace saccades (RHs). Finally, we grouped the subpopulation of saccades that started from the center (0) to left-directed centrifugal saccades (LDCs) and to right-directed centrifugal saccades (RDCs). Then we estimated the mean latency of each saccade group. We used these means to estimate three preponderance indices as follows: 1. The directional index for saccades (DIs) DIs ¼ ððRDsÞ  ðLDsÞ=ðRDsÞ þ ðLDsÞÞ  100 2. The hemispace index for saccades (HIs) HIs ¼ ððRHsÞ  ðLHsÞ=ðRHsÞ þ ðLHsÞÞ  100

Antisaccade task Each trial started with the appearance of a FP (white cross 0.30.3 of visual angle). After a variable period of 1–2 s, the FP was extinguished and a peripheral target (same white cross) appeared randomly at one of nine target distances (2–10 at 2 intervals) either to the left or to the right of FP. The subjects were instructed to make an eye movement to the opposite direction from that of the peripheral target as soon as possible. A calibration procedure was performed before the task, similar to the previous one. Each subject performed several practice trials and then 90

3. The directional index for centrifugal saccades (DICs) DICs ¼ ððRDCsÞ  ðLDCsÞ=ðRDCsÞ þ ðLDCsÞÞ  100 Positive values for each index indicate right preponderance and the negative values left preponderance. Antisaccades For each subject we computed the distractibility factor for antisaccades (percentage of errors) for left-target presentations

445

Fig. 1 Porac-Coren questionnaire score distribution in the population. Solid bars indicate the score obtained from the four hand laterality items (HLI), and open bars the global score from all items (GLI); x-axis values of laterality indices, y-axis percentage of subject in the population (N=676) (DFLa) (correct antisaccade to the right) and right-target presentations (DFRa). There were extremely few cases among all subjects where a corrective movement was not performed after an error movement towards the target. We also computed for each subject the mean response latency for left-directed correct antisaccades (LDa) and right-directed correct antisaccades (RDa). We then computed for each subject the following two antisaccade preponderance indices: 1. The distractibility factor index for antisaccades (DFIa) DFIa ¼ ððDFRa  DFLaÞ=ðDFRa þ DFLaÞÞ  100 2. The directional index for antisaccades (DIa) DIa ¼ ððRDaÞ  ðLDaÞ=ðRDaÞ þ ðLDaÞÞ  100

Fig. 2A, B Saccadic latency distributions (upper parts) and latency preponderance index distributions (lower parts). A Distribution for the left (LDs), right direction groups (RDs) and the directional preponderance index (DIs). B Distributions for the left (HLs), right (HRs) hemispace groups and the hemispace preponderance index (HIs). For the preponderance index distributions a vertical line indicates zero preponderance, the positive values indicate preponderance to the right and the negative preponderance to the left. Upper x-axis saccade latency in milliseconds, lower x-axis saccade latency preponderance indices. For all histograms, y-axis represents percentage of subjects (N=676)

Lateral preferences The Porac-Coren questionnaire was used to measure individual lateral preferences. This questionnaire uses four sets of four items each (total 13) to estimate four indices of lateral preferences corresponding to hand, foot, eye and ear. An index is computed by means of the formula (RL)/4, where R and L represent the number of ‘right’ and ‘left’ responses, respectively. For the purposes of our analysis we used the hand laterality index (HLI) and a global laterality index (GLI) where all 13 items where used. Statistical analysis The Wilcoxon matched pairs test was used to compare right- and left-related saccade and antisaccade latencies and DF for all subjects. Spearman rank order correlations were used to compare the saccade and antisaccade preponderance indices with the corresponding lateral preference indices. Furthermore, we separated our sample subjects into two groups according to their GLI and HLI scores, the pure dextrals and the pure sinisters. Pure dextrals were characterized as all those subjects with a GLI and HLI score >0.8; pure sinisters were all individuals with corresponding scores <0.8. The cut-off point of €0.8 was used based on the observation of the GLI and HLI distributions (Fig. 1). The Wilcoxon matched pairs test was used to compare right- and left-related saccade and antisaccade latencies and DF within each group. Finally we repeated the group analysis by dividing our sample into righthanded (HLI >0) and non-right-handed subjects (HLI <0).

