Combined eye-head gaze shifts in the primate. III. Contributions to the accuracy of gaze saccades

1990 ◽  
Vol 64 (6) ◽  
pp. 1873-1891 ◽  
Author(s):  
R. D. Tomlinson
Keyword(s):  
Head Movement ◽  
Head Movements ◽  
Whole Body ◽  
Midsagittal Plane ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
The Gaze ◽  
Gaze Stability ◽  
Gaze Saccade ◽  
Gaze Saccades

1. The behavior of the combined eye-head gaze saccade mechanism was investigated in the rhesus monkey under both normal circumstances and in the presence of perturbations delivered to the head by a torque motor. Animals were trained to follow a target light that stepped at regular intervals through an angle of 68 degrees (+/- 34 degrees with respect to the midsagittal plane). Thus all primary saccades were center crossing. On randomly occurring trials the torque motor was pulsed so as to perturb the trajectory of the head, thus allowing us to assess both the functional state of the vestibuloocular reflex (VOR) and the effects of such perturbations on gaze saccade accuracy (gaze is defined as the sum of eye-in-head plus head-in-space, and a gaze saccade as a combined eye-head saccadic gaze shift). 2. Gaze shifts can be divided into two discrete sections: the portion during which the gaze angle is changing (the saccadic portion), and the portion during which the gaze is stationary but the head continues to move (the terminal head-movement portion). For the system to accurately acquire eccentric targets, at least two criteria must be met: 1) the saccadic portion must be accurate, and 2) the compensatory eye movement that occurs during the terminal head-movement portion must be equal and opposite to the head movement, thereby maintaining gaze stability. Perturbations delivered during the terminal head-movement portion of the gaze shift indicated that VOR was functioning normally, and thus we concluded that the compensatory eye movements that accompany head movements were vestibular in origin. 3. As reported previously, during the saccadic portion of large-amplitude gaze saccades, the VOR ceases to function. In spite of this observation, the accuracy of the gaze saccade is not affected by perturbations delivered to the head. Gaze accuracy is maintained both by changing the duration of the saccadic portion and by altering the head trajectory. 4. Because rhesus monkeys often make very rapid head movements (1,200 degrees/s), we wished to discover the velocity range over which the monkey VOR might be expected to operate. Accordingly, in a second series of experiments, VOR function was assessed during passive whole-body rotations with the head fixed. By the use of spring-assisted manual rotations, peak velocities up to 850 degrees/s were achieved. When VOR gain was measured during such rotations, it was found to be equal to 0.9 up to the maximum velocities used.(ABSTRACT TRUNCATED AT 400 WORDS)

Rapid horizontal gaze movement in the monkey

1995 ◽  
Vol 73 (4) ◽  
pp. 1632-1652 ◽  
Author(s):  
J. O. Phillips ◽  
L. Ling ◽  
A. F. Fuchs ◽  
C. Siebold ◽  
J. J. Plorde
Keyword(s):  
Eye Movement ◽  
Head Movement ◽  
Vertical Axis ◽  
Head Position ◽  
Head Movements ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
The Gaze ◽  
Small Correction ◽  
Horizontal Gaze

1. We studied horizontal eye and head movements in three monkeys that were trained to direct their gaze (eye position in space) toward jumping targets while their heads were both fixed and free to rotate about a vertical axis. We considered all gaze movements that traveled > or = 80% of the distance to the new visual target. 2. The relative contributions and metrics of eye and head movements to the gaze shift varied considerably from animal to animal and even within animals. Head movements could be initiated early or late and could be large or small. The eye movements of some monkeys showed a consistent decrease in velocity as the head accelerated, whereas others did not. Although all gaze shifts were hypometric, they were more hypometric in some monkeys than in others. Nevertheless, certain features of the gaze shift were identifiable in all monkeys. To identify those we analyzed gaze, eye in head position, and head position, and their velocities at three points in time during the gaze shift: 1) when the eye had completed its initial rotation toward the target, 2) when the initial gaze shift had landed, and 3) when the head movement was finished. 3. For small gaze shifts (< 20 degrees) the initial gaze movement consisted entirely of an eye movement because the head did not move. As gaze shifts became larger, the eye movement contribution saturated at approximately 30 degrees and the head movement contributed increasingly to the initial gaze movement. For the largest gaze shifts, the eye usually began counterrolling or remained stable in the orbit before gaze landed. During the interval between eye and gaze end, the head alone carried gaze to completion. Finally, when the head movement landed, it was almost aimed at the target and the eye had returned to within 10 +/- 7 degrees, mean +/- SD, of straight ahead. Between the end of the gaze shift and the end of the head movement, gaze remained stable in space or a small correction saccade occurred. 4. Gaze movements < 20 degrees landed accurately on target whether the head was fixed or free. For larger target movements, both head-free and head-fixed gaze shifts became increasingly hypometric. Head-free gaze shifts were more accurate, on average, but also more variable. This suggests that gaze is controlled in a different way with the head free. For target amplitudes < 60 degrees, head position was hypometric but the error was rather constant at approximately 10 degrees.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Dissociation of Eye and Head Components of Gaze Shifts by Stimulation of the Omnipause Neuron Region

