PubMed Health⌕ Search

Biomedical subjects

J Schlag

Publications and source records attributed to J Schlag.

At least 19 recordsLinked to original sources
PubMed1997

Perceived geometrical relationships affected by eye-movement signals.

To determine the location of visual objects relative to the observer, the visual system must take account not only of the location of the stimulus on the retina, but also of the direction of gaze. In contrast, the perceived spatial relationship between visual stimuli is normally assumed to depend on retinal information alone, and not to require information about eye position. We now show, however, that the perceived alignment of three dots-tested by a vernier alignment task-is systematically altered in the period immediately preceding a saccade. Thus, information about eye position can modify not only the perceived relationship of the entire retinal image to the observer, but also the relations between elements within the image. The processing of relative position and of egocentric (observer-centred) position may therefore be less distinct than previously believed.

Fixation, Ocular↗
PubMed1997

Colliding saccades evoked by frontal eye field stimulation: artifact or evidence for an oculomotor compensatory mechanism underlying double-step saccades?

What happens when the goal is changed before the movement is executed? Both the double-step and colliding saccade paradigms address this issue as they introduce a discrepancy between the retinal images of targets in space and the commands generated by the oculomotor system necessary to attain those targets. To maintain spatial accuracy under such conditions, transformations must update "retinal error' as eye position changes, and must also accommodate neural transmission delays in the system so that retinal and eye position information are temporally aligned. Different hypotheses have been suggested to account for these phenomena, based on observations of dissociable cortical and subcortical compensatory mechanisms. We now demonstrate how a single compensatory mechanism can be invoked to explain both double-step and colliding saccade paradigm results, based on the use of a damped signal of change in position that is used in both cases to update retinal error and, thereby, account for intervening movements. We conclude that the collision effect is not an artifact, but instead reveals a compensatory mechanism for saccades whose targets appear near the onset of a preceding saccade.

Brain Stem↗
PubMed1995

The use of egocentric and exocentric location cues in saccadic programming.

Theoretically, the location of a visual target can be encoded with respect to the locations of other stimuli in the visual image (exocentric cues), or with respect to the observer (egocentric cues). Egocentric localization in the oculomotor system has been shown to rely on an internal representation of eye position that inaccurately encodes the time-course of saccadic eye movements, resulting in the mislocalization of visual targets presented near the time of a saccade. In the present investigation, subjects were instructed to localize perisaccadic stimuli in the presence or absence of a visual stimulus that could provide exocentric location information. Saccadic localization was more accurate in the presence of the exocentric cue, suggesting that localization is based on a combination of exocentric and egocentric cues. These findings indicate the need to reassess previously reported neurophysiological studies of spatial accuracy and current models of oculomotor control, which have focused almost exclusively on the egocentric localization abilities of the brain.

Cues↗
PubMed1995

Illusory localization of stimuli flashed in the dark before saccades.

A photic stimulus flashed just before a saccade in the dark tends to be mislocalized in the direction of the saccade. This mislocalization is not only perceptual; it is also expressed by errors of ocular targeting. A particular situation arises if the point of light is flashed twice at the same place, the second time, just before a saccade. The point of light may appear at two different places even though neither the site of its retinal image nor the direction of gaze change between the flashes. Experiments were run on five human subjects, head fixed in the dark, with flashes repeated at the site of the saccade goal or at the initial point of fixation. In both cases, the test stimulus was mislocalized. However, its apparent displacement never produced the perception of a streak. Streaks were reported only when there was an actual stimulus movement on the retina (e.g. by flashing the stimulus during the saccade). Mislocalization did not occur if the two flashes were not separated by a dark interval. This implies that, as long as a steady stimulus remains continually visible, there is no updating of the internal representation of eye position assumed to be used for stimulus localization.

Fixation, Ocular↗
PubMed1992

Supplementary eye field: influence of eye position on neural signals of fixation.

Single units were recorded in the supplementary eye field of monkeys performing visual-oculomotor tasks. Two patterns of unit activity were observed while the animals fixated photic stimuli. One consisted of a step in tonic firing frequency (either an increase or a decrease, depending on the cell), lasting as long as fixation. The occurrence of this pattern did not require the continuous presence of a stimulus but the animal had to be provided with an incentive to fixate a given location. The other pattern was a monotonically varying firing rate dependent on the eye orientation along a particular axis, indicative of eye eccentricity in orbit.

