Diurnal variation of c-Fos expression in subdivisions of the dorsal raphe nucleus of the Mongolian gerbil (Meriones unguiculatus)

Recent studies suggest that the dorsal raphe nucleus (DRN) of the brainstem contains several subdivisions that differ both anatomically and neurochemically. The present study examined whether variation of c-Fos expression across the 24-hour light-dark cycle may also be different in these subdivisions. Animals were kept on a 12:12 light-dark cycle, were perfused at seven different time points, and brain sections were processed by using c-Fos immunocytochemistry. At all coronal levels of the DRN, c-Fos expression reached a peak 1 hour after the light-dark transition (lights-off) and reached its lowest levels in the middle of the light period. In contrast to the light-dark transition, c-Fos levels did not change significantly after the dark-light transition (lights-on). One-way analysis of variance (ANOVA) revealed that the diurnal variation of c-Fos expression was highly significant in the caudal ventral DRN. Similar variation in c-Fos expression also was observed in the other DRN subdivisions, but this variation appeared to gradually diminish in the caudal-to-rostral and ventromedial-to-dorsomedial directions. Double-label immunocytochemistry revealed that, 1 hour after lights-off, only 11% of c-Fos-positive neurons in the caudal ventral DRN were serotonin (5-HT)-immunoreactive. These results suggest that DRN subdivisions may differ functionally with regard to the diurnal cycle, and that these differences may be reflected in the activity of nonserotonergic cells in the DRN.

GABAergic visual pathways in the frog Rana pipiens

Gamma-aminobutyric acid (GABA) is the most prevalent inhibitory neurotransmitter in the vertebrate brain. It can exert its influence either as GABAergic projection pathways or as local interneurons, which play an essential role in many visual functions. However, no GABAergic visual pathways have been studied in frogs so far. In the present study, GABAergic pathways in the central visual system of Rana pipiens were investigated with double-labeling techniques, combining immunocytochemistry for GABA with Rhodamine microspheres for retrograde tracing. Three GABAergic visual pathways were identified: (1) a retino-tectal projection, from retina to the contralateral optic tectum (OT); (2) an ipsilateral projection from the nucleus of the basal optic root (nBOR) to the pretectal nucleus lentiformis mesencephali (nLM); and (3) a second-order pathway from the nucleus isthmi (NI), bilaterally, to the optic tectum. These results indicate that GABA is involved in both first-order (retina to optic tectum) as well as second-order (nucleus isthmi to optic tectum) visual projections in Rana pipiens, and may play a major role in mediating visuomotor reflexs such as optokinetic nystagmus or other visually guided behaviors.

Retinal projection to the dorsal raphe nucleus in the Chilean degus (Octodon degus)

A substantial projection from the retina to the dorsal raphe nucleus (DRN) has been demonstrated in the Chilean degus, a diurnal/crepuscular hystricomorph rodent. Following intraocular injection of cholera toxin subunit B (CTB), immunocytochemically labeled CTB-positive axons and terminals were observed in all major retinorecipient nuclei as well as in the DRN and periaqueductal gray (PAG) of the mesencephalon. Two streams of optic axons to the DRN were observed: one descending from the optic tract at the level of the pretectum and anterior superior colliculus, the other emerging as a small fascicle at the anterior pole of the inferior colliculus and descending bilaterally through the PAG. Contralateral retinal afferents in the DRN appeared to terminate primarily in the dorsomedial and lateral subdivisions of the DRN, and a less extensive ipsilateral component also was observed. Axonal arborizations were characterized by short branches and multiple varicosities, both in the DRN and in the PAG. The extent and density of DRN retinal afferents were not as extensive as previously observed in Mongolian gerbils using identical techniques, but the retinal-DRN projection is considerably larger in degus than in rats. The functional significance of the retinal-DRN pathway remains to be determined, although a variety of evidence indicates that light may directly affect the activity of neurons and serotonin levels in the DRN.

Retinal afferents to the dorsal raphe nucleus in rats and Mongolian gerbils

A direct pathway from the retina to the dorsal raphe nucleus (DRN) has been demonstrated in both albino rats and Mongolian gerbils. Following intraocular injection of cholera toxin subunit B (CTB), a diffuse stream of CTB-positive, fine-caliber optic axons emerged from the optic tract at the level of the pretectum/anterior mesencephalon. In gerbils, CTB-positive axons descended ventromedially into the periaqueductal gray, moving caudally and arborizing extensively throughout the DRN. In rats, the retinal-DRN projection comprised fewer, but larger caliber, axons, which arborized in a relatively restricted region of the lateral and ventral DRN. Following injection of CTB into the lateral DRN, retrogradely labeled ganglion cells (GCs) were observed in whole-mount retinas of both species. In gerbils, CTB-positive GCs were distributed over the entire retina, and a nearest-neighbor analysis of CTB-positive GCs showed significant regularity (nonrandomness) in their distribution. The overall distribution of gerbil GC soma diameters ranged from 8 to 22 micrometer and was skewed slightly towards the larger soma diameters. Based on an adaptive mixtures model statistical analysis, two Gaussian distributions appeared to comprise the total GC distribution, with mean soma diameters of 13 (SEM +/-1.7) micrometer, and 17 (SEM +/-1.5) micrometer, respectively. In rats, many fewer CTB-positive GCs were labeled following CTB injections into the lateral DRN, and nearly all occurred in the inferior retina. The total distribution of rat GC soma diameters was similar to that in gerbils and also was skewed towards the larger soma diameters. Major differences observed in the extent and configuration of the retinal-DRN pathway may be related to the diurnal/crepuscular vs. nocturnal habits of these two species.

Topographic organization of serotonergic dorsal raphe neurons projecting to the superior colliculus in the Mongolian gerbil (Meriones unguiculatus)

Recent evidence suggests that the dorsal raphe nucleus (DRN) of the brainstem is a collection of neuronal clusters having different neurochemical characteristics and efferent projection patterns. To gain further insight into the neuroanatomic organization of the DRN, neuronal populations projecting to the superior colliculus (SC) were mapped in a highly visual rodent, the Mongolian gerbil (Meriones unguiculatus). Retrograde tracers Fluoro-Gold (FG) or cholera toxin subunit-B (CTB) were injected into the superficial layers of the SC, and serotonin (5-hydroxytryptamine, 5-HT) -positive cells were identified by using immunocytochemistry in the FG-injected animals. Based on its projections to the SC, the DRN was divided into five rostrocaudal levels. In the rostral and middle levels of the DRN, virtually all FG-filled cells occurred in the lateral DRN, and 36-55% of 5-HT-immunoreactive (5-HT-ir) cells were also double-labeled with FG. Caudally, FG-filled cells occurred in the lateral, ventromedial, and interfascicular DRN; and 44, 12, and 31% of 5-HT-ir cells, respectively, were also FG-filled. The dorsomedial DRN contained only a small proportion of FG-filled cells at its most caudal level and was completely devoid of FG-filled cells more rostrally. The CTB-injected animals showed a similar distribution of retrogradely labeled cells in the DRN. Topographically, the dorsal tegmental nucleus and the laterodorsal tegmental nucleus appeared to be closely associated with 5-HT-ir cells in the caudal DRN. These results suggest that the lateral DRN and the ventromedial/interfascicular DRN may be anatomically, morphologically, and neurochemically unique subdivisions of the gerbil DRN.