2007 ◽  
Vol 98 (1) ◽  
pp. 360-373 ◽  
Author(s):  
Neeraj J. Gandhi ◽  
David L. Sparks
Keyword(s):  
Head Movement ◽  
Displacement Vector ◽  
Movement Control ◽  
Head Movements ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
The Gaze ◽  
Movement Component ◽  
Neck Motoneurons ◽  
Stimulation Of

Natural movements often include actions integrated across multiple effectors. Coordinated eye-head movements are driven by a command to shift the line of sight by a desired displacement vector. Yet because extraocular and neck motoneurons are separate entities, the gaze shift command must be separated into independent signals for eye and head movement control. We report that this separation occurs, at least partially, at or before the level of pontine omnipause neurons (OPNs). Stimulation of the OPNs prior to and during gaze shifts temporally decoupled the eye and head components by inhibiting gaze and eye saccades. In contrast, head movements were consistently initiated before gaze onset, and ongoing head movements continued along their trajectories, albeit with some characteristic modulations. After stimulation offset, a gaze shift composed of an eye saccade, and a reaccelerated head movement was produced to preserve gaze accuracy. We conclude that signals subject to OPN inhibition produce the eye-movement component of a coordinated eye-head gaze shift and are not the only signals involved in the generation of the head component of the gaze shift.

Combined eye-head gaze shifts in the primate. I. Metrics

1986 ◽  
Vol 56 (6) ◽  
pp. 1542-1557 ◽  
Author(s):  
R. D. Tomlinson ◽  
P. S. Bahra
Keyword(s):  
Head Movement ◽  
Velocity Profiles ◽  
Hyperbolic Function ◽  
Total Change ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
Velocity Amplitude ◽  
The Gaze ◽  
Gaze Saccades ◽  
Gaze Angle

Gaze (eye-in-space) velocity-duration and velocity-amplitude curves were prepared for head-fixed and head-free gaze shifts in the rhesus monkey with an emphasis on large amplitudes. These plots revealed the presence of two distinct gaze reorientation mechanisms, one used when the gaze shift was small (less than 20 degrees) and the other utilized for large coordinated gaze shifts when the head was free. When head-free and head-fixed saccadic gaze shifts were compared in the same animal, no differences in the metrics were found for amplitudes less than 20 degrees. However, for large gaze shifts where contribution of the head to the change in gaze angle was considerable, head-free saccades were found to exhibit lower peak gaze velocities and greater durations than those recorded with the head-fixed paradigm. In order to differentiate between the eye saccades and combined saccadic eye-head gaze shifts, the latter have been termed gaze saccades. Change in head position and change in eye position were both measured during the actual gaze shift and were plotted against the gaze-shift amplitude to determine whether the head movement contributed significantly to the change in gaze angle. The results indicate that below 20 degrees the gaze shift is accomplished almost exclusively with the eyes and the head moves very little; however, for larger saccades, the head contributes approximately 80% of the total change in gaze angle with the eyes contributing only approximately 20%. Large saccadic eye-head gaze shifts do not exhibit 'bell-shaped' velocity profiles as do smaller head-fixed saccades; instead, gaze accelerates to reach a peak velocity after approximately 30-40 ms. This velocity is then maintained for the duration of the gaze shift. Close scrutiny of the fine structure of the velocity profiles of the eye, head, and gaze channels indicates that during gaze saccades, the eye and head movement motor programs interact to maintain gaze velocity nearly constant, unaffected by changes in head velocity. Previous authors had stated that when velocity-duration plots are obtained for oblique saccades of constant amplitude, the resulting points could be fitted with a hyperbolic function. These results were confirmed for head-free gaze saccades and extended to larger amplitudes. When an oblique saccade is made, the smaller component is stretched in duration to match the duration of the larger component. However, as the gaze shift becomes large (greater than 40 degrees), the relationship becomes more complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Action of the Brain Stem Saccade Generator During Horizontal Gaze Shifts. I. Discharge Patterns of Omnidirectional Pause Neurons

