Morphology,response properties,and collateral projections of trigeminothalamic neurons in brainstem subnucleus interpolaris of rat

Summary Intracellular recording, electrical stimulation and horseradish peroxidase (HRP) injection techniques were used to delineate the structural and functional characteristics of trigeminothalamic projection neurons in subnucleus interpolaris of the trigeminal brainstem nuclear complex in rat. Eleven such neurons were successfully characterized and recovered. All were medium to large multipolar neurons in the ventral part of interpolaris and all except one also projected to the superior colliculus. Six of these cells also sent axon collaterals to subnucleus principalis and the medullary parvicellular reticular formation and had local collaterals within interpolaris. None of these trigeminothalamic cells were antidromically activated from the cerebellum. All but one of the recovered cells were responsive to deflection of any one of a number (4–19) of vibrissae. The remaining cell was discharged by displacement of mystical guard hairs. Analysis of electrophysiological and anatomical data revealed significant correlations between receptive field size and dendritic area, thalamic conduction latency and axon diameter, and number of targets innervated and axon diameter.     

Projection from dorsal column nuclei to dorsal mesencephalon

This study investigated the projection from the dorsal column nuclei (DCN) to the dorsal mesencephalon. Single-unit extracellular recordings were obtained from the DCN of alpha-chloralose anesthetized cats. Neurons were identified by standard antidromic stimulation criteria as projecting to the dorsal mesencephalon (M neurons), the diencephalon (D neurons), or to both regions (MD neurons). Fifty-two neurons could be antidromically activated from the dorsal mesencephalon. Of these, 31 could also be antidromically activated by stimulation in the diencephalon. An additional 34 neurons were studied that could be antidromically activated only from the diencephalon. Stimulation sites within the dorsal mesencephalon effective in antidromically activating M and MD neurons were in the caudal ventrolateral superior colliculus, the intercollicular area, and external nucleus of the inferior colliculus. Effective diencephalic stimulation sites were in the ventroposterolateral nucleus, the zona incerta, and the magnocellular division of the medial geniculate. The antidromic latencies to stimulation in the dorsal mesencephalon of M and MD neurons spanned a similar but wide range of values in contrast to the latencies to stimulation in the diencephalon of D neurons which were all short. Conduction velocities along the mesencephalic and diencephalic collaterals of MD neurons were similar. Many of the neurons projecting to the mesencephalon had receptive fields located proximally on the body. Most of the neurons had rapidly adapting responses to low-intensity mechanical stimulation of the skin. The major difference between the mesencephalic M and MD projection neurons and diencephalic projection D neurons was the larger percentage of neurons having proximal receptive fields in the former group. These findings are the first electrophysiological demonstration of a direct somatosensory input to the dorsal mesencephalon arising in the DCN. This input is probably responsible for providing some of the somatosensory input to the deeper layers of the superior colliculus, the external nucleus of the inferior colliculus, and the intercollicular area, regions known to have neurons responding to somatosensory stimuli.     

Topographic organization of somatosensory corticotectal influences in cat

Using electrophysiological techniques, the present study demonstrated that substantial direct somatosensory cortical influences on the superior colliculus (SC) originate from three areas: a) SIV, b) para-SIV (the cortex adjacent to SIV but deeper in the anterior ectosylvian sulcus (AES) and for which no topography has yet been described), and c) the rostral suprasylvian sulcus. Influences also appeared to originate from SI and SII, but these may have been indirect. Detailed examination of the AES revealed that these corticotectal projections are topographically organized, and stimulation of a given cortical locus was observed to affect only those cells in the SC whose receptive fields overlapped those of cells at the stimulation site. A similar receptive-field register was found between the suprasylvian sulcus and the SC. Within this topographic pattern, considerable convergence was evident and an individual SC cell could be influenced from a surprisingly large cortical area. This was particularly evident within the representation of the forelimb. Thus, an SC cell with a receptive field covering the forelimb and paw could receive convergent input from many cortical cells with receptive fields covering all or restricted portions of this body region. Considerable corticotectal divergence also was observed within this general topographic scheme. For example, a given corticotectal site representing the digits sent projections to many different SC cells that included the digits within their receptive fields. These data are more consistent with a block-to-block than a point-to-point corticotectal projection. Somatosensory corticotectal projections excited only those SC cells that could also be activated by peripheral somatosensory stimuli. Similarly, the caudal AES, which contains auditory cells, excited only those SC cells activated also by peripheral auditory stimuli. Yet convergent influences from both auditory and somatosensory regions of the AES were observed in the SC cells that could be activated by both auditory and somatosensory stimuli. These data indicate that the AES is a major source of excitatory input to cells of the deep laminae of the SC. Since it is these deep laminae cells that project to premotor regions of the brain stem and the spinal cord, it is reasonable to suppose that the AES has a significant impact on the output signals of the SC that initiate the orientation responses to peripheral sensory stimulation.     

