Muscimol-induced inactivation of monkey frontal eye field: effects on visually and memory-guided saccades.

Muscimol-induced inactivation of the monkey frontal eye field: effects on visually and memory-guided saccades. Although neurophysiological, anatomic, and imaging evidence suggest that the frontal eye field (FEF) participates in the generation of eye movements, chronic lesions of the FEF in both humans and monkeys appear to cause only minor deficits in visually guided saccade generation. Stronger effects are observed when subjects are tested in tasks with more cognitive requirements. We tested oculomotor function after acutely inactivating regions of the FEF to minimize the effects of plasticity and reallocation of function after the loss of the FEF and gain more insight into the FEF contribution to the guidance of eye movements in the intact brain. Inactivation was induced by microinjecting muscimol directly into physiologically defined sites in the FEF of three monkeys. FEF inactivation severely impaired the monkeys' performance of both visually guided and memory-guided saccades. The monkeys initiated fewer saccades to the retinotopic representation of the inactivated FEF site than to any other location in the visual field. The saccades that were initiated had longer latencies, slower velocities, and larger targeting errors than controls. These effects were present both for visually guided and for memory-guided saccades, although the memory-guided saccades were more disrupted. Initially, the effects were restricted spatially, concentrating around the retinotopic representation at the center of the inactivated site, but, during the course of several hours, these effects spread to flanking representations. Predictability of target location and motivation of the monkey also affected saccadic performance. For memory-guided saccades, increases in the time during which the monkey had to remember the spatial location of a target resulted in further decreases in the accuracy of the saccades and in smaller peak velocities, suggesting a progressive loss of the capacity to maintain a representation of target location in relation to the fovea after FEF inactivation. In addition, the monkeys frequently made premature saccades to targets in the hemifield ipsilateral to the injection site when performing the memory task, indicating a deficit in the control of fixation that could be a consequence of an imbalance between ipsilateral and contralateral FEF activity after the injection. There was also a progressive loss of fixation accuracy, and the monkeys tended to restrict spontaneous visual scanning to the ipsilateral hemifield. These results emphasize the strong role of the FEF in the intact monkey in the generation of all voluntary saccadic eye movements, as well as in the control of fixation.

A pressure system for the microinjection of substances into the brain of awake monkeys.

We describe a method for placing pressure microinjections of drugs or anatomical tracers in physiologically defined sites in the brain of awake monkeys. This method provides a means to record neuronal activity from the tip of an injection cannula so that an injection can be made at a physiologically defined location. It uses pressure in a closed system to precisely control the amount of fluid injected and provides a visible means for monitoring injection volume. The injection cannula is easy to make with readily available components and can be used repeatedly for multiple recording sessions and injections.

The primate frontal eye field and the generation of saccadic eye movements: comparison of lesion and acute inactivation/activation studies.

The frontal eye field (FEF) of monkeys has been repeatedly implicated in the generation of saccadic eye movements by various experimental approaches. Electrical stimulation of most of the FEF produces saccadic eye movements, many cells have activities related to saccades, and it has anatomical connections with many other oculomotor areas. Surprisingly, complete lesions of the FEF have remarkably little effect on oculomotor behavior. Only when more cognitive aspects are tested is a deficit clearly detected. In contrast, acute inactivation of the FEF of monkeys with the GABA agonist muscimol produced much more severe oculomotor impairment. This difference is probably due to the acute nature of the muscimol effect, which does not allow time for reorganization of the control of eye movements before testing begins. In addition, acute activation of the FEF with the GABA antagonist bicuculline caused the monkey to make irrepressible saccades of the same dimensions as those electrically elicited at the site. These experiments further confirm the strong involvement of the FEF in the control of saccadic eye movements and fixation.

Acute activation and inactivation of macaque frontal eye field with GABA-related drugs.

1. This project tests the behavioral effects of reversible activation and inactivation of sites within the frontal eye field of rhesus monkeys with microinjections of the gamma-aminobutyric acid (GABA)-related drugs bicuculline and muscimol. 2. Muscimol injections impaired the monkeys' ability to make both visually and memory-guided saccades to targets at the center of the area represented by the injection site. The latencies of saccades to targets in regions flanking the injection were increased. For memory-guided saccades, saccades in the direction opposite to that represented by the injection site, were made with shorter latency than controls and often occurred before the movement cue. 3. Bicuculline injections produced irrepressible saccades equivalent to the saccade vector represented by the injection site, often in a staircase of several closely spaced movements. 4. Both substances decreased the accuracy of fixation of a central light. The distribution of points of fixation on different trials was diffuse, and the angle of gaze tended to deviate towards the side of the injection. 5. The results of these acute injections are similiar to those observed in the superior colliculus and are much more substantial than the effects observed in the long term after surgical removal of the frontal eye field. The results of this study promote a central role for the frontal eye field in the generation of all voluntary saccades and in the control of fixation.