Results Visually guided saccades Figure 2A shows the distribution of latencies according to the direction (right, left) and the corresponding directional index for saccades. There was no difference between right-directed saccades (RDs) and left-directed saccades (LDs) in the population (mean of 181€23 ms and 181€22 ms, respectively, z=0.22, P>0.82). The directional index for saccades (DIs) had a mean value of 0.04€4% and the 95% confidence intervals of the mean (0.33 to 0.26%) included zero, indicating that the directional index was not significantly different from zero. The distributions of latencies according to hemispace as well as the hemispace index for saccades are shown in Fig. 2B. There was no latency difference between right hemispace saccades (RHs) and left hemispace saccades (LHs) in the population (means of 182€24 ms and 182€23 ms, respectively, z=1.4, P>0.17). The hemispace index for saccades (HIs) had a mean of 0.17€3% and the 95% confidence intervals (0.12 to 0.33) included zero.

446 Table 1 Correlations among laterality preference indices and saccadic latency preponderance indices. Data represent the correlation coefficients and significance levels for the correlations among laterality preference indices (GLI global laterality index, HLI hand laterality index) and saccadic latency preponderance indices (DIs directional index for sacades, DICs directional index for centrifugal saccades, HIs hemispace index for saccades) Laterality index

Saccadic latency preponderance index DIs

DICs

HIs

GLI HLI

0.04 (P=0.33) 0.01 (P=0.73)

0.06 (P=0.10) 0.04 (P=0.25)

0.002(P=0.96) 0.01(P=0.84)

Finally, for centrifugal saccades there was no latency difference between right-directed centrifugal saccades (RDCs) and left-directed centrifugal saccades (LDCs) (means of 184€27 ms and 184€25 ms, respectively, z=0.18, P>0.88). The directional index for centrifugal saccades (DICs) had a mean of 0.06€7.5% with 95% confidence intervals (0.55 to 0.43) that included zero. Table 1 shows the correlations of global laterality index (GLI) and hand laterality index (HLI) with saccadic latency preponderance indices DIs, HIs and DICs. None of these correlations reached statistical significance. Repeating the comparison of latencies within the dextral group resulted again in non-significant differences in the population (RDs and LDS means 180€23 ms and 180€22 ms, respectively, z=0.19, P>0.84; RHs and LHs means 182€24 ms and 181€23 ms, respectively, z=1.5, P>0.13; RDCs and LDCs means 183€24 ms and 185€26 ms, respectively, z=1.6, P>0.09). The analysis of the right-handed group also did not show any differences between latencies. Finally, a similar lack of significant latency differences was found for the sinister subjects and the left-handers (GLI, HLI <0.8). Antisaccades Figure 3A shows the distributions of distractibility factor for right-target presentation (DFRa), distractibility factor for left-target presentation (DFLa) and the corresponding distractibility factor index for antisaccades (DFIa). There was no difference between DFRa and DFLa in the population (means of 26€19% and 24€19%, respectively, z=1.4, P>0.16). The DFIa had a mean value of 2.61€40.22 and the 95% confidence interval of the mean (4.57 to 0.46) included zero. The distributions of correct antisaccade latencies for left- and right-target presentation, as well as the directional index of antisaccade latencies, are depicted in Fig. 3B. Again, no differences were observed for mean response latency for right-directed correct antisaccades (RDa) and mean response latency for left-directed correct antisaccades (LDa) in the population (means of 274€43 ms and 276€44 ms respectively, z=1.7, P>0.08). DIa had a mean of 0.39€5.61 and the 95% confidence interval of the mean (0.81 to 0.04) included zero.