1999 ◽  
Vol 81 (3) ◽  
pp. 1284-1295 ◽  
Author(s):  
James O. Phillips ◽  
Leo Ling ◽  
Albert F. Fuchs
Keyword(s):  
Brain Stem ◽  
Eye Movement ◽  
Head Movement ◽  
Head Movements ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
The Gaze ◽  
Horizontal Gaze ◽  
Discharge Patterns ◽  
The Brain

Action of the brain stem saccade generator during horizontal gaze shifts. I. Discharge patterns of omnidirectional pause neurons. Omnidirectional pause neurons (OPNs) pause for the duration of a saccade in all directions because they are part of the neural mechanism that controls saccade duration. In the natural situation, however, large saccades are accompanied by head movements to produce rapid gaze shifts. To determine whether OPNs are part of the mechanism that controls the whole gaze shift rather than the eye saccade alone, we monitored the activity of 44 OPNs that paused for rightward and leftward gaze shifts but otherwise discharged at relatively constant average rates. Pause duration was well correlated with the duration of either eye or gaze movement but poorly correlated with the duration of head movement. The time of pause onset was aligned tightly with the onset of either eye or gaze movement but only loosely aligned with the onset of head movement. These data suggest that the OPN pause does not encode the duration of head movement. Further, the end of the OPN pause was often better aligned with the end of the eye movement than with the end of the gaze movement for individual gaze shifts. For most gaze shifts, the eye component ended with an immediate counterrotation owing to the vestibuloocular reflex (VOR), and gaze ended at variable times thereafter. In those gaze shifts where eye counterrotation was delayed, the end of the pause also was delayed. Taken together, these data suggest that the end of the pause influences the onset of eye counterrotation, not the end of the gaze shift. We suggest that OPN neurons act to control only that portion of the gaze movement that is commanded by the eye burst generator. This command is expressed by driving the saccadic eye movement directly and also by suppressing VOR eye counterrotation. Because gaze end is less well correlated with pause end and often occurs well after counterrotation onset, we conclude that elements of the burst generator typically are not active till gaze end, and that gaze end is determined by another mechanism independent of the OPNs.

Eye-Head Coordination During Head-Unrestrained Gaze Shifts in Rhesus Monkeys

1997 ◽  
Vol 77 (5) ◽  
pp. 2328-2348 ◽  
Author(s):  
Edward G. Freedman ◽  
David L. Sparks
Keyword(s):  
Head Movement ◽  
Rhesus Monkeys ◽  
Initial Position ◽  
Head Movements ◽  
Gaze Shifts ◽  
Unit Recording ◽  
Gaze Shift ◽  
The Gaze ◽  
Two Factors ◽  
Eye Velocity

Freedman, Edward G. and David L. Sparks. Eye-head coordination during head-unrestrained gaze shifts in rhesus monkeys. J. Neurophysiol. 77: 2328–2348, 1997. We analyzed gaze shifts made by trained rhesus monkeys with completely unrestrained heads during performance of a delayed gaze shift task. Subjects made horizontal, vertical, and oblique gaze shifts to visual targets. We found that coordinated eye-head movements are characterized by a set of lawful relationships, and that the initial position of the eyes in the orbits and the direction of the gaze shift are two factors that influence these relationships. Head movements did not contribute to the change in gaze position during small gaze shifts (<20°) directed along the horizontal meridian, when the eyes were initially centered in the orbits. For larger gaze shifts (25–90°), the head contribution to the gaze shift increased linearly with increasing gaze shift amplitude, and eye movement amplitude saturated at an asymptotic amplitude of ∼35°. When the eyes began deviated in the orbits contralateral to the direction of the ensuing gaze shift, the head contributed less and the eyes more to amplitude-matched gaze shifts. The relative timing of eye and head movements was altered by initial eye position; head latency relative to gaze onset increased as the eyes began in more contralateral initial positions. The direction of the gaze shift also affected the relative amplitudes of eye and head movements; as gaze shifts were made in progressively more vertical directions, eye amplitude increased and head contribution declined systematically. Eye velocity was a saturating function of gaze amplitude for movements without a head contribution (gaze amplitude <20°). As head contribution increased with increasing gaze amplitude (20–60°), peak eye velocity declined by >200°/s and head velocity increased by 100°/s. For constant-amplitude eye movements (∼30°), eye velocity declined as the velocity of the concurrent head movement increased. On the basis of these relationships, it is possible to accurately predict gaze amplitude, the amplitudes of the eye and head components of the gaze shift, and gaze, eye, and head velocities, durations and latencies if the two-dimensional displacement of the target and the initial position of the eyes in the orbits are known. These data indicate that signals related to the initial positions of the eyes in the orbits and the direction of the gaze shift influence separate eye and head movement commands. The hypothesis that this divergence of eye and head commands occurs downstream from the superior colliculus is supported by recent electrical stimulation and single-unit recording data.

