Bilateral Input Research Articles

Electrophysiological characterization of the superior paraolivary nucleus in the Mongolian gerbil

The superior paraolivary nucleus (SPN) of the superior olivary complex (SOC) though morphologically well described, has not been characterized physiologically. Here we report the basic response properties of SPN units acquired with extracellular recording techniques under monaural acoustic stimulation in the Mongolian gerbil. Poststimulus-time histograms corresponded to those described earlier for the cat’s cochlear nucleus (onset, chopper, primary-like), and partly to those previously acquired in other SOC nuclei (tonic, off/rebound). Two-thirds of the units responded solely to contralateral stimulation (40% excitatory [E], 19% inhibitory [I], 6% mixed [EI]). Most of the remainder responded equally to stimulation from either ear (18% I.I, 9% E.E). Overall, the monaural contralateral input was more effective than the ipsilateral and bilateral input. Characteristic frequencies and response areas covered the entire hearing range of the gerbil and the units mostly showed broad frequency-tuning. In combination, these properties suggest that the SPN might be a constituent of an afferent pathway encoding stimulus features across broad frequency ranges.

Ipsilateral and contralateral projections from upper cervical segments to the vestibular nuclei in the rat.

The present study examined uncrossed and crossed projections from upper cervical segments to the vestibular nuclei, and the relationship between the afferents and vestibulospinal neurons in the rat. Afferent axons were labeled following unilateral injections of biotinylated dextran into the C2 and C3 segments, while vestibulospinal neurons were labeled following unilateral injections of cholera toxin subunit B into the same segments. The terminals of uncrossed afferents were distributed in the entire area of the rostrocaudal extent of the lateral vestibular nucleus (LV). In the magnocellular part (MVmc) of the medial vestibular nucleus (MV), they were seen near the parvocellular part (MVpc) of the MV at caudal levels. In the MVpc, terminals were seen laterally and ventromedially, close to the border of the MVmc. At caudal levels of the caudal part (MVc) of the MV, they were distributed within the groups of labeled neurons in the middle and lateral areas. In the descending vestibular nucleus (DV), terminals were abundant dorsally and laterally in the rostral two-thirds. The distribution of contralateral cervical afferents was similar to that of ipsilateral afferents. The terminals of ipsilateral and contralateral cervical afferents were seen in contact with vestibulospinal neurons in the DV. The present study demonstrates bilateral input from upper cervical segments to the LV, DV, and all subdivisions of the MV. The input to the LV would contribute to the tonic neck reflex or cervicovestibulospinal reflex.

Ephrin-B3 is the midline barrier that prevents corticospinal tract axons from recrossing, allowing for unilateral motor control

Growing axons follow highly stereotypical pathways, guided by a variety of attractive and repulsive cues, before establishing specific connections with distant targets. A particularly well-known example that illustrates the complexity of axonal migration pathways involves the axonal projections of motor neurons located in the motor cortex. These projections take a complex route during which they first cross the midline, then form the corticospinal tract, and ultimately connect with motor neurons in the contralateral side of the spinal cord. These obligatory contralateral connections account for why one side of the brain controls movement on the opposing side of the body. The netrins and slits provide well-known midline signals that regulate axonal crossings at the midline. Herein we report that a member of the ephrin family, ephrin-B3, also plays a key role at the midline to regulate axonal crossing. In particular, we show that ephrin-B3 acts as the midline barrier that prevents corticospinal tract projections from recrossing when they enter the spinal gray matter. We report that in ephrin-B3(-/-) mice, corticospinal tract projections freely recross in the spinal gray matter, such that the motor cortex on one side of the brain now provides bilateral input to the spinal cord. This neuroanatomical abnormality in ephrin-B3(-/-) mice correlates with loss of unilateral motor control, yielding mice that simultaneously move their right and left limbs and thus have a peculiar hopping gait quite unlike the alternate step gait displayed by normal mice. The corticospinal and walking defects in ephrin-B3(-/-) mice resemble those recently reported for mice lacking the EphA4 receptor, which binds ephrin-B3 as well as other ephrins, suggesting that the binding of EphA4-bearing axonal processes to ephrin-B3 at the midline provides the repulsive signal that prevents corticospinal tract projections from recrossing the midline in the developing spinal cord.