Fig. 3A, B Antisaccade distractibility factor (DF) and latency distributions (upper parts) and antisaccade preponderance index distributions (lower parts). A Distributions of errors for left (DFL), right (DFR) target presentation and the DF preponderance index (DFI). B Distributions for the left (LDa), right (RDa) antisaccade latencies and the antisaccade latency preponderance index (DFIa). For the preponderance index distributions, a vertical line indicates zero preponderance, the positive values indicate preponderance to the right and the negative preponderance to the left. Upper left x-axis DF values (percentages), upper right x-axis antisaccade latencies in milliseconds, lower left x-axis DF preponderance index (DFI), lower right x-axis antisaccade latency preponderance index (DFIa). For all histograms, y-axis represents percentage of subjects (N=676) Table 2 Correlations among laterality indices and antisaccadic preponderance indices. Data represent the correlation coefficients and significance levels for the correlations among lateral preference indices (GLI global laterality index, HLI hand laterality index) and antisaccadic preponderance indices (DFIa distractibility factor index for antisaccades, DIa latency directional index for antisaccades) Laterality index

GLI HLI

Antisaccadic preponderance index DFIa

DIa

0.02 (P=0.62) 0.02 (P=0.66)

0.03 (P=0.50) 0.01 (P=0.70)

Table 2 shows the correlations of lateral preference indices GLI and HLI with antisaccade preponderance indices DFIa and DIa. There was no significant correlation among these indices. Comparing DF and latencies in the dextral group did not result in significant lateral differences in the popula-

447

tion (for DFRa and DFLa, means of 25€19% and 24€19%, respectively, z=1.7, P>0.08; for RDa and LDa, means of 273€43 ms and 275€43 ms, respectively, z=1.19, P>0.23). Similar results were obtained using the group of right-handed subjects. Finally, neither DF nor latency differences were observed in the subpopulation of sinisters and left-handed subjects (GLI or HLI <0.8). Idiosyncratic left/right asymmetries In this analysis we used the mean latency difference between left and right direction for centrifugal saccades, and the same measure for correct antisaccades. These were the same measures that were used in the study of idiosyncratic left/right asymmetries of Honda (2002). We then identified 44 subjects that had a difference in latency for saccades below the 2.5 percentile and 39 individuals with a difference above the 97.5 percentile of the distribution of differences for the whole population (676 subjects). Correspondingly, we identified 16 individuals below the 2.5 percentile and 16 individuals above the 97.5 percentile in the distribution of latency differences of correct antisaccades for the population. There were no individuals that belonged to the low 2.5 percentile group for saccade latency differences and at the same time to the low 2.5 percentile group of antisaccade latency differences. Only four individuals belonged to the high 97.5 percentile of saccade and antisaccade latency differences. These findings suggest that individuals with left/right asymmetries in saccade latencies do not also exhibit the same asymmetries in the antisaccade task.

Discussion We studied left/right asymmetries in saccade and antisaccade latencies as well as DF of antisaccades in a large sample of healthy young males. The salient finding from this study was that there were no differences in latencies and DF. Moreover, there was no correlation of laterality with left/right asymmetries for any of the saccade or antisaccade task parameters. We found no differences for the latencies of saccades performed to the right compared with saccades performed to the left direction or hemispace. Furthermore we confirmed that, even for subjects having a left hemispheric dominance (measured with the Porac-Coren questionnaire), the saccadic latencies did not show any lateral asymmetry. Our findings do not confirm previous reports (Pirozzolo and Rayner 1980; Hutton and Palet 1986) showing that individuals with left hemispheric dominance have shorter latencies for saccades to the right than for saccades to the left. There are several differences between this study and those mentioned previously that could explain this discrepancy, such as the experimental paradigm used, the small number of subjects in the previous studies, and the lack of clear assessment of the handedness in those studies. In our study, we used a very