scholarly journals Target modality determines eye-head coordination in nonhuman primates: implications for gaze control

2011 ◽  
Vol 106 (4) ◽  
pp. 2000-2011 ◽  
Author(s):  
Luis C. Populin ◽  
Abigail Z. Rajala
Keyword(s):  
Nonhuman Primates ◽  
Saccadic Eye Movements ◽  
Head Movements ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
The Gaze ◽  
Midbrain Structure ◽  
The Subject ◽  
Visual Targets ◽  
Time Of Presentation

We have studied eye-head coordination in nonhuman primates with acoustic targets after finding that they are unable to make accurate saccadic eye movements to targets of this type with the head restrained. Three male macaque monkeys with experience in localizing sounds for rewards by pointing their gaze to the perceived location of sources served as subjects. Visual targets were used as controls. The experimental sessions were configured to minimize the chances that the subject would be able to predict the modality of the target as well as its location and time of presentation. The data show that eye and head movements are coordinated differently to generate gaze shifts to acoustic targets. Chiefly, the head invariably started to move before the eye and contributed more to the gaze shift. These differences were more striking for gaze shifts of <20–25° in amplitude, to which the head contributes very little or not at all when the target is visual. Thus acoustic and visual targets trigger gaze shifts with different eye-head coordination. This, coupled to the fact that anatomic evidence involves the superior colliculus as the link between auditory spatial processing and the motor system, suggests that separate signals are likely generated within this midbrain structure.

Brain Stem Omnipause Neurons and the Control of CombinedEye-Head Gaze Saccades in the Alert Cat

1998 ◽  
Vol 79 (6) ◽  
pp. 3060-3076 ◽  
Author(s):  
Martin Paré ◽  
Daniel Guitton
Keyword(s):  
Gaze Control ◽  
Error Signal ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
The Gaze ◽  
Alert Cat ◽  
Motor Error ◽  
Omnipause Neurons ◽  
Gaze Saccades ◽  
Stimulation Of

Paré, Martin and Daniel Guitton. Brain stem omnipause neurons and the control of combined eye-head gaze saccades in the alert cat. J. Neurophysiol. 79: 3060–3076, 1998. When the head is unrestrained, rapid displacements of the visual axis—gaze shifts (eye-re-space)—are made by coordinated movements of the eyes (eye-re-head) and head (head-re-space). To address the problem of the neural control of gaze shifts, we studied and contrasted the discharges of omnipause neurons (OPNs) during a variety of combined eye-head gaze shifts and head-fixed eye saccades executed by alert cats. OPNs discharged tonically during intersaccadic intervals and at a reduced level during slow perisaccadic gaze movements sometimes accompanying saccades. Their activity ceased for the duration of the saccadic gaze shifts the animal executed, either by head-fixed eye saccades alone or by combined eye-head movements. This was true for all types of gaze shifts studied: active movements to visual targets; passive movements induced by whole-body rotation or by head rotation about stationary body; and electrically evoked movements by stimulation of the caudal part of the superior colliculus (SC), a central structure for gaze control. For combined eye-head gaze shifts, the OPN pause was therefore not correlated to the eye-in-head trajectory. For instance, in active gaze movements, the end of the pause was better correlated with the gaze end than with either the eye saccade end or the time of eye counterrotation. The hypothesis that cat OPNs participate in controlling gaze shifts is supported by these results, and also by the observation that the movements of both the eyes and the head were transiently interrupted by stimulation of OPNs during gaze shifts. However, we found that the OPN pause could be dissociated from the gaze-motor-error signal producing the gaze shift. First, OPNs resumed discharging when perturbation of head motion briefly interrupted a gaze shift before its intended amplitude was attained. Second, stimulation of caudal SC sites in head-free cat elicited large head-free gaze shifts consistent with the creation of a large gaze-motor-error signal. However, stimulation of the same sites in head-fixed cat produced small “goal-directed” eye saccades, and OPNs paused only for the duration of the latter; neither a pause nor an eye movement occurred when the same stimulation was applied with the eyes at the goal location. We conclude that OPNs can be controlled by neither a simple eye control system nor an absolute gaze control system. Our data cannot be accounted for by existing models describing the control of combined eye-head gaze shifts and therefore put new constraints on future models, which will have to incorporate all the various signals that act synergistically to control gaze shifts.

