Biomedical subjects

I Evdokimidis

Publications and source records attributed to I Evdokimidis.

16 recordsLinked to original sources

PubMed2003

Effects of direction on saccadic performance in relation to lateral preferences.

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.

Adult↗

PubMed2002

The antisaccade task in a sample of 2,006 young males. II. Effects of task parameters.

Antisaccade performance was investigated in a sample of 2,006 young males as part of a large epidemiological study investigating psychosis proneness. This report summarizes the effects of task parameters on performance using a sample of 55,678 antisaccade trials collected from a subpopulation of 947 individuals. Neither the amplitude nor the latency of an error prosaccade in the antisaccade task was correlated with the latency of the ensuing corrective antisaccade that almost always followed an error. However, the latency of the corrective antisaccade decreased with increasing stimulus distance. Concerning the effects of specific task parameters, trials with stimuli closer to the central fixation point and trials preceded by shorter fixation intervals resulted in more errors and longer latencies for the antisaccades. Finally, there were learning and fatigue effects reflected mainly in the error rate, which was greater at the beginning and at the end of the 5-min task. We used a model to predict whether an error or a correct antisaccade would follow a particular trial. All task parameters were significant predictors of the trial outcome but their power was negligible. However, when modeled alone, response latency of the first movement predicted 40% of errors. In particular, the smaller this latency was, the higher the probability of an error. These findings are discussed in light of current hypotheses on antisaccade production mechanisms involving mainly the superior colliculus.

Adolescent↗

PubMed2002

The antisaccade task in a sample of 2,006 young men. I. Normal population characteristics.

A population of 2,075 young men aged 18-25 years selected from the conscripts of the Greek Air Force performed an antisaccade task as part of a prospective study for the identification of risk factors in the development of psychoses. The aim of this study, which is ongoing, is to follow this population and investigate the possible predictive value of oculomotor, cognitive, and psychometric factors for the development of psychosis and other psychiatric conditions. In this report we present data concerning the antisaccade task in this population. We measured performance indices, including the percentage of errors (PE), the latencies of different eye movement responses (latency for correct antisaccades, errors, corrections), and performance in perseveration-prone trials. These indices were also evaluated with respect to IQ (measured by the Raven progressive matrices test) and educational level. Mean PE was 23%, with 17% variance. This large variance is of particular importance whenever the detection of a putative deviant behavior is explored. As mean latency of the first eye movement decreased, the PE increased, as did the latency variance. While the negative correlation between percentage of error and mean latency is well established, the relationship of the latency variability of the first response to error production has not been studied before. Thus, optimal performance appears to require both an intermediate mean latency and a small variability. Furthermore, performance seems to be affected by IQ (the higher the IQ score, the lower the percentage of errors). This report offers an analysis of the interindividual variation in the performance of the antisaccade task and discusses some of the sources of this variation.

Adolescent↗

PubMed2002

Frontal lobe dysfunction in amyotrophic lateral sclerosis.

The aim of the present study was to investigate the involvement of frontal lobe dysfunction in amyotrophic lateral sclerosis (ALS) using ocular motor paradigms and neuropsychological testing. Fifty-one patients with ALS participated in the following ocular motor tasks: (1) a three-choice task and (2) a remembered saccade task. The patients underwent a clinical and neuropsychological evaluation. One-third of ALS patients presented with signs of frontal dysfunction, as determined by their high distractibility factors (DF) in the three-choice task and their performances in both the Wisconsin and Stroop tests. ALS patients exhibited longer latencies to eye movement than controls in the performance of the remembered saccade task, specifically in performance of both remembered and delayed saccades, but saccade accuracy was not impaired. Finally, performance indices of the ocular motor tasks, in particular the DF, was correlated only with the degree of dysarthria.

Aged↗

PubMed2002

Higher scores of self reported schizotypy in healthy young males carrying the COMT high activity allele.

The gene for COMT is located on chromosome 22q11, an area that has been implicated in the pathogenesis of schizophrenia through linkage studies and through the detection of deletions in schizophrenics and velocardiofacial syndrome patients that often present psychotic symptomatology. Additionally catechol-O-methyl transferase activity has been found increased in schizophrenia and a functional polymorphism in the COMT gene itself has been associated with the disease, as well as with aggression in patients. We tested the hypothesis that COMT genotype for the functional Val158Met might contribute to the variance of self reported schizotypy and aggression scores in the normal population. We genotyped 379 healthy 18- to 24-year-old male individuals who had completed the PAS, SPQ and AQ questionnaires. Our results showed that self-reported schizotypy scores in both questionnaires were significantly related to COMT genotype (P = 0.028 for the PAS and P = 0.015 for the SPQ) with individuals homozygous for the high activity allele showing the highest scores. No significant differences were detected for AQ scores. We conclude that the COMT genotype for the functional Val158Met polymorphism is correlated to self-reported schizotypy in healthy males. This finding is in the same direction as reported findings on schizophrenia and it adds to the list of evidence that COMT or a nearby gene in linkage disequilibrium is involved in the pathogenesis of the disease.

