TYPES OF VISUAL RESPONSE FROM SINGLE UNITS IN THE OPTIC TECTUM AND OPTIC NERVE OF THE GOLDFISH

The structure and diversity of retinal ganglion cells in the masked greenling Hexagrammos octogrammus Pallas, 1814 (Pisces: Scorpaeniformes: Hexagrammidae)

The authors studied the structure and diversity of retinal ganglion cells (GC) in the masked greenling Hexagrammos octogrammus. In vivo labelling with horseradish peroxidase revealed GCs of various structures in retinal wholemounts. A total of 154 cells were camera lucida drawn, and their digital models were generated. Each cell was characterized by 17 structural and topological parameters. Using nine clustering algorithms, a variety of clusterings were obtained. The optimum clustering was found using silhouette analysis. It was based on a set of three variables associated with dendritic field size and dendrite stratification depth in the retina. A total of nine cell types were discovered. A number of non-parametric tests showed significant pair-wise between-cluster differences in at least four parameters with medium and large effect sizes. Three large-field types differed mainly in dendritic field size, total dendrite length, level of dendrite stratification in the retina and position of somata. Six medium- to small-field types differed mainly in the structural complexity of dendritic arbors and level of dendrite arborization. Cells similar and obviously homologous to types 1-4 were identified in many fish species, including teleosts. Potential homologues of type 5 cells were identified in fewer teleost species. Cells similar to types 6-9 in relative dendritic field size and dendrite arborization pattern were also described in several teleostean species. Nonetheless, their homology is more questionable as their stratification patterns do not match so well as they do in large types. Potential functional matches of the GC types were identified in a number of teleostean species. Type 1 and 2 cells probably match spontaneously active units with the large receptive field centre, so-called dimming and lightening detectors; type 4 may be a counterpart of changing contrast detectors with medium receptive field centre size preferring fast-moving stimuli. Type 3 (biplexiform) cells have no obvious functional matches. Probable functional matches of types 6, 8 and 9 belong to ON-centre elements with small receptive fields such as ON-type direction-selective cells, ON-type spot detectors or ON-type spontaneously active units. Type 5 and 7 cells may match ON-OFF type units, in particular, changing contrast detectors or orientation-selective units. Potential functional matches of GC types presently described are involved in a wide spectrum of visual reactions related to adaptation to gradual change in illumination, predator escape, prey detection and capture, habitat selection and social behaviour.

The tectum/superior colliculus as the vertebrate solution for spatial sensory integration and action

The superior colliculus, or tectum in the case of non-mammalian vertebrates, is a part of the brain that registers events in the surrounding space, often through vision and hearing, but also through electrosensation, infrared detection, and other sensory modalities in diverse vertebrate lineages. This information is used to form maps of the surrounding space and the positions of different salient stimuli in relation to the individual. The sensory maps are arranged in layers with visual input in the uppermost layer, other senses in deeper positions, and a spatially aligned motor map in the deepest layer. Here, we will review the organization and intrinsic function of the tectum/superior colliculus and the information that is processed within tectal circuits. We will also discuss tectal/superior colliculus outputs that are conveyed directly to downstream motor circuits or via the thalamus to cortical areas to control various aspects of behavior. The tectum/superior colliculus is evolutionarily conserved among all vertebrates, but tailored to the sensory specialties of each lineage, and its roles have shifted with the emergence of the cerebral cortex in mammals. We will illustrate both the conserved and divergent properties of the tectum/superior colliculus through vertebrate evolution by comparing tectal processing in lampreys belonging to the oldest group of extant vertebrates, larval zebrafish, rodents, and other vertebrates including primates.

Reduction of visual stimulus artifacts using a spherical tank for small, aquatic animals

Delivering appropriate stimuli remains a challenge in vision research, particularly for aquatic animals such as zebrafish. Due to the shape of the water tank and the associated optical paths of light rays, the stimulus can be subject to unwanted refraction or reflection artifacts, which may spoil the experiment and result in wrong conclusions. Here, we employ computer graphics simulations and calcium imaging in the zebrafish optic tectum to show, how a spherical glass container optically outperforms many previously used water containers, including Petri dish lids. We demonstrate that aquatic vision experiments suffering from total internal reflection artifacts at the water surface or at the flat container bottom may result in the erroneous detection of visual neurons with bipartite receptive fields and in the apparent absence of neurons selective for vertical motion. Our results and demonstrations will help aquatic vision neuroscientists on optimizing their stimulation setups.

On the organization of receptive fields of retinal spot detectors projecting to the fish tectum: Analogies with the local edge detectors in frogs and mammals

Responses of ON- and OFF-ganglion cells (GCs) were recorded extracellularly from their axon terminals in the medial sublamina of tectal retino-recipient layer of immobilized cyprinid fish (goldfish and carp). These units were recorded deeper than direction selective (DS) ones and at the same depth where responses of orientation selective (OS) GCs were recorded. Prominent responses of these units are evoked by small contrast spots flickering within or moving across their visual field. They are not selective either to the direction of motion or to the orientation of stimuli and are not characterized by any spontaneous spike activity. We refer to these fish GCs as spot detectors (SDs) by analogy with the frog SD. Receptive fields (RFs) of SDs are organized concentrically: the excitatory center (about 4.5°) is surrounded by opponent periphery. Study of interactions in the RF has shown that inhibitory influences are generated already inside the central RF area. This fact suggests that RFs of SDs cannot be defined as homogeneous sensory zone driven by a linear mechanism of response generation. Physiological properties of fish SDs are compared with the properties of frog SDs and analogous mammalian retinal GCs-local edge detectors (LEDs). The potential role of the SDs in visually guided fish behavior is discussed.

Updated functional segregation of retinal ganglion cell projections in the tectum of a cyprinid fish-further elaboration based on microelectrode recordings

Single-unit responses of retinal ganglion cells (GCs) were recorded extracellularly from their axonal terminals in the tectum opticum (TO) of the intact fish (goldfish, carp). The depths of retinal units consecutively recorded along the track of the microelectrode were measured. At the depth of around 50 μm, the responses of six types of direction-selective (DS) GCs were regularly recorded. Responses of two types of orientation-selective (OS) GCs and detectors of white and black spots occurred approximately 50 μm deeper. Responses of GCs with dark- and light-sustained activity were recorded deeper than all others, at about 200 μm. The receptive fields of consecutively recorded units overlap, so they analyze the same fragment of the visual scene, focused by eye optic on the photoreceptor raster. The responses of pairs of DS GCs (ON and OFF units that preferred same direction of stimulus movement) and OS GCs (detectors of vertical and horizontal lines) were often simultaneously recorded at one position of the microelectrode. (The paired recordings of certain units amounted about fourth part of all recordings.) This suggests that their axonal arborizations are located close to each other in the tectal retinorecipient layer. Electrophysiological method, thus, allows to indirectly clarify and make precise the morphology of the retino-tectal connections and to establish a morpho-physiological correspondence.