Animals↗
PubMed1992

The frontal eye field provides the goal of saccadic eye movement.

Microstimulation of oculomotor regions in primate cortex normally evokes saccadic eye movements of stereotypic directions and amplitudes. The fixed-vector nature of the evoked movements is compatible with the creation of either an artificial retinal or motor error signal. However, when microstimulation is applied during an ongoing natural saccade, the starting eye position of the evoked movement differs from the eye position at stimulation onset (due to the latency of the evoked saccade). An analysis of the effect of this eye position discrepancy on the trajectory of the eventual evoked saccade can clarify the oculomotor role of the structure stimulated. The colliding saccade paradigm of microstimulation was used in the present study to investigate the type of signals conveyed by visual, visuomovement, and movement unit activities in the primate frontal eye field. Colliding saccades elicited from all sites were found to compensate for the portion of the initial movement occurring between stimulation and evoked movement onset, plus a portion of the initial movement occurring before stimulation. This finding suggests that activity in the frontal eye field encodes a retinotopic goal that is converted by a downstream structure into the vector of the eventual saccade.

Animals↗
PubMed1992

How the frontal eye field can impose a saccade goal on superior colliculus neurons.

Saccades were electrically evoked from the frontal eye field (FEF) of two trained monkeys while saccade-cells were recorded from the intermediate layers of the superior colliculus (SC). We found that FEF microstimulation, eliciting saccades of a given vector, excited SC saccade-cells encoding the same vector and inhibited all others. Such a mechanism can prevent competing commands from arising simultaneously in different structures.

Animals↗
PubMed1991

Primate supplementary eye field. II. Comparative aspects of connections with the thalamus, corpus striatum, and related forebrain nuclei.

The supplementary eye field (SEF) was defined electrophysiologically in behaving monkeys to study its connections with the diencephalon and corpus striatum. The specificity of SEF pathways was determined with horseradish peroxidase (HRP) histochemistry to compare its connections with those of the arcuate frontal eye field (FEF), contiguous dorsocaudal area 6 (6DC), and primary motor cortex (M1, arm/hand region). Results indicate that patterns of SEF connectivity were similar to the FEF and markedly different from areas 6DC and M1. Primary reciprocal thalamic pathways of the SEF were with the magnocellular ventral anterior (VA) nucleus, medial parvicellular VA, medial area X, and paralaminar medialis dorsalis (multiformis and parvicellularis). FEF showed similar connections but its most robust pathway was with MD rather than VA. In contrast, area 6DC showed the most extensive reciprocal connections with lateral VApc and lateral area X with only sparse connections with paralaminar MD. Area 6DC also exhibited reciprocal connections with the ventral lateral (VL) complex and the ventral posterior lateral nucleus, pars oralis (VPLo). M1 showed dense bidirectional connections with VPLo, and to a lesser extent, with VL. M1 pathways with the medial dorsal nucleus were negligible. All areas exhibited connections with the paracentral and central lateral nuclei and only M1 lacked connections with the central superior lateral nucleus. SEF and FEF exhibited similar efferent projections to the caudate and putamen. In the caudate, terminal fields were restricted to a central longitudinal core while those from area 6DC were more widely distributed. Eye field efferents were restricted to the putamen's face region while 6DC projections were more exuberant. The arm/hand region of M1 projected to the arm/hand region of the putamen. Pathways are discussed with respect to their significance in oculomotor control.

Animals↗
PubMed1990

Primate supplementary eye field: I. Comparative aspects of mesencephalic and pontine connections.

WGA-HRP was used to examine projections to the brainstem from the supplementary eye field (SEF). The SEF was defined electrophysiologically in awake, behaving monkeys and connections were compared to those of the arcuate frontal eye field (FEF), area 6DC, and primary motor cortex. The SEF was found to have either direct or indirect connections with almost every known pre- and paraoculomotor structure of the brainstem. The SEF was found to project bilaterally to layers I and IV of a tangentially widespread region of the superior colliculus. Terminal label was evident in the pretectal olivary nucleus, nucleus of the optic tract, nucleus raphe interpositus (omnipause region), nucleus prepositus hypoglossi, the perioculomotor cap of the central gray, dorsal central gray, nucleus reticularis tegmenti pontis, nucleus reticularis pontis oralis, and to multiple nuclei of the basis pontis (most densely to the dorsomedial nucleus). Bilateral projections were found in the parvicellular red nucleus. Reciprocal connections were present in the nucleus limitans, the mesencephalic reticular formation, locus coeruleus, and the serotonergic nuclei of the raphe complex (dorsalis and central superior). Overall patterns of connectivity were similar to those of the FEF and markedly different from those of the contiguous dorsocaudal area 6 or primary motor cortex. It was concluded that observed patterns of SEF-brainstem connectivity further justifies viewing this region as a distinct eye field that is likely to serve preparatory and trigger functions in the generation of saccadic eye movements.

