Electrophysiological investigations on the pigeon's optic tectum

Topography of visual and somatosensory inputs to the pontine nuclei in zebra finches (Taeniopygia guttata)

Birds have a comprehensive network of sensorimotor projections extending from the forebrain and midbrain to the cerebellum via the pontine nuclei, but the organization of these circuits in the pons is not thoroughly described. Inputs to the pontine nuclei include two retinorecipient areas, nucleus lentiformis mesencephali (LM) and nucleus of the basal optic root (nBOR), which are important structures for analyzing optic flow. Other crucial regions for visuomotor control include the retinorecipient ventral lateral geniculate nucleus (GLv), and optic tectum (TeO). These visual areas, together with the somatosensory area of the anterior (rostral) Wulst, which is homologous to the primary somatosensory cortex in mammals, project to the medial and lateral pontine nuclei (PM, PL). In this study, we used injections of fluorescent tracers to study the organization of these visual and somatosensory inputs to the pontine nuclei in zebra finches. We found a topographic organization of inputs to PM and PL. The PM has a lateral subdivision that predominantly receives projections from the ipsilateral anterior Wulst. The medial PM receives bands of inputs from the ipsilateral GLv and the nucleus laminaris precommisulis, located medial to LM. We also found that the lateral PL receives a strong ipsilateral projection from TeO, while the medial PL and region between the PM and PL receive less prominent projections from nBOR, bilaterally. We discuss these results in the context of the organization of pontine inputs to the cerebellum and possible functional implications of diverse somato-motor and visuomotor inputs and parcellation in the pontine nuclei.

Features of the retinotopic representation in the visual wulst of a laterally eyed bird, the zebra finch (Taeniopygia guttata)

The visual wulst of the zebra finch comprises at least two retinotopic maps of the contralateral eye. As yet, it is not known how much of the visual field is represented in the wulst neuronal maps, how the organization of the maps is related to the retinal architecture, and how information from the ipsilateral eye is involved in the activation of the wulst. Here, we have used autofluorescent flavoprotein imaging and classical anatomical methods to investigate such characteristics of the most posterior map of the multiple retinotopic representations. We found that the visual wulst can be activated by visual stimuli from a large part of the visual field of the contralateral eye. Horizontally, the visual field representation extended from -5° beyond the beak tip up to +125° laterally. Vertically, a small strip from -10° below to about +25° above the horizon activated the visual wulst. Although retinal ganglion cells had a much higher density around the fovea and along a strip extending from the fovea towards the beak tip, these areas were not overrepresented in the wulst map. The wulst area activated from the foveal region of the ipsilateral eye, overlapped substantially with the middle of the three contralaterally activated regions in the visual wulst, and partially with the other two. Visual wulst activity evoked by stimulation of the frontal visual field was stronger with contralateral than with binocular stimulation. This confirms earlier electrophysiological studies indicating an inhibitory influence of the activation of the ipsilateral eye on wulst activity elicited by stimulating the contralateral eye. The lack of a foveal overrepresentation suggests that identification of objects may not be the primary task of the zebra finch visual wulst. Instead, this brain area may be involved in the processing of visual information necessary for spatial orientation.

An in vitro study of retinotectal transmission in the chick: role of glutamate and GABA in evoked field potentials