Putative targets of direction-selective retinal ganglion cells in the tectum opticum of cyprinid fish

Responses of direction selective (DS) units of retinal and tectal origin were recorded extracellularly from the tectum opticum (TO) of immobilized fish. The data were collected from three cyprinid species - goldfish, carp and roach. Responses of the retinal DS ganglion cells (GCs) were recorded from their axon terminals in the superficial layers of TO. According to their preferred directions DS GCs, characterized by small receptive fields (3-8°), can be divided in three distinct groups, each group containing ON and OFF subtypes approximately in equal quantity. Conversely, direction-selective tectal neurons (DS TNs), recorded at two different tectal levels deeper than the zone of retinal DS afferents, are characterized by large receptive fields (up to 60°) and are indifferent to any sign of contrast i.e. can be considered as ON-OFF type units. Fish DS TNs unlike the retinal DS GCs, select four preferred directions. Three types of tectal DS units prefer practically the same directions as those already selected on the retinal level - caudo-rostral, dorso-ventral and ventro-dorsal. The fact that three preferred directions of DS GCs and DS TNs coincide allows us to assume that three types of DS GCs are input neurons for corresponding types of DS TNs. The fourth group of DS TNs has the emergent rostro-caudal preference not explicitly present in any of the DS GC inputs. These units are recorded in deep TO layers exclusively. Receptive fields of these DS neurons could be entirely formed on the tectal level. Possible interrelations between retinal and tectal DS units are discussed.

Direction-selective units in goldfish retina and tectum opticum - review and new aspects

The output units of fish retina, i.e., the retinal ganglion cells (detectors), send highly processed information to the primary visual centers of the brain, settled in the midbrain formation tectum opticum (TO). Axons of different fish motion detectors terminate in different tectal levels. In the superficial layer of TO, axons of direction-selective ganglion cells (DS GCs) are terminated. Single unit responses of the DS GCs were recorded in intact fish from their axon terminals in TO. Goldfish DS GCs projecting to TO were shown to comprise six physiological types according to their selectivity to sign of stimulus contrast (ON and OFF units) and their preferred directions: three directions separated by 120[Formula: see text]. These units, characterized by relatively small receptive fields and remarkable spatial resolution should be classified as local motion detectors. In addition to the retinal DS GCs, other kinds of DS units were extracellularly recorded in the superficial and deep sublaminae of tectum. Some features of their responses suggested that they originated from tectal neurons (TNs). Contrary to DS GCs which are characterized by small RFs and use separate ON and OFF channels, DS TNs have extra-large RFs and ON-OFF type responses. DS TNs were shown to select four preferred directions. Three of them are compatible with those already selected on the retinal level. Complementary to them, the fourth DS TN type with rostro-caudal preference (lacking in the retina) has been revealed. Possible functional interrelations between DS GCs and DS TNs are discussed.

Functional segregation of retinal ganglion cell projections to the optic tectum of rainbow trout

The interpretation of visual information relies on precise maps of retinal representation in the brain coupled with local circuitry that encodes specific features of the visual scenery. In nonmammalian vertebrates, the main target of ganglion cell projections is the optic tectum. Although the topography of retinotectal projections has been documented for several species, the spatiotemporal patterns of activity and how these depend on background adaptation have not been explored. In this study, we used a combination of electrical and optical recordings to reveal a retinotectal map of ganglion cell projections to the optic tectum of rainbow trout and characterized the spatial and chromatic distribution of ganglion cell fibers coding for increments (ON) and decrements (OFF) of light. Recordings of optic nerve activity under various adapting light backgrounds, which isolated the input of different cone mechanisms, yielded dynamic patterns of ON and OFF input characterized by segregation of these two fiber types. Chromatic adaptation decreased the sensitivity and response latency of affected cone mechanisms, revealing their variable contributions to the ON and OFF responses. Our experiments further demonstrated restricted input from a UV cone mechanism to the anterolateral optic tectum, in accordance with the limited presence of UV cones in the dorsotemporal retina of juvenile rainbow trout. Together, our findings show that retinal inputs to the optic tectum of this species are not homogeneous, exhibit highly dynamic activity patterns, and are likely determined by a combination of biased projections and specific retinal cell distributions and their activity states.

Color properties of the motion detectors projecting to the goldfish tectum: II. Selective stimulation of different chromatic types of cones

Sensitivity to the sign of contrast of direction-selective (DS) and orientation-selective (OS) ganglion cells (GCs) was investigated with selective stimulation of different chromatic types of cones. It was shown that the DS GCs that were classified with the use of achromatic stimuli as belonging to the ON type responded to selective stimulation of the long-wave cones as the ON type also, while the stimulation of middle-wave or short-wave cones elicited the OFF type responses. Character of the responses of DS GCs of the OFF type was exactly the opposite. OS GCs, which responded to achromatic stimuli as the ON-OFF type, responded to selective stimulation of the long-wave cones as the ON-OFF type as well, responded to middle-wave stimulation as the OFF type and to stimulation of short-wave cones it responded mainly as the ON type. At the same time, under color-selective stimulation, both DS and OS GCs retained the directional and orientation selectivity with the same preferred directions. The results obtained are in favor of the idea that the signals from the different chromatic types of cones are combined in the outer synaptic layer of the retina at the inputs of bipolar cells using sign-inverting and/or sign-conserving synapses, while specific spatial properties of motion detectors are formed in the inner synaptic layer.

Color properties of the motion detectors projecting to the goldfish tectum: I. A color matching study

Responses of direction-selective and orientation-selective motion detectors were recorded extracellularly from the axon terminals of ganglion cells in the superficial layers of the tectum opticum of immobilized goldfish, Carassius gibelio (Bloch, 1782). Color stripes or edges moving on some color background (presented on the CRT monitor with known emission spectra of its phosphors) served as stimuli. It was shown that stimuli of any color can be more or less matched with the background by varying their intensities what is indicative of color blindness of the motion detectors. Sets of stimuli which matched the background proved to represent planes in the three-dimensional color space of the goldfish. A relative contribution of different types of cones to the spectral sensitivity was estimated according to orientation of the plane of color matches. The spectral sensitivity of any motion detector was shown to be determined mainly by long-wave cones with a weak negative (opponent) contributions of middle-wave and/or short-wave ones. This resulted in reduced sensitivity in the blue-green end of the spectrum, what may be considered as an adaptation to the aquatic environment where, because of the substantial light scattering of a blue-green light, acute vision is possible only in a red region of the spectrum.

