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Biomedical subjects

M Biscaldi

Publications and source records attributed to M Biscaldi.

14 recordsLinked to original sources
PubMed2002

The initiation of smooth pursuit eye movements and saccades in normal subjects and in "express-saccade makers".

A vast knowledge exists about saccadic reaction times (RT) and their bi- or multimodal distributions with very fast (express) and regular RT. Recently, there has been some evidence that the smooth pursuit system may show a similar RT behavior. Since moving targets usually evoke a combined pursuit/saccade response, we asked which processes influence the initiation of pursuit and saccadic eye movements. Furthermore, we investigated whether and how the pursuit and saccadic system interact during the initiation of eye movements to moving targets. We measured the RT of the initial smooth pursuit (iSP) response and of the first corrective saccade and compared the RT behavior of both. Furthermore we compared the behavior of the corrective saccades to moving targets to that of saccades to stationary targets, known from the literature. The stimulus consisted of a target that moved suddenly at constant velocity (ramp). In addition, prior to the movement, a temporal gap, a position step or a combination of both could occur (gap-ramp, step-ramp, gap-step-ramp, respectively). Differently from most previous studies, we chose step and ramp with the same direction to provoke competition between the pursuit and saccade system. For the first time we investigated pursuit initiation in "express-saccade makers" (ES makers), a subject group known to produce an abnormally high percentage of short-latency saccades in saccade tasks. We compared their results with subject groups who were either naive or trained with respect to saccade tasks. The iSP started at approximately 100 ms, which corresponds to express saccade latencies. These short iSP-RT occurred reflex-like and almost independent of the experimental task. A bimodal frequency distribution of RT with a second peak of longer iSP-RT occurred exclusively in the ramp paradigm. The RT of the first corrective saccades in a pursuit task were comparable with that in a saccade task and depended on the stimulus. The ability of ES makers to produce a high number of express saccades was transferred to corrective saccades in the pursuit task, but not to pursuit initiation. In summary, short-latency pursuit responses differ from express saccades with respect to their independence of experiment and subject group. Therefore, a simple analogy to express saccades cannot be drawn, although some mechanisms seem to act similarly on both the pursuit and the saccade system (such as disengagement of attention with the gap effect). Furthermore, we found evidence that the initial pursuit response and the first corrective saccade are processed independently of each other. The first corrective saccades to moving targets behave like saccades to stationary targets. Normal pursuit but abnormal saccade RT of ES makers can be explained by recent theories of superior colliculus (SC) function in terms of retinal error handling.

Brain↗
PubMed2000

Voluntary saccadic control in dyslexia.

The role of eye-movement control in dyslexia is still unclear. Recent studies, however, confirmed that dyslexics show poor saccadic control in single and sequential target tasks. In the present study we investigated whether dyslexic subjects are impaired on an antisaccade task requiring saccades against the direction of a stimulus. Altogether, 620 subjects between the ages of 7 and 17 years were classified as dyslexics (N = 506) or control subjects (N = 114) on the grounds of the discrepancy between their intellectual abilities and reading/spelling achievements. All subjects performed an overlap prosaccade and a gap antisaccade task with 100 trials to each side of stimulation in random order. Variables analysed were the overall saccadic reaction time of both tasks; and from the antisaccade task the number of errors (prosaccades), the number of corrected errors, and the number of trials in which the subjects still failed to reach the side opposite the stimulus even after two saccades. An analysis of variance was carried out taking into account the development of saccadic behaviour with age and the differences between the groups. The results confirm development of saccade control with age, especially in the voluntary component (a frontal-lobe function) for both groups, but indicate that the antisaccade task performance, as measured by the error and the correction rate, is significantly worse in the dyslexic group at ages above 8 years. Up to 50% of the dyslexics performed the antisaccade task 1.5 standard deviations below the mean of the controls.

Adolescent↗
PubMed1998

Poor saccadic control correlates with dyslexia.

A large group of subjects, either average readers or reading/spelling disabled subjects (n = 185; age between 8-25 years; M = 13 +/- 4 years), were tested in various standardized cognitive tasks including reading/spelling assessment and in non-cognitive saccadic eye movement tasks. Dyslexics were separated into a subgroup (D1) with deficits in the serial auditory short-term memory and a subgroup (D2) with an isolated low achievement in reading/writing. Control subjects had no relevant cognitive deficit of any type. Saccadic eye movements were measured in a single target and in a sequential-target task. A significant correlation was found between abnormal saccadic control and reading disability. The two dyslexic groups showed only slight differences. As compared to the control group, the mean values of the standard deviations of the saccadic reaction times (SRT) and the amount of late saccades (SRT > 700) were significantly increased in both dyslexic groups and especially in group D1 who also showed an increased amount of anticipatory saccades. The number of express saccades (SRT = 80-134 ms) was increased, but not significantly, in D2 dyslexics. Both dyslexic groups produced significantly more regressive saccades in the sequential-target task. The correlation between saccadic variables and "reading factor" was 0.4. Significant deviations from normal performance of the saccadic variables were found in an estimated 50% of the dyslexics as compared to 20% of the control subjects. In spite of their worse level in saccadic control, dyslexics also developed with age in the eye movement performance as the control subjects did. Yet, the development was slower in group D1. It is suggested that reading process and saccade system are both controlled by visuo spatial attention and fixation systems that maybe impaired or develop slowly in many dyslexic subjects.

