Behavior of luminance neurons in the pretectal olivary nucleus during the pupillary near response

In cats and monkeys, extrastriate visual areas that have been reported to be involved in the near triad of pupilloconstriction, convergence, and accommodation have well-defined projections to the pretectal olivary nucleus (PON), the retinorecipient pretectal nucleus mediating the pupillary light reflex in mammals. We have therefore used alert, behaving primates to investigate the possibility that PON neurons are involved in the pupillary near response in addition to the pupillary light reflex. Single-unit recording revealed that PON luminance neurons significantly increased their firing rate with increases in retinal illumination and the resultant pupilloconstriction. In contrast, their activity did not significantly increase during pupilloconstriction elicited by near viewing. Thus the behavior of PON luminance neurons is appropriate for their participation in the pupillary light reflex, but is inappropriate for any proposed role in the pupillary near response. This result strongly suggests that neurons in the primate PON are solely related to the pupillary light reflex and that the cortical projections to this pretectal nucleus are related to this reflex and do not play a role in the pupillary near response.     

Both striate cortex and superior colliculus contribute to visual properties of neurons in superior temporal polysensory area of macaque monkey

Although the tectofugal system projects to the primate cerebral cortex by way of the pulvinar, previous studies have failed to find any physiological evidence that the superior colliculus influences visual activity in the cortex. We studied the relative contributions of the tectofugal and geniculostriate systems to the visual properties of neurons in the superior temporal polysensory area (STP) by comparing the effects of unilateral removal of striate cortex, the superior colliculus, or of both structures. In the intact monkey, STP neurons have large, bilateral receptive fields. Complete unilateral removal of striate cortex did not eliminate visual responses of STP neurons in the contralateral visual hemifield; rather, nearly half the cells still responded to visual stimuli in the hemifield contralateral to the lesion. Thus the visual properties of STP neurons are not completely dependent on the geniculostriate system. Unilateral striate lesions did affect the response properties of STP neurons in three ways. Whereas most STP neurons in the intact monkey respond similarly to stimuli in the two visual hemifields, responses to stimuli in the hemifield contralateral to the striate lesion were usually weaker than responses in the ipsilateral hemifield. Whereas the responses of many STP neurons in the intact monkey were selective for the direction of stimulus motion or for stimulus form, responses in the hemifield contralateral to the striate lesion were not selective for either motion or form. Whereas the median receptive field in the intact monkey extended 80 degrees into the contralateral visual field, the receptive fields of cells with responses in the contralateral field that survived the striate lesions had a median border that extended only 50 degrees into the contralateral visual field. Removal of both striate cortex and the superior colliculus in the same hemisphere abolished the responses of STP neurons to visual stimuli in the hemifield contralateral to the combined lesion. Nearly 80% of the cells still responded to visual stimuli in the hemifield ipsilateral to the lesion. Unilateral removal of the superior colliculus alone had only small effects on visual responses in STP. Receptive-field size and visual response strength were slightly reduced in the hemifield contralateral to the collicular lesion. As in the intact monkey, selectivity for stimulus motion or form were similar in the two visual hemifields. We conclude that both striate cortex and the superior colliculus contribute to the visual responses of STP neurons. Striate cortex is crucial for the movement and stimulus specificity of neurons in STP.(ABSTRACT TRUNCATED AT 400 WORDS)     

Characteristics of the pupillary light reflex in the macaque monkey: discharge patterns of pretectal neurons