Trans-neuronal modification of anterior piriform cortical circuitry in the rat

Long–Evans rats received unilateral naris closure on postnatal day 1 (PN1) or sham surgery. On PN30, brains were processed for anterograde horseradish peroxidase (HRP) labeling of lateral olfactory tract (LOT) fibers in anterior piriform cortex (aPCX) Layer Ia, Timm staining of association/commissural fibers in aPCX Layer Ib, or Golgi staining for reconstruction of aPCX semilunar cell dendrites. The results demonstrate that the width of aPCX Layer Ia was reduced ipsilateral to the sealed naris compared to undeprived controls. No significant change in Layer Ib was detected. Furthermore, semilunar cell dendrites were reduced by unilateral deprivation compared to undeprived controls. The reduction in dendritic tree size was localized to distal dendritic segments, roughly corresponding to Layer Ia. These results suggest an activity-dependent change in both the distribution of cortical afferents and in the dendritic field of their target cells. While these results are similar to those reported for other sensory systems, the relatively simple architecture and laminated organization of bilateral inputs to the aPCX make the olfactory system an ideal model system to examine experience-dependent synaptic reorganization and its functional consequences.

Speaking speed effects on delayed auditory feedback disruption of speech fluency.

24 Italian medical students performed a task of verbal fluency. 12 students (the control group) receiving Normal Auditory Feedback and 12 students receiving Delayed Auditory Feedback (delay of 200 msec.) performed six trials in six different experimental settings: normal or increased speaking rate, and, for each condition, once with bilateral input of the auditory feedback, once to the right ear, and once to the left ear. At the normal speaking rate, the disruptive effect of delayed feedback was confirmed. As the speaking rate increased, the total number of errors increased within the control group but decreased within the group given delayed feedback, although the total number of errors was always greater for the latter. In addition, speech was more disrupted when the auditory input was returned to the right ear (left hemisphere) for all the different conditions: Normal and Delayed Auditory Feedback, normal and increased speaking rate. In particular, the left hemisphere was less resistant to the disruptive effect of the delayed feedback than the right hemisphere. From these results, we suggest that, when speaking more quickly, one uses more central mechanisms of movement programming (cortical-cerebellum-thalamus-cortical, cortical-corpus striatum-thalamus-cortical, and cortical-thalamus-cortical circuits), or attentional control (cortico-reticular-cortical circuits) than peripheral mechanisms (tactile, proprioceptive, and acoustic circuits). This may explain the decreased disruptive influence of delayed auditory feedback on speed, fluency, and quality at increased speaking rates. Hemispheric specialization processes, however, may explain the more pronounced susceptibility of the left hemisphere or the less pronounced susceptibility of the right hemisphere during the delayed feedback condition. In fact, the former processes phonemic, grammatical, and lexical features of words whilst the latter is competent in using metaphors and prosody in controlling the emotional aspects of language. Moreover, the right hemisphere is more active on attentional tasks.

Trigeminal projections to thalamus and subthalamus in the hedgehog tenrec

The objective of the present study was the identification and characterization of the trigemino-diencephalic target areas in the Madagascan lesser hedgehog tenrec in order to get a more comprehensive view on the mammalian somatosensory thalamus, its evolution and representation in different species. Such an analysis has been considered important because in lower mammals the head and face are relatively well represented, but their ascending trigeminal projections have scarcely been analysed. Following injections of different tracer substances into the rostral and caudal portions of the trigeminal nuclear complex the most prominent area of termination was found in the medial ventroposterior nucleus. These projections were patchy and scarcely overlapped the region previously shown to receive spinal and dorsal column nuclear afferents. On the basis of the laterality and the intensity of the projections, two subdivisions were distinguished, the principal portion and the accessory portion receiving a dense contralateral and a weak bilateral input, respectively. They were considered equivalents to the magnocellular and parvocellular subdivisions of the medial ventroposterior nucleus in more differentiated mammals. In the latter species, however, the overlap between trigeminal and parabrachial fibres appears less extensive than in the tenrec. In addition, a weak bilateral projection was shown from the caudal trigeminal nucleus to the caudal and dorsal subdivision of the nucleus submedius. There was little, if any evidence for a trigeminal projection to the intralaminar nuclei and we failed to identify a correlate to the posterior nuclear complex of higher mammals. On the other hand, there was a distinct contralateral projection to the ventral portion of the zona incerta. This projection was of similar strength as the projection to the medial ventroposterior nucleus; it supports the notion that the zona incerta may play a crucial role in relaying trigeminal information.

