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Biomedical subjects

J Schlag

Publications and source records attributed to J Schlag.

At least 37 records · Page 2Linked to original sources
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.

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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.

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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)

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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.

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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.

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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).

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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.

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PubMed1985

Role of intralaminar thalamus in gaze mechanisms: evidence from electrical stimulation and fiber-sparing lesions in cat.

Information on eye movement related activity in the internal medullary lamina (IML) of the thalamus is consistent with an IML role in the control of eye movements, or with its serving to convey information to the forebrain on impending events in the oculomotor system. Interpretation is impeded by the fact that eye movements evoked by electrical stimulation of the IML might be triggered by antidromic activation of projections to the IML from brainstem preoculomotor centers. In order to address this issue, thresholds for the elicitation of eye movements and cortical recruiting responses (as a marker for IML integrity) were studied before and after destruction of IML neuronal populations by the fiber-sparing neurotoxin ibotenic acid. Experiments were performed in alert cats chronically implanted with a scleral search coil for the monitoring of eye movements and an electrode-cannula assembly that permitted threshold determinations and drug injections at the stimulating site without any movement of probes in the tissue. Drastic elevation of the threshold for cortical recruiting responses, but not for eye movements following ibotenic acid destruction of IML neurons uncomplicated by myelin damage at the site of stimulation suggests an antidromic route for eye movements elicited by IML stimulation.

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PubMed1985

Unit activity related to spontaneous saccades in frontal dorsomedial cortex of monkey.

Single unit activity was studied in the dorsomedial edge of the frontal lobe, above the superior arcuate sulcus in three trained monkeys (Macaca nemestrina). Gaze and head movements were recorded with two magnetic search coils. Discharges preceding spontaneous eye movements in a preferred direction were consistently observed in light and in dark, in a limited cortical territory at the anterior border of the supplementary motor area. Microstimulation at these sites elicited saccades in the unit preferred direction. Five presaccadic units were studied head fixed and head free and showed the same saccade-related activity under both conditions. Preliminary data suggest that the area studied may be a supplementary eye field distinct from the arcuate frontal eye field.

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PubMed1984

Unit activity related to head and eye movements in central thalamus of cats.

Single-unit activity was recorded with stereotaxically guided microelectrodes in the central thalamus of five alert cats. The animals were studied with the head either fixed or free to move in a horizontal plane. They were trained to make eye and/or head movements toward discrete visual targets presented on a screen. Unit activity was analyzed in relation to triggered and spontaneous gaze displacements with head fixed and free successively. Four groups of cells were found, all within the thalamic internal medullary lamina: 20 cells were active with eye but not head movements, 49 with head but not eye movements, 36 with head or eye movements, and 17 responding to visual stimuli in the absence of movement. The patterns of firing during gaze shifts are described. It is hypothesized that eye- or head-related units carry a signal representing gaze driving.

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PubMed1984

Visuomotor functions of central thalamus in monkey. I. Unit activity related to spontaneous eye movements.

The region in and around the thalamic internal medullary lamina (IML) in the cat recently has been shown to contain neurons active with ocular saccades and responding to visual stimuli. In the present study, single-unit microelectrode recordings were made in the corresponding thalamic region of the alert monkey in order to determine whether neurons with similar properties existed. Our objective was to specify the functional characteristics of these thalamic cells in the monkey, since 1) cell populations in the central thalamus form an important link between brain stem structures, such as superior colliculus and paramedian pontine reticular formation, and cortical areas, such as frontal eye field and inferior parietal lobule; and 2) most neurophysiological information on these structures with regard to gaze mechanisms has been obtained in primates. In this first part of the study we report observations on 164 thalamic units whose activity was related to the performance of spontaneous eye movements, head fixed. The animals had been trained on a visual discrimination task but photic stimuli were used only for calibrating the eye-position recording and for inducing small saccades and smooth pursuit. The experiments were performed in dim red light and in total darkness. Three types of units were found: 67 saccadic burst units, 58 saccade pause-rebound units, and 39 eye-position units. Sixty-two of the burst units had a directional preference. Most of the on-directions were contraversive, and it was in such units that the lead time of firing before saccades was the longest (up to at least 400 ms). Some of the burst units had a movement field, others fired more intensively and with a longer lead time, depending on the eccentricity of the eye position reached in orbit. The five units with no directional preference were the ones showing the best relation of burst duration with saccade duration. Three types of pause-rebound units were distinguished, depending on whether the saccadic pause or the postsaccadic burst was the most conspicuous event or the pause occurred after saccade offset. The three types were called, respectively, omnipausers, omnirebound cells, and late pausers. Omnipausers and omnirebound cells had no directional preference but their typical firing patterns occurred very consistently with all saccades, even less than 2 degrees. In a few units, the rebound progressively faded away in total darkness. The relation of firing rate of eye-position units with eccentricity of the eyes in orbit was analyzed. Fluctuations in time and a hysteresis effect were found to affect this relation.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1984

Visuomotor functions of central thalamus in monkey. II. Unit activity related to visual events, targeting, and fixation.