Organization of retinal axons within the optic nerve, optic chiasm, and the innervation of multiple central nervous system targets Rana pipiens

Light microscopic analysis of the optic nerve, chiasm, and optic tracts of Rana pipiens after the anterograde and retrograde transport of horseradish peroxidase has shown that retinal ganglion-cell axons reach the optic nerve head in chronotopically organized fascicles that form bands across the intraocular optic nerve. These bands of fascicles are divided along the midline in a "zone of reorganization" to create two full maps of the retinal surface; however, this map is discontinuous in that nasal and temporal quadrants are adjacent to one another. In the intracranial portion of the optic nerve, axons undergo another reorganization such that peripheral retinal axons shift position and become localized laterally and ventrally, whereas centrally placed axons become localized dorsally. Within this reorganization, the nerve is reconfigured into laminae of axons, and each lamina consists of age-related axons organized into two retinal maps. In the ipsilateral chiasm, axons diverge to form three central, optic tracts: the medial optic tract, the projection to the corpus geniculatum, and the basal optic root. Ipsilateral axons leave the chiasm at the same level of the chiasm as do their contralateral counterparts. The remaining axons converge in the lateral diencephalon to form a fourth fascicle, the marginal optic tract. Thus, within the optic chiasm, a sequence of positional transformations occur that result in the formation of multiple optic pathways. The various changes in axonal trajectory always coincide with changes in the orientation of cell groups that lie within the nerve and optic chiasm.

Distribution of GABA-like immunoreactive neurons and fibers in the central visual nuclei and retina of frog, Rana pipiens

Immunocytochemistry was used to study the distribution of gamma-aminobutyric acid (GABA) throughout the central visual nuclei and retina in Rana pipiens. In the retina, GABA immunoreactivity (both somata and fibers) was observed in all layers except the outer nuclear layer (ONL). Contrary to earlier reports, about 30% of total neurons within ganglion cell layer (GCL) expressed GABA immunoreactivity. Double-labeling studies indicated that about half of the GABA-containing perikarya in the GCL were retinal ganglion cells (RGCs). In the diencephalon, intensely labeled GABA-immunoreactive neurons and nerve fibers were observed within the neuropil of Bellonci (nB) and corpus geniculatum (CG), while only immunoreactive puncta were found in the rostral visual nucleus (RVN). In the pretectal region, the posterior thalamic nucleus (nPT) contained the most intensely labeled GABA immunoreactive perikarya and nerve fibers in the entire brain. Lightly immunoreactive perikarya were also found in the large-celled nucleus lentiformis mesencephali (nLM), as well as in the pretectal gray, which contains neurons postsynaptic to the retinal terminal zones within nLM. In the optic tectum (OT), both immunoreactive perikarya and fibers were found within superficial layers 8 and 9, whereas only densely packed immunoreactive perikarya were evident in the deep tectal layers (i.e. 2, 4, 6). The nucleus of the basal optic root (nBOR) contained a small number of lightly labeled GABA-immunoreactive perikarya, mostly located in the dorsal half of the nucleus. A large number of perikarya within the nucleus isthmi (NI) were also immunostained.

NMDAR1-like immunoreactive fibers appear in the ipsilateral optic tract during optic nerve regeneration in Rana pipiens

N-Methyl-D-aspartate receptor subunit 1-like immunoreactivity (NMDAR1-LI) was investigated in the brain of Rana pipiens during optic nerve regeneration. Following unilateral optic-nerve crush, frogs were tested for prey-catching and optokinetic nystagmus responses to assess return of visual function. At 1, 2, 3 and 5 months after the surgery, NMDAR1-LI was assessed in central visual pathways. At 3 and 5 months, conspicuous ipsilateral NMDAR1-LI fibers were detected in the thalamic and pretectal nuclei, and the time of their appearance coincided with the onset of behavioral recovery. Also, only ipsilateral retinorecipient layers in the optic tectum showed increased NMDAR1-LI during optic nerve regeneration. These results suggest that NMDA receptors may be present on retinal ganglion cell axons and terminals that have been misrouted during regeneration.

Distribution of the GABAA receptor complex beta 2/3 subunits in the brain of the frog Rana pipiens

This report describes the distribution of labeling of the monoclonal antibody bd-17 against the beta 2/3 subunits of the mammalian GABAA receptor complex throughout the brain of the frog Rana pipiens. The distribution matches quite closely those in homologous brain regions as previously described for this antibody in fishes, birds, and mammals, indicating that this antibody also labels beta 2/3 subunits of frog. A semiquantitative analysis of the distribution of labeling throughout the brain is based upon relative optical densities with respect to the structure showing maximal optical density in each brain, using standard illumination conditions. Comparison with distributions in birds and mammals suggests that these GABAA receptor complex subunits are strongly conserved in vertebrate evolution and play an important role in the visual, auditory, olfactory and motor systems.

Single-unit responses to whole-field visual stimulation in the pretectum of Rana pipiens

Visually responsive single units were recorded from the pretectal region that includes the large-celled nucleus lentiformis mesencephali (nLM) in the leopard frog, Rana pipiens. During monocular stimulation of the contralateral eye, 60 single units responding to movement of a large-field, random-dot pattern were quantitatively analyzed using horizontal and vertical directions at each of four pattern velocities (0.4-40 degrees/s). All units were spontaneously active, motion sensitive, and the majority showed 'on'-'off' responses. Several different response profiles were observed, including velocity-sensitive units with peak response at 10 degrees/s, most of which showed directional selectivity, and speed-sensitive units that showed increasing spike frequencies as pattern velocity increased, but little or no directional selectivity. About one-third of all unit analyzed were direction-selective, and 55% of those responded optimally to the temporal-to-nasal (T-N) direction of motion. T-N units were recorded primarily from an area that lies dorsolaterally between nLM and the optic tectum, in the 'peri-nLM' region. The pronounced monocular optokinetic nystagmus (OKN) response asymmetry that occurs in anurans appears to be reflected in the response profiles of the T-N direction selective units.