Lateralization and functional organization of the locus coeruleus projection to the trigeminal somatosensory pathway in rat

The primary goals of this study were to (1) examine the distribution of locus coeruleus (LC) neurons, which project to cortical and subcortical sites along the trigeminal somatosensory pathway in rats, and (2) determine the extent to which different regions within this ascending sensory system receive collateral projections from the same LC neuron. Long-Evans hooded rats received unilateral pressure injections of different combinations of retrograde fluorescent tracers into whisker-related regions of primary (SI) and secondary (SII) somatosensory cortices, the ventrobasal (VB) and posterior group (POm) nuclei of the thalamus, and the principalis nucleus of the trigeminal complex (PrV). Coronal sections (40–100 μm) through the LC were examined by fluorescence microscopy, and the distribution of retrogradely labeled cells was recorded. The major finding was that whisker-related regions of the cortex receive efferent projections from neurons concentrated in the caudal portion of the ipsilateral LC, whereas subcortical trigeminal somatosensory structures receive bilateral input from both LC nuclei. Despite the bilateral nature of the LC projection to subcortical sites, the majority of LC efferents to VB and POm thalamus originate in the ipsilateral LC nucleus, whereas projections to PrV originate primarily from the contralateral LC. An additional finding was that a relatively large proportion of LC cells, which project to a single somatosensory structure, also send axon collaterals to other relay sites along the same ascending somatosensory pathway. Taken together, these results suggest that the LC–noradrenergic system maintains a more selective relationship with functionally related efferent targets than has been previously appreciated. J. Comp. Neurol. 385:135–147, 1997. © 1997 Wiley-Liss, Inc.

Lateralization and functional organization of the locus coeruleus projection to the trigeminal somatosensory pathway in rat

The primary goals of this study were to (1) examine the distribution of locus coeruleus (LC) neurons, which project to cortical and subcortical sites along the trigeminal somatosensory pathway in rats, and (2) determine the extent to which different regions within this ascending sensory system receive collateral projections from the same LC neuron. Long-Evans hooded rats received unilateral pressure injections of different combinations of retrograde fluorescent tracers into whisker-related regions of primary (SI) and secondary (SII) somatosensory cortices, the ventrobasal (VB) and posterior group (POm) nuclei of the thalamus, and the principalis nucleus of the trigeminal complex (PrV). Coronal sections (40-100 microm) through the LC were examined by fluorescence microscopy, and the distribution of retrogradely labeled cells was recorded. The major finding was that whisker-related regions of the cortex receive efferent projections from neurons concentrated in the caudal portion of the ipsilateral LC, whereas subcortical trigeminal somatosensory structures receive bilateral input from both LC nuclei. Despite the bilateral nature of the LC projection to subcortical sites, the majority of LC efferents to VB and POm thalamus originate in the ipsilateral LC nucleus, whereas projections to PrV originate primarily from the contralateral LC. An additional finding was that a relatively large proportion of LC cells, which project to a single somatosensory structure, also send axon collaterals to other relay sites along the same ascending somatosensory pathway. Taken together, these results suggest that the LC-noradrenergic system maintains a more selective relationship with functionally related efferent targets than has been previously appreciated.

Ventral nucleus of the lateral lemniscus in guinea pigs: cytoarchitecture and inputs from the cochlear nucleus.

Cytoarchitectonic criteria were used to distinguish three subdivisions of the ventral nucleus of the lateral lemniscus in guinea pigs. Axonal tracing techniques were used to examine the projections from the cochlear nucleus to each subdivision. Based on the cell types they contain and their patterns of input, we distinguished ventral, dorsal, and anterior subdivisions of the ventral nucleus of the lateral lemniscus. All three subdivisions receive bilateral inputs from the cochlear nucleus, with contralateral inputs greatly outnumbering ipsilateral inputs. However, the relative density of the inputs varies: the ventral subdivision receives the densest projection, whereas the anterior subdivision receives the sparsest projection. Further differences are apparent in the morphology of the afferent axons. Following an injection of Phaseolus vulgaris-leucoagglutinin into the ventral cochlear nucleus, most of the axons on the contralateral side and all of the axons on the ipsilateral side are thin. Thick axons are present only in the ventral subdivision contralateral to the injection site. The evidence from both anterograde and retrograde tracing studies suggests that the thick axons originate from octopus cells, whereas the thin axons arise from multipolar cells and spherical bushy cells. The differences in constituent cell types and in patterns of inputs suggest that each of the three subdivisions of the ventral nucleus of the lateral lemniscus makes a distinct contribution to the analysis of acoustic signals.