Adolescent↗
PubMed1997

On the development of voluntary and reflexive components in human saccade generation.

The saccadic performance of a large number (n = 281) of subjects of different ages (8-70 years) was studied applying two saccade tasks: the prosaccade overlap (PO) task and the antisaccade gap (AG) task. From the PO task, the mean reaction times and the percentage of express saccades were determined for each subject. From the AG task, the mean reaction time of the correct antisaccades and of the erratic prosaccades were measured. In addition, we determined the error rate and the mean correction time, i.e. the time between the end of the first erratic prosaccade and the following corrective antisaccade. These variables were measured separately for stimuli presented (in random order) at the right or left side. While strong correlations were seen between variables for the right and left sides, considerable side asymmetries were obtained from many subjects. A factor analysis revealed that the seven variables (six eye movement variables plus age) were mainly determined by only two factors, V and F. The V factor was dominated by the variables from the AG task (reaction time, correction time, error rate) the F factor by variables from the PO task (reaction time, percentage express saccades) and the reaction time of the errors (prosaccades!) from the AG task. The relationship between the percentage number of express saccades and the percentage number of errors was completely asymmetric: high numbers of express saccades were accompanied by high numbers of errors but not vice versa. Only the variables in the V factor covaried with age. A fast decrease of the antisaccade reaction time (by 50 ms), of the correction times (by 70 ms) and of the error rate (from 60 to 22%) was observed between age 9 and 15 years, followed by a further period of slower decrease until age 25 years. The mean time a subject needed to reach the side opposite to the stimulus as required by the antisaccade task decreased from approximately 350 to 250 ms until age 15 years and decreased further by 20 ms before it increased again to approximately 280 ms. At higher ages, there was a slight indication for a return development. Subjects with high error rates had long antisaccade latencies and needed a long time to reach the opposite side on error trials. The variables obtained from the PO task varied also significantly with age but by smaller amounts. The results are discussed in relation to the subsystems controlling saccade generation: a voluntary and a reflex component the latter being suppressed by active fixation. Both systems seem to develop differentially. The data offer a detailed baseline for clinical studies using the pro- and antisaccade tasks as an indication of functional impairments, circumscribed brain lesions, neurological and psychiatric diseases and cognitive deficits.

Adolescent↗
PubMed1996

Fixation and saccade control in an express-saccade maker.

In express-saccade makers a large incidence of express saccades (latencies around 100 ms) is paralleled by a reduced ability to suppress saccade generation when required. Such a behavior occurs frequently in dyslexics. We studied the latencies and the metrical properties of saccades in the very rare case of an adult, nondyslexic express-saccade maker (male, age 29 years). The subject produced 65-95% express saccades in the gap (fixation point removed 200 ms before target onset) as well as in the overlap (fixation point not removed) paradigm, which qualified the subject as the most clear case of an express-saccade maker found so far. The number of express saccades increased rather than decreased when fixation foreperiod, gap duration, and target location were randomized from trial to trial as compared to when they remained constant. In the memory-guided saccade and in the antisaccade paradigms in which immediate saccade execution to a visual target had to be suppressed, the subject often reacted to the target with express saccades in an involuntary way. The amplitudes of express saccades were--in some conditions--found to progressively decrease with increasing latency, giving rise to amplitude transition functions. The present findings disprove the notion that express saccades are generated based on the prediction of the time and location of target appearance and support the notion that they are the result of an optomotor reflex. It is argued that the operation of the reflex is gated by a separate fixation system. Express-saccade makers are described as subjects with a dysfunction of the fixation system. Recent neurophysiological findings suggest that the subject studied in the present study has a selective dysfunction of the fixation system at the level of the superior colliculus.

Eye Movements↗
PubMed1996

Human express saccade makers are impaired at suppressing visually evoked saccades.