Visual receptive field properties of cells in the optic tectum of the archer fish

The archer fish is well known for its extreme visual behavior in shooting water jets at prey hanging on vegetation above water. This fish is a promising model in the study of visual system function because it can be trained to respond to artificial targets and thus to provide valuable psychophysical data. Although much behavioral data have indeed been collected over the past two decades, little is known about the functional organization of the main visual area supporting this visual behavior, namely, the fish optic tectum. In this article we focus on a fundamental aspect of this functional organization and provide a detailed analysis of receptive field properties of cells in the archer fish optic tectum. Using extracellular measurements to record activities of single cells, we first measure their retinotectal mapping. We then determine their receptive field properties such as size, selectivity for stimulus direction and orientation, tuning for spatial frequency, and tuning for temporal frequency. Finally, on the basis of all these measurements, we demonstrate that optic tectum cells can be classified into three categories: orientation-tuned cells, direction-tuned cells, and direction-agnostic cells. Our results provide an essential basis for future investigations of information processing in the archer fish visual system.

Cardinal difference between the orientation-selective retinal ganglion cells projecting to the fish tectum and the orientation-selective complex cells of the mammalian striate cortex

Responses from two types of orientation-selective units of retinal origin were recorded extracellularly from their axon terminals in the medial sublaminae of tectal retinorecipient layer of immobilized cyprinid fish Carassius gibelio. Excitatory and inhibitory interactions in the receptive field were analyzed with two narrow stripes of optimal orientation flashing synchronously, one in the center and the other in different parts of the periphery. The general pattern of results was that the influence of the remote peripheral stripe was inhibitory, irrespective of the polarity of each stripe (light or dark). In this regard, the orientation-selective ganglion cells of the fish retina differ from the classical orientation-selective complex cells of the mammalian cortex, where the remote paired stripes of the opposite polarity (one light and one dark) interact in a facilitatory fashion. The consequence of these differences may be a weaker lateral inhibition in the latter case in response to stimulation by periodic gratings, which may contribute to a better spatial frequency tuning in the visual cortex.

Morphology and spatial arrangement of large retinal ganglion cells projecting to the optic tectum in the perciform fish Pholidapus dybowskii

Using retrograde HRP labeling from the optic nerve (ON) or optic tectum (OT), we have visualized large ganglion cells (LGCs) in wholemounted retinas of the teleost Pholidapus dybowskii and studied their morphology and spatial properties. In all, three LGC types were distinguished. In a previous paper, detailed data were provided on one type, biplexiform cells [Pushchin, I. I., & Kondrashev, S. L. (2003). Biplexiform ganglion cells in the retina of the perciform fish Pholidapus dybowskii revealed by HRP labeling from the optic nerve and optic tectum. Vision Research, 43, 1117-1133]. Here, we present data on the other two confirmed types, alpha(a) and alpha(ab) cells. The types differed in the level of dendrite stratification, dendrite arborization pattern, dendritic field size, and other features, and formed in the retina significantly non-random, spatially independent mosaics. Both types were labeled from the OT, indicating their participation in OT-mediated visual reactions. The comparison of spatial properties of alpha(a) and alpha(ab) mosaics labeled from the ON and OT suggests that the OT is the major or one of the major projection areas of both types. We also describe the morphology of cells resembling alpha(c) cells of other fishes, which were only labeled from the ON. The LGC types presently revealed were similar in their morphology to LGCs found in other teleosts supporting the hypothesis of LGC homology across the teleost lineage.

Direction selectivity in the goldfish tectum revisited

Responses of direction-selective (DS) ganglion cells (GCs) were recorded extracellularly from their axon terminals in the superficial layer of the tectum opticum (TO) of immobilized goldfish, Carassius auratus gibelio (Bloch). Directional tuning curves were measured with contrast edges moving in 12 or more different directions across the receptive field (RF). All directional tuning curves had cardioid-like appearance, their acceptance angles amounted to somewhat more than 180 degrees . According to their preferred directions DS GCs proved to comprise three distinct groups, each group containing DS GCs of ON and OFF subtypes approximately in equal quantity. Thus, this gives six physiological types of DS GCs in total. The preferred direction of a DS GC does not depend to some extent on a value of contrast, speed, size, and form of the stimuli. Coincidence in number of preferred directions with number of semicircular canals implies that DS GCs projecting to tectum are involved in some multimodal sensory integration in postural, locomotor, and oculomotor control in the three-dimensional aquatic world. DS neurons of the TO itself respond independently of the sign of stimulus contrast, have enormous receptive fields, and seem likely to collect signals from the retinal DS units of both ON and OFF subtypes with the same preferred direction.

Visual cues eliciting the feeding reaction of a planktivorous fish swimming in a current

The visual plankivorous feeding behaviour of the shiner perch(Cymatogaster aggregata) was investigated by means of a flow tank operated at various current speeds. Artemia salina was used as prey. In a second set of experiments, Artemia was darkened with black ink,to compare the visually mediated behaviour of C. aggregata while feeding on dark prey vs feeding on natural (i.e. semi-transparent)prey. The positions of the fish and its prey at the time of the feeding reaction of C. aggregata were measured in three dimensions. Prey were on average closer and more in line with the fish's axis when feeding reactions to darkened Artemia were considered, in comparison with natural Artemia. Three potential mechanisms triggering the feeding reaction of C. aggregata were explored: the prey may trigger a reaction in C. aggregata when it reaches a threshold (1) angular size, (2)angular velocity, or (3) rate of change of the angular size (i.e. loom) of the prey as it is carried passively by the current towards the fish. Our results show that angular velocity may trigger the fish's reaction when using semi-transparent prey, while loom may trigger the reaction to darkened prey. This suggests that feeding behaviour of planktivorous fish is flexible and can use different cues to trigger a motor reaction to prey with different visual characteristics. The feeding reaction appeared to occur at longer distances for semi-transparent rather than darkened Artemia. We suggest that semi-transparent Artemia were visible at greater distances because of their higher scattering (i.e. diffuse reflectance) that made them appear brighter when viewed against a dark background.