Anatomical and physiological data have implicated the pretectal olivary nucleus (PON) as the midbrain relay for the pupillary light reflex in a variety of species. To determine the nature of the discharge of pretectal light reflex relay neurons, we recorded their activity in monkeys that were fixating a stationary spot while a full-field random-dot stimulus was flashed on for 1 s. Based on their discharge patterns, neurons in or near the PON came in two varieties. The most prevalent neuron discharged a burst of spikes 56 ms (on average) after the light came on followed by a sustained rate for the duration of the stimulus (burst-sustained neurons). When the light went off, nearly all neurons (33/34) ceased firing, and then all the neurons with a resting response in the dark (n = 15) resumed firing. Both the firing rate within the burst and the sustained discharge rate increased with log light intensity and the latency of the burst decreased. The burst and cessation of firing were better aligned with the stimulus occurrence than with the onset of pupillary constriction or dilation. Taken together, these data suggest that burst-sustained neurons respond to the visual stimulus eliciting the pupillary change rather than dictating the metrics of the subsequent pupillary response. Electrical stimulation at the site of four of five burst-sustained neurons elicited pupillary constriction at low stimulus strengths after a latency of approximately 100 ms. When the electrode was moved 250 microm away from the burst-sustained neuron, the elicited response disappeared. Reconstructions of the locations of burst-sustained luminance neurons place them in the PON or its immediate vicinity. We suggest that PON burst-sustained neurons constitute the pretectal relay for the pupillary light reflex. A minority of our recorded pretectal neurons discharged a burst of spikes at both light onset and light offset. For most of these transient neurons, neither the burst rate nor the interburst rate was significantly related to light intensity. We conclude that these neurons are not involved in the light reflex but subserve some other pretectal function.     

Luminance neurons in the pretectal olivary nucleus mediate the pupillary light reflex in the rhesus monkey

In humans and other primates, an increase in luminance in either eye elicits bilateral pupilloconstriction that is essentially equal in both eyes. Current models of the neural substrate for this clinically important light reflex propose that a retinorecipient pretectal nucleus projects bilaterally to the Edinger-Westphal nucleus (EW), which contains the parasympathetic, preganglionic neurons controlling pupilloconstriction. Based on single-unit recording studies in anesthetized cats and rats, it has been further suggested that luminance neurons in only one pretectal nucleus, the pretectal olivary nucleus, mediate this reflex. However, to our knowledge, there have been no comparable electrophysiological studies in primates of the pupillary light reflex or the pretectal luminance neurons that mediate this reflex. To address this issue, single-unit recording and electrical microstimulation studies were carried out in the pretectum of alert, trained, rhesus monkeys. These studies demonstrated that the primate pretectum contains luminance neurons with the characteristics appropriate for mediating the pupillary light reflex and that these neurons are located in one retinorecipient pretectal nucleus, the pretectal olivary nucleus. Electrical microstimulation at the site of these neurons often elicited pupilloconstriction. Our results provide clear evidence for the involvement of the pretectum, and more specifically the pretectal olivary nucleus, in mediating the pupillary light reflex in primates.     

Lithium alters mu-opioid receptor expression in the rat brain

The pretectal nucleus lentiformis mesencephali (LM) receives direct input from the contralateral retina and is dedicated to the analysis of optic flowfields resulting from self-motion. The activity of 126 LM neurons in response to optic flow stimuli was recorded. As with previous studies, it was found that most neurons (approximately 90%) exhibited direction-selectivity to large-field stimuli moving in the contralateral hemifield. However, some neurons (approximately 10%) responded to stimulation of both eyes and had receptive field structures conducive for detection of particular patterns of optic flow resulting from either self-translation or self-rotation. These binocular neurons were maximally responsive to panoramic optic flowfields simulating either translational or rotational optic flow.     

Central core control of developmental plasticity in the kitten visual cortex: I. Diencephalic lesions

Summary In five, dark-reared, 4-week-old kittens the posterior two thirds of the corpus callosum were split, and a lesion comprising the intralaminar nuclei was made of the left medial thalamic complex. In addition, the right eye was closed by suture. Postoperatively, the kittens showed abnormal orienting responses, neglecting visual stimuli presented in the hemifield contralateral to the side of the lesion. Sudden changes in light, sound, or somatosensory stimulation elicited orienting responses that all tended toward the side of the lesion. These massive symptoms faded within a few weeks but the kittens continued to neglect visual stimuli in the hemifield contralateral to the lesion when a second stimulus was presented simultaneously in the other hemifield. Electrophysiologic analysis of the visual cortex, performed after the end of the critical period, revealed marked interhemispheric differences. In the visual cortex of the normal hemisphere most neurons were monocular and responded exclusively to stimulation of the open eye, but otherwise had normal receptive field properties. In the visual cortex of the hemisphere containing the thalamic lesion, the majority of the neurons remained binocular. In addition, the selectivity for stimulus orientation and the vigor of responses to optimally aligned stimuli were subnormal on this side. Thus, the same retinal signals, which in the control hemisphere suppressed the pathways from the deprived eye and supported the development of normal receptive fields, failed to do either in the hemisphere containing the thalamic lesion. Apparently, experience-dependent changes in the visual cortex require both retinal stimulation and the functioning of diencephalic structures which modulate cortical excitability and control selective attention.     