1. We report the oculomotor behavior of human subjects who produce unusually high numbers (> 30%) of express saccades (latency range 85-135 ms) in the overlap saccade task, where express saccades are usually absent or small in number (< 15%). We refer to these subjects as "express saccade makers" (ES makers). 2. We tested the hypothesis that ES makers have difficulties in maintaining fixation and in suppressing unwanted saccades to a suddenly appearing peripheral target by comparing the performances of 10 ES makers and 10 control subjects in gap and overlap antisaccade tasks and in a memory-guided saccade task. 3. The ES makers produced between 35% and 95% incorrect saccades toward the stimulus (prosaccades) in the antisaccade tasks, compared with control subjects, who produced < 20%. Their correct antisaccades appeared to be normal. 4. We further tested the ability of ES makers to maintain fixation and to avoid reflexive saccades to the onset of a target in the memory-guided saccade task. ES makers tended to glance to the briefly presented cue in many trials (4 of them in 50-80% of the trials) instead of delaying the saccade until fixation point offset. Most of the inappropriate saccades had latencies in the range of express saccades. 5. These results can be associated with the finding of fixation related neurons in different cortical and subcortical brain regions (e.g., inferior-parietal and frontal cortex, basal ganglia, superior colliculus). The unusual number of express saccades made by the ES makers in the standard overlap and gap tasks, and their unwanted short-latency reflexive saccades to the target in the memory-guided saccade task, are reminiscent of the performance in these tasks of monkeys whose collicular fixation neurons were chemically deactivated. The collicular fixation neurons are probably the final common pathway in the control of active fixation, and are in mutual inhibitory relationship with the saccade cells. 6. The decreased saccadic control observed in the ES makers suggests that saccade execution in humans is also gated by a fixation system. These ES makers may have reduced voluntarily control over saccade generation as a result of a defect or poor development of their fixation system.

Adolescent↗
PubMed1995

Intertrial effects of randomization on saccadic reaction times in human observers.

We investigated the effect of randomizing different spatial and temporal parameters on saccadic reaction times (SRTs) in five human subjects, to explore the relative occurrence of express and regular saccades. Parameters randomized in various test sessions were: target direction (right/left), intertrial interval, fixation foreperiod and gap duration (two or three 400 msec) in gap and overlap trials. For the sake of comparison the parameters under consideration were kept constant in non-random control sessions. We found that express saccades were still present in the random test sessions but their relative frequency (and the number of regular saccades) obtained in a given test session depends on the type of randomized parameters: randomizing the intertrial interval or the fixation foreperiod in the gap task yielded modest but significant changes in the SRT distributions, express and fast regular saccades being present in both the control and the random conditions. Randomization of the fixation foreperiod in the overlap task, on the other hand, caused a quite drastic increase in the SRTs. Randomization of gap and overlap trials did not cause considerable effects on express and fast regular saccades in the gap trials, and the SRTs in the overlap trials were significantly increased only in two subjects. When two or three gap durations were randomly interleaved, we found effects that ranged from "negligible" (usually for the longest gap in a given test session) to highly significant differences as compared with the corresponding control condition. The results suggest that express saccades--as fast regular saccades--are visually guided saccades which occur when a certain state of saccade preparation has been reached before target onset. This state depends on the amount of activation in the brain structures involved in the control of attention and fixation, and the decision processes involved in saccade preparation.

Adult↗
PubMed1995

Oculomotor activity and visual spatial attention.

Subjects made a horizontal or vertical saccade in response to a non-lateralized auditory stimulus. Simple manual reaction time (RT) for the detection of light targets at extrafoveal locations was modulated by the intention to make the saccade insofar as RT to targets presented at the saccadic goal location or in the hemifield containing that location was faster than RT to targets presented at the opposite, mirror-symmetric location. This RT difference was maximal prior to the beginning of the saccade and vanished after saccade termination, indicating that the effect was caused by the neural activity leading to the saccade rather than to the eye movement or the eye position per se. The results have implications for the understanding of the relations between visual spatial attention and oculomotor control, especially with regard to inhibitory phenomena arising from the non-correspondence between the line of sight and the focus of attention.

Acoustic Stimulation↗
PubMed1994

Saccadic eye movements of dyslexic and normal reading children.

Twenty-four children made saccades in five noncognitive tasks. Two standard tasks required saccades to a single target presented randomly 4 deg to the right or left of a fixation point. Three other tasks required sequential saccades from the left to the right. 75 parameters of the eye-movement data were collected for each child. On the basis of their reading, writing, and other cognitive performances, twelve children were considered dyslexic and were divided into two groups (D1 and D2). Group statistical comparisons revealed significant differences between control and dyslexic subjects. In general, in the standard tasks the dyslexic subjects had poorer fixation quality, failed more often to hit the target at once, had smaller primary saccades, and had shorter reaction times to the left as compared with the control group. The control group and group D1 dyslexics showed an asymmetrical distribution of reaction times, but in opposite directions. Group D2 dyslexics made more anticipatory and express saccades, they undershot the target more often in comparison with the control group, and almost never overshot it. In the sequential tasks group D1 subjects made fewer and larger saccades in a shorter time and group D2 subjects had shorter fixation durations than the subjects of the control group.