Adolescent↗

PubMed2001

An early transient 40 Hz activity discriminates a following pro-saccade from a no-move and anti-saccade choice.

We studied the oscillatory activity of the scalp-recorded EEG in healthy humans performing a task that required a particular eye-movement response choice according to the shape of a visual target. We observed a significant stimulus-aligned activity at the 40 Hz frequency band 100 ms after the appearance of the target only when that target was the end point for the subsequent eye movement (pro-saccade). This activity was most prominent over the central-parietal area of the right hemisphere. When the target indicated a movement to the opposite direction (anti-saccade) or indicated that no movement was required (no-move), this 40 Hz activity was nearly absent. This difference in activity between the pro-saccade and the other two tasks was evident in the single subject ERPs for four of the six subjects studied. In contrast, the movement-aligned 40 Hz activity for the pro-saccade and anti-saccade was almost identical. We speculate that this early stimulus-aligned 40 Hz activity might reflect a fast transformation of a visual stimulus to a motor response (eye movement) that can be performed for the pro-saccade task where stimulus-response compatibility is strong compared to the anti-saccade and no-move tasks. The movement-aligned 40 Hz activity might be related to the motor response preparation per se. We conclude that this task specific transient oscillatory activity could be used as a probe in the study of the temporal dynamics of visuomotor transformations.

Analysis of Variance↗

PubMed2001

Systematic errors of planar arm movements provide evidence for space categorization effects and interaction of multiple frames of reference.

Healthy humans performed arm movements in a horizontal plane, from an initial position toward remembered targets, while the movement and the targets were projected on a vertical computer monitor. We analyzed the mean error of movement endpoints and we observed two distinct systematic error patterns. The first pattern resulted in the clustering of movement endpoints toward the diagonals of the four quadrants of an imaginary circular area encompassing all target locations (oblique effect). The second pattern resulted in a tendency of movement endpoints to be closer to the body or equivalently lower than the actual target positions on the computer monitor (y-effect). Both these patterns of systematic error increased in magnitude when a time delay was imposed between target presentation and initiation of movement. In addition, the presence of a stable visual cue in the vicinity of some targets imposed a novel pattern of systematic errors, including minimal errors near the cue and a tendency for other movement endpoints within the cue quadrant to err away from the cue location. A pattern of systematic errors similar to the oblique effect has already been reported in the literature and is attributed to the subject's conceptual categorization of space. Given the properties of the errors in the present work, we discuss the possibility that such conceptual effects could be reflected in a broad variety of visuomotor tasks. Our results also provide insight into the problem of reference frames used in the execution of these aiming movements. Thus, the oblique effect could reflect a hand-centered reference frame while the y-effect could reflect a body or eye-centered reference frame. The presence of the stable visual cue may impose an additional cue-centered (allocentric) reference frame.

Adult↗

PubMed2001

Frontal-parietal activation differences observed before the execution of remembered saccades: an event-related potentials study.

Healthy subjects performed saccadic eye movements in one memory (MEM) and two delay tasks (delay, DEL and modified delay, M-DEL) while we recorded scalp event-related potentials (ERPs) from 25 electrode sites. In the MEM task the subjects were instructed to retain in memory the location of a visual target for a delay of 1-6 s and then perform a remembered saccade at the go signal. In the DEL task the target remained on until movement completion and in the M-DEL task the target, that was visible during the delay period, disappeared synchronously with the go signal. A reduction in response latency and an increase in the percentage of dysmetric movements were observed for the MEM task compared to the two delay tasks. An increased ERP activity at the central-frontal electrode sites compared to the parietal sites was significant only for the MEM task early on during the delay period (500-1000 ms). During the period preceding the onset of the saccade, a parietal increase of activity was observed for all tasks. Furthermore the activity was smaller for the frontal compared to the parietal areas only for the memory task thus indicating a near reversal of the previous pattern of activity observed during the early delay period. This specific activation pattern of frontal and parietal areas, observed for the MEM task only, requires further investigation focusing on the temporal pattern of activation of large brain areas involved in working memory processing.

Electroencephalography↗

PubMed2000

Speed-accuracy trade-off in the performance of pointing movements in different directions in two-dimensional space.