Asymmetric temporal properties in the receptive field of retinal transient amacrine cells

The speed of signal conduction is a factor determining the temporal properties of individual neurons and neuronal networks. We observed very different conduction velocities within the receptive field of fast-type On-Off transient amacrine cells in carp retina cells, which are tightly coupled to each other via gap junctions. The fastest speeds were found in the dorsal area of the receptive fields, on average five times faster than those detected within the ventral area. The asymmetry was similar in the On- and Off-part of the responses, thus being independent of the pathway, pointing to the existence of a functional mechanism within the recorded cells themselves. Nonetheless, the spatial decay of the graded-voltage photoresponse within the receptive field was found to be symmetrical, with the amplitude center of the receptive field being displaced to the faster side from the minimum-latency location. A sample of the orientation of varicosity-laden polyaxons in neurobiotin-injected cells supported the model, revealing that approximately 75% of these processes were directed dorsally from the origin cells. Based on these results, we modeled the velocity asymmetry and the displacement of amplitude center by adding a contribution of an asymmetric polyaxonal inhibition to the network. Due to the asymmetry in the conduction velocity, the time delay of a light response is proposed to depend on the origin of the photostimulus movement, a potentially important mechanism underlying direction selectivity within the inner retina.

Tectotectal connectivity in goldfish

The vertebrate optic tectum is a functionally coupled bilateral structure which plays a major role in the generation of motor commands for orienting responses. However, the characteristics of the tectotectal connectivity are unknown in fish, and have been reported only to a limited extent in other vertebrates. The purpose of the present study was to determine the anatomical basis underlying the functional coupling between tecta in goldfish, and to identify both similarities and differences to those features reported in other vertebrate species. The present experiments used the bidirectional tracer biotinylated dextran amine to map the distribution of labeled cells and synaptic boutons in the contralateral tectum following injections into identified tectal sites. Fibers that interconnect both tecta coursed through the tectal commissure. The cells of origin of these fibers, the tectotectal cells, and their synaptic endings were located in the deep layers, mainly in the strata periventricular and griseum central, respectively. Corresponding sites throughout the two tecta were interconnected in a symmetrical point-to-point fashion. The tectal commissure was composed of at least two distinct bundles of axons, which differed in their dorsoventral location, fiber diameter, and projection targets. The dorsal axons were tectotectal axons, they were thinner in diameter and profusely branched, and gave off en passant and terminal boutons in the deep layers of the contralateral tectum. The ventral axons were thicker in diameter, and formed the contralateral tectofugal-descending tract. Such fibers had few axon collaterals and boutons in the contralateral tectum. Boutons adjacent to retrogradely labeled tectotectal cells were very scarce. The data are discussed in terms of the coupling between tecta generating the motor commands required for orienting movements.

Axonal conduction velocities of functionally characterized retinal ganglion cells in goldfish

1. Visual response properties and conduction velocities of retinal ganglion cells were studied by extracellular recordings in the intact goldfish eye. Visually responsive single units were confirmed as ganglion cells by collision testing, and their receptive fields were mapped. 2. From compound action potentials, we identified groups I-V in the optic nerve, with overall conduction velocities of 11.5 +/- 1.17, 7.1 +/- 0.79, 4.4 +/- 0.56, 3.1 +/- 0.31 and 2.3 +/- 0.18 m s-1 (mean +/- S.D.) at 23 degrees C. 3. Ganglion cells were classified by their receptive fields as off-, on-off- or on-centre. Nearly all confirmed ganglion cells had axonal conduction velocities in groups II, III and IV; none fell in the fastest group, I. 4. Off-centre ganglion cells had conduction velocities only in the fast group, II. On-off-centre cells fell mainly in group III, with some in group, II. On-centre cells fell in groups II-V, but mainly in groups III and IV. 5. Receptive field centre diameters were 5-30 deg measured with a photopic background. The mean diameters for off-, on-off- and on-centres were 24, 15 and 18 deg, respectively. The relatively larger diameter and higher rate of spontaneous firing of the off-centre cells were maintained under different adaptation conditions. 6. The off-centre cells can be identified with an anatomical class of large, alpha-like ganglion cells in the goldfish retina.

Tectal codification of eye movements in goldfish studied by electrical microstimulation. f

This work compares the tectal codification of eye movements in goldfish with those reported for other vertebrate groups. Focal electrical stimulation was applied in various tectal zones and the characteristics of evoked eye movements were examined as a function of (i) the position of the stimulation over the tectal surface, (ii) the initial position of the eyes and (iii) the parameters (pulse rate, current strength, duration) of the stimulus. In a large medial zone, stimulation within the intermediate and deep layers of the tectum evoked contraversive saccades of both eyes, whose direction and amplitude were roughly congruent with the retinotopic representation of the visual world within overlying layers. These saccades were minimally influenced by the initial position of the eye in the orbit. The topographical arrangement of evoked saccades and body movements suggests that this tectal zone triggers orienting responses in a similar way to those described in other vertebrates. Stimulations applied within the caudal tectum also evoked contraversive saccades, but in disagreement with the overlying retinotopic map--the vertical component was absent. Taken together with electrically evoked body movements reported in free-swimming fish, these saccades could reveal that this zone is involved in escape responses. When stimulations were applied within the anteromedial zone of the tectum, contraversive movements of both eyes appeared much more dependent on initial eye position. Saccades elicited from this area displayed characteristics of "goal-directed saccades" which were similar to those described in the cat. The generation of goal-directed movements from the anteromedial zone suggests that this portion of the goldfish optic tectum has a different intrinsic organization or is connected with the brainstem saccade generator in a different fashion than the medial zone. Finally, stimulation of the extreme anteromedial zone evoked convergent eye movements. These movements and those reported in free-swimming fish following electrical stimulation of this tectal area suggest that this zone could be involved in feeding responses. The relationships between the parameters of electrical stimulation and the characteristics of elicited saccades suggest that the stimulated location within the tectum determines a constant direction in the evoked saccade, whereas the amount and duration of tectal activity, as mimicked by changes in stimulus parameters, together with the tectal locus, determine the velocity and amplitude of the evoked saccade.