Afferent and efferent connections of the lateral and medial pallia of the silver lamprey.

Injections of the carbocyanine dye, DiI, into the lateral pallium of the silver lamprey reveal that this pallial region receives bilateral inputs from the olfactory bulbs, dorsomedial telencephalic neuropil, and the habenular nuclei, and ipsilateral inputs from the septum, preoptic area, medial pallium, thalamus, and, possibly, the striatum. The efferent projections of the lateral pallium form dorsal (olfacto-habenular tract of Heier) and ventral (olfacto-thalamic and hypothalamic tracts of Heier) bundles. The dorsal bundle terminates ipsilaterally in the dorsal pallium, medial pallium, habenular nuclei, and pretectum and contralaterally in the habenular nuclei and, possibly, the dorsal pallium. The ventral bundle terminates ipsilaterally in the septum, striatum, and preoptic areas and bilaterally within the hypothalamus. Injections of DiI into the medial pallium reveal bilateral inputs to this pallial formation from the olfactory bulbs, the dorsomedial telencephalic neuropil, septum, habenular nuclei, thalamic nuclei, preoptic area and hypothalamus, as well as ipsilateral inputs from the lateral pallium, dorsal isthmal grey and midbrain tegmentum. The efferent projections of the medial pallium form dorsal, ventral and descending bundles. The dorsal bundle terminates ipsilaterally in the dorsal and lateral pallia and in the olfactory bulb. The ventral bundle terminates ipsilaterally in the dorsal pallium and bilaterally within the lateral pallium and in preoptic and hypothalamic areas. The descending bundle terminates bilaterally in thalamic and hypothalamic areas and in the pretectum and optic tectum. These data support a number of earlier hypotheses concerning pallial homologues in lampreys and other vertebrates but suggest that the earlier hypothesis of an olfactory origin of the telencephalon of craniates should be rejected.

GABAergic inputs to the nucleus rotundus (pulvinar inferior) of the pigeon (columba livia).

The avian nucleus rotundus, a nucleus that appears to be homologous to the inferior/ caudal pulvinar of mammals, is the major target of an ascending retino-tecto-thalamic pathway. Further clarification of the inputs to the rotundus and their functional properties will contribute to our understanding of the fundamental role of the ascending tectal inputs to the telencephalon in all vertebrates, including mammals. We found that the rotundus contains a massive plexus of glutamic acid decarboxylase (GAD)-immunoreactive axons using antibodies against GAD. The cells within the rotundus, however, were not immunoreactive for GAD. The retrograde tracer cholera toxin B fragment was injected into the rotundus to establish the location of the afferent neurons and determine the source of the gamma-aminobutyric acid (GABA) inputs into the rotundus. In addition to the recognized bilateral inputs from layer 13 of the tectum, we found intense retrograde labeling of neurons within the ipsilateral nuclei subpretectalis (SP), subpretectalis-caudalis (SPcd), interstitio-pretecto-subpretectalis (IPS), posteroventralis thalami (PV), and reticularis superior thalami (RS). All the neurons of the SP, SPcd, IPS, and PV were intensely GAD-immunoreactive. The neurons of layer 13 of the tectum were not immunoreactive for GAD. Following the destruction of the ipsilateral SP/IPS complex, we found a major reduction in the intensity of the GAD axonal immunoreactivity within the ipsilateral rotundus, but this destruction did not diminish the intensity of the GAD-immunoreactivity within the contralateral rotundus. Our studies indicated that the source of the massive GAD-immunoreactive plexus within the rotundus was from the ipsilateral SP, SPcd, IPS, and PV nuclei. These nuclei, in turn, received ipsilateral tectal input via collaterals of the neurons of layer 13 in the course of their projections upon the rotundus. We suggest that the direct bilateral tecto-rotundal projections are excitatory, whereas the indirect ipsilateral projections from the SP/IPS and PV are mainly inhibitory, possibly acting via a GABA-A receptor.