simple ocular motor paradigm in a large number of subjects, and we did not find any asymmetry in saccadic latencies. It could be argued that asymmetries might exist in more complex eye movement tasks such as the antisaccade task. Nevertheless, we did not find any latency difference between rightward versus leftward antisaccades. In addition, this finding persisted even in subjects with left cerebral dominance. Finally, measuring errors in performance, we also did not find any right/left asymmetry in the antisaccade task, and individuals with left cerebral dominance did not exhibit an error bias towards a particular hemifield. In a recent study it was reported that, although saccadic latencies in saccadic and antisaccadic tasks do not exhibit right/left asymmetries in the total sample, there were specific individuals with an idiosyncratic asymmetry for either the right or left hemifield that was present for both saccades and antisaccades (Honda 2002). We did not confirm this finding in our sample. Thus, individuals at the extreme of the distribution of right minus left saccadic latency (showing a large right/left latency asymmetry) were not the ones that exhibited the same asymmetry for antisaccade latencies, except for four individuals. Thus an idiosyncratic left/right latency asymmetry observed in an individual for visually triggered saccades pattern is not present for the latencies of antisaccades in the same individual. Our findings confirm that saccade and antisaccade latency is not affected by hemifield, but other measures might still be affected. In a study of visually triggered saccades, the authors reported a left/right asymmetry in the amplitude of the evoked potential preceding the saccadic eye movement, but this was not accompanied by a similar difference in response latency (Hollants-Gilhuijs et al. 2000). Thus, the hemispheric dominance for eye movements may be observed in measures of neural cortical activity such as evoked potentials. Our data do not exclude the possibility of asymmetries in other ocular motor tasks. For example it would be interesting to search for possible latency asymmetries for saccades larger than 15. It is well known that saccades performed in everyday activities never exceed the range of 15 (Bahill et al. 1975). Consequently, the execution of such large saccades could be considered as a more demanding ocular motor task. Another possibility could be to study saccade latencies in tasks with more lateralized cognitive functions such as memory-guided saccades. Furthermore, the introduction of a gap in the antisaccade task, namely a time delay between fixation point offset and stimulus onset, revealed left/right asymmetries in the error rate (Fischer et al. 1997). Finally, it could be the case that hemispheric dominance is not present in the saccadic system, in contrast to the findings for the control of arm movements. In a study of aiming arm movements or finger movements, it was found that moving with the right arm resulted in shorter latencies when subjects performed leftward movements (Savage and Thomas 1993). In a recent positron emission

448

tomography (PET) study, both right- and left-handers performed finger movements in response to a visual stimulus. Increased left hemisphere activation was observed whether the contralateral or ipsilateral hand was used, leading the authors to the conclusion that left hemisphere is the dominant one not only for speech but also for action (Schluter et al. 2001). In summary, our findings indicate that there is no left/ right latency asymmetry for visually guided saccades (range of €10) and antisaccades that were executed by healthy young male subjects. In addition, the percentage of errors for antisaccade was also not affected by the hemifield of target presentation. Finally, we showed that cerebral dominance, as measured with laterality questionnaires, did not influence the saccadic latency or the error rate to rightward or leftward saccades or antisaccades. It remains an open question whether such lateralization does not exist for the saccadic system suggesting that the ocular motor system escapes the “rule” of cerebral dominance or, alternatively, that this dominance is reflected in other measures or other oculomotor tasks. Acknowledgments This work was supported by the grant EKBAN 97 to Professor C.N. Stefanis from the General Secretariat of Research and Technology of the Greek Ministry of Development. The technical support for this project was provided by Intrasoft Co.

References Bahill AT, Adler D, Stark L (1975) Most naturally occurring human saccades have magnitudes of 15 degrees or less. Invest Ophthalmol Vis Sci 14:468–469 Becker W (1989) Metrics. In: Wurtz R, Goldberg M (eds) The neurobiology of saccadic eye movements. Elsevier, Amsterdam, pp 13–67 Braun D, Weber H, Mergner T, Shulte-Montig J (1992) Saccadic reaction times in patients with frontal and parietal lesions. Brain 115:1359–1386 Doma H, Hallett PE (1988a) Rod-cone dependence of saccadic eye movement latency in a foveating task. Vision Res 28:899–913