Monocular eye movement in the cat: its corticotectal pathway

The corticotectal pathway from the fundus of the cat's coronal sulcus (CORo) from which monocular movements of contralateral eye were evoked was studied using electrophysiological and anatomical techniques. Neurons in the CORo were activated antidromically by electrical stimulation of the deep layer of the superior colliculus (SC). Labeled cells were found in the CORo following horseradish peroxidase injection in the SC.     

Some topographical connections of the striate cortex with subcortical structures in Macaca fascicularis

Summary Subcortical connections of the striate cortex with the superior colliculus (SC), the lateral pulvinar (Pl), the inferior pulvinar (Pi) and the dorsal lateral geniculate nucleus (LG) were studied in the macaque monkey, Macaca fascicularis, following cortical injections of tritiated proline and/or horseradish peroxidase. All four structures were shown to receive topographically organized projections from the striate cortex. The exposed surface of the striate cortex was found to be connected to the rostral part of the SC and the caudal part of the LG. Injections of the exposed striate cortex close to its rostral border resulted in label in adjoining parts of the Pl and Pi. The ventral half and dorsal half of the calcarine fissure were connected with the medial and lateral parts of the SC, the ventrolateral and dorsomedial portions of the Pl and Pi and the lateral and medial parts of the LG, respectively. Injections located at the lateral posterior extreme of the calcarine fissure resulted in label at the optic disc representation in the LG. The horseradish peroxidase material demonstrated that LG neurons in all laminae and interlaminar zones project to the striate cortex.Abbreviations BIC brachium of the inferior colliculus - BSC brachium of the superior colliculus - C cerebellum - CG central grey - i interlaminar zone(s) of the dorsal lateral geniculate nucleus - IC inferior colliculus - ICc central nucleus of the inferior colliculus - LG dorsal lateral geniculate nucleus - m magnocellular layer(s) of the dorsal lateral geniculate nucleus - MG medial geniculate body - p parvocellular layer(s) of the dorsal lateral geniculate nucleus - P pulvinar complex - Pi inferior pulvinar - PG pregeniculate nucleus - Pl lateral pulvinar - Pm medial pulvinar - s superficial layer(s) of the dorsal lateral geniculate nucleus - SC superior colliculus - sgs stratum griseum superficiale of the superior colliculus - R reticular nucleus of the thalamus - VP ventroposterior group - 17 Area 17 Supported by NEI Grants EY-07007 (J. Graham) and EY-02686 (J.H. Kaas)     

Intracellular recordings from binocularly activated cells in the cat's dorsal lateral geniculate nucleus

Five binocularly activated cells near the interlaminar layers of the dorsal lateral geniculate nucleus have been studied with intracellular recording techniques. Four neurons were relay cells and antidromically activated from the visual cortex. They received monosynaptic excitation and disynaptic inhibition from Y type retinal ganglion cells in both eyes and disynaptic recurrent inhibition. The fifth cell was similar to perigeniculate neurons. It received disynaptic excitation from retinal ganglion cells in both eyes and monosynaptic excitation from antidromically activated relay cell axons. It was also inhibited from all these sources after an additional synaptic delay. The cell had a large receptive field, about twice the center size of neighboring relay cells, and gave on-off responses from the entire response area. Such displaced perigeniculate like cells may explain why relay cells issue occasional axon collaterals within the dorsal lateral geniculate nucleus.     