Response characteristics of neurons in the pulvinar of awake cats to saccades and to visual stimulation

We studied responses of pulvinar neurons in awake cats that were allowed to execute spontaneous eye movements. Extracellular cell activity during saccades, saccade-like image shifts, and various stationary visual stimuli was recorded together with the animals' eye positions. All neurons analyzed had receptive fields that covered most of the central 80x80 degrees of the animals' visual field and did only respond to large (>20 degrees) visual stimuli. According to their response properties, recorded neurons were divided into three populations. The first group, termed "S neurons" (16%), responded when the animals performed saccades but were unresponsive to any of the visual stimuli tested. These neurons do not seem to receive a visual input that is strong enough to drive them. The second group, termed "V neurons" (51%), responded to various visual stimuli including saccade-like image motion when the eyes were stationary, but not when the animals executed saccades. V neurons therefore distinguish retinal image movements that are generated externally from internally generated image motion. Finally, "SV neurons" (31%) responded when the animals made saccades as well as to saccade-like image motion or to stationary stimuli. Although these neurons do not distinguish self-induced retinal image motion from motion generated by external stimulus movements, they must receive non-retinal motion-related input, because responses elicited by saccades had shorter latencies than responses to saccade-like stimulus movements. Only SV neurons resemble response properties of pretectal neurons that project to the pulvinar and that comprise the major subcortical visual input. The functional significance of pulvinar neuronal populations for visual and visuomotor information processing is discussed.     

Midbrain connections of the olivary pretectal nucleus in the marmoset (<Emphasis Type="Italic">Callithrix jacchus</Emphasis>): implications for the pupil light reflex pathway

Midbrain projections of the pretectal olivary nucleus (PON) were studied in the marmoset, a New World primate. The fluorescent retrograde tracers Fluoro-Gold (FG) and Fast Blue (FB) were injected into the Edinger-Westphal (EW) nucleus and the lateral terminal nucleus (LTN), respectively. EW nucleus injections resulted in retrograde labeling of significant numbers of FG-positive neurons of the PON as well as a small number of cells in the LTN. LTN injections led to labeling of a population of singly-labeled cells seen dispersed through the larger population of FG-labeled somata within the contralateral PON. The ipsilateral PON was devoid of FB-labeled somata, whereas the adjacent nucleus of the optic tract (NOT) contained FB-labeled cells. These findings show that a large number of PON neurons project directly to the oculomotor complex. Additionally, the study shows the presence of a separate population of PON neurons projecting to the contralateral LTN. This, combined with our earlier observation that LTN neurons project to the EW nucleus in the marmoset (see main text for reference), lends support to the presence of separate direct and indirect pupillary light reflex pathways from the PON to the nucleus of EW.     

Discharge patterns of neurons in the pretectal nucleus of the optic tract (NOT) in the behaving primate

1. To determine the possible role of the primate pretectal nucleus of the optic tract (NOT) in the generation of optokinetic and smooth-pursuit eye movements, we recorded the activity of 155 single units in four behaving rhesus macaques. The monkeys were trained to fixate a stationary target spot during visual testing and to track a small moving spot in a variety of visual environments. 2. The majority (82%) of NOT neurons responded only to visual stimuli. Most units responded vigorously for large-field (70 x 50 degrees) moving visual stimuli and responded less, if at all, during smooth-pursuit eye movements in the dark; many of these units had large receptive fields (greater than 10 x 10 degrees) that included the fovea. The remaining visual units responded more vigorously during smooth-pursuit eye movements in the dark than during movement of large-field visual stimuli; all but one had small receptive fields (less than 10 x 10 degrees) that included the fovea. For all visual units that responded during smooth pursuit, extinction of the small moving target so briefly that pursuit continued caused the firing rates to drop to resting levels, confirming that the discharge was due to visual stimulation of receptive fields with foveal and perifoveal movement sensitivity and not to smooth-pursuit eye movements per se. 3. Eighteen percent of all NOT units ceased their tonic discharge in association with all saccades including the quick phases accompanying optokinetic or vestibular nystagmus. The pause in firing began after saccade onset, was unrelated to saccade duration, and occurred even in complete darkness. 4. Most (90%) of the visual NOT units were direction selective. They exhibited an increase in firing above resting during horizontal (ipsilateral) background movement and/or during smooth pursuit of a moving spot and a decrease in firing during contralateral movement. 5. The firing rates of NOT units were highly dependent on stimulus velocity. All had velocity thresholds of less than 1 degree/s and exhibited a monotonic increase in firing rate with visual stimulus velocity over part (n = 90%) or all (n = 10%) of the tested range (i.e., 1-200 degrees/s). Most NOT units exhibited velocity tuning with an average preferred velocity of 64 degrees/s.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effect of lesions to cortical areas V4 or AIT on pupillary responses to chromatic and achromatic stimuli in monkeys