Differences in hippocampal volume among food storing corvids

The hippocampal complex (hippocampus and parahippocampalis) is known to play a role in spatial memory in birds and is known to be larger in food-storing versus non-storing birds. In the present study, we investigated the relative volume of the hippocampal complex in four food-storing corvids: gray-breasted jays (Aphelocoma ultramarina), scrub jays (Aphelocoma coerulescens), pinyon jays (Gymnorhinus cyanocephalus), and Clark's nutcrackers (Nucifraga columbiana). The results show that Clark's nutcrackers have a larger hippocampal complex, relative to both body and total brain size, than the other three species. Clark's nutcrackers rely more extensively on stored food in the wild than the other three species. Clark's nutcrackers also perform better during cache recovery and operant tests of spatial memory than scrub jays. Thus, greater hippocampal volume is associated with better performance in laboratory tests of spatial memory and with stronger dependence on food stores in the wild.

Segregation of optic axons based on central target: the medial optic tract in Rana pipiens

The retinofugal projection to the nucleus of Bellonci (nB) was examined in Rana pipiens using both anterograde and retrograde transport of horseradish peroxidase (HRP). Following HRP injection into the nB, retrogradely labeled optic axons formed a discrete fascicle that crossed the lateral margin of the anterior diencephalon. We have designated this branch of the retinofugal pathway as the "medial optic tract." HRP-positive, medial optic tract axons projecting to nB occupied the rostral and most dorsal portion of the optic chiasm. The present findings indicate that within the optic chiasm, retinal axons are sorted according to their final destinations.

Distribution of GAD-like immunoreactivity in the retina and central visual system of Rana pipiens

Immunohistochemistry was used to survey the distribution of glutamic acid decarboxylase (GAD), the synthesizing enzyme for gamma-aminobutyric acid (GABA), throughout the visual system of the frog Rana pipiens. GAD-like immunoreactivity (GAD-LI) was assessed in the retina, in primary retinorecipient targets, and in thalamic nuclear groups postsynaptic to primary retinal terminal zones. Within the retina, the inner plexiform layer displayed intense GAD-LI, but immunoreactivity was absent in the ganglion cell layer (GCL). Putative amacrine, bipolar, and horizontal cell somata were also labeled. Centrally, GAD-LI was observed in all primary visual nuclei. Dense, GAD-like immunoreactive punctate structures (puncta), presumably terminals, were observed in the pretectal nucleus lentiformis mesencephali, posterior thalamic neuropil, and uncinate neuropil. GAD-like immunoreactive puncta were noted in several laminae of the optic tectum, with the highest concentrations located within the 9th and 8th laminae. Moderate numbers of GAD-like immunoreactive puncta were found in the mesencephalic nucleus of the basal optic root, and two thalamic neuropils--corpus geniculatum and neuropil of Bellonci. The ventrolateral area, posterocentral nucleus, and posterolateral nucleus all contained sparse amounts of GAD-LI. These observations suggest that GABA plays an important physiological role in all central visual areas of Rana pipiens.

Drusen-like deposits in the outer retina of Japanese quail

Histological examination of the outer retina of Japanese quail (Coturnix coturnix Japonica) ranging in age from 4-48 months revealed the presence of drusen-like deposits which closely resemble those described in primate and human retinas. At both light and electron microscopic levels, these deposits were characterized by randomly distributed, granular and heterogenous materials. Larger deposits with pleomorphic inclusions, often globular in shape, occurred more frequently in older quail, particularly in males. An increase in the incidence and size of drusenoid deposits occurred with increasing age, with a greater rate of accumulation in males than in females. Previously, we have shown that retinal pigment epithelial lipofuscin increases more rapidly with age in female quail and that cumulative light damage is associated with increased lipofuscin in females but not in males. In the present study, no increase in drusen-like deposits was observed in single light-damaged or double light-damaged retinas of either sex. Thus, lipofuscin and the frequency and size of drusenoid deposits do not appear to be directly linked, although both increase with age in both sexes.

Experimental light damage increases lipofuscin in the retinal pigment epithelium of Japanese quail (Coturnix coturnix japonica)

Lipofuscin accumulation in the retinal pigment epithelium (RPE) of Japanese quail was investigated in normal and light-damaged animals at 4 months and 12 months of age following a single, 18-hr light-damaging exposure (3000-3200 lx) evaluated 6 weeks postexposure. Quail in a second experimental group light-damaged at 4 months of age were exposed to a second, identical light-damaging exposure at 12 months of age. RPE lipofuscin was quantitatively assessed in three ways: (1) total lipofuscin, expressed as a percentage of the basal RPE cell area, (2) mean number of lipofuscin granules per RPE cell, and (3) mean area of lipofuscin granules. At 12 months of age, control female RPE cells showed substantially more lipofuscin than did males. Animals that received a single damaging light-exposure showed no significant differences in lipofuscin at either age when compared with controls, and none were observed between males and females. However, in the double light-exposure condition, both total lipofuscin and the number of lipofuscin granules were significantly higher in females compared to controls and also, when compared with animals in the single light-exposure condition. Mean lipofuscin granule size decreased in females in the double light-exposure condition when compared with control values. Histopathological evaluation of photoreceptor outer segments and outer nuclear layer indicated that double-exposed, 12-month-old females showed the most severe effects, including a substantial decrease (70%) in rod photoreceptor densities. However, only a small change was observed in cone densities for either sex (12-15%) in this condition. Overall, a strong, negative correlation (r = -0.78) was obtained between total lipofuscin and average rod density across age, sex, control and experimental conditions. A somewhat greater correlation (r = -0.82) was obtained for females, alone. These results suggest that rods, when severely damaged by light exposure, contribute to increased RPE lipofuscin. Age, sex and light-exposure history appear to be critical variables that can influence both the amount of lipofuscin present in RPE cells as well as the relative vulnerability of rod and cone photoreceptors to cumulative light-exposure damage. These results have implications for the development of a variety of age-related changes and diseases of the outer retina, including age-related macular degeneration.