Nine healthy subjects performed 2D pointing movements using a joystick that controlled a screen cursor. Continuous visual feedback was provided until movement completion. Three variables were systematically manipulated: (1) target distance, (2) target size and (3) target direction. A four-way factorial ANOVA was used to analyze the effects of these fixed factors and of the random factor of subject on several movement parameters. Movement time increased with increasing distance and decreasing target size and as predicted from Fitts' law. The target direction did not affect movement time. In contrast the direction, distance and size of the target significantly affected the movement time until the first zero crossing on the speed record reflecting the time to bring the arm into the vicinity of the target. Movements on the lateral axis of the horizontal plane (horizontal movements) resulted in a decrease in initial movement time compared to movements on the anterior axis of the horizontal plane (vertical movements). A significant effect of target distance and direction but not target size was observed for the magnitude of maximum acceleration, maximum speed and maximum deceleration. Horizontal movements had a larger maximum acceleration, speed and deceleration. Furthermore the maximum speed and deceleration occurred earlier in time for these horizontal movements. Finally the number of secondary peaks on the speed record increased with decreasing target size and was not affected by the target distance or target direction. In conclusion our results indicate that different movement parameters are affected by target distance, size and direction. The crucial distinction was between parameters affected by target size and direction. These parameters did not overlap. Target direction affects the first part of movement execution while target size affects the final part of movement execution. Thus a clear segmentation of movement execution in two phases is supported by these results. The implications of these results for theoretical models of speed-accuracy trade-off are discussed.

Acceleration↗

PubMed2000

A systematic directional error in 2-D arm movements increases with increasing delay between visual target presentation and movement execution.

Forty-seven normal subjects performed two-dimensional arm movements on a digitizer board using a mouse device. The movements were projected on a computer monitor. Subjects were instructed to move the mouse using the whole arm from a center position to a peripheral target so that the projected movement would pass over the target without stopping on the target. A large number of targets (360) were used to cover the entire directional continuum. The direction of the arm movement was the parameter of interest, which was measured at an initial position, at one third of the distance towards the target, and at the vicinity of the target. Four conditions of delay between target presentation and movement execution were used (0, 2, 4, 6 s). A systematic directional error was observed at the initial portion of the trajectory. This error resulted from a clustering of movement directions on an axis that was perpendicular to the axis of the resting forearm before movement onset. This pattern of errors can be explained by the initial inertial anisotropy of the arm. As the trajectory evolved, a different directional error emerged, resulting from a clustering of movement directions in two orthogonal axes. This pattern of directional error increased in amplitude as the delay increased, in contrast to the error at the initial portion of the trajectory which remained invariant with increasing delay. Finally, the information transmitted by the movement direction was shown to increase with the evolution of the trajectory. The increase in delay resulted in a decrease in directional-information transmission. It is proposed that the directional bias towards the end of the movement trajectory might reflect the action of "movement primitives", that is patterns of muscle activation resulting from spinal interneuronal activation. It is further proposed that the directional bias observed at the vicinity of the target might reflect a loss of cortical directional information with increasing delay between target presentation and movement onset.

Acceleration↗

PubMed1997

Changes of presaccadic cortical activity when performing horizontal, visually guided saccades.

When a visually guided saccade task is running, the presaccadic potential obtained in the initial period of the task differs from those obtained later, while the subject's oculomotor performance remains unaffected. These time-related changes of cortical activity consist both of an overall decreasing electrical activity as well as a selective one over certain cortical areas. The generalised reduced activity already described in earlier studies is considered as an unspecified effect such as fatigue or decreased motivation. On the contrary, the pronounced selective changes of cortical activity obtained over cortical areas such as the centro-parietal and frontal cortices, should be related with more specific, that is, visuomotor function. We assume that at the beginning of the task of the performance of the saccade needs the activation of several cortical areas but later on the same oculomotor plan runs sufficiently under subcortical control.

Adult↗

PubMed1997

Gaze stabilization by optokinetic reflex (OKR) and vestibulo-ocular reflex (VOR) during active head rotation in man.

Vestibulo-ocular reflex (VOR)-optokinetic reflex (OKR) interaction was studied in normal human subjects during active sine-like head movements in the horizontal plane for a variety of vestibular-optokinetic stimulus combinations (frequency range, 0.05-1.6 Hz). At low to mid frequencies (< 0.2 Hz) the eyes tended to be stabilized on the optokinetic pattern, independently of whether the head, the pattern, or both were rotated. At higher frequencies, the OKR gain was attenuated and, in each of the differing stimulus combinations, the eyes became increasingly stabilized in space. Qualitatively similar results were obtained when, for the same visual-vestibular combinations, the head was passively rotated at 0.05 and 0.8 Hz. The data could be simulated by a model which assumes a linear interaction of vestibular and optokinetic signals. It considers the OKR with its negative feedback loop of primordial importance for image stabilization on the retina and the VOR only as a useful addition which compensates for the limited bandwidth of the OKR during high frequency/velocity head rotations in a stationary visual environment.

