Biomedical subjects

I Evdokimidis

Publications and source records attributed to I Evdokimidis.

7 recordsLinked to original sources

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↗