Morphophysiological study on the divergent projection of axon collaterals of medial vestibular nucleus neurons in the cat

(1) Spikes of single neurons were extracellularly recorded in the medial vestibular nucleus (MVN) in decerebrate cats and were functionally identified as secondary type I neurons by observing their responses to horizontal rotation and monosynaptic activation after stimulation of the ipsilateral vestibular nerve. Axonal projection of these neurons was examined by their antidromic responses to stimulation of the contralateral abducens nucleus, the spinal cord, and the ascending and descending MLF. (2) Almost all secondary type I vestibular neurons which sent their axon to the contralateral abducens nucleus were antidromically activated from the descending MLF at the level of the obex as well. Nearly half of these neurons sent their collateral axon to the level of C1 segment in the spinal cord and approximately one third to the ascending MLF close to the oculomotor complex. (3) The mean conduction velocity was 29 m/s for descending collateral axons and 30 m/s for ascending collateral axons. (4) Systematic tracking for antidromic microstimulation in the contralateral abducens nucleus and spinal gray matter at C2-C3 suggested that collateral axons of single type I vestibular neurons gave off local branches in the abducens nucleus and the motoneuron pool in the upper cervical gray matter. Existence of terminal branches in the neck motoneuron pool was confirmed by intraaxonal staining with horseradish peroxidase (HRP). (5) Neurons which projected to both the contralateral abducens nucleus and the spinal cord were located in a fairly localized region in the ventrolateral part of the rostral MVN. Neurons which projected to the contralateral abducens nucleus and not to the spinal cord were located in a rostrocaudally wider area in the ventrolateral MVN. Neurons projecting to the spinal cord and not to the contralateral abducens nucleus were located in the widest area in the rostrocaudal direction, covering almost the whole extent of the rostral half of the MVN.     

Reorganization of the corticotectal projections introduced by neonatal monocular enucleation in the Monodelphis opossum and the influence of serotoninergic depletion

The influence of neonatal serotoninergic lesion (performed with s.c. injection of 5,7-dihydroxytryptamine) on the plasticity of the developing corticotectal projection was studied in the gray short-tailed opossum (Monodelphis domestica). As a first step, the placement and density of neurons projecting from the visual cortical areas to the superior colliculus was established in the adult opossum. Injections of retrogradely transported fluorescent dyes into the superior colliculus of intact three-month-old animals labeled neurons of cortical layer V. In this species, there are three visual areas: the striate area and two secondary areas, the laterally placed peristriate area and the medial visual area. The population of the labeled neurons was denser in peristriate and medial visual areas than in the striate area. Secondly, the influence of neonatal monocular enucleation on the extent of this projection was investigated, alone or in combination with a serotoninergic lesion. Injection of dyes into the superior colliculi of three-month-old animals that were unilaterally enucleated on the second postnatal day also labeled neurons of cortical layer V. However, the density of the cortical neurons projecting to the superior colliculus contralateral to the remaining eye was much lower. This reduction was most profound in the striate visual area. No significant modifications of this projection were found on the side ipsilateral to the remaining eye. In another group of opossums, unilateral enucleation on the second postnatal day was combined with serotoninergic lesion. Brains of some of the treated pups were immunostained for serotonin on the fifth postnatal day. At this age, 70-80% of serotoninergic axons in the brain were missing. However, in about three weeks these axons had regrown, and their density in the neocortex was approximately the same as in the control animals. We conclude that severe reduction of the serotoninergic innervation during the early postnatal period did not influence the plastic changes induced in the corticotectal projection by unilateral enucleation.     

A neurophysiological study of prepositus hypoglossi neurons projecting to oculomotor and preoculomotor nuclei in the alert cat