Child↗
PubMed1993

The time of secondary saccades to primary targets.

When a first saccade is made in response to a single, suddenly appearing stimulus it often misses the target. The retinal error may be very large, in particular in those cases where the subject anticipates the target location and initiates a saccade to a wrong position. We have analyzed the time of the occurrence of the secondary saccades by which the subject corrects these errors. Using the gap task with random target locations we found that large errors after anticipatory saccades--especially those after direction errors--can be corrected very fast. The latencies of these corrective saccades (being measured from target onset, not from the end of the primary saccade) form bimodal distributions with a first peak at 100 ms. It is therefore concluded that large errors can be corrected by express secondary saccades.

Attention↗
PubMed1993

Separate populations of visually guided saccades in humans: reaction times and amplitudes.

The saccadic eye movements of 20 naive adults, 7 naive teenagers, 12 naive children, and 4 trained adult subjects were measured using two single target saccade tasks; the gap and the overlap task. In the gap task, the fixation point was switched off before the target occurred; in the overlap task it remained on until the end of each trial. The target position was randomly selected 4 degrees to the left or 4 degrees to the right of the fixation point. The subjects were instructed to look at the target when it appeared, not to react as fast as possible. They were not given any feedback about their performance. The results suggest that, in the gap task, most of the naive subjects exhibit at least two (the teenagers certainly three) clearly separated peaks in the distribution of the saccadic reaction times. The first peak occurs between 100 and 135 ms (express saccades), the second one between 140 and 180 ms (fast regular), and a third peak may follow at about 200 ms (slow regular). Other subjects did not show clear signs of two modes in the range of 100 to 180 ms, and still others did not produce any reaction times below 135 ms. In the overlap task as well three or even more peaks were obtained at about the same positions along the reaction time scale of many, but not all subjects. Group data as well as those of individual subjects were fitted by the superposition of three gaussian functions. Segregating the reaction time data into saccades that over- or undershoot the target indicated that express saccades almost never overshoot. The results are discussed in relation to the different neural processes preceding the initiation of visually-guided saccades.

Adolescent↗
PubMed1993

Saccadic eye movements of dyslexic adult subjects.

The characteristics of visually guided saccadic eye movements were compared in 12 normal adult subjects and 12 test subjects of normal intelligence, but with problems in reading and writing. All subjects were examined psychometrically for different cognitive abilities, and for their reading and writing capabilities. The anamnestic reports about their reading and writing problems earlier in their lives were analysed. Based on scores of the writing and reading tests, the test group was subdivided into two subgroups: A and B (group A reached medium, group B very low scores in both tests). Five different non-cognitive eye movement tasks were applied: two single tasks (gap and overlap) requiring single saccades from a fixation point to a peripherally appearing target and three sequential tasks (overlap, synchronous, and simultaneous) requiring sequences of saccades to four equally spaced targets presented sequentially to the right side from an initial fixation point. Many parameters of the subjects' eye movement performance were determined and their mean values were calculated for each subject. The Student t-test revealed that the eye movement data of the two test groups deviated differently from the data of the control group. Group B had the largest deviation of the eye movement parameter from the control group. The differentiating parameters were the consistency of target acquisition, the saccadic reaction time, and the number of anticipatory responses in the single target tasks. In the sequential tasks these differences were in the amplitude, the number of saccades, and the fixation durations. The incidence of regressive saccades did not discriminate between test subjects and controls. The possible underlying deficits in the attentional control over the saccadic system and their implications for reading are discussed.

Adult↗
PubMed1989

Ipsilateral inhibition and contralateral facilitation of simple reaction time to non-foveal visual targets from non-informative visual cues.

Orienting to an extrafoveal light cue without foveating it induces a temporary inhibition of responses to subsequent targets presented in the same visual hemifield, as evinced from the fact that reaction time (RT) to targets ipsilateral to the cue relative to fixation is longer than RT to targets contralateral to the cue. This study has tested the hypothesis that ipsilateral RT inhibition is associated with contralateral RT facilitation by attempting to divide the difference between ipsilateral and contralateral RTs into costs and benefits. A neutral condition suited to this purpose should involve a cue that does not require a lateral orientation. Such neutral condition was provided by measuring RT to lateralized light targets following a central overhead auditory cue (experiment 1) or a foveal visual cue (experiment 2). In both experiments RT in the neutral condition was intermediate between ipsilateral and contralateral RTs, and the differences reaches significance in the second experiment. Benefits over the neutral condition measured in the contralateral condition were thus associated with costs in the ipsilateral condition. These results suggest that a reciprocal antagonism between opposite turning tendencies underlies the organization of covert orienting. They also agree with general multi-channel theories of selective attention according to which the facilitation of given channels is an obligatory accompaniment of the inhibition of other competing channels and vice versa.

Adult↗