Adult↗

PubMed1996

Cortical potentials with antisaccades.

The term antisaccade refers to saccades that are performed towards the side opposite to that of target appearance. The performance of antisaccades is considered to be determined by intact frontal inhibitory areas as patients with frontal, and especially prefrontal, lesions show a striking impairment in suppressing an unwanted protarget saccade. We recorded cortical slow potentials from subjects performing saccades and antisaccades in a task, antitask and no move conditions in order to investigate possible topographic differences between these two types of eye movement. Our main findings concern both movement related as well as sensory related potentials. With regard to the saccadic potentials, performance of an antisaccade is preceded by a much more pronounced activity during the last 100 ms prior to the eye movement onset over central-anterior leads with a slight ipsilateral lateralization. As for the sensory potentials, the target related with antisaccade performance is followed by smaller, but nonstatistically significant, exogenous responses while at 300-350 ms after target appearance, the activity associated with the antisaccade's target is clearly larger over central midline leads. Although we could not precisely relate the electrical activity obtained with well circumscribed cortical function, the results support the view that the anterior and slightly ipsilateral cortical activation which precedes the performance of an antisaccade could reflect the frontal mechanisms of suppression of the unwanted saccade.

Adult↗

PubMed1992

Dependence of presaccadic cortical potentials on the type of saccadic eye movement.

Premovement cortical potentials were studied with 4 types of saccadic eye movement: (a) visually triggered saccades of normal reaction time (RT; regular saccades); (b) visually triggered saccades of extremely short RT (express saccades); (c) saccades towards predicted target locations (anticipatory saccades); (d) saccades back towards predicted location of fixation point (refixation saccades). With all 4 saccade types a "presaccadic negativity" with the maximum at the vertex (Cz) was observed. A bilaterally symmetrical component contained in this potential (being smallest with almost unconsciously performed refixation saccades and smaller in trained than in naive subjects) appeared to be related mainly to the subjects' volitional effort. In addition, anticipatory and refixation saccades were preceded by an early, widespread contralateral negativity, which we relate to cortical activities that prepare, in general terms, action within or towards the hemifield containing the saccade goal. During the 60 msec before anticipatory saccades, a negativity occurred over the contralateral central lead, which may reflect neural activation in the frontal eye field (FEF) and premotor cortex. In contrast, regular saccades were preceded 30 msec before onset by a negativity over the contralateral parietal cortex, which probably reflects an activation of parietal visuo-motor neurons. No lateralization of the cortical potentials was observed before express saccades, which suggests that these saccades are generated in a reflex-like way mainly by subcortical mechanisms.

Adult↗

PubMed1991

Cortical potentials preceding centrifugal and centripetal self-paced horizontal saccades.

Cortical potentials preceding self-paced centrifugal and centripetal saccades were recorded in 15 subjects from F3, Fz, F4, C3, Cz, C4, P3, Pz and P4 versus linked mastoid electrodes. A negative potential starting about 1.0 sec prior to saccade onset, reaching a peak amplitude of 6.8 microV on average, preceded centrifugal saccades. In contrast the negativity preceded centripetal saccades by only 500 msec, and its peak amplitude was smaller (4.6 microV). We conclude that these differences reflect the fact that less 'effort' is needed with centripetal as compared to centrifugal saccades.

Adult↗

PubMedDate not supplied

The increased reaction time of antisaccades. What makes the difference?

The aim of this study was the detection of the parameters involved in an already defined phenomenon, namely that the reaction time of antisaccades is greater than that of pro-target saccades. Thus, we performed four different experimental paradigms: (i) The target location was unpredictable (to the left or to the right) but the type of saccade (pro-or anti-target) was predictable. The corresponding mean values of the reaction times were 231 +/- 40 ms for anti- and 179 +/- 50 ms for pro-target saccades. (ii) Both the target position and the type of saccade were unpredictable (437 +/- 91 and 412 +/- 85 for anti-and pro-target saccades, respectively). (iii) The target location remained predictable while the type of saccade was unpredictable (397 +/- 104 and 385 +/- 90 ms) and (iv) Both the target position and the type of saccade were predictable (185 +/- 67 and 180 +/- 61). The statistical analysis (ANOVA and post hoc comparisons) revealed significant differences only in the first two experiments. Our results suggest that the antisaccades present increased latency, compared to that of pro-target saccades, only under certain experimental conditions and especially when the target location is unpredictable. We presume that the antisaccade's latency prolongation is due not to the frontal lobe inhibition but to the double interference of the parietal lobe, which has to re-reconstruct the target location in space.

Adult↗