We have developed two brain slice preparations for studying tectofugal visual pathways in the chick: conventional, 400-microns slices ("thin slices"), and "thick slices" which encompass the rostral pole of the optic tectum and the contralateral optic nerve. Stimulation was delivered with a bipolar electrode positioned in stratum opticum in thin slices and in the contralateral optic nerve in thick slices. While the latter preparation provided a means of exclusively and unambiguously activating retinal afferents, several lines of evidence also indicated that the evoked field potentials in thin slices were chiefly consequent to retinal afferent excitation: (1) the similarity of evoked field potentials in thin slices to those in thick slice preparations; (2) their precise localization in retinorecipient layers as shown by prelabeling from retina with FITC-coupled cholera toxin; (3) transmission delays appropriate for retinal afferents as established with the thick slice preparation; (4) patterns of labeled afferents resulting from applications of Dil crystals to slices fixed after recording; and (5) the similarity in transmitter pharmacology between thin and thick slice preparations. Pharmacological manipulations carried out with bath-applied antagonists indicated that glutamate is the principal retinotectal transmitter. The broadly active glutamate receptor blocker, kynurenic acid, reversibly eliminated the postsynaptic component of the field potential as confirmed with 0 Ca2+ saline. A complete block was also effected by the non-NMDA antagonists CNQX and DNQX. The specific NMDA antagonist, AP5, caused a smaller and variable reduction in response amplitude. The GABA antagonist, bicuculline, caused a prolongation of the monosynaptic field epsp in retinorecipient layers and an enhancement of the long-latency, negative wave in cellular layers below, supporting a late, excitation-limiting role for this inhibitory transmitter.

Conduction velocity groups in the retino-tectal and retino-thalamic visual pathways of the pigeon (Columbia livia)

The anatomical characteristics of the avian visual system are well known. However, there are wide gaps in our knowledge with respect to the physiological characteristics of their visual system. For example, we lack both an operational identification of the different ganglion cell types present in the retinae of birds, and a description of their presumptive differential central projections. The results presented here address this latter point by classifying the conduction velocity groups of fibers present in the optic tract of the pigeon. We report the existence of at least 5 groups of axons in the optic tract of the pigeon, with conduction velocities of 22-18 m/s, 12-10 m/s, 8 m/s, 6 m/s and less than 2.5 m/s. All five groups project to the tectum but only the four fastest groups project to the dorsal thalamic complex. The homologies with the populations of retinal axons found in cats are discussed.

Retinal afferents to the tectum opticum and the nucleus opticus principalis thalami in the pigeon

The retinal afferents of the tectum opticum and the n. opticus principalis thalami (OPT) were studied with fluorescent tracers in pigeons. Injections into the tectum opticum revealed topographically related areas of high density labelling in the contralateral retina. In these areas up to 15,000 cells/mm2 were labelled. After tectal injections the soma sizes of labelled retinal ganglion cells in the area centralis ranged from 5 to 23 microns with a mean of 7.5 microns. Afferents from the ipsilateral retina could not be demonstrated. Injections into the OPT labelled neurons throughout the retina without a clear topographical relation to the locus of injection. The density never exceeded 150 cells per mm2. The soma size range was 8 to 35 microns with a mean of 14.6 microns. Independently of the injection area within the OPT, the red field in the dorsotemporal retina was always extremely sparsely labelled. The number of labelled ganglion cells in this area never exceeded 25 neurons/mm2. After OPT injections the average density of labelling per unit area was six times higher in the yellow than in the red field. The results confirm previous reports of a massive and topographically organized retinal projection onto the optic tectum. The projection onto the OPT was clearly smaller and with the retrograde tracing techniques in use, an orderly topography has not been demonstrated. The paucity of red field projections onto the OPT suggests that the role of the thalamofugal pathway in binocular integration is very limited.

Retinotopic representation of the bifoveate eye of the kestrel (Falco spraverius) on the optic tectum

Like many diurnal raptors, the American kestrel or sparrow hawk, Falco sparverius, possesses two foveae in each eye. In this study, we used fundus photography and reversed ophthalmoscopy to plot the projection of these foveae onto the visual field, together with other retinal landmarks such as the pecten and ora terminali. From such data, it was determined that the central monocular fovea (CMF) and temporal binocular fovea (TBF) were separated by 36 deg and that the kestrel has 58 deg of binocular overlap when the eyes are appropriately converged. Single-cell and multi-unit recordings were used to systematically explore the tectal surface and map receptive fields (RF) onto a hemispheric screen. A retinotopic map of the tectal surface was produced from such data and revealed an expanded representation for each fovea on the tectum and a systematic increase in RF size from fovea to periphery. The functional significance of this organization is discussed.