Metabolic correlates of optokinetic stimulation in the central visual system of the frog, Rana pipiens

The [14C]2-deoxyglucose (2-DG) method was used to identify those structures in the central visual system of Rana pipiens showing increased metabolic activity during binocular and monocular optokinetic stimulation at two pattern velocities (2 deg/sec and 9 deg/sec). Analysis of autoradiograms made with computer-assisted microdensitometry and pseudocolor image-enhancement techniques revealed that the greatest uptake of 2-DG occurred in the pretectal region, which included the large-celled nucleus lentiformis mesencephali (nLM), nucleus pretectalis, and pretectal gray. Both temporal-to-nasal (T-N) and nasal to temporal (N-T) directions of pattern motion were correlated with high levels of 2-DG uptake. However, the nucleus of the basal optic root (nBOR) showed the greatest uptake of 2-DG for the N-T direction, which evokes little or no optokinetic (OKN) response in this species. These results suggest that the major efferent projection from nBOR to nLM may exert its greatest effect upon the ipsilateral pretectum during N-T stimulation. Other regions showing substantial 2-DG uptake included a large region of the ventral thalamus, for all stimulus conditions tested, including those cell groups associated with the retinorecipient neuropil, corpus geniculatum. The pattern of uptake was less closely associated with specific parameters of optokinetic stimulation than was observed in the pretectum and accessory optic nucleus. Like nBOR, the auricular lobe of the cerebellum also showed greatest uptake of 2-DG for the N-T, monocular stimulus condition. The neural circuitry that underlies OKN and its directional asymmetry during monocular stimulation appears to involve a number of structures whose functional interrelationships are yet to be described.

Anatomical evidence for an intergeniculate leaflet in Rana pipiens

The projections of the nucleus of Bellonci and the anterior thalamic nucleus in Rana pipiens appear to be remarkably similar to those that have been described for the mammalian intergeniculate leaflet. The connections of these nuclei were examined using both the anterograde and retrograde transport of horseradish peroxidase. Afferents to the neuropil of Bellonci and its nucleus include bilateral projections from the retina, the contralateral nucleus of Bellonci, and anterior thalamic nucleus as well as bilateral projections from the pretectum and the ipsilateral suprachiasmatic nucleus. Efferent projections observed following HRP injections in the anterior thalamus consist of three components: (1) a ventral hypothalamic-suprachiasmatic and commissural projection, (2) a dorsal descending tract to the pretectum and tectum, and (3) a ventral descending tract to the somatomotor brainstem.

Organization of ascending projections from the optic tectum and mesencephalic pretectal gray in Rana pipiens

The ascending projections from the dorsal mesencephalon to the thalamus and pretectum in Rana pipiens were investigated by using the anterograde and retrograde transport of HRP with regard to two major issues: (1) the degree of tectotopic organization in the projections, and (2) their cells of origin. The results indicate that the spatial organization of the tecto-thalamic tract is specifically related to the laminar organization of the contributing tectal efferent neurons. Axons of neurons in the superficial portion of tectal layer 8 exit the tectum through layer 9 and travel in the superficial portion of the dorsal and ventral tecto-thalamic tracts and innervate the nucleus lentiformis mesencephali, the posterior lateral dorsal nucleus, and corpus geniculatum. The distribution of terminals within these structures varied with the tectal HRP-injection site. HRP injections in the ventral tecto-thalamic tract retrogradely labeled neurons in the superficial portion of tectal layer 8 across the lateral and caudal portion of the tectal lobe. HRP injections into the dorsal tecto-thalamic tract, at the level of the pretectum, retrogradely labeled pyriform neurons in the superficial portion of tectal layer 8 in the rostral and medial portions of the tectal lobe. With regard to the deep tectal layers, axons from pyramidal neurons in layer 6 and ganglionic neurons in layer 8 leave the tectum through layer 7, travel in both the dorsal and ventral tecto-thalamic tracts, and are located internal to the axons of the pyriform neurons of superficial tectal layer 8. The majority of the ganglionic neurons project to the posterior lateral ventral nucleus and the anterior lateral nucleus. The distribution of terminals within these nuclei did not display a tectotopic organization. A second major projection to the thalamus originates from the mesencephalic pretectal gray and innervates the nucleus lentiformis mesencephali, the posterior lateral dorsal nucleus, the anterior lateral nucleus, dorsal and ventral divisions of the ventral lateral thalamus, and the nucleus of Bellonci. Other axons from the mesencephalic pretectal gray terminate in the contralateral, medial portions of the posterior lateral dorsal thalamus, the ventral lateral thalamus, and the anterior lateral nucleus. The isthmo-tectal projection was also retrogradely labeled following tectal injections of HRP. This pathway travels in the most ventral portion of the ventral tecto-thalamic tract; its axons passed over the lateral margin of the endopeduncular nucleus bilaterally, and crossed the midline in the caudal portion of the optic chiasm. Extensive, bead-like varicosities were observed on these axons both in the endopeduncular nucleus and in the posterior optic chiasm.(ABSTRACT TRUNCATED AT 400 WORDS)

Age-dependent changes in visual acuity and retinal morphology in pigeons

The visual acuities of 17 pigeons that ranged in age from 2 to 17 years were tested with high-contrast, square-wave gratings. A systematic decline in visual acuity was observed that was well described by a logarithmic function. Pupillary diameter also declined with age, which decreased retinal illumination, but increased depth of focus. A small amount of presbyopia also was observed. Both the decrease in retinal illumination and the presbyopia accounted for only a trivial proportion of the acuity loss. No relationship between corneal or lenticular density and age was observed. Ophthalmoscopic examination of the optic media revealed no abnormalities associated with age. Microscopic examination of the area dorsalis of the retina (the high-density region specialized for frontal vision) revealed age-related losses of up to 33% of photoreceptors and 23% of cells in the ganglion-cell layer. A study of the photoreceptor layer within area dorsalis indicated that single-cone densities were unaffected by aging whereas the double-cone densities, which are the predominant photoreceptor type in the area dorsalis, were reduced in number by about one third. Calculation of the Nyquist limit both for photoreceptors and ganglion cells suggested that the decreased retinal density together with the decreased retinal illumination and presbyopia could not account for all of the observed acuity loss.

Aging and sex-related changes in the outer retina of Japanese quail

Age-related changes in the outer retina of Japanese quail 3 months to 3 years of age have been assessed with light and electron microscopy. A major difference was observed between males and females in the accumulation of lipofuscin in retinal pigment epithelial (RPE) cells. Females showed greater densities of lipofuscin granules, larger granules, and more lipofuscin per RPE cell than did males of comparable ages. In addition, a small but significant decrease (14-16%) in photoreceptor nuclear densities occurred in the temporal retina of both 1-year females and 3-year males. An overall correlation of -.77 between photoreceptor densities and amount of lipofuscin was observed, with a correlation of -.88 for females, alone. No male/female differences were observed with regard to age-related changes in height of RPE cells. Major differences in calcium metabolism and demand associated with egg-laying in females may underlie the observed sex-related differences that occur in the outer retina of this relatively short-lived, domestic species.