Effect of stimulus position and velocity upon the maintenance of smooth pursuit eye velocity.

The relative contributions of retinal slip velocity and position errors to the generation of smooth pursuit eye movements were examined in three rhesus monkeys. Recognizing the unlikelihood of producing a pure retinal slip velocity or position error signal, these two stimulus parameters were combined under open-loop conditions. Both slip velocity and position error were used by the monkey to maintain an established eye velocity. Both parameters had the greatest effects upon eye velocity when they were in the same direction, enabling the monkey to maintain an established pursuit velocity. When slip velocity and position error were in the direction opposite to the initial pursuit, eye velocity reversed direction and moved very quickly towards zero. When the two parameters were in opposite directions, their effect upon eye velocity was minimized.

Primate frontal eye field activity during natural scanning eye movements.

1. As we scan an image, saccadic eye movements direct our vision to features that attract our attention. Although it is likely that the frontal eye field (FEF) cortex is an important component of the system generating those movements, most studies of FEF neuronal activity have relied upon visuomotor tasks where the experimental subjects are constrained to look from one spot of light to another. In this study, single-unit activity was recorded in the FEF while monkeys freely scanned a variety of projected images, and that activity was compared with activity evoked during conventional visuomotor tasks. 2. FEF neurons with visual activity in conventional tasks increased their activity during scanning when a portion of the image within their receptive field was targeted for the next saccade, but decreased their activity when a target was chosen outside of the receptive field. 3. FEF neurons with movement-related activity during conventional tasks were also active in association with saccades made during scanning. 4. Visual and movement activity were also studied by creating a task that approximated the conditions during the scanning paradigm (rescan task). This was done by superimposing a moveable spot of light onto the image that had been scanned, and rewarding the monkey for following the light as it recreated the original scan's spatial and temporal pattern of eye fixations. In contrast to the visual activity of neurons during the scanning paradigm, visual activity during the rescan task was unaffected by portions of the image within the cell's receptive field, but increased in response to the appearance of the target light.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship of monkey frontal eye field activity to saccade dynamics.

1. In this study, we compared the temporal waveforms of the activity of monkey frontal eye field movement neurons with the dynamics of saccadic eye movements. 2. Movement neurons in the frontal eye field were selected according to previously published criteria. They had little or no response to visual stimuli in a fixation task, and equivalent activity before visually guided and memory-guided saccades. We studied corticotectal neurons and corticopontine neurons identified by antidromic excitation, as well as neurons whose projections were not identified. 3. These neurons had a peak activation at a mean of 13 ms before the saccade began. However, rather than falling off rapidly as the saccade ended, most neurons continued to fire after the saccade, returning to baseline at a mean of 93 ms after the end of the saccade. 4. We measured the decrement in activity for these neurons during the saccade. Although a few neurons showed decrements of > 60% of their peak activity level, the average activity dropped only 16.9%, with some neurons actually showing a rise in activity during the saccade. If we ignored the latency between peak in activity and saccade start and measured the fall in activity for a period equal to one saccade duration after the peak, the average drop in activity was still only 34.9%. Thus the activity of these neurons did not appear to be closely related to dynamic motor error, which falls from its maximum value to zero over the time course of a saccade. 5. These results suggest that a focus of movement activity within the topographic map in the frontal eye field specifies the amplitude and direction for an impending saccade, whereas the peak of movement activity signals the time to initiate a saccade. 6. Unlike the superior colliculus, the activity of frontal eye field movement neurons does not appear to be related to dynamic events that occur during the saccade, such as motor error.

Activity of monkey frontal eye field neurons projecting to oculomotor regions of the pons.