Morphological Correlates of Bilateral Synchrony in the Rat Cerebellar Cortex

Simultaneous recordings of the left and right crus IIA of the cerebellar cortex in the rat have demonstrated that Purkinje cells of both sides can be activated synchronously by their climbing fibers. Because climbing fibers arise exclusively from the contralateral inferior olive (IO), this physiological finding seems to contradict the anatomy. To define the structural basis responsible for the bilateral synchrony, we examined the possibilities that bilateral common afferent inputs to the IO and interolivary connections form the underlying mechanisms. The bilaterality of the major afferents of the olivary regions that project to crus IIA was studied using Phaseolus vulgaris leucoagglutinin as an anterograde tracer. We found that the excitatory and inhibitory projections from the spinal trigeminal nucleus and dorsolateral hump of the interposed cerebellar nucleus to the transition area between the principal olive and dorsal accessory olive were bilateral. A second possible mechanism for bilateral synchrony, which is the possibility that axons of olivary neurons provide collaterals to the contralateral side, was investigated using biotinylated dextran amine as an anterograde tracer. Labeled axons were traced and reconstructed from the principal olive and dorsal and medial accessory olive up to the entrance of the contralateral restiform body. None of these axons gave rise to collaterals. The possibility that neurons in the left and right IO are electronically coupled via dendrodendritic connections was investigated by examining the midline region of the IO. The neuropil of the left and right IO is continuous in the dorsomedial cell column. Examination of Golgi impregnations of this subdivision demonstrated that (1) many dendrites cross from one side to the other, (2) neurons close to the midline give rise to dendrites that extend into both olives, and (3) dendrites of neurons in the dorsomedial cell column frequently traverse into adjacent olivary subdivisions such as the medial accessory olive and the transition area between the principal olive and dorsal accessory olive. Sections immunostained for dendritic lamellar bodies or GABAergic terminals showed the same pattern: the neuropils of the dorsomedial cell columns on both sides form a continuum with each other as well as with the neuropil of other adjacent olivary subdivisions. Ultrastructural examination of the dorsomedial cell column demonstrated that the midline area includes many complex glomeruli that contain dendritic spines linked by gap junctions. To verify whether the complex spike synchrony observed between left and right crus IIA could indeed be mediated in part through coupled neurons in the dorsomedial cell column, we recorded simultaneously from crus IIA areas and from left and right vermal lobule IX, which receives climbing fibers from the dorsomedial cell column. In these experiments we demonstrated that the climbing fibers of all four areas, i.e., the left and right crus IIA as well as the left and right lobule IX, can fire synchronously. The present results indicate that synchronous climbing fiber activation of the left and right crus IIA in the rat can be explained by (1) bilateral inputs to the transition areas between the principal olive and dorsal accessory olive and (2) dendrodendritic electrotonic coupling between neurons of the left and right dorsomedial cell column and between neurons of the dorsomedial cell column and adjacent olivary subdivisions.

Prefrontal cortex inputs of the nucleus accumbens-nigro-thalamic circuit.

The functional organization of the cortico-nucleus accumbens-substantia nigra pars reticulata circuit was investigated in the rat using combined anatomical and electrophysiological approaches. The nucleus accumbens neurons which project to the substantia nigra pars reticulata are located in a circumscribed region of the core immediately adjacent and extending dorsally to the anterior commissure. As shown by retrograde and anterograde transports of wheatgerm agglutinin conjugated to horseradish peroxidase, the region of the nucleus accumbens related to the substantia nigra was found to receive bilateral inputs from restricted areas of the medial and lateral prefrontal cortex, i.e., prelimbic/medial orbital and dorsal agranular insular areas. The electrical stimulation of these medial and lateral prefrontal cortical areas induced excitatory responses in nucleus accumbens neurons projecting to the dorsomedial substantia nigra pars reticulata. Interestingly, an important proportion (61%) of the nucleus accumbens-nigral cells responding to the stimulation of the lateral prefrontal cortex were also excited by the stimulation of the medial prefrontal cortex, demonstrating the existence of a convergent influence of these cortical areas on single nucleus accumbens cells. Furthermore, the present data also show that the stimulation of the medial prefrontal cortex results in a powerful inhibition of the tonic firing of the substantia nigra pars reticulata neurons. In conclusion, this study reveals the existence of a functional link between the prefrontal cortex (prelimbic/medial orbital and agranular insular areas) and the nucleus accumbens neurons which innervate the dorsomedial region of the substantia nigra pars reticulata. Since the dorsomedial region of substantia nigra pars reticulata is known to project to subfields of the mediodorsal and ventromedial thalamic nuclei related to the prefrontal cortex, the present data further demonstrate the existence of a prefrontal-nucleus accumbens-thalamo-cortical circuit involving the substantia nigra pars reticulata.