Retinotopic organization of central optic projections in Rana pipiens

The retinotopic organization of the anuran visual system has been investigated with the method of selective anterograde transport of horseradish peroxidase (HRP) following retinal lesions. The course of optic axons to specific structures was also confirmed by retrograde transport in the optic tract following HRP injections in the tectum and pretectum. As the optic nerve reaches the optic chiasm, the fibers from each of the four retinal quadrants appear as bands with the nasal (n) quadrant entering the chiasmal anterior pole, followed by ventral (v), temporal (t), and dorsal (d) quadrants. The preoptic nucleus is the first structure to be innervated, followed by the suprachiasmatic nucleus; both are innervated directly from fibers in the dorsal part of the optic nerve, which contains fibers from all the retinal quadrants. Each quadrant expands across the dorsoventral extent of the chiasm at the point where it enters. At this level the quadrants are arrayed along the rostrocaudal axis (as they are later in the marginal optic tract) in the sequence n-v-t-d. Optic fibers then spread across the chiasm, the nasal quadrant splits, taking up positions in the rostral and caudal margins of the optic radiation. Following the split in the nasal representation, the optic tract is transformed into topographically arranged sheets in the marginal optic tract. In the other retinorecipient nuclei, the sheet of optic axons is transformed back into the shape of the retinal hemisphere. Topographic maps of this kind display one of two possible orientations: (1) in the tectum and the nucleus lentiformis mesencephali (nLM), the temporal retina is represented in the anterior portion of the nucleus, whereas the nasal quadrant is found in the posterior portion; (2) in the thalamus, the retinotopic map is organized as a mirror-image reversal of that seen in the tectum and nLM (i.e., the nasal pole is anterior, whereas the temporal pole is in the posterior portion of the nucleus). Structures with this type of retinal map include the rostral visual nucleus, the corpus geniculatum, the nucleus of Bellonci, and the posterior thalamic nucleus. A third type of innervation occurs in the nucleus of the basal optic root (nBOR), which is the only mesencephalic visual nucleus not innervated by the marginal optic tract. The basal optic root is formed by the fibers exiting most caudally from the optic chiasm. All the retinal quadrants contribute to the basal optic root, but no evidence of retinotopy was found in nBOR.4+ target nuclei.

Neurophysiological investigation of the pretectal nucleus lentiformis mesencephali in Rana pipiens

Previous studies have demonstrated that the large-celled, optic pretectal nucleus lentiformis mesencephali (nLM) is essential for horizontal optokinetic nystagmus, yet little is known about its neurophysiology. In the present study, single-unit analysis of nLM utilized a large-field, patterned stimulus presented for 8 directions and 3 velocities of movement. All units localized in nLM were spontaneously active, motion sensitive, with response profiles that ranged from strongly directional and narrowly tuned to asymmetric and broadly tuned. Only about one-third of the units could be classified as directional, and no response bias for horizontal or temporal-to-nasal motion was observed. The majority of directional units showed greatest responsiveness at the lowest stimulus velocity, while the reverse occurred for many broadly tuned units. These low-velocity, highly directional units may be comparable to the 'retinal slip' neurons recently described in the large-celled pretectal nucleus of mammals. Directional information in the nLM of Rana pipiens thus appears to be represented in the activity of a large population of motion-sensitive units which includes both narrowly and broadly tuned individual response profiles. These results are consistent with the population-coding hypothesis recently advanced to account for directional coding in other sensorimotor systems, including primate motor cortex and superior colliculus.

Anuran accessory optic system: evidence for a dual organization

The accessory optic system of Rana pipiens consists of lateral and medial fascicles of the basal optic root (BOR1, BORm) and a single terminal nucleus, nBOR. The present study provides new evidence that these two fascicles differ not only in their trajectories but in fiber spectra and innervation patterns as well. BOR1 contains a substantially higher percentage of large, myelinated axons than does BORm, and the mean diameter of axons in BOR1 is greater than that of BORm. BOR1 innervates the entire terminal field of nBOR, while BORm innervates only the central and mediodorsal portions of nBOR. The ventrolateral portion of nBOR is uniquely innervated by BOR1 and contains several types of neurons not found in the central and medial regions of nBOR which are innervated by both fascicles. Cytoarchitectural analysis of nBOR with Golgi techniques has revealed a number of similarities between the anuran nBOR and the mammalian medial terminal nucleus (MTN) with regard to cellular morphology, dendritic geometry, and retinofugal arborization patterns. In frog, nBOR appears comparable to the ventral subdivision of the mammalian MTN, while the peri-nBOR region, which contains neurons postsynaptic to nBOR, may represent a more primordial version of the mammalian dorsal MTN.

Enhanced integumental and ocular amelanosis following the termination of cyclosporine administration

The Smyth delayed amelanotic line of chickens display symptoms commonly associated with human vitiligo. Administration of the immunosuppressive compound, cyclosporine, significantly delayed the mean age of onset and incidence of integumental pigment losses in this mutant line of vitiliginous chickens. Associated ocular pathology was also less severe in treated chicks. Termination of cyclosporine administration resulted in enhanced integumental and choroidal amelanosis, choroidal inflammation, and chorioretinal damage beyond that observed in nontreated controls. These results suggest that withdrawal of cyclosporine in treatment of this spontaneous autoimmune disease may exacerbate associated symptoms.

Effects of cyclosporine in spontaneous, posterior uveitis

Cyclosporin A (CsA) was administered to chicks of the Smyth delayed-amelanotic (SDA) line from day of hatch to 4, 8 or 12 weeks of age. Animals were evaluated at 8, 12, 16 or 20 weeks with regard to major features of the SDA-line disorder, including extent of feather and choroidal amelanosis, choroidal inflammation, and histopathology of the retinal pigment epithelium and outer retina. A suppression and delay in the onset of both amelanosis and ocular histopathology occurred during CsA administration. However, a rebound enhancement of symptoms occurred 4-8 weeks after withdrawal of CsA that was closely associated with the duration of CsA treatment. These results indicate that CSA may yield therapeutic effects during the period of treatment, but its withdrawal may lead to more severe symptoms that would have occurred without treatment.