1. This study identified neurons in the rhesus monkey's frontal eye field that projected to oculomotor regions of the pons and characterized the signals sent by these neurons from frontal eye field to pons. 2. In two behaving rhesus monkeys, frontal eye field neurons projecting to the pons were identified via antidromic excitation by a stimulating microelectrode whose tip was centered in or near the omnipause region of the pontine raphe. This stimulation site corresponded to the nucleus raphe interpositus (RIP). In addition, electrical stimulation of the frontal eye field was used to demonstrate the effects of frontal eye field input on neurons in the omnipause region and surrounding paramedian pontine reticular formation (PPRF). 3. Twenty-five corticopontine neurons were identified and characterized. Most frontal eye field neurons projecting to the pons were either movement neurons, firing in association with saccadic eye movements (48%), or foveal neurons responsive to visual stimulation of the fovea combined with activity related to fixation (28%). Corticopontine movement neurons fired before, during, and after saccades made within a restricted movement field. 4. The activity of identified corticopontine neurons was very similar to the activity of neurons antidromically excited from the superior colliculus where 59% had movement related activity, and 22% had foveal and fixation related activity. 5. High-intensity, short-duration electrical stimulation of the frontal eye field caused omnipause neurons to stop firing. The cessation in firing appeared to be immediate, within < or = 5 ms. The time that the omnipause neuron remained quiet depended on the intensity of the cortical stimulus and lasted up to 30 ms after a train of three stimulus pulses lasting a total of 6 ms at an intensity of 1,000 microA. Low-intensity, longer duration electrical stimuli (24 pulses, 75 microA, 70 ms) traditionally used to evoke saccades from the frontal eye field were also followed by a cessation in omnipause neuron firing, but only after a delay of approximately 30 ms. For these stimuli, the omnipause neuron resumed firing when the stimulus was turned off. 6. The same stimuli that caused omnipause neurons to stop firing excited burst neurons in the PPRF. The latency to excitation ranged from 4.2 to 9.8 ms, suggesting that there is at least one additional neuron between frontal eye field neurons and burst neurons in the PPRF. 7. The present study confirms and extends the results of previous work, with the use of retrograde and anterograde tracers, demonstrating direct projections from the frontal eye field to the pons.(ABSTRACT TRUNCATED AT 400 WORDS)

Evaluation of salivary and vaginal electrical resistance for determination of the time of ovulation.

The usefulness of changes in salivary and vaginal electrical resistance (SER and VER) measurements for timing ovulation was evaluated in 15 cycles. A peak in mean SER was observed seven to eight days before the LH peak (five to nine days before the thermal nadir). The nadir of mean VER coincided with the day of maximum LH and was significantly correlated with the day of the thermal nadir. Use of SER peak and the rise of VER from its nadir in a protocol for timing insemination yielded correct timing in 93.3% (14/15) of cycles. These findings substantiate earlier reports indicating that there is a useful relationship between SER and VER trends and ovulation, which may be employed for accurate scheduling of inseminations and other procedures.

The visual and frontal cortices.

The saccadic system uses a muscular apparatus and motor programs that evolved long before the cerebral cortex assumed the dominant role in the generation of behavior that it occupies in the primate. The cortical role in eye movements therefore is to contribute aspects of sophisticated processing to the basic apparatus for rapid eye movements. Thus visual cortex is necessary for the integration of visual motion information into the saccadic system, because the superior colliculus in the primate cannot do adequate motion processing. Similarly, frontal cortex is necessary for performing saccades to remembered stimulus positions, whereas visually driven saccades can be performed by the colliculus alone. To generate saccade-related information, the cortex has activity that reflects all levels of processing, from the registration of the stimulus and the selection of a stimulus for a saccade, to the elaboration of the motor command for the saccade. Presumably the cortex also contains the decision mechanism, whereby a primate decides to make a saccade to a certain stimulus. How, and where that decision is made, or even if the decision occurs at a single place, is totally unknown.

Functional properties of corticotectal neurons in the monkey's frontal eye field.