In alert monkeys, single-unit responses to visual stimuli were recorded in the central thalamic region where eye movement-related activity has been observed (33). Usually, the stimuli were 1 degree annulus patterns of dim light presented at unpredictable locations on a tangent screen. The animals were trained on two tasks: one in which they delivered the stimulus themselves by pressing a panel that they had to release immediately when the stimulus shape changed to a square, and another one in which the stimulus was turned on by the experimenter and the monkeys were rewarded for fixating this target for a predetermined length of time. In both tasks, continuous stimulus fixation was required. Receptive fields were tested with and without a fixation point. Retinal coordinates of stimuli were obtained by subtracting eye-position coordinates from stimulus coordinates in space, the monkey's head being fixed. Unit responses in the cases where targeting occurred or did not occur were analyzed separately. Transient responses were observed in 63 units and sustained responses in 44 units. Among the 63 units responding transiently, 42 did so irrespective of targeting. Their receptive fields were very large, generally including the fovea, and predominantly contralateral when the fields were asymmetric. The responses of the other 21 units depended on the occurrence of targeting. They were called visually triggered eye movement-related responses (VTEM). VTEM units were further subdivided in 9 units active only with targeting and 12 units showing the classical phenomenon of "response enhancement" under this condition. VTEM units were contrasted to six units that were both passively visually responsive and bursting with saccades, either spontaneous or visually triggered. The latencies of passive visual and VTEM responses to stimulus onset were comprised between 77 and 135 ms in 80% of the units. VTEM units also fired prior to retargeting saccades. Presaccadic units active with spontaneous saccades also discharged with visually elicited saccades. The earliest sign of activation after stimulus onset eliciting a saccade appeared between 80 and 100 ms, that is, in the same range of latencies as passive visual and VTEM units. Sustained visual responses consisted of activation in 18 units and inactivation in 26 units. The occurrence of these patterns of firing was related to stimulus fixation. In the majority of cases, the changes in discharge frequency started before fixation was achieved by a targeting saccade. They terminated before fixation was broken by a saccade away from the stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1983

Vestibular-oculomotor interaction in cat eye-head movements.

When cats make slow scanning head movements, intersaccadic counterrotary eye movements are driven by the vestibulo-ocular reflex, but reset or forward saccades are not directly affected by vestibular afference. When the movements are rapid (approximately 200 deg./sec), large (greater than 40 degrees), and executed in single step shifts gaze (whether in the dark or during visual fixation shifts between known targets), there is no longer any clear vestibular effect on any eye movements during the gaze shift. The vestibulo-ocular reflex is active only at the beginning of head rotation and again at its termination as the gaze reaches its goal, even in the absence of vision. It is postulated that head-in-space and eye-in-orbit movements are perfectly monitored to adjust the amplitude of gaze shifts, despite the lack of overt vestibular effects on eye movements.

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PubMed1983

Thalamic units firing upon refixation may be responsible for plasticity in visual cortex.

In alert cat and monkey, a unit type recorded in the region of the thalamic internal medullary lamina seems to provide the extraretinal signal postulated by Singer (1982) to explain the development of mature receptive field properties in cortical visual neurons. These thalamic units are silent (or silenced) during saccades in all directions; they discharge as soon as the eyes assume a new position. The abolition of this discharge by thalamic lesion, or conversely, its elicitation by electrical stimulation could respectively prevent or facilitate plastic changes in visual cortical areas of kittens.

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PubMed1983

Comparison of EOG and search coil techniques in long-term measurements of eye position in alert monkey and cat.

In cat and monkey a comparison was made of simultaneous records of eye movements obtained by the electro-oculographic and the magnetic search coil techniques. The major drawback of the electro-oculographic method is the slow d.c. drift, still present despite careful preparation of the electrodes. The d.c. level varies considerably with lighting conditions over long periods of time. In addition, the EOG picks up blink artifacts in monkey and other movement artifacts which are described in detail. The objective of this study was to document and evaluate the relative importance of these technical problems. Because of its simplicity and convenience the EOG method still has some applications in neurophysiological experiments. Its use requires the possibility of making frequent calibrations of gain and d.c. level.

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