The topographical projection of the nucleus isthmi pars parvocellularis (Ipc) onto the tectum opticum in the pigeon

The projection of the nucleus isthmi pars parvocellularis (Ipc) onto the tectum opticum was studied with tectal injections of three different fluorescent tracers and wheat germ-agglutinated horseradish peroxidase (WGA-HRP) in 21 pigeons. In frontal sections obtained, the relationship between position and size of the tectal injection and the location of the retrogradely labelled Ipc neurons were analyzed. The results reveal a topographically organized system with reciprocal projections in which the dorsoventral axis of the tectum receives afferents from the lateromedial axis of the Ipc. The caudorostral tectal axis is innervated by neurons situated along the caudorostral axis of the Ipc. Specific points on the tectal surface do not correspond to single points within the Ipc, but to column-like strings of Ipc neurons extending through the dorsoventral aspect of the nucleus.

Sensory properties and afferents of the N. dorsolateralis posterior thalami of the pigeon

According to previous studies, the avian n. dorsolateralis posterior thalami (DLP) receives visual and somatosensory afferents. While some authors (e.g., Gamlin and Cohen: J. Comp. Neurol. 250:296-310, '86) proposed a distinction between a visual caudal (DLPc) and a somatosensory rostral (DLPr) part, other authors (e.g., Wild: Brain Res. 412:205-223, '87) could not confirm such a differentiation. The aim of the present experiment was to study with physiological and anatomical methods the proposed parcellation of the DLP into various components dealing with different modalities. The physiological properties of the DLP of the pigeon were analysed with extracellular single unit recordings. With the same approach, neurons of the n. dorsalis intermedius ventralis anterior (DIVA), a somatosensory relay nucleus in the dorsal thalamus, were also analysed. The afferents of the DLP were studied by using anatomical tract tracing techniques with retrograde and anterograde tracers. The sensory properties of DLP cells revealed that somatosensory, visual, and auditory modalities affect the neuronal firing frequency in this nucleus. All three modalities were present throughout the full caudorostral extent of the DLP. Cells recorded in DIVA responded nearly exclusively to somatosensory stimulation. Unlike the DLP, single units in DIVA generally had smaller receptive fields encompassing only one extremity. The analysis of afferent connections of the DLP by using injections of retrograde and anterograde tracers (HRP, WGA-HRP, Fast Blue, and Rhodamine-beta-isothiocyanate) demonstrated extensive projections from the nuclei gracilis et cuneatus (GC) and more sparse projections from the nucleus tractus descendens trigemini (TTD), and the nucleus cuneatus externus (CE). Brainstem afferents of the DLP came from different vestibular nuclei, various areas of the brainstem reticular formation, and the optic tectum. Prosencephalic afferents originated in the n. posteroventralis thalami (PV), the n. ventromedialis posterior thalami (VMP), the n. dorsalis intermedius ventralis anterior (DIVA), and the nucleus reticularis superior pars dorsalis and ventralis (RSd and RSv). Telencephalic afferents of the DLP came from the hyperstriatum accessorium (HA) and a group of cells at the borderline between the hyperstriatum intercalatus superior (HIS) and the hyperstriatum dorsale (HD). The somatosensory afferents of the DLP probably originate from the GC, TTD, and CE, whereas it is likely that the visual input is mediated by the optic tectum. The anatomical source for the acoustic input is unclear. The very long latencies of auditory DLP neurons make it likely that the acoustic input originates at least partly in the reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Near-field acuity after visual system lesions in pigeons. I. Thalamus