We measured the pupillary response to achromatic and chromatic grating stimuli in left and right visual hemifields of two rhesus monkeys, who were trained to fixate the centre of a screen. After removing the rostral inferior temporal cortex of one hemisphere, the response to chromatically modulated gratings in the contralateral hemifield was abolished, whereas the response to the luminance modulated grating was unaffected. In one of the monkeys, in which area V4 of the other hemisphere was also removed, there was no effect on the pupillary response to either kind of grating presented in the hemifield contralateral to the V4 lesion. The results show that the cortical contribution to the response of the pupil to purely chromatic changes is mediated by rostral temporal cortex, not by area V4.     

Position selectivity in scene- and object-responsive occipitotemporal regions

Complex visual scenes preferentially activate several areas of the human brain, including the parahippocampal place area (PPA), the retrosplenial complex (RSC), and the transverse occipital sulcus (TOS). The sensitivity of neurons in these regions to the retinal position of stimuli is unknown, but could provide insight into their roles in scene perception and navigation. To address this issue, we used functional magnetic resonance imaging (fMRI) to measure neural responses evoked by sequences of scenes and objects confined to either the left or right visual hemifields. We also measured the level of adaptation produced when stimuli were either presented first in one hemifield and then repeated in the opposite hemifield or repeated in the same hemifield. Although overall responses in the PPA, RSC, and TOS tended to be higher for contralateral stimuli than for ipsilateral stimuli, all three regions exhibited position-invariant adaptation, insofar as the magnitude of adaptation did not depend on whether stimuli were repeated in the same or opposite hemifields. In contrast, object-selective regions showed significantly greater adaptation when objects were repeated in the same hemifield. These results suggest that neuronal receptive fields (RFs) in scene-selective regions span the vertical meridian, whereas RFs in object-selective regions do not. The PPA, RSC, and TOS may support scene perception and navigation by maintaining stable representations of large-scale features of the visual environment that are insensitive to the shifts in retinal stimulation that occur frequently during natural vision.     

Area MT neurons respond to visual motion distant from their receptive fields

Neurons in cortical area MT have localized receptive fields (RF) representing the contralateral hemifield and play an important role in processing visual motion. We recorded the activity of these neurons during a behavioral task in which two monkeys were required to discriminate and remember visual motion presented in the ipsilateral hemifield. During the task, the monkeys viewed two stimuli, sample and test, separated by a brief delay and reported whether they contained motion in the same or in opposite directions. Fifty to 70% of MT neurons were activated by the motion stimuli presented in the ipsilateral hemifield at locations far removed from their classical receptive fields. These responses were in the form of excitation or suppression and were delayed relative to conventional MT responses. Both excitatory and suppressive responses were direction selective, but the nature and the time course of their directionality differed from the conventional excitatory responses recorded with stimuli in the RF. Direction selectivity of the excitatory remote response was transient and early, whereas the suppressive response developed later and persisted after stimulus offset. The presence or absence of these unusual responses on error trials, as well as their magnitude, was affected by the behavioral significance of stimuli used in the task. We hypothesize that these responses represent top-down signals from brain region(s) accessing information about stimuli in the entire visual field and about the behavioral state of the animal. The recruitment of neurons in the opposite hemisphere during processing of behaviorally relevant visual signals reveals a mechanism by which sensory processing can be affected by cognitive task demands.     