Animals↗
PubMed1990

Colliding saccades may reveal the secret of their marching orders.

Saccades are fast eye movements by which objects of interest are sought and captured. Their brevity means that they can often be executed within the time taken by visual impulses to reach the brain. Microstimulation, used in conjunction with single-unit recording, has become a powerful tool for analysing the role of central visuomotor structures in producing accurate saccades. This review presents the rationale of a test in which saccades are artificially elicited (evoked) while the eyes are moving. The presumed role in saccade production played by stimulated local neurons is inferred from the way trajectories of evoked saccades are deviated. The results suggest that the forebrain structures tested designate a goal expressed in retinocentric coordinates. To take into account an eye displacement that occurs during visual processing, the goal location must be recomputed; in this article, current views on how this might be accomplished are discussed in relation to observations made by unit recording.

Animals↗
PubMed1990

Saccades can be aimed at the spatial location of targets flashed during pursuit.

1. If an eccentric, stationary target is flashed while a subject is performing an eye movement in the dark, can this subject make a saccade to the location in space where the target briefly appeared? Different predictions result from alternative hypotheses regarding the manner in which saccade goals are determined. Retinal error being defined as the vector from the eye position at the time of the flash to the position of the target, the retinal-error hypothesis predicts that the saccade vector will be equal to the retinal-error vector. This hypothesis assumes that the oculomotor system ignores the eye displacement between target presentation and saccade. If so, the target will be missed. In contrast, the spatial-error hypothesis predicts that the eye displacement is taken into account by the brain to calculate the target's physical location to which, therefore, a correct saccade could be aimed. 2. At issue is the generality of a fundamental principle of ocular targeting. Previous studies have established that, if the movement is saccadic, eye displacement is used by the oculomotor system to calculate the target's physical location. In the case of pursuit, perceptual experiments on humans suggest that eye displacement is taken into account although its velocity is underestimated. However, in a recent study McKenzie and Lisberger reported that saccade trajectories starting during pursuit conform to the retinal error hypothesis. In other words, velocity underestimation is close to 100%. 3. Although McKenzie and Lisberger's results are very clear, they might have depended on particular experimental conditions. The issue was reinvestigated in a situation facilitating the discrimination of stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Interactions between natural and electrically evoked saccades. I. Differences between sites carrying retinal error and motor error signals in monkey superior colliculus.

Fixed-vector saccades evoked by electrical stimulation may result from the elicitation of a retinal error signal directing the eyes toward a goal, or from the elicitation of a motor error signal determining the vector itself. Theoretically, the two mechanisms can be differentiated by delivering the stimulation while the eyes are already in motion (colliding saccade paradigm), thereby changing the eye position from which the evoked saccade starts. Only in the first case is the trajectory of the evoked saccade expected to be modified to compensate for part of the ongoing eye movement. An attempt was made to distinguish retinal vs. motor error mechanisms by applying the colliding saccade paradigm of stimulation to 29 sites throughout the superior colliculus (SC) of two trained monkeys. Compensatory evoked saccades, as predicted by the retinal error hypothesis, were obtained consistently in the superficial layers and at deeper sites where visual unit responses could be recorded. Conversely, in deep layers where only presaccadic activity was found, evoked saccades either were not affected by collision or summed their vectors with that of the ongoing movement. These last observations are both consistent with the hypothesis that the signal produced from deep sites was an initial motor error. A second observation was incidentally made: when stimulation was applied to the most superficial SC region, it definitively erased the goal of the ongoing saccade, and the latter did not resume its interrupted course. The colliding saccade paradigm may be useful in clarifying the role of structures involved in oculomotor function.

Animals↗
PubMed1989

Interactions between natural and electrically evoked saccades. II. At what time is eye position sampled as a reference for the localization of a target?