The activity of 62 antidromically identified prepositus hypoglossi neurons was recorded in 10 alert cats during spontaneous, vestibular or visually induced eye movements. Neurons were antidromically activated from stimulating electrodes implanted in the ipsilateral medial longitudinal fasciculus (n = 24), the ipsilateral interstitial nucleus of Cajal (n = 6), the ipsilateral parabigeminal nucleus (n = 2), the contralateral superior colliculus (n = 6) and the contralateral cerebellar posterior peduncle (n = 24). Neurons were identified as eye-movement-related when their rate-position and/or rate-velocity plots showed correlation coefficients greater than or equal to 0.6. They were further classified as "position", "position-velocity" and "velocity-position" according to their relative eye position and velocity coefficients. However, they seemed to be distributed as a continuum in which a progressive decrease of eye velocity sensitivity was accompanied by a proportional increase in eye position sensitivity. "Position-velocity" neurons (n = 9) were mainly horizontal type II neurons projecting to the vicinity of the oculomotor complex; two of these neurons with vertical sensitivity were also activated from the interstitial nucleus of Cajal. Mean position and velocity sensitivity of these neurons were 5.2 spikes/s per degree and 0.62 spikes/s per degree per second, respectively. Pure "position" neurons (n = 7) also showed activation during ipsilateral eye fixations; their mean position gain was 7.3 spikes/s per degree and they projected to the ipsilateral oculomotor and Cajal nuclei, and to the contralateral superior colliculus. "Velocity-position" neurons (n = 18) were type I or II neurons with rather irregular tonic firing rates and a mean velocity gain of 0.75 spikes/s per degree per second. Type II "velocity-position" neurons projected mainly to the oculomotor area, while type I neurons projected preferentially to the cerebellum. A special type of "pause" neuron (n = 5), with very low firing rate and pausing mainly for contralateral saccades, was activated exclusively from the contralateral posterior peduncle. Many neurons with weak eye movement sensitivity (n = 22) were activated mainly (73%) from the cerebellum. It can be concluded that the prepositus hyperglossi nucleus distributes specific eye movement related signals to motor and premotor brainstem and cerebellar structures. The variability of interspike intervals of representative prepositus hypoglossi neurons of each class was compared to the discharge variability of identified abducens motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Large layer VI cells in macaque striate cortex (Meynert cells) project to both superior colliculus and prestriate visual area V5

Summary Layer VI of macaque striate cortex contains a number of large solitary neurones called Meynert cells. It has been shown earlier that these Meynert cells project to the posterior bank of the superior temporal sulcus (area V5), but it has also been shown that they project to the superior colliculus. In retrograde fluorescent double-labelling experiments, it was found that Meynert cells represent a class of neurones which distribute divergent axon collaterals to the posterior bank of the superior temporal sulcus and to the superior colliculus, i.e. to a distant cortical and a subcortical structure. This feature appears to be unique among projecting neurones in monkey visual cortex.     

Branching cortical neurons in cat which project to the colliculi and to the pons: a retrograde fluorescent double-labeling study