Doma H, Hallett PE (1988b) Dependence of saccadic eyemovements on stimulus luminance and an effect of task. Vision Res 28:915–924 Evdokimidis I, Smyrnis N, Constantinidis TS, Stefanis NC, Avramopoulos D, Paximadis C, Theleritis C, Efstratiadis C, Kastrinakis G, Stefanis CN (2002) The antisaccade task in a sample of 2,006 young males. I. Normal population characteristics. Exp Brain Res 147:45–52 Everling S, Fischer B (1998) The antisaccade: a review of basic research and clinical studies. Neuropsychologia 36:885–899 Fischer B, Ramsperger E (1984) Human express saccades: extremely short reaction times of goal directed eye movements. Exp Brain Res 57:191–195 Fischer B, Biscaldi M, Gezeck S (1997) On the development of voluntary and reflexive components in human saccade generation. Brain Res 754:285–297 Fuller JH (1996) Eye position and target amplitude effects on human visual saccadic latencies. Exp Brain Res 109:457–466 Hollants-Gilhuijs MAM, Munck JC, Kubova Z, Royen E, Spekreijse H (2000) The development of hemispheric asymmetry in human motion VEP. Vision Res 40:1–11 Honda H (2002) Idiosyncratic left–right asymmetries of saccadic latencies: examination in a gap paradigm. Vision Res 42:1437– 1445 Hutton JT, Palet J (1986) Lateral saccadic latencies and handedness. Neuropsychologia 24:449–451 Leigh RJ, Zee DS (1999) The neurology of eye movements. Oxford University Press, Oxford Pirozzolo F, Rayner K (1980) Handedness, hemispheric specialization and saccadic eye movement latencies. Neuropsychologia 18:225–229 Porac C, Coren S (1981) Lateral preferences and human behavior. Springer-Verlag, Berlin Heidelberg New York Savage CR, Thomas DG (1993) Information processing and interhemispheric transfer in left- and right-handed adults. Int J Neurosci 71:201–219 Schluter ND, Krams M, Rushworth MFS, Passingham RE (2001) Cerebral dominance for action in the human brain: the selection of actions. Neuropsychologia 39:105–113 Smyrnis N, Evdokimidis I, Stefanis NC, Constantinidis TS, Avramopoulos D, Theleritis C, Paximadis C, Efstratiadis C, Kastrinakis G, Stefanis CN (2002) The antisaccade task in a sample of 2,006 young males: II. Effects of task parameters. Exp Brain Res 147:53–63 Weber H, Fischer B (1995) Gap duration and location of attention focus modulate the occurrence of left/right asymmetries in the saccadic reaction times of human subjects. Vision Res 35:987– 998

Effects of direction on saccadic performance in relation ...

Received: 27 September 2002 / Accepted: 26 February 2003 / Published online: 25 April 2003 ... good indicators of lateral preferences in these tasks. Other oculomotor tasks ... programming and execution of saccadic eye movements has been studied both ..... human saccades have magnitudes of 15 degrees or less. Invest.

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ABCD is a cyclic quadrilateral such that AB is a diameter of the circle circumscribing it and ADC = 140º, then BAC is equal to: (A). 80º. (B) 50º. (C) 40º. (D) 30º. 4. The linear equation 2x – 5y = 7 has. (A). A unique solution. (B) Two soluti

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Mar 25, 2010 - ized or not, shapes health care providers' performance of cognitive work .... therapies and how the patient is reacting), the comparison of performance to ...... hall and takes out his cell phone and calls the nurse. He can't give ...

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are exponential random variables. (RVs) with parameter ij. For example, the data link between. S and D denoted by ℎsd has the parameter sd. To facilitate.

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012 Performance Effects Of Different Audit Staff Assignment Strategies.pdf. 012 Performance Effects Of Different Audit Staff Assignment Strategies.pdf. Open.

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Keywords. Web Servers, Persistent Connection, Performance Evaluation ..... but the maintenece of them are cheap - has been weakened. ... Internet draft, 1997.