Visual acuity determinations were made for 14 pigeons trained to discriminate high-contrast, square-wave gratings of successively higher spatial frequencies from blank stimuli of equal average luminances. The stimuli were presented according to the method of constant stimuli. Video-taped motion pictures of the key-pecking response provided a measure of the cornea-to-stimulus distance. The preoperative results indicated that the mean minimal-separable visual angle was 2.3 min (range 1.6-3.6) under conditions of photopic adaptation with the stimulus luminance at 70 cd/m2. When performance was stable, bilateral electrolytic lesions were placed in: nucleus rotundus (RT), nucleus opticus principalis thalami (the OPT complex); and RT plus the OPT complex. Lesions confined to the OPT complex did not impair resolution threshold while lesions confined to the nucleus rotundus produced permanent threshold elevations. When the destruction of OPT and RT was combined, the deficit was more severe. The results indicated that the integrity of the tectofugal pathway of the pigeon is required for normal performance of spatial frequency discrimination near threshold. These observations suggest that nuclei containing neurons with wide receptive fields are capable of processing visual information with fine spatial detail.

Receptive field properties of single cells in the pigeon's optic tectum during cooling of the 'visual wulst'

In birds, efferents from the visual telencephalon (visual wulst) terminate in the ipsilateral and contralateral optic tectum. This study concerns the influence of a bilateral cryogenic block of the wulst on the receptive field properties of the visual tectal cells in the pigeon. Tectal units were tested for their responses to static and moving stimuli before, during and after cooling the wulst. For some units the cryogenic block of the wulst was repeated twice. The responsiveness to static and moving stimuli was decreased in most of the tectal cells when the neural activity of the wulst was blocked. In contrast, in some units cooling the wulst provokes an increase of responsiveness. These results indicate that the wulst-tectum path is able to convey both excitatory and inhibitory influences. Other receptive field properties such as the spatial location of the light and dark excitatory regions in the field, the effect of the surround, the size and shape of the excitatory region, the relative responsiveness to static and moving stimuli and the 'spontaneous activity' were not affected by wulst cooling. Directional tuning curves were obtained in 18 directionally selective cells before, during and after wulst cooling. In 6 of them the cryogenic block provoked a reduction in directional selectivity either by way of a reduction of the preferred response (4 cells) or by way of an increase of the non-preferred responses (2 cells). In two others directionally selective cells, cooling the wulst provoked a total loss of directional selectivity due to a reduction of the response to the preferred direction together with an increase of the response to the null direction. These results show: (1) that the retinal directional selective input to the tectum is affected by the cryogenic block of the wulst; and (2) that the visual wulst provokes a sharpening of the directional tuning at the optic tectum level.

Regional differences in pigeon optic tract, chiasm, and retino-receptive layers of optic tectum

Electron-microscopic examination of the pigeon optic chiasm, tract, stratum opticum, and retino-receptive layers of the optic tectum revealed regional differences at each level. Axonal size in the fiber pathways paralleled that previously reported for pigeon optic nerve, with mean diameter values of 0.96 micrometer for optic chiasm and 1.06 micrometer for optic tract. The dorsolateral aspects of these pathways contained a heterogeneous population of fibers (mean diameter congruent to 1.44 micrometer) similar to that found in the nasal portion of optic nerve, while the ventromedial regions were occupied by a more homogeneous population of smaller fibers (mean diameter congruent to 0.82 micrometer) resembling those observed in the temporal portion of the nerve. The retino-receptive layers of anteroventral optic tectum (avT) differed ultrastructurally from those of posterodorsal tectum (pdT) with respect to the thickness of horizontal dendrites in layer 2-3, the size of optic terminals in layers 2-7, and the number of synaptic contacts per terminal. These findings point towards a regional variation in the processing of visual information throughout the retino-tectal system and suggest that neurons in avT vs. pdT should show differences in the way they modify the neurophysiological characteristics of their respective optic inputs.