Motion detection in goldfish investigated with the optomotor response is "color blind"

The action spectrum of the optomotor response in goldfish was measured to investigate which of the four cone types involved in color vision contributes to motion detection. In the dark-adapted state, the action spectrum showed a single maximum in the range of 500-520 nm, and resembled the rod spectral sensitivity function. Surprisingly, the action spectrum measured in the light-adapted state also revealed a single maximum only, located in the long wavelength range between 620 and 660 nm. A comparison with spectral sensitivity functions of the four cone types suggests that motion detection is dominated by the L-cone type. Using a two colored, "red-green" cylinder illuminated with two monochromatic lights separately adjustable in intensity, it could be shown that motion vision is "color-blind": the optomotor response disappeared whenever "isoluminant" red and green stripes were offered. Under this condition, calculations revealed that the L-cones were only slightly modulated by the "red-green" stimulus.

Activity-driven sharpening of the retinotectal projection in goldfish: development under stroboscopic illumination prevents sharpening

Blocking or synchronizing activity during regeneration of the retinotectal projection prevents both the sharpening of the retinotopic map recorded on tectum and the refinement of the structure of individual arbors within the plane of the map, and this refinement is triggered by N-methyl-D-aspartate (NMDA) receptors. We tested whether activity-driven refinement also occurs during development of the projection in larval and young adult goldfish. Shortly after hatching, larval goldfish were placed into tanks within light-tight chambers illuminated by a xenon strobe at 1 Hz for 14 h of each daily cycle. Fish were reared for 1.5-2 years, until large enough to record in our retinotectal mapping apparatus (6 cm length). Age- and size-matched controls had normal maps with multiunit receptive fields (MURFs) recorded at each tectal point of 10.8 degrees (0.16 S.E.M., n = 5), whereas the strobe-reared fish had only roughly retinotopic maps with much enlarged MURFs averaging 26.7 degrees (1.41 S.E.M., n = 5). This enlargement represents an abnormal convergence onto each tectal point, as the maps failed to sharpen during development. The arbors of individual retinal axons were stained with horseradish peroxidase (HRP) in larval fish and in adult strobe-reared and control fish. They were drawn with camera lucida from tectal whole mounts, and analyzed for spatial extent in the plane of the retinotopic map, order of branching, number of branch endings, depth of termination, and caliber of the parent axon. Arbors from larval fish (1-2 weeks) were small (approximately 50 x 40 microns) with less than 10 branches, occupied a single strata, and could not be separated into different classes by caliber of axon. The 87 arbors stained in control adult fish (6 cm long) were much like previously examined adult arbors, with those from fine, medium, and coarse axons averaging 115, 166, and 194 microns in extent, respectively, and having 17-24 branch endings. The 110 arbors from 12 strobe-reared fish were often abnormal. Although the fasciculation was normal, the extrafascicular routes were abnormal with reversing turns. The axons often had branches along their course, and these branches were scattered across a wider extent, rather than forming a distinct cluster. In contrast, neither the number of branches nor the depths of termination was significantly changed in any group. The coarse caliber arbors were most abnormal, being 64% longer and 30% wider than controls. The fine caliber arbors were also significantly larger by about 20%, but the medium caliber arbors were not enlarged.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuroanatomical substrate for the dorsal light response. I. Differential afferent connections of the lateral lobe of the valvula cerebelli in goldfish (Carassius auratus)

The afferent connections of the lateral lobe of the valvula cerebelli in goldfish were investigated by retrograde labeling with horseradish peroxidase (HRP). Retrogradely labeled neurons were observed ipsilaterally in the lateral nucleus of the valvula and contralaterally in the inferior olivary nucleus after HRP injection into all parts of the lateral valvula. The valvulopetal projections from these nuclei were topographically arranged. After HRP injection confined to the rostral half of the lateral valvula, labeled neurons were also found ipsilaterally in the octavolateral and trigeminal cell groups: the eminentia granularis, the medial nucleus of the octavolateralis column, and the isthmic primary sensory trigeminal nucleus. HRP injection confined to the caudal half of the lateral valvula resulted in retrograde labeling of the following vision-related cell groups; the central pretectal nucleus, nucleus paracommissuralis, and the nucleus isthmi. However, the octavolateral and trigeminal cell groups did not project to the caudal half of the lateral valvula. These data provide insight into central nervous integration of visual and vestibular information, and help reveal the mechanism of the dorsal light response (DLR). Bilateral lesions of either the valvula cerebelli or pretectal area completely abolish this visually-guided response, but lesions of the optic tectum have no such effect. The pretectal nuclei (central pretectal nucleus, nucleus paracommissuralis) project to the lateral valvula directly, not via the optic tectum. These direct projections from the pretectal accessory optic nuclei to the lateral valvula may control the DLR. On the other hand, the lateral valvula receives vestibular and lateral line inputs indirectly, via the eminentia granularis and the medial nucleus of the octavolateralis column, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Orientation and direction tuning of goldfish ganglion cells

Orientation and direction tuning were examined in goldfish ganglion cells by drifting sinusoidal gratings across the receptive field of the cell. Each ganglion cell was first classified as X-, Y-, or W-like based on its responses to a contrast-reversal grating positioned at various spatial phases of the cell's receptive field. Sinusoidal gratings were drifted at different orientations and directions across the receptive field of the cell; spatial frequency and contrast of the grating were also varied. It was found that some X-like cells responded similarly to all orientations and directions, indicating that these cells had circular and symmetrical fields. Other X-like cells showed a preference for certain orientations at high spatial frequencies suggesting that these cells possess an elliptical center mechanism (since only the center mechanism is sensitive to high spatial frequencies). In virtually all cases, X-like cells were not directionally tuned. All but one Y-like cell displayed orientation tuning but, as with X-like cells, orientation tuning appeared only at high spatial frequencies. A substantial portion of these Y-like cells also showed a direction preference. This preference was dependent on spatial frequency but in a manner different from orientation tuning, suggesting that these two phenomena result from different mechanisms. All W-like cells possessed orientation and direction tuning, both of which depended on the spatial frequency of the stimulus. These results support past work which suggests that the center and surround components of retinal ganglion cell receptive fields are not necessarily circular or concentric, and that they may actually consist of smaller subareas.