Summary The fluorescent double-labeling technique has been used to determine whether the corticopontine and the corticotectal fibers in the cat are derived from two different sets of neurons or whether they are derived from branching neurons which distribute collaterals to the pontine grey and the colliculi. After unilateral DY.2HCl injections in the pontine grey and FB injections in the ipsilateral colliculi, large numbers of FB-DY.2HCl double-labeled neurons were present in the cortex of the ipsilateral hemisphere. However, the labeled neurons in its rostral part may have represented pyramidal tract neurons which were labeled retrogradely because their fibers descended through the DY.2HCl injection area. Therefore, also DY.2HCl injections were made in the pyramid (i.e. caudal to the pons) and the cortical pyramidal tract area, containing the retrograde DY.2HCl-labeled neurons, was delineated. In the rest of the experiments only the DY.2HCl-labeled neurons in the caudal two thirds of the hemisphere (outside the pyramidal tract area) were taken into account because only these neurons could, with confidence, be regarded as corticopontine neurons. In some anterograde HRP transport experiments the trajectories of the corticotectal and the corticopontine fibers were visualized. On the basis of the findings the DY.2HCl injections in the pontine grey were placed such that they could not involve any of the corticotectal fibers passing from the cerebral peduncle to the colliculi. Thus artifactual doublelabeling of cortical neurons was avoided. However, also under these circumstances many double-labeled neurons were present in the caudal two thirds of the hemisphere. This led to the conclusion that in the cat a large proportion of the corticopontine neurons in the caudal two thirds of the hemisphere represent branching neurons which also distribute collaterals to the colliculi. The parietal (anterior part of the lateral gyrus, middle and posterior suprasylvian gyri) and the cingulate areas together contained three quarters of all labeled corticopontine neurons outside the pyramidal tract area. In the parietal areas roughly 25% of them were double-labeled and in the cingulate area 14%. However, in the visual areas 18 and 19 a much larger percentage (30–60%) was doublelabeled. In a recent study from our laboratory it was found that in the cat the pyramidal tract fibers distribute an abundance of collaterals to the pontine grey. Therefore, a large proportion of all corticopontine connections in this species appear to be established by branching neurons which also distribute fibers to other cell groups in the brain stem and the spinal cord.Abbreviations A.E. anterior ectosylvian sulcus - a.e.s. anterior ectosylvian sulcus - BC brachium conjunctivum - BCI brachium colliculus inferior - BP brachium pontis - cor. sulc. coronal sulcus - CP cerebral peduncle - CR. cruciate sulcus - CUN cuneiform nucleus - DBC decussation brachium conjunctivum - DLP dorsolateral pontine nucleus - IC inferior colliculus - inf. coll. inferior colliculus - INS. insula cortex - IO inferior olive - IP interpeduncular nucleus - LAT. lateral sulcus - l.s. lateral sulcus - MG medial geniculate body - LL lateral lemniscus - ML medial lemniscus - MLF medial longitudinal fascicle - NdG dorsal nucleus of Gudden - NLL nucleus lateral lemniscus - NRTP reticular tegmental pontine nucleus - ORB. orbital sulcus - P pyramid - PAG periaqueductal grey - P.E. posterior ectosylvian sulcus - RF reticular formation - PG pontine grey - RB restiform body - RN red nucleus - S. sylvian sulcus - SC superior colliculus - SN substantia nigra - SO superior olive - SPV spinal trigeminal complex - S.S. suprasylvian sulcus - s.syl.s. suprasylvian sulcus - S.SPL. suprasplenial sulcus - SPL. splenial sulcus - spl.s. splenial sulcus - sup. coll. superior colliculus - syl.s. sylvian sulcus - TB trapezoid body - VC vestibular complex - Vm trigeminal motor nucleus - Vs trigeminal principle nucleus - III oculomotor nucleus - IV trochlear nucleus - VI abducens nucleus - VII facial nerve - VIII vestibulo-trochlear nerve Supported in part by grant 13-46-91 of FUNGO/ZWO (Dutch Organization for Fundamental Research in Medicine)     

Postnatal development of the corticotectal projection from the visual cortex of the mouse.

The postnatal development of the corticotectal projection was investigated by injecting the axon tracer DiI into the visual cortex of mouse pups. It was found that DiI-labeled axons arrive at the ipsilateral superior colliculus and enter the optic nerve layer of this structure on postnatal days 3 and 4 (P3-P4). These corticotectal axons extend into the caudal end of the superior colliculus on P4 and give off small collateral branches that ascend vertically to the superficial gray layer. During the first two postnatal weeks, the collateral branches do not form a demarcated terminal zone, but rather diffusely spread within the superficial gray layer of the superior colliculus. These collateral branches continue to dichotomize and form a bright terminal zone within the superficial gray layer on P11. The terminal zone decreases in size during the second and third postnatal weeks, and appears to be of the same size when compared with the adult counterpart by P19. The terminal zone of the corticotectal axons from the visual cortex is established by P19. In parallel with the maturation of the terminal zone of the corticotectal projection, the distal segment of the corticotectal axons is lost during the second postnatal week. We conclude that the growing tips of the corticotectal axons do not strictly project to their future terminal zone within the superior colliculus, and 'misdirected' axons are eliminated during the early postnatal period.     

Electrophysiological and anatomical demonstration of an overlapping striate and tectal projection to the lateral posterior-pulvinar complex of the cat