Eye-head coordination in cats

1984 ◽  
Vol 52 (6) ◽  
pp. 1030-1050 ◽  
Author(s):  
D. Guitton ◽  
R. M. Douglas ◽  
M. Volle
Keyword(s):  
Eye Movements ◽  
Eye Movement ◽  
Head Movement ◽  
Maximum Velocity ◽  
Head Movements ◽  
Visual Axis ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
Rapid Eye Movements ◽  
Motor Strategy

Gaze is the position of the visual axis in space and is the sum of the eye movement relative to the head plus head movement relative to space. In monkeys, a gaze shift is programmed with a single saccade that will, by itself, take the eye to a target, irrespective of whether the head moves. If the head turns simultaneously, the saccade is correctly reduced in size (to prevent gaze overshoot) by the vestibuloocular reflex (VOR). Cats have an oculomotor range (OMR) of only about +/- 25 degrees, but their field of view extends to about +/- 70 degrees. The use of the monkey's motor strategy to acquire targets lying beyond +/- 25 degrees requires the programming of saccades that cannot be physically made. We have studied, in cats, rapid horizontal gaze shifts to visual targets within and beyond the OMR. Heads were either totally unrestrained or attached to an apparatus that permitted short unexpected perturbations of the head trajectory. Qualitatively, similar rapid gaze shifts of all sizes up to at least 70 degrees could be accomplished with the classic single-eye saccade and a saccade-like head movement. For gaze shifts greater than 30 degrees, this classic pattern frequently was not observed, and gaze shifts were accomplished with a series of rapid eye movements whose time separation decreased, frequently until they blended into each other, as head velocity increased. Between discrete rapid eye movements, gaze continued in constant velocity ramps, controlled by signals added to the VOR-induced compensatory phase that followed a saccade. When the head was braked just prior to its onset in a 10 degrees gaze shift, the eye attained the target. This motor strategy is the same as that reported for monkeys. However, for larger target eccentricities (e.g., 50 degrees), the gaze shift was interrupted by the brake and the average saccade amplitude was 12-15 degrees, well short of the target and the OMR. Gaze shifts were completed by vestibularly driven eye movements when the head was released. Braking the head during either quick phases driven by passive head displacements or visually triggered saccades resulted in an acceleration of the eye, thereby implying interaction between the VOR and these rapid-eye-movement signals. Head movements possessed a characteristic but task-dependent relationship between maximum velocity and amplitude. Head movements terminated with the head on target. The eye saccade usually lagged the head displacement.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Frames of Reference for Gaze Saccades Evoked During Stimulation of Lateral Intraparietal Cortex

2007 ◽  
Vol 98 (2) ◽  
pp. 696-709 ◽  
Author(s):  
A. G. Constantin ◽  
H. Wang ◽  
J. C. Martinez-Trujillo ◽  
J. D. Crawford
Keyword(s):  
Three Dimensional ◽  
Saccadic Eye Movements ◽  
Head Movements ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
Lateral Intraparietal Cortex ◽  
The Gaze ◽  
The Right ◽  
Gaze Position ◽  
Stimulation Of