Effects of caloric vestibular stimulation on prepositus hypoglossi neurons in rats.

In our previous study, we found that the majority of locus coeruleus (LC) neurons were inhibited by caloric stimulation and that the inhibition was mediated by gamma aminobutyric acid (GABAA) receptors. It has been reported that the nucleus prepositus hypoglossi (PrH) sends GABAergic inputs to the LC. In the present study, an attempt was made to determine whether PrH neurons contribute to the LC neuronal inhibition caused by caloric stimulation. We characterized the neuronal response in the PrH to caloric stimulation in rats. About three-fourths of PrH neurons showed various responses to both ipsi- and contralateral caloric stimulation, suggesting that PrH neurons receive bilateral vestibular inputs. However, these neurons were not activated by antidromic stimulation of the LC. The remaining one-fourth of the PrH neurons that were antidromically activated did not respond to caloric stimulation. Lesions in the PrH did not attenuate the LC neuronal inhibition caused by caloric stimulation. These findings suggest that the inhibition of LC neurons caused by caloric stimulation is not mediated by the PrH.

The visual system of the Florida garfish, Lepisosteus platyrhincus (Ginglymodi). IV. Bilateral projections and the binocular visual field.

Infusion of cobaltous-lysine into the optic nerve of juvenile Florida garfish reveals that the preoptic area, pretectum, thalamus and the mediorostral and ventrolateral poles of the optic tectum each receive bilateral, monosynaptic input. The bilateral input to the mediorostral pole of the tectum projects via the dorsal optic tract and terminates in the rostral half of the tectum. The bilateral input to the ventrolateral tectum projects via the ventral optic tract and extends the whole length of the tectum. The incidence of direct ipsilateral input to the tectum suggests binocular vision may play a functional role in the survival of this species. In order to test this hypothesis, we examined the retinal distribution of ipsilateral retinal ganglion cells that project to the mediorostral tectum and the size and location of the binocular visual field. Cell distribution was examined by retrograde labelling with rhodamine-conjugated dextran amine delivered by microinjections into the mediorostral pole of the right optic tectum. In the ipsilateral retina, ganglion cells are distributed in a narrow temporoventral area with a mean of 0.44 +/- 0.14 x 10(3) cells per mm2 apposing the retinal margin. In the contralateral retina, ganglion cells are also distributed within this temporoventral region with a mean peak density of 2.33 +/- 0.47 x 10(3) cells per mm2. Three broad classes of ipsilaterally projecting retinal ganglion cells (orthotopic cells within the ganglion cell layer, displaced cells within the inner nuclear layer, and giant cells within the ganglion and inner nuclear layers) are identified, intermingled in both the ipsilateral and contralateral retinae after retrograde labelling from the mediorostral pole of the tectum. Ophthalmoscopic mapping of the monocular and binocular visual fields reveals two small frontal wedges of binocular overlap. A dorsal wedge (12 degrees wide) extends from approximately 7 degrees above the horizontal axis to 10 degrees beyond the vertical axis, and a ventral wedge (20 degrees wide) extends approximately 10 degrees below the horizontal axis to 15 degrees beyond the vertical axis. The dorsal binocular visual field is subtended by the temporoventral region of the retina possessing both ipsilaterally and contralaterally projecting ganglion cells which terminate in the mediorostral pole of the optic tectum. Therefore, the partial decussation of retinal ganglion cell axons at the optic chiasm brings information from corresponding regions of the binocular visual field into register in the mediorostral pole of the optic tectum.(ABSTRACT TRUNCATED AT 250 WORDS)