Histochemical localization of cytochrome oxidase in the retina and optic tectum of normal goldfish: a combined cytochrome oxidase-horseradish peroxidase study

Cytochrome oxidase (C.O.) was histochemically localized in the normal retina and optic tectum of goldfish in order to examine the laminar and cellular oxidative metabolic organization of these structures. In the optic tectum, C.O. exhibited a distinct laminar, regional, and cellular distribution. The laminae with highest C.O. levels were those that receive optic input, suggesting a dominant role for visual activity in tectal function. This was demonstrated by colocalizing C.O. and HRP-filled optic fibers in the same section. However, the distribution of C.O. within the optic laminae was not uniform. Within the main optic layers, the SFGS, four metabolically distinct sublaminae were distinguished and designated from superficial to deep as sublaminae a, b, c, and d. The most intense reactivity was localized within SFGSa and SFGSd, followed by SFGSb, then SFGSc. In SFGSd, intense reactivity was found to occur specifically within a class of large diameter axons and terminals that were apparently optic since these were also labeled with HRP and cobaltous lysine applied to the optic nerve. Regional C.O. differences across the tectum were also noted. Low levels were found in neurons and optic terminals along the growing immature medial, lateral, and posterior edges of tectum, but were higher at the more mature anterior pole and central regions of tectum. This suggests that the oxidative metabolic activity is initially low in newly formed tectal neurons and optic axons, but gradually increases with neuronal growth and functional axon terminal maturation. Most C.O. staining was localized within neuropil, whereas the perikarya of most tectal neurons were only lightly reactive. Only a few neuron classes, mostly the relatively larger projection neurons, had darkly reactive perikarya. In the retina, intense C.O. reactivity was localized within the inner segments of photoreceptors, the inner and outer plexiform layers, and within certain classes of bipolar and ganglion cells. The large ganglion cells in particular were intensely reactive. Like the large diameter optic terminals in SFGSd, the large ganglion cells were preferentially filled with HRP, suggesting that they may project to tectum and are the source of the darkly reactive large diameter axons and terminals in sublamina SFGSd. We propose a new scheme to describe tectal lamination that integrates laminar differences in C.O. reactivity with classical histological work.(ABSTRACT TRUNCATED AT 400 WORDS)

Staining of regenerated optic arbors in goldfish tectum: progressive changes in immature arbors and a comparison of mature regenerated arbors with normal arbors

Individual optic arbors, normal and regenerated, were stained via anterograde transport of HRP and viewed in tectal whole mounts. Camera lucida drawings were made of 119 normal optic arbors and of 242 regenerated arbors from fish 2 weeks to 14 months postcrush. These arbors were analyzed for axonal trajectory, spatial extent in the horizontal plane, degree of branching, number of branch endings, average depth, and degree of stratification. Normal optic arbors ranged in size from roughly 100 to 400 microns across in a continuous distribution, had an average of 20 branch endings with average of fifth-order branching, and were highly stratified into one of three planes within the major optic lamina (SO-SFGS). Small arbors arising from fine-caliber axons terminated in the most superficial plane of SO-SFGS; large arbors from coarse axons terminated in the superficial and middle planes; and medium arbors from medium-caliber axons terminated in the middle and deep planes of SO-SFGS, as well as deeper in the central gray and deep white layers. Arbors from central tectum tended to be much more tightly stratified than those in the periphery. No other differences between central and peripheral arbors were noted. Mature regenerated arbors (five months or more postcrush) were normal in their number of branch endings, order of branching, and depth of termination. Their branches covered a wider area of tectum, partially because of their early branching and abnormal trajectories of branches. Axonal trajectories were often abnormal with U-turns and tortuos paths. Fine-, medium-, and coarse-caliber axons were again present and gave rise to small, medium, and large arbors at roughly the same depths as in the normals. There was frequently a lack of stratification in the medium and large arbors, which spanned much greater depths than normal. Overall, however, regenerates reestablished nearly normal morphology except for axonal trajectory and stratification. Early in regeneration, the arbors went through a series of changes. At 2 weeks postcrush, regenerated axons had grown branches over a wider-than-normal extent of tectum, though they were sparsely branched and often tipped with growth cones. At 3 weeks, the branches were more numerous and covered a still wider extent (average of five times normal), many covering more than half the tectal length or width. At 4-5 weeks smaller arbors predominated, although a few enlarged arbors were present for up to 8 weeks. Additional small changes occurred beyond 8 weeks as the arbors became progressively more normal in appearance.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinal topography in the optic tract of adult goldfish

The present study was aimed at determining how transformations in fiber order establish a retinal topography in the optic tract of adult Carassius auratus. Horseradish peroxidase was applied to the optic nerve or retina, and the pathways of labeled axons originating from retinal annuli or wedges were analysed in reconstructed serial-sections and wholeamounts of the optic pathway. The age-related fiber order of the optic tract involves a rotation of the optic pathway that begins near the chiasm, continues through the optic tract as it wraps around the brain, and extends through the brachia. The relative order of laminae, in which each lamina is composed of age-related axons, is maintained in the optic pathway. The laminae add systematically onto the optic tract in a mediolateral direction with the oldest lamina forming the medial margin. Retinal sector order in the optic tract is established by the rearrangement of axons from each lamina. These rearrangements begin at the chiasm and, in part, involve transposition of axons originating from the ventrotemporal and dorsonasal sectors of the retina. The transformations achieve a fiber order in the optic tract that is appropriate for entry into the tectum. It is proposed that the final retinal topography of the optic tract is determined by the combined influences of selective affinities along the neural axis and substrate guidance mechanisms, the latter being mediated largely by the oldest axons of the fasciculus medialis tractus opticus.

Retinotectal synapses formed by ipsilaterally projecting fibers in the doubly innervated goldfish tectum

When one tectum of an adult goldfish is removed, the severed retinal fibers regenerate ipsilaterally into the remaining tectal lobe. Initially fibers from the two eyes overlap in the tectum but EM-HRP data suggest that few mature retinal synapses are formed between the ipsilateral eye and tectum at this time. At longer time periods, when some fibers appear to segregate into eye-specific termination bands, our data suggest that a significant number of synapses from the ipsilateral eye are present. These findings have important implications for how eye-specific termination bands are formed in doubly innervated tecta.

Inner plexiform layer of jack mackerel retina: participation of amacrine and ganglion cells in its spatial organization

In the jack mackerel retina (Trachurus mediterraneus ponticus) the inner plexiform layer demonstrates a very high degree of differentiation and contains not less than 25 sublayers. Investigation with Golgi method revealed many varieties of neurons, which are responsible for the structural organization of the inner plexiform layer. There are 8 types of bipolar cells, 24 types of amacrine cells and 7 types of ganglion cells with layered processes. The branching levels of the processes of these neurons were determined. Several varieties of neurons are described for the first time.