1. We examined the activity of identified corticotectal neurons in the frontal eye field of awake behaving rhesus monkeys (Macaca mulatta). Corticotectal neurons were antidromically excited using biphasic current pulses passed through monopolar microelectrodes within the superior colliculus. The activity of single corticotectal neurons was studied while the monkey performed behavioral tasks designed to test the relation of the neuron's discharge to visual and oculomotor events. 2. Fifty-one frontal eye field corticotectal neurons were examined in two monkeys. Current thresholds for antidromic excitation ranged from 6 to 1,200 microA, with a mean of 330 microA. Antidromic latencies ranged from 1.2 to 6.0 ms, with a mean of 2.25 ms. 3. Fifty-three percent of the identified corticotectal neurons were classified as having movement-related activity. They had little or no response to visual stimuli, but very strong activity before both visually guided and memory-guided saccades. An additional 6% of corticotectal neurons had visuomovement activity, combining both a visual- and a saccade-related response. In each case, visuomovement neurons antidromically excited from the superior colliculus had movement-related activity, which was much stronger than the visual component of their response. 4. Twenty-two percent of the corticotectal neurons were primarily responsive to visual stimulation of the fovea. These included both neurons responding to the onset and neurons responding to the disappearance of a light flashed on the fovea. 5. The remaining 20% of the corticotectal neurons were a heterogeneous group whose activity could not be classified as movement, visuomovement or foveal. Their responses included postsaccadic, anticipatory, and reward-related activity, as well as activity modulated during certain directions of smooth-pursuit eye movements. One neuron was unresponsive during all of the behavioral tasks used. There were no corticotectal neurons that could be classified as primarily responsive to peripheral visual stimuli. 6. Histological reconstructions of electrode penetrations localized corticotectal neurons to layer V of the frontal eye field. For 22 corticotectal neurons tested, each had its minimum threshold for antidromic excitation within the superior colliculus, as judged by either histological confirmation, or surrounding neuronal responses recorded through the stimulation microelectrode. The majority of these neurons had minimum threshold sites within the intermediate layers, a few minimum threshold sites were located within the superficial or deep collicular layers.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of striate cortex in the guidance of eye movements in the monkey.

We studied the effect of unilateral striate cortical ablations on smooth pursuit and saccadic eye movements in the monkey. The monkeys made quite accurate saccades to stationary stimuli in the field contralateral to the lesion, and they readily pursued foveal targets moving in all directions. However, when visual stimuli were stepped into the field contralateral to the lesion and then began to move, thus insuring that the moving stimulus was confined to the impaired visual hemifield, several oculomotor abnormalities emerged. Saccades to moving stimuli presented in the impaired field consistently undershot targets that moved away from the central fixation point after the step, and overshot targets that moved back towards the central fixation point. There was little or no smooth pursuit eye velocity generated in any direction to moving stimuli in the impaired field, and the monkeys could not generate smooth pursuit to stimuli maintained a few degrees from the fovea in the impaired field, although they were able to pursue such stimuli held in the normal field. Ablation of striate cortex also affected the latencies of saccades. When step-ramp stimuli were presented in the normal field, the monkeys delayed the initiation of saccades to targets moving towards the central fixation point, and hastened the initiation of saccades to targets moving away from the central fixation point. By contrast, changes in the direction of target movement did not affect the latencies of saccades into the impaired field. The deficits seemed permanent, lasting as long as the monkeys were tested--over 2 years in one case--but they were not total. Each monkey could use stimuli moving into the affected field to develop some eye velocity, although this residual ability had a much longer latency and lower gain than that provided by the intact visual system. These results show that striate cortex is intimately involved in the estimation of stimulus velocity critical to the genesis of smooth pursuit and saccadic eye movements.

Visuospatial and motor attention in the monkey.

Visuospatial attention involves the selection of stimuli from the environment for further neural processing. The attention-related enhancement of visual responses in posterior parietal cortex is a possible neural substrate for visuospatial attention. By analogy with the selection process in the spatial domain, motor attention is postulated to involve a selection among simultaneous upper motor signals. Selection of motor programs within the oculomotor system is used as an example of this attentional process. Since attentive fixation modulates the effect on the oculomotor system of electrical stimulation of the frontal eye fields, a given upper motor neuronal signal need not necessarily invoke a movement. That the brain has multiple simultaneous motor signals is apparent from the profusion of sensory-driven upper motor neurons. The frontal cortex is probably important in selecting which upper motor signals actually evoke movements, by elaborating motor programs for purposive behavior, but not for all movements.

Comparison of the distributions of ipsilaterally and contralaterally projecting corticocortical neurons in cat visual cortex using two fluorescent tracers.