The Visual System of the Florida Garfish, <i>Lepisosteus platyrhincus</i>(Ginglymodi) (Part 2 of 2)

Infusion of cobaltous-lysine into the optic nerve of juvenile Florida garfish reveals that the preoptic area, pretectum, thalamus and the mediorostral and ventrolateral poles of the optic tectum each receive bilateral, monosynaptic input. The bilateral input to the mediorostral pole of the tectum projects via the dorsal optic tract and terminates in the rostral half of the tectum. The bilateral input to the ventrolateral tectum projects via the ventral optic tract and extends the whole length of the tectum. The incidence of direct ipsilateral input to the tectum suggests binocular vision may play a functional role in the survival of this species. In order to test this hypothesis, we examined the retinal distribution of ipsilateral retinal ganglion cells that project to the mediorostral tectum and the size and location of the binocular visual field. Cell distribution was examined by retrograde labelling with rhodamine-conjugated dextran amine delivered by microinjections into the mediorostral pole of the right optic tectum. In the ipsilateral retina, ganglion cells are distributed in a narrow temporoventral area with a mean of 0.44±0.14 x 10³ cells per mm² apposing the retinal margin. In the contralateral retina, ganglion cells are also distributed within this temporoventral region with a mean peak density of 2.33 ± 0.47 x 10³ cells per mm². Three broad classes of ipsilaterally projecting retinal ganglion cells (orthotopic cells within the ganglion cell layer, displaced cells within the inner nuclear layer, and giant cells within the ganglion and inner nuclear layers) are identified, intermingled in both the ipsilateral and contralateral retinae after retrograde labelling from the mediorostral pole of the tectum. Ophthalmoscopic mapping of the monocular and binocular visual fields reveals two small frontal wedges of binocular overlap. A dorsal wedge (12° wide) extends from approximately 7° above the horizontal axis to 10° beyond the vertical axis, and a ventral wedge (20° wide) extends approximately 10° below the horizontal axis to 15° beyond the vertical axis. The dorsal binocular visual field is subtended by the temporoventral region of the retina possessing both ipsilaterally and contralaterally projecting ganglion cells which terminate in the mediorostral pole of the optic tectum. Therefore, the partial decussation of retinal ganglion cell axons at the optic chiasm brings information from corresponding regions of the binocular visual field into register in the mediorostral pole of the optic tectum. In addition, the size of the binocular visual field matches the relative proportion of the retina containing ipsilaterally projecting ganglion cells. The ontogeny and phylogeny of ipsilateral retinofugal projections in actinopterygians is discussed in relation to the putative secondary pathways mediating binocular vision.

Intergeniculate leaflet: An anatomically and functionally distinct subdivision of the lateral geniculate complex

The intergeniculate leaflet (IGL) in the rat is a distinctive subdivision of the lateral geniculate complex that participates in the regulation of circadian function through its projections to the circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. The present investigation was undertaken to provide a precise definition of the IGL and a characterization of its neuronal organization including neuronal morphology, chemical phenotype, connections, and synaptic organization. The IGL extends the entire rostrocaudal length of the geniculate complex and contains a distinct population of small to medium neurons. In Golgi preparations, the neurons are multipolar with dendrites largely confined to the IGL. The neurons can be subdivided into three groups on the basis of neurotransmitter content and projections: (1) neurons that contain GABA and neuropeptide Y and project to the SCN; (2) neurons that contain GABA and enkephalin and project to the contralateral IGL; and (3) a small group of neurons that projects to the SCN but not characterized as yet by neurotransmitter content. The IGL receives dense, bilateral input from retinal ganglion cells and dense substance P input of unknown origin. A number of neurons in the anterior hypothalamic area and, particularly, the retrochiasmatic area project to the IGL, and there are sparse projections from brainstem monoamine and cholinergic neurons. The synaptic organization of the IGL is complex with afferents terminating in glomerular complexes that include axoaxonic synaptic interactions. Virtually all IGL afferents synapse upon dendrites and spines, with the densest synaptic input occurring on the distal portions of the dendritic arbor. The organization of the IGL and its connections as revealed in this analysis is in accord with its role in the integration of visual input with other information to provide feedback regulation of the SCN pacemaker.