Diameters and terminal patterns of retinofugal axons in their target areas: an HRP study in two teleosts (Sebastiscus and Navodon)

Studies in various vertebrate classes, particularly amphibians and mammals, have revealed that retinal ganglion cells with different functional properties project by means of axons of correspondingly different diameters onto specific target regions. Whether a similar pattern exists in teleosts is partly investigated in the present study. HRP was injected into the optic nerve of Sebastiscus and Navodon. The calibers of intraretinal HRP-labeled axons were classed as fine (ca. 0.8 micron), medium (ca. 1.3 micron), and coarse (ca. 2.5 microns). The calibers of HRP-labeled retinofugal axons were then determined in their target areas, and these can be summarized as follows: Optic hypothalamus: fine, medium. Lateral geniculate nucleus: fine. Dorsolateral thalamic nucleus: fine, medium. Area pretectalis: fine. Nucleus of the posterior commissure: fine, medium. Area ventralis lateralis, contralateral: fine, medium, coarse; ipsilateral: coarse. Optic tectum, stratum opticum: fine, medium; stratum fibrosum et griseum superficiale: fine, medium, coarse, segregated in sublayers; stratum album centrale: fine, medium, coarse. Therefore, fine fibers were found to reach all target areas except the ipsilateral area ventralis lateralis, and these were the only fibers found in the lateral geniculate nucleus, area pretectalis, and stratum griseum centrale of the optic tectum. Coarse fibers, on the other hand, were found only in the area ventralis lateralis and the optic tectum (stratum fibrosum et griseum superficiale and stratum album centrale). Terminal patterns of these fibers were also studied. Most fine fibers take tortuous courses giving off a few branches and terminate with many varicosities, and medium and coarse fibers give off several finer branches and terminate with bulbous swellings. The physiological significance of these findings is discussed. In addition, retrogradely labeled (retinopetal) cells were found in the olfactory bulb and the area ventralis pars ventralis of the telencephalon, as well as in the preoptic area and the dorsolateral thalamic nucleus.

Rules for retinotectal terminal arborizations in the goldfish optic tectum: a whole-mount study

Retinal axons were labeled in the retina and optic nerve with horseradish peroxidase and traced in tectal whole-mounts. The typical network formed by retinal fibers in the five retinorecipient layers of tectum is illustrated in camera lucida drawings. Three size classes of terminal arbors were identified in the Stratum fibrosum et griseum superficiale (SFGS)-ca. 34 X 52, ca. 103 X 150, and ca. 158 X 274 micron. Terminal arbors are flattened and occupy three sublayers of SFGS. Passing an HRP-coated needle through the optic nerve labeled ganglion cells in retina and axons and terminal arbors in tectum. Terminal arbors of axons that originated in retinal annuli lay in distinct annular regions in SFGS, with old generations from central retina lying central to younger generations from peripheral retina. The tectal annuli were concentric with one another and agreed with the retinotopic map as it had been described before. The youngest terminal arbors from peripheral retina were next to the path of their fascicle along the tectal periphery, connected to their fascicle by short, centrally directed extrafascicular axons. The oldest terminal arbors from central retina were caudally displaced from their rostral fascicle of entrance, at the end of long, caudally directed extrafascicular axons. Terminal arbors from intermediate retina occupied intermediate positions in the tectum. Rostrally, they arose from centrocaudally directed extrafascicular axons but caudally from axons of various orientations. Terminal arbors arising from those extrafascicular axons exhibited different orientations and shapes depending on their tectal position. The spatial order of intratectal paths and terminal arbor sites, and the variability of terminal arbor orientation and shape, are consistent with an earlier model on shifting retinotectal terminals (Easter and Stuermer, '84).

Use of a lectin-peroxidase conjugate (WGA-HRP) to assess the retinotopic precision of goldfish optic terminals

The lectin wheat-germ agglutinin (WGA), conjugated to horseradish peroxidase (HRP), is an effective tracer for goldfish optic axons. Under suitable conditions, WGA-HRP injected iontophoretically into the normal goldfish tectum is taken up by retinotopic optic terminals (but not by axons of passage) and labels a small, discoid patch of retinal ganglion cells. Under identical conditions, unconjugated HRP is mainly taken up by injured axons of passage. WGA-HRP injected into the tectum after regeneration of the contralateral optic nerve again labels a small patch of ganglion cells, demonstrating the extent to which regenerated terminals are also retinotopic.

Functional anatomy of the tectum mesencephali of the goldfish. An explorative analysis of the functional implications of the laminar structural organization of the tectum

The present paper is aimed at an exploration of the possible functional significance of the laminar organization of the goldfish tectum at both the cellular and the synaptic level. For this purpose (1) the data concerning the structure of the teleostean tectum are surveyed, (2) a conceptual framework of the intratectal connectivity in the goldfish is proposed, (3) the electrophysiological data concerning the teleosteam tectum are surveyed and (4) the degree of correlation between the structural and physiological data available is discussed. Apart from the retina, tectal afferents originate from at least 10 other brain centers. At least 5 of these projections appear to be topographically organized. Tectal afferents, neurons as well as synapses reveal a characteristic intratectal lamination pattern. Tectal efferents project to at least 10 brain centers, and have until now been shown to arise from 6 cell types. The structural data surveyed allow the construction of a conceptual framework of tectal circuitry on the basis of 3 starting points. (1) The existence of at least 8 presynaptic zones or laminae, each containing a characteristic set of presynaptic structures (afferents and axons of interneurons). (2) The fact that the tectal postsynaptic structures (somata and dendrites of tectal neurons) each have a characteristic location, extension and synaptic density, which determines the relative importance of the different presynaptic zones for each cell type. (3) The laminar specificity hypothesis, which implies that presynaptic structures that coexist in a particular presynaptic zone terminate without preference on all types of postsynaptic structures within that zone. The conceptual framework of tectal circuitry is quantified in terms of connectivity index and connective importance. Analysis of the framework constructed leads to a detailed description of the intratectal pathways involved in the processing of the 4 main streams of tectal input (i.e. visual, toral, telencephalic and 'deep' input). It was concluded that the laminar organization of the tectum is primarily relevant for multimodal integration and that the tectal cell types each receive a characteristic sample out of the multimodal information available in the different tectal layers.(ABSTRACT TRUNCATED AT 400 WORDS)

Some topological considerations of the optic pathway

The contralateral projection of the vertebrate retinotopic map has a component of a mirror or uniaxial inversion in it. Here, a simple hypothesis is proposed which explains how this can come about. The emphasis is on topological considerations, in a global sense, of the overall map. An important feature of the hypothesis is that the optic nerve fibres follow pathways such that they retain their nearest neighbour relationships till they terminate in the optic tectum, i.e. no criss-crossing of the fibres is envisaged. Anatomical evidence for this is already available in the case of the optic tract of frog. Some speculations are also suggested concerning the role of this uniaxial inversion in information processing. The observations of optic tracts of other amphibian and some lower vertebrate systems are also considered.