Previous studies suggest that stimulation of lateral intraparietal cortex (LIP) evokes saccadic eye movements toward eye- or head-fixed goals, whereas most single-unit studies suggest that LIP uses an eye-fixed frame with eye-position modulations. The goal of our study was to determine the reference frame for gaze shifts evoked during LIP stimulation in head-unrestrained monkeys. Two macaques ( M1 and M2) were implanted with recording chambers over the right intraparietal sulcus and with search coils for recording three-dimensional eye and head movements. The LIP region was microstimulated using pulse trains of 300 Hz, 100–150 μA, and 200 ms. Eighty-five putative LIP sites in M1 and 194 putative sites in M2 were used in our quantitative analysis throughout this study. Average amplitude of the stimulation-evoked gaze shifts was 8.67° for M1 and 7.97° for M2 with very small head movements. When these gaze-shift trajectories were rotated into three coordinate frames (eye, head, and body), gaze endpoint distribution for all sites was most convergent to a common point when plotted in eye coordinates. Across all sites, the eye-centered model provided a significantly better fit compared with the head, body, or fixed-vector models (where the latter model signifies no modulation of the gaze trajectory as a function of initial gaze position). Moreover, the probability of evoking a gaze shift from any one particular position was modulated by the current gaze direction (independent of saccade direction). These results provide causal evidence that the motor commands from LIP encode gaze command in eye-fixed coordinates but are also subtly modulated by initial gaze position.

Contribution of the Rostral Fastigial Nucleus to the Control of Orienting Gaze Shifts in the Head-Unrestrained Cat

1998 ◽  
Vol 80 (3) ◽  
pp. 1180-1196 ◽  
Author(s):  
Denis Pélisson ◽  
Laurent Goffart ◽  
Alain Guillaume
Keyword(s):  
Head Movements ◽  
Visual Signals ◽  
Fastigial Nucleus ◽  
Retinal Eccentricity ◽  
Visual Axis ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
The Gaze ◽  
Eye Rotation ◽  
Fixation Offset

Pélisson, Denis, Laurent Goffart, and Alain Guillaume. Contribution of the rostral fastigial nucleus to the control of orienting gaze shifts in the head-unrestrained cat. J. Neurophysiol. 80: 1180–1196, 1998. The implication of the caudal part of the fastigial nucleus (cFN) in the control of saccadic shifts of the visual axis is now well established. In contrast a possible involvement of the rostral part of the fastigial nucleus (rFN) remains unknown. In the current study we investigated in the head-unrestrained cat the contribution of the rFN to the control of visually triggered saccadic gaze shifts by measuring the deficits after unilateral muscimol injection in the rFN. A typical gaze dysmetria was observed: gaze saccades directed toward the inactivated side were hypermetric, whereas those with an opposite direction were hypometric. For both movement directions, gaze dysmetria was proportional to target retinal eccentricity and could be described as a modified gain in the translation of visual signals into eye and head motor commands. Correction saccades were triggered when the target remained visible and reduced the gaze fixation error to 2.7 ± 1.3° (mean ± SD) on average. The hypermetria of ipsiversive gaze shifts resulted predominantly from a hypermetric response of the eyes, whereas the hypometria of contraversive gaze shifts resulted from hypometric responses of both eye and head. However, even in this latter case, the eye saccade was more affected than the motion of the head. As a consequence, for both directions of gaze shift the relative contributions of the eye and head to the overall gaze displacement were altered by muscimol injection. This was revealed by a decreased contribution of the head for ipsiversive gaze shifts and an increased head contribution for contraversive movements. These modifications were associated with slight changes in the delay between eye and head movement onsets. Inactivation of the rFN also affected the initiation of eye and head movements. Indeed, the latency of ipsiversive gaze and head movements decreased to 88 and 92% of normal, respectively, whereas the latency of contraversive ones increased to 149 and 145%. The deficits induced by rFN inactivation were then compared with those obtained after muscimol injection in the cFN of the same animals. Several deficits differed according to the site of injection within the fastigial nucleus (tonic orbital eye rotation, hypermetria of ipsiversive gaze shifts and fixation offset, relationship between dysmetria and latency of contraversive gaze shifts, postural deficit). In conclusion, the present study demonstrates that the rFN is involved in the initiation and the control of combined eye-head gaze shifts. In addition our findings support a functional distinction between the rFN and cFN for the control of orienting gaze shifts. This distinction is discussed with respect to the segregated fastigiofugal projections arising from the rFN and cFN.

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