Extracellular unit activity was recorded in the lateral posterior (LP)-pulvinar complex. The responses of 254 neurons after electrical stimulation of the central-paracentral part of cortical area 17 and of 84 neurons after stimulation of both area 17 and the superior colliculus (SC) were investigated. Neurons in the LP-pulvinar complex responded to area 17-stimulation with excitatory-inhibitory effects; in some cases only with inhibition. Neurons affected by striate stimulation were found in the caudal region of the complex in a region that extended widely into the medial part of the lateralis posterior nucleus (LPm), the so-called tectorecipient part of the lateral posterior nucleus. Accordingly, 26 of the 84 neurons in which electrical stimulation of area 17 and of the SC was tested, were found to react to both types of stimulation. Cells responding only to SC-stimulation were found in the ventral region of the anterior LP-Pulvinar complex. Anatomical studies supported the finding that striate and tectal inputs overlap considerably in the LP-pulvinar complex. After depositing horseradish peroxidase (HRP) into various regions of the LP-pulvinar complex, retrogradely labeled cells were found in area 17 (layer V) as well as in the superficial layers of the SC. These results were confirmed by orthograde transport autoradiography after injection of labeled amino-acids into area 17. Our findings indicate that cortical and collicular inputs into the caudal part of the LP-pulvinar complex overlap considerably and that, in these overlapping regions, individual neurons may receive converging afferent excitation from both regions.     

Influence of the superior colliculus on visual responses of cells in the rabbit's lateral posterior nucleus

Summary The lateral posterior-pulvinar (LP-P) complex of mammals receives a major input from the superior colliculus (SC). We have studied the response properties of LP cells and investigated the effects of reversible inactivation of the colliculus on the visual responses of LP units in anesthetized and paralyzed rabbits. Cells in LP had large receptive fields responsive to either stationary or moving stimuli. One third of the motion-sensitive cells were direction selective. The size of the receptive fields increased with eccentricity and there was a retinotopic organization along the dorso-ventral axis. Comparison of the LP and superior colliculus properties revealed substantial differences in visual response characteristics of these two structures such as the size of the receptive fields and the number of direction-selective cells. Electrical stimulation of the LP evoked antidromic action potentials in tectal cells that were motion sensitive. We found a dorsoventral gradient in the projections of collicular cells. Units located more dorsally in the colliculus sent their axons to LP while cells lying more ventrally sent axons toward the region lying posterior to LP. A micropipette filled with lidocaine hydrochloride was lowered into the superficial layers of the superior colliculus in order to reversibly inactivate a small population of collicular cells. Rendering the superior colliculus inactive produced a sharp attenuation of visual responses in the majority of LP cells. Some neurons ceased all stimulus-driven activity after collicular blockade while a few cells exhibited increased excitability following collicular inactivation. These experiments also indicate that the tecto-LP path is topographically organized. An injection in the colliculus failed to influence the thalamic response when it was not in retinotopic register with the LP cells being recorded. Our results demonstrate that the superior colliculus input to LP is mainly excitatory in nature.     

Grating detection and visual acuity after lesions of striate cortex in hooded rats

Summary The ability of rats to detect high-contrast square-wave gratings over a range of spatial frequencies was measured before and after ablation of striate cortex. The animals relearnt to detect low-frequency gratings very quickly after operation, and their acuity was reduced from 1.0 c/deg to about 0.7 c/deg. These effects were in striking contrast to those produced by larger posterior cortical ablations, which included both striate and prestriate cortex (Dean 1978); after the larger lesions, rats required many weeks of retraining to detect even low-frequency gratings and their acuity was reduced to 0.3 c/deg. The difference in the effects of the two lesions suggested that the rats with striate ablation were using information about spatial contrast that was relayed either by spared remnants of the geniculo-cortical pathway, or by the pathway from superior colliculus to prestriate cortex via the lateral posterior nucleus. To try and distinguish between these possibilities, the destriate rats were given a further lesion of the superior colliculus. This second lesion severely disrupted contrast detection: the animals made about as many errors as rats with large posterior cortical removal in relearning to detect a low-frequency grating, which is about 20 to 30 times as many as after either striate cortex or superior colliculus lesions alone. This result suggests that rats, like other mammals, can use spatial information conveyed in the tectocortical path when striate cortex has been destroyed.This work was supported by MRC Grant G987/429/N     

Corticogeniculate neurons, corticotectal neurons, and suspected interneurons in visual cortex of awake rabbits: receptive-field properties, axonal properties, and effects of EEG arousal