Electrical microstimulation was applied at brain sites (thalamic internal medullary lamina complex and superficial layers of superior colliculus) of alert, trained monkeys to evoke fixed-vector saccades. When the stimulation was timed to occur during or after an eye movement, the evoked saccade had a modified trajectory, compensating for, at least, the last portion of the ongoing eye movement. The hypothesis proposed to explain this compensatory effect (Schlag-Rey et al. 1989) is that the electrical stimulation produces a saccade by generating a signal, equivalent to a retinal error, specifying the saccade goal at a fixed location with respect to some eye position (called reference eye position). If the eyes are moving at the time of stimulation, the reference eye position lies somewhere along the trajectory of the ongoing movement. In the present study, we tried to determine this reference eye position, and deduce from it the instant at which the goal was specified. A significant timing difference was observed between thalamic and collicular stimulations. The goal appeared to be referred to an eye position existing at stimulation onset in superior colliculus (SC), and 35-65 ms before stimulation onset in central thalamus. In the latter case, the results suggest that the evoked saccade was aimed at the spatial location that the brain computed by summing a retinal error signal (evoked by stimulation) with the eye position at the time such a signal would have been elicited by a real target. In contrast, the collicular results suggest that the evoked saccade was directed to the retinal location specified by the retinal error signal. The findings imply that if the eyes are not steady while the target position is calculated, signals conveyed in the superficial layers of SC (in contrast to the thalamus) cannot direct the eyes correctly to a visual target.

Animals↗
PubMed1987

Does microstimulation evoke fixed-vector saccades by generating their vector or by specifying their goal?

Electrical stimulation was performed at several sites of the monkey's forebrain producing fixed-vector saccades when the eyes were steady. When the same stimulation was applied during or immediately after a spontaneous eye movement, the saccade trajectory was considerably modified: the eyes were driven, from wherever they were deviated by the spontaneous movement, to the point where the fixed-vector saccade would have brought them if the spontaneous movement had not occurred. This finding implies first, that saccades evoked from these sites are directed toward a goal and, a second, that the goal is defined with respect to an eye position sampled long in advance (before the spontaneous eye movement). This is consistent with the hypothesis that the electrical stimulation evoked the retinotopic representation of a target whose spatial coordinates were then computed at further stages to produce a saccade. Using the present paradigm, it may be possible to distinguish brain sites processing retinal error (at the visual stage) from those processing motor error (at the motor stage).

Animals↗
PubMed1987

Evidence for a supplementary eye field.

Electrical microstimulation and unit recording were performed in dorsomedial frontal cortex of four alert monkeys to identify an oculomotor area whose existence had been postulated rostral to the supplementary motor area. Contraversive saccades were evoked from 129 sites by stimulation. Threshold currents were lower than 20 microA in half the tests. Response latencies were usually longer than 50 ms (minimum: 30 ms). Eye movements were occasionally accompanied by blinks, ear, or neck movements. The cortical area yielding these movements was at the superior edge of the frontal lobe just rostral to the region from which limb movements could be elicited. Depending on the site of stimulation, saccades varied between two extremes: from having rather uniform direction and size, to converging toward a goal defined in space. The transition between these extremes was gradual with no evidence that these two types were fundamentally different. From surface to depth of cortex, direction and amplitude of evoked saccades were similar or changed progressively. No clear systematization was found depending on location along rostrocaudal or mediolateral axes of the cortex. The dorsomedial oculomotor area mapped was approximately 7 mm long and 6 mm wide. Combined eye and head movements were elicited from one of ten sites stimulated when the head was unrestrained. In the other nine cases, saccades were not accompanied by head rotation, even when higher currents or longer stimulus trains were applied. Presaccadic unit activity was recorded from 62 cells. Each of these cells had a preferred direction that corresponded to the direction of the movement evoked by local microstimulation. Presaccadic activity occurred with self-initiated as well as visually triggered saccades. It often led self-initiated saccades by more than 300 ms. Recordings made with the head free showed that the firing could not be interpreted as due to attempted head movements. Many dorsomedial cortical neurons responded to photic stimuli, either phasically or tonically. Sustained responses (activation or inhibition) were observed during target fixation. Twenty-one presaccadic units showed tonic changes of activity with fixation. Justification is given for considering the cortical area studied as a supplementary eye field. It shares many common properties with the arcuate frontal eye field. Differences noted in this study include: longer latency of response to electrical stimulation, possibility to evoke saccades converging apparently toward a goal, and long-lead unit activity with spontaneous saccades.

Animals↗