Microiontophoresis and single-unit analysis of cholinergic drugs in the optic tectum of frog

Microiontophoresis of acetylcholine (ACh), atropine, or curare was accompanied by single-unit analysis of visual neuronal responses in the optic tectum of Rana nigromaculata. In 71% of the units tested, ACh enhanced responses to visual stimuli, while atropine suppressed visual responses. Twenty-two percent of the remainder showed no ACh- or atrophin-induced effects and were recorded within 125 microns of the pial surface. The majority of ACh-enhanced, atropine-suppressed units were recorded from visually responsive units localized in the postsynaptic cellular layer 8. The iontophoretic effects of curare were considerably more varied, with approximately equal numbers of units showing an increase (33%), a decrease (35%) or no change (32%) in visually activated responses. The specific effects of curare were also related to the depth from which unit recordings were obtained. These findings indicate that ACh functions as a modulatory neurotransmitter in the frog optic tectum, with a predominantly muscarinic mode of action at postsynaptic levels.

Development of the avian accessory optic system: effects of embryonic optic lesions

Representative cross-sections of the nuclei ectomammillaris (EM) from both normal and optically lesioned chick embryos (45 h of incubation, stage 12), were analyzed and compared on days 6, 8, 10, 12, 14 and 16 of incubation. An identifiable EM is clearly present at 8 days, in both normal and lesioned embryos, and increases in cell number and area up to embryonic day 12. However, embryos with partial or complete unilateral optic ablations demonstrate an apparent acceleration in cell death rate when compared with normals, from days 12-16, when a relatively mature and stable form of EM is apparently reached. Thus, early optic lesions do not affect the morphology of EM until day 12. These data also indicate that embryonic ipsilateral pathways to EM may persist and even expand when one eye primordium is removed or partially lesioned.

The pretectal nucleus lentiformis mesencephali of Rana pipiens

The pretectal nucleus lentiformis mesencephali (nLM) of Rana pipiens was investigated with autoradiographic, horseradish peroxidase (HRP), and Golgi techniques. Retinal afferents to nLM originate primarily from the central retina. The primary projection is contralateral with a small ipsilateral component. Following optic nerve transection and HRP impregnation, contralateral retinal afferents show a restricted, dense core of HRP label in the superficial portion of the nucleus with sparser HRP label in the surround. Ipsilateral retinal afferents arborize throughout nLM, except in the dense-core region. Additional afferents to nLM originate from the ipsilateral tectum, the nucleus rotundus, the mesencephalic pretectal gray, the contralateral nLM, and the nucleus of the basal optic root. Afferents from the accessory optic system arborize only in the dense-core region, following HRP injections into the nucleus of the basal optic root, while afferents from the mesencephalic pretectal gray arborize in all parts of nLM except the dense core. Afferents from the tectum and anterior thalamus appear to arborize throughout the nucleus without discernible pattern. The lamination of afferent terminals in nLM was correlated with Nissl-stained cytoarchitectural material in which the majority of large neurons cluster around the dense core of nLM. Three types of neurons occur in nLM: large neurons (25-micron dia.), fusiform neurons (12.5-micron dia.), and stellate neurons (10-micron dia.). Additionally, two cell groups outside nLM which send dendrites into the nucleus were observed: cells of the posterior lateral nucleus and cells of the posterior thalamic pretectal gray. Both large and fusiform neurons project to the deep layers of the optic tectum as well as to the ventral rhombencephalon superficial to the abducens nucleus. While a small number of fusiform neurons project to the nucleus of the basal optic root, the stellate neurons appear to be intrinsic to nLM. The anuran nLM strongly resembles the nucleus of the optic tract in mammals in terms of the site of origin of its retinal afferents, lamination of afferent terminations, its central connections, and its demonstrated involvement in horizontal optokinetic nystagmus.

Retinal pigment epithelial correlates of avian retinal degeneration: electron microscopic analysis

The delayed amelanotic (DAM) strain of domestic chicken is characterized by an early, developmental onset of choroidal inflammation and destruction of both feather and choroidal melanocytes. Secondarily, retinal pigment epithelial (RPE) cells in the peripapillary region develop abnormalities, and a series of progressive histopathological changes ensues which includes reduction and ultimate loss of RPE-melanin granules and RPE-cell atrophy. The earliest sign of RPE-cell abnormality is a dramatic alteration in the distribution of intracellular melanin granules. Apical processes also show a lessening of contact with photoreceptor outer segments, leading in more advanced stages to their retraction and development of retinal detachments. Other progressive alterations in RPE cells include disorganization and loss of basal infoldings; size reductions and density increases in both mitochondria and myeloid bodies from early to advanced stages; appearance of large macrophages in the subretinal space; Loss of intercellular junctional complexes; and progressive reduction in the density of melanin granules. These abnormalities appear to spread in a cell-by-cell, radial pattern, until widespread areas of the retina become severely pathologic and atrophic. The DAM chorioretinal disorder appears to show many of the histopathologic features which characterize experimentally induced uveitis and other ocular diseases which may result from hypersensitivity to, or autoimmune reaction against, pigments of the uveal tract.

Pretectal and accessory-optic visual nuclei of fish, amphibia and reptiles: theme and variations

The organization of the accessory optic and pretectal circuits is surveyed in a variety of poikilotherms and the conclusion drawn that a common organizational pattern exists. This pattern consists of the presence of three optic pretectal nuclei located rostrocaudally in lateral-superficial, central and dorsomedial (or periventricular) positions. Furthermore, a well-defined basal optic tract and ventrolaterally placed terminal field at the level of the oculomotor nucleus appears to be a consistent feature among bony fish, amphibians and reptiles. The possible role of these circuits in in visuomotor behaviors is discussed.

The accessory optic system and temporal correlates of visuomotor orientation

Surgical transection of the basal optic root in Rana pipiens leads to substantial increases in prey-orientation latencies for stimuli presented in the contralateral visual field. In general, the greater the reduction in retinal innervation of nBOR, the greater the postoperative increase in prey-orientation latencies. The results support Herrick's earlier suggestion that the accessory optic system may be substantially involved in early activation and temporal modulation of visuomotor behaviors mediated via mesencephalic circuits.

Optokinetic nystagmus in progressive retinal degeneration

Optokinetic nystagmus (OKN) was studied in both normally pigmented and delayed amelanotic (DAM) strains of domestic chicken. The DAM line is characterized by postnatal feather and ocular depigmentation accompanied by progressive retinal degeneration that occurs, initially and most severely, in the central retina. A close association exists between the extent of ocular pigment loss and relative reduction in OKN responsiveness in DAMs. The directional asymmetry of OKN responses, which normally occurs with monocular temporal-to-nasal (T-N) but not to nasal-to-temporal (N-T) stimulation, was altered in relation to the extent of ocular amelanosis among DAMs. In particular, T-N OKN responses were progressively reduced as amelanosis of the central retina increased in severity. In DAMs with moderate to severe reductions in T-N responsiveness, relatively little reduction occurred in N-T responsiveness. The central retina, therefore, appears to play a major role in mediating responses to T-N stimulation, whereas the peripheral retina mediates both directions of response. Optokinetic nystagmus also provides a useful index of the extent of retinal degeneration and the progressive loss of retinal pigment epithelium and photoreceptors which occurs in this mutant strain.