Giant neurons in the rat reticular formation: a sensorimotor interface in the elementary acoustic startle circuit?

The mammalian acoustic startle response (ASR) is a relatively simple motor response that can be elicited by sudden and loud acoustic stimuli. The ASR shows several forms of plasticity, such as habituation, sensitization, and prepulse inhibition, thereby making it an interesting model for studying the underlying neuronal mechanisms. Among the neurons that compose the elementary startle circuit are giant neurons in the caudal pontine reticular nucleus (PnC), which may be good candidates for analyzing the neuronal basis of mammalian behavior. In a first step of this study, we employed retrograde and anterograde tracing techniques to identify the possible sources of input and the efferent targets of these neurons. In a second step, we performed intracellular recordings in vivo, followed by subsequent injections of HRP for morphological identification, thereby investigating whether characteristic features of the ASR are reflected by physiological properties of giant PnC neurons. Our observations demonstrate convergent, bilateral input from several auditory brainstem nuclei to the PnC, predominantly originating from neurons in the cochlear nuclear complex and the superior olivary complex. Almost no input neurons were found in the nuclei of the lateral lemniscus. As the relatively long neuronal response latencies in several of these auditory nuclei appear to be incompatible with the primary ASR, we conclude that neurons in the cochlear root nuclei most likely provide the auditory input to PnC neurons that is required to elicit the ASR. The giant PnC neurons have a remarkable number of physiological features supporting the hypothesis that they may be a neural correlate of the ASR: (1) they receive short-latency auditory input, (2) they have high firing thresholds and broad frequency tuning, (3) they are sensitive to changes in stimulus rise time and to paired-pulse stimulation, (4) repetitive acoustic stimulation results in habituation of their response, and (5) amygdaloid activity enhances their response to acoustic stimuli. Anterograde tracing showed that most giant PnC neurons are reticulospinal cells. Axon collaterals and terminal arbors were found in the reticular formation as well as in cranial and spinal motoneuron pools. The results of this study indicate that giant PnC neurons form a sensorimotor interface between the cochlear nuclear complex and cranial and spinal motoneurons. This neuronal pathway implies that the elementary acoustic startle circuit is composed of only three central relay stations and thus appears to be organized more simply than assumed in the past.

Mirror-image matching and mental rotation problem solving by baboons (Papio papio): unilateral input enhances performance.

Three experiments, using a matching-to-sample procedure, were conducted to examine hemispheric specialization in mirror-image discrimination and mental rotation in baboons (Papio papio). In Experiment 1, no significant difference was found in discrimination of mirror-image and asymmetric pattern stimuli. In Experiment 2, orientation discrimination was assessed within the left (LVF) and right (RVF) visual half-fields. An RVF advantage was found in accuracy for asymmetric patterns, whereas an LVF advantage was found for discrimination of mirror-image stimuli. No significant relation was found between angular disparity of the stimuli and response time. Experiment 3 examined the effect of bilateral visual input on accuracy and response time. Significantly lower accuracy and longer response times were found for bilateral compared with unilateral visual input.

The oculo-auricular phenomenon. Findings in normals and patients with brainstem lesions.

The oculo-auricular phenomenon consists of bilateral coactivation of external ear muscles during lateral gaze. The electromyogram of the transverse auricular muscle was recorded in 25 healthy volunteers and 1186 patients. In normal subjects bilateral coactivation was observed with lateral gaze (96%), convergence (61%), active and intended head rotation (100%) and passive head rotation (50%). Uni- and bilateral labyrinthine excitation (60 and 80%) and proprioceptive input from the neck muscles (38%) are also effective. In patients with brainstem disease abnormal transverse auricular muscle coactivation is characterized by absence of activity in one or both ear muscles during lateral gaze in either or both directions. The most frequent abnormality was absence of transverse auricular muscle activity homolateral to right or left gaze (type Ia pattern). It was related to homolateral impairment of the blink reflex R1-response (90%) and the caloric response (90%), and to contralateral masseter reflex abnormality (70%). Electrophysiological data, clinical signs and imaging findings indicate that the type Ia pattern is caused by homolateral pontine or contralateral midbrain lesions. It is suggested that the supranuclear organization of the oculo-auricular phenomenon is based on descending tracts crossing at a mid-pontine level.