The intrinsic stability of the rabbit eye was exploited to enable receptive-field analysis of antidromically identified corticotectal (CT) neurons (n = 101) and corticogeniculate (CG) neurons (n = 124) in visual area I of awake rabbits. Eye position was monitored to within 1/5 degrees. We also studied the receptive-field properties of neurons synaptically activated via electrical stimulation of the dorsal lateral geniculate nucleus (LGNd). Whereas most CT neurons had either complex (59%) or motion/uniform (15%) receptive fields, we also found CT neurons with simple (9%) and concentric (4%) receptive fields. Most complex CT cells were broadly tuned to both stimulus orientation and velocity, but only 41% of these cells were directionally selective. We could elicit no visual responses from 6% of CT cells, and these cells had significantly lower conduction velocities than visually responsive CT cells. The median spontaneous firing rates for all classes of CT neurons were 4-8 spikes/s. CG neurons had primarily simple (60%) and concentric (9%) receptive fields, and none of these cells had complex receptive fields. CG simple cells were more narrowly tuned to both stimulus orientation and velocity than were complex CT cells, and most (85%) were directionally selective. Axonal conduction velocities of CG neurons (mean = 1.2 m/s) were much lower than those of CT neurons (mean = 6.4 m/s), and CG neurons that were visually unresponsive (23%) had lower axonal conduction velocities than did visually responsive CG neurons. Some visually unresponsive CG neurons (14%) responded with saccadic eye movements. The median spontaneous firing rates for all classes of CG neurons were less than 1 spike/s. All neurons synaptically activated via LGNd stimulation at latencies of less than 2.0 ms had receptive fields that were not orientation selective (89% motion/uniform, 11% concentric), whereas most cells with orientation-selective receptive fields had considerably longer synaptic latencies. Most short-latency motion/uniform neurons responded to electrical stimulation of the LGNd (and visual area II) with a high-frequency burst (500-900 Hz) of three or more spikes. Action potentials of these neurons were of short duration, thresholds of synaptic activation were low, and spontaneous firing rates were the highest seen in rabbit visual cortex. These properties are similar to those reported for interneurons in several regions in mammalian central nervous system. Nonvisual sensory stimuli that resulted in electroencephalographic arousal (hippocampal theta activity) had a profound effect on the visual responses of many visual cortical neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

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)     

Electrophysiological evidence of collateral projections of parabrachio-thalamic relay neurons

We studied axonal branching of 14 parabrachial nucleus neurons, activated antidromically from 2 of the 3 stimulation sites, i.e. the ipsi- and contralateral thalamic taste area (TTAs) and the ipsilateral central nucleus of the amygdala (CA). Making use of antidromic latencies, collision times and refractory periods at the 2 sites of stimulation, the conduction times were calculated for the distance between the branching point of the axon and the stimulation sites or the recording site at the soma. Nine of these 14 neurons had a significant length of axon branches terminating at 2 of the 3 sites of stimulation. Five neurons sent axon branches to the bilateral TTAs, 3 to both the CA and the ipsilateral TTA and the remaining one to both the CA and the contralateral TTA. Four of these 9 neurons with collateral branches responded to taste stimulation.     

Corticotectal circuit in the cat: a functional analysis of the lateral geniculate nucleus layers of origin

1. The dorsal lateral geniculate nucleus (LGN) of the cat is a major thalamic relay between the retina and several visual cortical areas. These cortical areas in turn project to the superior colliculus (SC). The aim of the present experiment was to determine which LGN layers provide a necessary input to the corticotectal circuit. 2. Individual layers of the LGN were reversibly inactivated by microinjection of cobalt chloride during recording of visual responses in the retinotopically corresponding part of the superior colliculus. 3. For cells driven through the contralateral eye, inactivation of layer A or the medial interlaminar nucleus (MIN) had little effect on visual responsiveness in the superior colliculus. In contrast, inactivation of layer C abolished visual responses at one-quarter of the SC recording sites, reduced responses at another quarter, and left half of the recording sites unaffected. 4. For cells driven through the ipsilateral eye, inactivation of layer C1 or the MIN had no effect. Inactivation of layer A1 uniformly reduced visual responses in the superior colliculus and usually abolished them entirely. 5. These results are compatible with previous work showing that cortical input to the SC originates from Y-cells. They indicate that two of the five Y-cell containing layers (A1 and C) provide major inputs to the corticotectal circuit. The results suggest that layer A1 is functionally allied to layer C as well as to layer A.