Midbrain connections of the olivary pretectal nucleus in the marmoset (Callithrix jacchus): implications for the pupil light reflex pathway

Midbrain projections of the pretectal olivary nucleus (PON) were studied in the marmoset, a New World primate. The fluorescent retrograde tracers Fluoro-Gold (FG) and Fast Blue (FB) were injected into the Edinger-Westphal (EW) nucleus and the lateral terminal nucleus (LTN), respectively. EW nucleus injections resulted in retrograde labeling of significant numbers of FG-positive neurons of the PON as well as a small number of cells in the LTN. LTN injections led to labeling of a population of singly-labeled cells seen dispersed through the larger population of FG-labeled somata within the contralateral PON. The ipsilateral PON was devoid of FB-labeled somata, whereas the adjacent nucleus of the optic tract (NOT) contained FB-labeled cells. These findings show that a large number of PON neurons project directly to the oculomotor complex. Additionally, the study shows the presence of a separate population of PON neurons projecting to the contralateral LTN. This, combined with our earlier observation that LTN neurons project to the EW nucleus in the marmoset (see main text for reference), lends support to the presence of separate direct and indirect pupillary light reflex pathways from the PON to the nucleus of EW.Abbreviations PON Pretectal olivary nucleus - NOT Nucleus of the optic tract - NPC Nucleus of the posterior commissure - EW Edinger-Westphal (nucleus) - LTN Lateral terminal nucleus - FG Fluoro-Gold - FB Fast Blue - PAG Periaqueductal gray - AON Accessory oculomotor nucleiResearch supported by grants awarded to RJC through the Brazilian Councils for Science and Technology (CNPq and FINEP).     

Responses of primate visual cortical V4 neurons to simultaneously presented stimuli

We report here results from 45 primate V4 visual cortical neurons to the preattentive presentations of seven different patterns located in two separate areas of the same receptive field and to combinations of the patterns in the two locations. For many neurons, we could not determine any clear relationship for the responses to two simultaneous stimuli. However, for a substantial fraction of the neurons we found that the firing rate was well modeled as the maximum firing rate of each stimulus presented separately. It has previously been proposed that taking the maximum of the inputs ("MAX" operator) could be a useful operation for neurons in visual cortex, although there has until now been little direct physiological evidence for this hypothesis. Our results here provide direct support for the hypothesis that the MAX operator plays a significant (although certainly not exclusive) role in generating the receptive field properties of visual cortical neurons.     

Binocular single vision and depth discrimination. Receptive field disparities for central and peripheral vision and binocular interaction on peripheral single units in cat striate cortex

Summary Of binocularly-activated striate neurons only a proportion have their two receptive fields in exactly corresponding positions in the contralateral hemifield. Those which are not corresponding are said to show receptive field disparity. Because the eyes diverge in the anaesthetized and paralyzed preparation, the binocular receptive fields are horizontally separate. With increasing retinal eccentricity there is a gradual decrease in this horizontal separation as well as progressive changes in the local receptive field disparities. With increasing horizontal retinal eccentricity there is a progressive increase in horizontal receptive field disparities together with a smaller decrease in vertical disparities. Receptive field disparities are relatively unaffected by increasing vertical retinal eccentricity. A neurophysiological theory for binocular single vision and depth discrimination is put forward as a theoretical framework for the construction of the horopter for the cat as well as a region analogous to Panum's fusional area in man. Observations have been made on the responses, particularly to moving slit stimuli, of units with peripherally-located receptive fields. For several binocular units it was possible to study the full range of the binocular interaction when the two receptive fields were moved from exact correspondence to positions of increasing non-alignment.     

Luminance detectors in the olivary pretectal nucleus and their relationship to the pupillary light reflex in the rat. II. Studies using sinusoidal light