Topographic projections of the retina and optic tectum upon the ventral lateral geniculate nucleus in the chick

The topographic projections of the retina upon the optic tectum and ventral lateral geniculate nucleus (GLv) of the chick were investigated by making small intraretinal injections of 3H-proline. The retinotectal projection pattern was similar to that described for the pigeon. The retinal projection to the GLv was also topographic and was restricted to the outermost lamina of the nucleus. The anteroposterior retinal axis was reversed in the GLv relative to its orientation in the tectum but the superoinferior axis was oriented identically in both. Furthermore, the posterior retina had an enlarged area of projection in the GLv similar to the enlarged area of retinotectal projection for the "red field" found in pigeons. The tectogeniculate projection was topographic and was confined to the outermost geniculate lamina. The second-order retinotopic map made by the tectogeniculate projections was in register with the retinogeniculate projection. Although the retinal and tectal projection areas were coextensive in the outermost geniculate lamina, the grain density distributions peaked at different points along a radial path through the geniculate laminae. Injections of HRP into the optic tectum led to very light retrograde labeling of a small population of GLv cells topographically corresponding to the tectogeniculate projection zone of the injection site. The data suggest that the chick GLv is comparable to the GLv of other non-primate mammals.

Functional role of efferents to the avian retina. I. Analysis of retinal ganglion cell receptive fields

Receptive fields of retinal ganglion cells were analyzed during extracellular microelectrode recordings in the optic tract of the lightly anesthetized pigeon. Four major types of receptive field can be distinguished among the 359 fibers studied. Twenty-five percent of the receptive fields are relatively simple, responding at on and at off to stationary spots of light in the central region. All of the receptive fields have inhibitory surrounds of varying strength that do not produce a response when illuminated alone, but antagonize responses from the central region. Motion sensitive units comprise 15% of the recorded population; they are similar to the on-off center type except that responses to stationary stimuli are absent or very weak while responses to moving stimuli are virorous. Directionally selective units also have the basic features of on-off, inhibitory surround cells, but respond to moving stimuli well from the preferred direction and not at all from the null direction. Directional cells have a broad range of null directions; in about one-third of the units the range becomes broader when the stimulus involves both center and surround of the receptive field, thus enhancing directional selectivity. Directionally selective units are common, comprising 38% of the units studied. Cells unresponsive to stimuli moving from anterior in the visual field are much more common than other types, while cells unresponsive to stimuli from posterior in the field are rare. A few units (11%) respond only at on or at off to stationary stimuli in their receptive field centers; they also have antagonistic but unresponsive receptive field surrounds. The area of the visual field sampled is uniform in regard to the relative numbers of the four major receptive field types.

The connections and laminar organization ofthe optic tectum in a reptile (lguana iguana)

The goals of this study were: (1) to describe the total pattern of projections from the optic tectum of Iguana iguana and Pseudemys scripta; and (2) to describe the contributions of particular lamina of the Iguana's optic tectum to this total pattern. Lesions were made in the optic tectum of the Iguana which damaged either all or only certain tectal laminae and, for comparison with the Iguana, lesions in the turtle's optic tectum were made which involved all laminae. The anterograde degeneration resulting from these lesions was stained with the Fink-Heimer ('67) method. The total pattern of projections from the optic tectum in the Iguana and the turtle is similar to that reported for representatives of other vertebrate classes. That is, the optic tectum gives rise to ipsilateral ascending projections to pretectal nuclei, to nucleus rotundus and to nucleus geniculatus lateralis pars ventralis of the diencephalon and, in addition, to a contralateral ascending pathway which courses via the supraoptic decussation to the contralateral diencephalon. Tectotectal connections and several descending pathways were also recognized in each species. The descending pathways include ipsilateral tectobulbar and tecto-isthmi pathways and a contralateral predorsal bundle. Lesions which damaged only certain tectal laminae in the Iguana revealed a laminar organization of the efferent projections. A lesion restricted to the superficial retinal-recipient layers, stratum griseum et album superficiale, resulted in degeneration in only nucleus isthmi pars magnocellularis and nucleus geniculatus lateralis pars ventralis. A lesion which involved both the retinal-recipient layers and stratum griseum centrale resulted in degeneration in only one additional structure, nucleus rotundus. A small lesion involving the deep periventricular layers as well as the superficial layers produced degeneration in the predorsal bundle and the ipsilateral tectobulbar tract as well as in the structures receiving input from the more superficial layers. These results are compared to the results of similar analyses of the superior colliculus in mammals.