Visual cells of zebrafish optic tectum: mapping with small spots

The zebrafish optic tectum is anatomically similar to those of goldfish and other teleosts, both in its laminar structure and the morphology of intrinsic neurons as studied with Golgi stains. We have applied standard electrophysiological techniques to study the visual properties of tectal cells, utilizing a computer system for stimulus control and data recording. All tectal cells have very large receptive fields, averaging 25-39 degrees in linear dimensions. Retinal receptive fields are smaller, averaging 7-13 degrees. In many cases the receptive fields of tectal cells, but never of retinal cells, consist of two parts (main field and accessory field) separated by tens of degrees. The two parts are differentially adapted by background illumination, accessory fields becoming unresponsive under lit conditions while main fields do not. This may reflect separate retinal input channels. Four types of tectal cells are described, which differ in their spontaneous activity in the dark and response to stationary spots. Type I are not spontaneously active in the dark, but respond phasically at response to ON and OFF. Type T are tonically active and give more prolonged phasic responses to ON and OFF. They may also have pure-inhibitory receptive fields in which spot ON suppresses the spontaneous firing with no phasic excitation. Type S are also silent in the dark, but give sustained firing as long as a spot is ON in the receptive field. Cells of type B fire spontaneously in bursts; the burst rate may be raised or lowered by stationary spots, but there is no phasic response. Each of the four physiological types is found to occur among the cells of the periventricular layer, all of which share a stereotyped overall morphology. Tectal cells do not exhibit spatially separated ON and OFF areas or orientation specificity.

Some determinants of optic terminal localization and retinotopic polarity within fibre populations in the tectum of goldfish

1. The reorganization of the retinotectal projection which results after partial ablation of tectal tissue was examined in goldfish using electrophysiological methods. 2. Regardless of the size of a unilateral ablation of caudal tectum, an orderly and virtually complete, 'compressed', visual projection re-formed on the remaining tectum after crushing the optic nerve and allowing it to regenerate. 3. If the optic nerve was left intact after ablations of caudal tectum, compressed projections were only found when the ablations were small. Large caudal ablations involving half or more of the dorsal tectum resulted in the cut fibres transposing onto the remaining tectum and forming an overlaid, 'duplicate', projection on the remaining intact projection. 4. In approximately one third of cases the duplicate projection lay in a reversed polarity along the rostrocaudal axis of the tectum. In the remaining cases the polarity of the duplicate projection was normal. 5. Transposed projections of reversed rostrocaudal polarity could be consistently obtained by ablating temporal retina and caudal tectum, leaving an intact strip of fibres terminals along the caudal edge of the tectal remnant. 6. Compression and duplication occurred in the same way if fish were maintained in constant light. 7. After ablations of lateral tectum, leaving the optic nerve intact, compression and some disorderly duplications were found. 8. Reversed projections could be induced across the mediolateral axis of dorsal tectum by denervating the medial tectum and ablating a strip of lateral tectum. 9. Projections of normal polarity were found after the optic nerve was allowed to regenerate into tecta which had previously supported reversed polarity projections.

Sensory representation in reptilian optic tectum: some comparisons with mammals

The sensory representations in the tectum of Iguana iguana were studied with electrophysiological recording techniques, and visual, somatic, and auditory cells were found to be represented here. These cells were not equally distributed throughout the tectal laminae. Upper tectal laminae were populated exclusively by visual cells, and deeper laminae were primarily nonvisual. The intermediate laminae had nonvisual, as well as visual, cells. Maps of the visual and somatic representations were constructed, and both representations were topographic and in register with no another. When electrical stimulation was presented via implanted electrodes, orientation responses were evoked that were predictable on the basis of the visuotopic and somatotopic maps. The organizational features of the iguana tectum are strikingly similar to those described in various mammalian species. It is suggested that the pattern of sensory and motor representation used in the midbrain of mammals is an ancient scheme that was retained during the transition from reptilian to mammalian forms more than 180 million years ago.

A Golgi-electron microscopic study of goldfish optic tectum. II. Quantitative aspects of synaptic organization

The size, density, and number of the synaptic contacts of three types of interneurons (types I, III, and XIV of Meek and Schellart, '78) and three types of efferent neurons (types VI, XII, and XIII) of the goldfish optic tectum were quantified by means of a quantitative stereological study of Golgi-EM serial sections. Furthermore, an estimation was made of the percentage of optic terminals on these six cell types and of the ratio between terminals with pleomorphic and terminals with round vesicles. The mean density of contacts per receptive component (i.e. the cell body and the different parts of the dendritic tree) varies from 0 to 100 per 100/micrometer2 surface, corresponding to 0-8% receptive surface. Each cell type has a characteristic average density as well as a characteristic density distribution along the distinct components. This suggests that the receptive components of the tectal cell types investigated have a predetermined density and that a morphological classification of tectal cells has functional relevance. The mean length of the contact zones in the ultrathin sections varies from 213 to 332 nm for identified postsynaptic elements and from 188 to 293 nm for identified presynaptic elements. The size of the contacts on the distinct receptive components appears to be primarily related to the tectal lamination pattern. Distinct types of axons, however, have characteristic mean sizes of contacts. This might suggest that the size of the contacts, contrary to their density, is primarily determined by the presynaptic elements. The mean number of synaptic contacts calculated per cell type is as follows: type XIV, 200; type III, 450; type VI, 1,400; type I, 2,100; type XII, 4,200; and type XIII, 5,400. Multiplication of these numbers with the number of cells per tectal half shows that the population of type XIV cells has by far the most synaptic contacts, since their low number of synaptic contacts is clearly overruled by their high frequency of occurrence. Optic terminals, identified by their characteristic mitochondria and large round vesicles, appear to contribute to about 10-20% of the contacts on identified post-synaptic elements in layer 5. The ratio between presynaptic elements with pleomorphic vesicles and those with round vesicles shows a slight tendency to increase when the distance to the origin of the axon decreases. It is concluded that a combination of the Golgi-EM technique with quantitative stereological methods appears well suited to the study of the synaptic organization of brain centers, and that combination of quantitative Golgi-EM with neuronal tracing methods (degeneration, HRP, autoradiography) offers good prospects for detailed investigations of neuronal connectivity.