Optokinetic nystagmus in progressive retinal degeneration

Optokinetic nystagmus (OKN) was studied in both normally pigmented and delayed amelanotic (DAM) strains of domestic chicken. The DAM line is characterized by postnatal feather and ocular depigmentation accompanied by progressive retinal degeneration that occurs, initially and most severely, in the central retina. A close association exists between the extent of ocular pigment loss and relative reduction in OKN responsiveness in DAMs. The directional asymmetry of OKN responses, which normally occurs with monocular temporal-to-nasal (T-N) but not to nasal-to-temporal (N-T) stimulation, was altered in relation to the extent of ocular amelanosis among DAMs. In particular, T-N OKN responses were progressively reduced as amelanosis of the central retina increased in severity. In DAMs with moderate to severe reductions in T-N responsiveness, relatively little reduction occurred in N-T responsiveness. The central retina, therefore, appears to play a major role in mediating responses to T-N stimulation, whereas the peripheral retina mediates both directions of response. Optokinetic nystagmus also provides a useful index of the extent of retinal degeneration and the progressive loss of retinal pigment epithelium and photoreceptors which occurs in this mutant strain.

Progressive cytologic changes during the development of delayed feather amelanosis and associated choroidal defects in the DAM chicken line. A vitiligo model

Newly hatched Gallus domesticus chicks of the delayed amelanotic (DAM) line have phenotypically normal down pigmentation. Functioning pigment cells are present in the down plumage, choroid, and retinal pigment epithelium. However, histologic and ultrastructural studies reveal that after hatching regenerating feather melanocytes synthesize melanosomes with abnormal, irregularly shaped surfaces and pigmented extensions. Eventually retraction of melanocytic dendrites and clumping of pigment occurs concomitantly with intracellular compartmentalization of the abnormal melanosomes. Melanocyte degeneration is accompanied by the appearance of mononuclear leukocytes (MNLs) in the pulp of the regenerating feathers. Concurrently, melanocytes cease to migrate into the regenerating feather epithelium, and the result is amelanosis. Changes in choroidal melanocytes are first evident as swelling of cell bodies and associated dendrites. Ultrastructurally, the choroidal melanocytes demonstrate increased cytoplasmic material, melanosomal irregularities, retraction of dendrites, melanosome compartmentalization, and eventual necrosis. Concurrently, MNLs arrive and remove the pigment from the choroid. The authors conclude that a basic melanocyte defect precedes the arrival of immunocytes in the delayed cutaneous and choroidal amelanosis in the genetic DAM vitiligo model of the chicken.

Behavioral and central visual correlates of inherited retinal degeneration in the domestic chicken (Gallus domesticus)

A developmental, posthatch loss of cutaneous and ocular melanocytes in the mutant DAM (delayed amelanotic) chicken is accompanied by severe retinal degeneration. Using a food-location task, increased ocular pigment loss was associated with increased latency of response, both developmentally in young DAMS, and among adults with differing degrees of amelanosis. Analysis of optic nerve fiber composition in normals and DAMs showed a reduction in the small diameter optic axon population in severely amelanotic DAMs. With increasing severity of ocular amelanosis among animals, reduction in the density of anterogradely transported horseradish peroxidase (HRP) reaction product was seen first in retinorecipient thalamic nuclei, subsequently in the optic tectum, and ultimately in the accessory optic nucleus. An inverse relationship was also observed between the density of HRP reaction product occurring in retinal terminal fields and density of HRP label seen in the optic tracts of the same animal. No changes in either the volume or extent of central visual nuclei were apparent in partially sighted and blind DAMs.

Photomechanical responses of visual receptors in the retina of the bullfrog (Rana catesbeiana)

All photoreceptor types in the bullfrog (Rana catesbeiana) retina exhibit changes of position within the receptor layer, depending upon the state of retinal adaptation, and the season in which the displacement was measured. The most light-adapted position of each photoreceptor type following monochromatic adaptation was not related specifically to the lambda-max of its own photopigment. In general, however, the response profiles were similar for photoreceptors having the same lambda-max. The red rod, accessory member of the double cone, green rod and the miniature cone showed similar response profiles, while the single cone and the principal member of the double cone showed similar patterns of movement as a function of adapting wavelength. The response profiles for the short and middle lambda-max photoreceptors obtained in summer months appeared similar to the action spectrum of the retinal pigment epithelium (RPE) as previously described by Liebman and co-workers. Seasonal differences were also found. The observed photomechanical response profiles may be determined by complex interactions between adjacent photoreceptors, as well.

Retinal degeneration in the delayed amelanotic (DAM) chicken: an electroretinographic study

The retinal function of the delayed amelanotic (DAM) chicken that has progressive retinal degeneration, was investigated electroretinographically (ERG). Subjects showing a gradation of abnormalities of retinal morphology and impairment of visual behaviors were tested. Dark-adapted a-, b-, and c-wave responses to full-field stimuli were studied. In general, the more severe the anatomical abnormalities, the greater the compromise of sensitivity and amplitude of ERG responses. The DAM diseases appears to affect c-waves more than a-waves, and a-waves more than b-waves. Dark-adapted responses are affected more than light-adapted responses. In DAM's with moderately to severely impaired visual behaviors, ERG studies should permit analysis of the distal retinal pathophysiology caused by this disease.

Neural correlates of optokinetic nystagmus in the mesencephalon of Rana pipiens: a functional analysis

The effects of lesions of the anuran mesencephalic retinal terminal fields on horizontal optokinetic nystagmus (OKN) were examined. Lesion sites which produced effects upon OKN responses were as follows: BOR, nBOR, peri-nBOR, the large-celled pretectal nucleus, and the dorsal tegmental gray and deep tectal layers. Transection of BOR generally resulted in an increase in saccadic frequently at the lower stimulus velocities. Lesions of nBOR produced a decrease in the frequency of both head and eye saccades in the middle to high range of stimulus velocities. The only lesions which totally abolished horizontal OKN were those located medical to nBOR, in the peri-nBOR region. Lesions of the large-celled pretectal nucleus and dorsal tegmental gray substantially reduced both head and eye saccades at all stimulus velocities. Small lesions in the deep tectal layers also depressed OKN frequency. These studies indicate that horizontal OKN may be mediated by a number of structures within the anuran mesencephalon.