Summary The luminance detectors in the olivary pretectal nucleus, which are likely candidates mediating the pupillary light reflex, responded to all frequencies of sinusoidally modulated light up to 12–25 Hz. At low frequencies (0.05–4.0 Hz) the luminance detectors responded with modulated firing to different stimulation rates. The modulation depth of the cell response increased with the increase in stimulation frequency up to 20 Hz, then rapidly fell. There was a delay between the peak intensity of the stimulus and the peak firing of cell response of about 30–40 ms. The amplitudes of the consensual pupil responses to the same sinusoidal stimulus, on the other hand, decreased with an increase in frequency and no discernible response was recorded above 2 Hz. The pupil responses were little affected by sympathectomy. The differences in the frequency response characteristics of luminance detectors and the pupil were attributed to the sluggish dynamic properties of the pupil muscles. This was demonstrated using an electronic model of the iris muscle which modified the responses of the luminance detectors giving output waveforms broadly resembling pupil responses to square and sinusoidally modulated lights.The work was supported by a grant from the Special Trustees of St. Thomas' Hospital to H.I.; R.J.C. was a holder of a studentship of the Science Research Council of Great Britain (SERC)     

Primate nucleus gracilis neurons: responses to innocuous and noxious stimuli

1. Recordings were made from 67 neurons in the nucleus gracilis (NG) of anesthetized macaque monkeys. All of the cells were activated antidromically from the ventral posterior lateral (VPL) nucleus of the contralateral thalamus. Stimuli used to activate the cells orthodromically were graded innocuous and noxious mechanical stimuli, including sinusoidal vibration and thermal pulses. 2. The latencies of antidromic action potentials following stimulation in the VPL nucleus were significantly shorter for cells in the caudal compared with the rostral NG. The mean minimum afferent conduction velocity of the afferent conduction velocity of the afferent fibers exciting the NG cells was 52 m/s, as judged from the latencies of the cells to orthodromic volleys evoked by electrical stimulation of peripheral nerves. The overall conduction velocity of the pathway from peripheral nerve to thalamus was approximately 40 m/s. 3. Cutaneous receptive fields on the distal hindlimb usually occupied an area equivalent to much less than a single digit. However, a few cells had receptive fields up to or exceeding the area of the foot. 4. NG cells were classified by their responses to graded mechanical stimulation of the skin as low threshold (LT) or wide dynamic range (WDR). No high-threshold NG cells were found. A special subcategory of pressure-sensitive LT (SA) neurons was recognized. Many of these cells were maximally responsive to maintained indentation of the skin. The sample of NG cells differed from the population of primate spinothalamic and spinocervicothalamic pathways so far examined, in having a larger proportion of LT neurons and a smaller proportion of WDR cells. A few NG cells responded best to manipulation of subcutaneous tissue. 5. Discriminant analysis permitted the NG cells to be assigned to classes determined by a k-means cluster analysis of the responses of a reference set of 318 primate spinothalamic tract (STT) cells. There were four classes of cells based on normalized responses of individual neurons and another four classes based upon responses compared across the population of cells. The NG cells were allocated to the various categories in different proportions than either primate STT cells or spinocervicothalamic neurons, consistent with the view that the functional roles of these somatosensory pathways differ. 6. Some of the pressure-sensitive NG cells were excited when the skin was stretched, suggesting an input from type II slowly adapting (Ruffini) mechanoreceptors.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interhemispheric interactions and redundancy gain: tests of an interhemispheric inhibition hypothesis

In simple reaction time (RT) tasks, responses are faster when stimuli are presented to both the left and right visual hemifields than when a stimulus is presented to a single hemifield. Paradoxically, this redundancy gain with bilateral stimuli is enhanced in split-brain individuals relative to normals. This article reports three experiments testing an account of that enhancement in which normals' responses to bilateral stimuli are slowed by interhemispheric inhibition. In simple RT tasks, normal participants responded bimanually to left, right, or bilateral visual stimuli. In choice RT tasks, they responded to each stimulus with one hand, responding bimanually only when both stimuli were presented. Measurements of response forcefulness (Experiment 1) and electroencephalographic activity (Experiments 2 and 3) showed no evidence of the correlation patterns predicted by the hypothesis of interhemispheric inhibition. The results suggest that such inhibition is unlikely to be the explanation for enhanced redundancy gain in split-brain individuals.     

Neuronal activity related to visually guided saccades in the frontal eye fields of rhesus monkeys: comparison with supplementary eye fields.