Using the retrograde fluorescent tracers Fast Blue and Diamidino Yellow we have studied the callosal and ipsilateral corticocortical connections between the cat's area 17/18 border region and the posteromedial lateral suprasylvian visual area (PMLS), as well as the callosal connections of each of these regions with its contralateral homologue. The main goal was to determine whether single cortical neurons project with branching axons to more than one cortical target. In addition, the double-labeling technique enabled us to examine, within a single section of cortical tissue, the relative distributions of neurons with different cortical targets. Most corticocortical neurons labeled in the area 17/18 border region and in area PMLS projected to only one of the cortical injection sites tested. When two callosal neuron types were labeled in the same area, no double-labeled neurons were found. When ipsilateral corticocortical and callosal neurons were labeled in combination, a few double-labeled neurons were found in both cortical regions examined. The most common type of double-labeled neuron was located in area PMLS and projected bilaterally to the area 17/18 border region. Our findings regarding the laminar distributions of ipsi- and contralaterally projecting neurons are in agreement with previous studies. In addition, we have found that, for callosal neurons within the upper layers of areas 17 and 18, neurons projecting to the contralateral area 17/18 border are located in the lower half of layer II/III and in upper layer IV, whereas neurons projecting to contralateral area PMLS are restricted to the lower portion of layer II/III. In addition, for callosal neurons within the deep layers of area PMLS, neurons projecting to contralateral area PMLS are located throughout layers V and VI, whereas neurons projecting to the contralateral area 17/18 border are restricted to layer VI. There are numerous other possible targets for axon collaterals not examined in this paper. However, the scarcity of neurons with multiple projections demonstrated in this study reflects the high degree of specificity of cortical connectivity. This anatomical organization may be the basis for a precise channeling of differential information at the single neuron level.

The distribution of the cells of origin of callosal projections in cat visual cortex.

The distribution of neurons projecting through the corpus callosum (callosal neurons) was examined in retinotopically defined areas of cat visual cortex. As many callosal neurons as possible were labeled in a single animal by surgically dividing the posterior two-thirds of the corpus callosum and exposing the cut ends of callosal axons to horseradish peroxidase. The distribution of callosal neurons within a visual field representation was related to standard electrophysiological maps as well as to recording sites marked by electrolytic lesions. Callosal neurons were found in every retinotopically defined cortical area. The portion of the visual field representation that contained callosal neurons increased progressively from the area 17/18 border to area 19, to areas 20 and 21, and to the lateral suprasylvian visual areas. In area 17, the portion of the visual field representation containing callosal neurons extended from the vertical meridian out to a maximum of 10 degrees azimuth. In the posteromedial lateral suprasylvian visual area, callosal neurons were present in a region extending from the vertical meridian representation out to a representation of 60 degrees azimuth. Most callosal neurons were medium to large pyramids at the border of layers III and IV. A few layer IV stellates were among the callosal neurons of areas 17 and 18. In area 19 and even more so in the lateral suprasylvian visual areas, callosal neurons included pyramidal and fusiform-shaped cells in layers V and VI. The laminar distributions of callosal neurons in areas 20 and 21 were similar to those of area 19 and the lateral suprasylvian visual areas. The widespread distribution of callosal neurons in areas 20 and 21 and in the lateral suprasylvian visual areas suggests that the regions of peripheral visual field representation in cat cortex, as well as the representations of the vertical meridian, have access to the opposite cerebral hemisphere. This finding is significant in light of demonstrations of the importance of some of these cortical areas in the interhemispheric transfer of visual learning.

The afferent and efferent callosal connections of retinotopically defined areas in cat cortex.

We compared the callosal afferent and efferent connections of different retinotopic loci within a given visual cortical area as well as the connectivity patterns among similar retinotopic loci in different visual areas. Small injections (75 nl) of a mixture of horseradish peroxidase and [3H]leucine were made through a recording pipette at injection sites identified by retinotopic mapping. A small locus of cortex within a callosally connected region had precise reciprocal connections with the homotopic locus in the contralateral hemisphere. This small locus also was callosally connected with a variable number of heterotopic loci. Both reciprocal and nonreciprocal heterotopic callosal connections were found. Homotopic and heterotopic connections appeared to have a high degree of retinotopic fidelity. Precisely homotopic connections were present not only between locations on the vertical meridian representations at the left and right area 17/18 borders but also, for example, between mirror-symmetrical points on the peripheral horizontal meridian representation in the left and right posteromedial lateral suprasylvian areas. In several experiments, we found that both callosal neurons and terminals in the homotopic cortex were grouped into two to three distinct clusters ranging from 600 to 900 micrometers in width. Callosal neurons with homotopic connections were primarily pyramidal cells in lower layer III and upper layer IV. Outside of areas 17 and 18, there was a significant number of pyramidal and fusiform-shaped callosal neurons in layers V and VI. The majority of callosal terminals were located in layers II, III, and IV.