1. The purpose of this study was to analyze the response properties of neurons in the frontal eye fields (FEF) of rhesus monkeys (Macaca mulatta) and to compare and contrast the various functional classes with those recorded in the supplementary eye fields (SEF) of the same animals performing the same go/no-go visual tracking task. Three hundred ten cells recorded in FEF provided the data for this investigation. 2. Visual cells in FEF responded to the stimuli that guided the eye movements. The visual cells in FEF responded with a slightly shorter latency and were more consistent and phasic in their activation than their counterparts in SEF. The receptive fields tended to emphasize the contralateral hemifield to the same extent as those observed in SEF visual cells. 3. Preparatory set cells began to discharge after the presentation of the target and ceased firing before the saccade, after the go/no-go cue was given. These neurons comprised a smaller proportion in FEF than in SEF. In contrast to their counterparts in SEF, the preparatory set cells in FEF did not respond preferentially in relation to contralateral movements, even though most responded preferentially for movements in one particular direction. The time course of the discharge of the FEF set cells was similar to that of their SEF counterparts, except that they reached their peak level of activation sooner. The few preparatory set cells in FEF tested with both auditory and visual stimuli tended to respond preferentially to the visual targets, whereas, in contrast, most set cells in SEF were bimodal. 4. Sensory-movement cells represented the largest population of cells recorded in FEF, responding in relation to both the presentation of the targets and the execution of the saccade. Although some of these sensory-movement cells resembled their counterparts in SEF by exhibiting a sustained elevation of activity, most of the FEF sensory-movement cells gave two discrete bursts, one after the presentation of the target and another before and during the saccade. Like their counterparts in SEF, the sensory-movement cells tended to be tuned for saccades into the contralateral hemifield, but this tendency was more pronounced in FEF than in SEF. The FEF sensory-movement cells discharged more briskly, with a shorter latency relative to the presentation of the target, than their counterparts in SEF. In addition, the FEF sensory-movement neurons reached their peak activation sooner than SEF sensory-movement neurons. Most FEF sensory-movement cells exhibited different patterns of activation in response to visual and auditory targets.(ABSTRACT TRUNCATED AT 400 WORDS)     

The ultrastructure of the olivary pretectal nucleus in rats. A tracing and GABA immunohistochemical study

 The olivary pretectal nucleus is a primary visual centre, involved in the pupillary light reflex. In the present study an ultrastructural analysis was made of the olivary pretectal nucleus by means of separate, anterograde and retrograde tracing techniques and immunohistochemistry of gamma-aminobutyric acid. Large-projection neurons and two types of gamma-aminobutyric acid-immunoreactive (GABA-ir) neurons are observed in the olivary pretectal nucleus. The primary dendrites of the projection neurons have a dichotomous appearance, the secondary dendrites a multipolar appearance. At the ultrastructural level the projection neurons have well-developed Golgi fields, abundant rough endoplasmic reticulum and the nucleus is always heavily indented. Numerous small GABA-ir neurons and a few medium-sized GABA-ir neurons are found. The small GABA-ir neurons contain a few stacks of rough endoplasmic reticulum and the nucleus is oval-shaped. The medium-sized GABA-ir neurons have well-developed Golgi fields, a moderate number of rough endoplasmic reticulum stacks and an indented nucleus. GABA-positive dendritic profiles containing vesicles also are observed. In the neuropil of the olivary pretectal nucleus, retinal terminals are found that contain round clear vesicles and electron-lucent mitochondria. They make asymmetric synaptic contacts (Gray type I) with dendritic profiles and with profiles containing vesicles. Terminals originating from the contralateral olivary pretectal nucleus exhibit small, round clear vesicles, electron-dense mitochondria and make asymmetric synaptic contacts (Gray type I) mainly with dendritic profiles. Two types of GABA-ir terminals were found. One type is incorporated in glomerulus-like arrangements, whereas the other type is not. GABA-ir terminals contain pleomorphic vesicles, electron-dense mitochondria and make symmetric synaptic contacts (Gray type II). Retinal terminals, terminals originating from the contralateral olivary pretectal nucleus and GABA-ir terminals are organized in glomerulus-like structures, in which dendrites of the large projection neurons form the central elements. Triadic arrangements are observed in these structures; a retinal terminal contacts a dendrite and a GABA-ir terminal and the GABA-ir terminal also contacts the dendrite. The complexity of the synaptic organization and the abundancy of inhibitory elements in the olivary pretectal nucleus suggest that the olivary pretectal nucleus is strongly involved in processing visual information in the pupillary light reflex arc. Received: 17 July 1996 / Accepted: 24 September 1996