The accessory optic system of Rana pipiens: neuroanatomical connections and intrinsic organization

The accessory optic system of Rana pipiens was investigated by autoradiographic, horseradish peroxidase, and Golgi techniques, revealing a complexity of neuroanatomical organization previously unrecognized. Retinal afferents project to the nucleus of the basal optic root (nBOR) via a primary bundle and more diffuse, medial bundle of optic axons, both of which contain large- and small-diameter fibers. At least six types of retinal ganglion cell contribute to the basal optic root (BOR), including giant ganglion cells, two intermediate-sized ganglion cell types, small ganglion cells, and two types of displaced ganglion cell. The major retinal projection is contralateral, but a small, ipsilateral component also exists. Afferents from neurons which are postsynaptic to the thalamic retinal terminal fields also reach nBOR. Four distinct cell types were identified within the terminal field of nBOR: stellate neurons (63%), amacrine cells (19%), elongate neurons (14%), and large ganglionic neurons (4%). Both stellate and amacrine cells appear to be intrinsic neurons, while elongate and ganglionic neurons constitute the efferent neuron population of nBOR. In addition, cells which lie medial to the terminal field, pyriform and commissural neurons, send dendrites into nBOR. Pyriform neurons project to the nucleus of the medial longitudinal fasciculus (nMLF) and cranial nerve nuclei III and IV, while commissural neurons project to the contralateral nBOR. Large reticular neurons of the nMLF also send dendrites into nBOR. Efferent projections from nBOR were observed in the large-celled pretectal nucleus and in nucleus lateralis profundus. A second major projection originates from the peri-nBOR region and is associated with the oculomotor system and with the nMLF. Efferent projections from the nMLF to the vestibular nuclei and to the rostral spinal cord were also observed, as well as projections which reach the brainstem from the large-celled pretectal nucleus, the posterior thalamic and anterior mesencephalic central gray.

Retinal dystrophy associated with a postnatal amelanosis in the chicken

Melanin pigmentation changes were studied in a mutant (delayed amelanotic) line of chickens characterized by a postnatal, spontaneous cutaneous amelanosis and a high incidence of blindness. Cutaneous pigment loss was accompanied by destruction of the choroidal melanocytes throughout the orbit. The presence of blindness appeared to be correlated with the histopathologic finding of severe degenerative changes in the pigment epithelium and neural retina first seen near the base of the pecten and progressing radially in irregular patterns.

The DAM chicken: a model for spontaneous postnatal cutaneous and ocular amelanosis

A mutant line of chickens (DAM line) has been developed that is characterized by a high incidence of a spontaneous, postnatal, cutaneous amelanosis. Amelanotic individuals also have a high incidence of blindness and a low incidence of an integumentary defect expressed as a variable loss of feathers. A low incidence of hypothyroidism is also present but is not limited to the amelanotic phenotype. All line associated traits appear to be controlled by multiple autosomal genes. Selection experiments showed that the amelanosis and eye defect are highly heritable and partially share a common genetic basis. However, it also was shown that the incidence of blindness could be readily altered within the amelanotic population by selection. It is suggested that the blindness represents the most severe expression of the amelanosis syndrome. The amelanosis equally affected both sexes and both eumelanin and phaeomelanin pigments. A relationship was found between the presence of the back plumage color gene, E, and an increase incidence of feather amelanosis in segregating DAM line populations. This was not accompanied by an increase in blindness. The DAM line appears to be a valuable animal model for vitiligo, as well as several melanin-related eye diseases.

Displaced ganglion cells and the accessory optic system of pigeon

The central projection and retinal distribution of displace ganglion cells (DGC's) are described for the pigeon. Discrete, localized injections of horseradish peroxidase (HRP) into the nucleus of the basal optic root (nBOR) complex labeled as many as 4,800 DGC's in the contralateral retina. The greatest densities of DGC's were observed in the more peripheral regions of the middle and inferior temporal regions of the retina, with lowest densities occurring in the inferior nasal, red field, and foveal areas. Large HRP injections of the tectal lobes, which did not include the pretectal, accessory optic (nBOR), hypothalamic, or thalamic visual nuclei, labeled only ganglion cells within the ganglion cells layer. An HRP injection centered within the nucleus lentiformis mesencephali, also including portions of the optic tectum and optic tract, labeled only ganglion cells within the ganglion cell layer of the contralateral retina. DGC's thus appear to be the primary, if not exclusive, source of retinal afferents to the nBOR complex in pigeon. The observed retinal distribution of DGC's indicates that the areas of retina with the greatest density of cells in the receptor layer, inner nuclear layer, and ganglion cell layer are relatively devoid of DGC's. Since the nBOR complex projects directly upon the vestibulocerebellum and oculomotor nuclei, DGC's would thus appear to be involved in neural circuits that mediate oculomotor reflexes and visuomotor behavior.

Bifoveal vision in anolis lizards

The retinas of 14 ecologically diverse species of Anolis lizards have been examined neuroanatomically. Both central and temporal foveas were observed in each eye of all species: however, the two foveas differed with regard to a number of structural and morphological characteristics. All central foveas were deep and convexiclivate, while temporal foveas were shallower and more variable both in shape and retinal location across species. Central foveas had higher cell densities and a broader foveal clivus than did temporal foveas both within and across species. As eye size increased across species, densities of receptor nuclei per visual degree2 (vis. deg.2) increased more rapidly than did ganglion cell densities/vis, deg.2 in the central fovea. In contrast, both cell types increased at approximately the same rate in the temporal fovea as eye size increased. Several of these relationships have been previously reported for diurnal birds of prey, which are also bifoveate. of stomach contents revealed that Anolis species which feed upon small prey items have temporal foveas with a relatively deep clivus. Foveal characteristics may influence both species typical foraging behaviors and the type of prey which can be effectively utilized by a given species.

Optokinetic nystagmus in the domestic pigeon. Effects of foveal lesions

Using both binocular and monocular viewing conditions, optokinetic nystagmus (OKN) frequency-velocity functions were measured before and after ophthalmic laser-produced retinal lesions. With binocular viewing conditions, the range of effective pattern velocities and the upper velocity threshold increased by 20--30 degrees/sec in subjects with both foveas lesioned, and by 12 degrees/sec in a subject with only a single foveal lesion. Subjects with parafoveal lesions showed no change in postlesion binocular OKN response functions. Prelesion monocular OKN functions were obtained for both temporal-to-nasal (T-N) and nasal-to-temporal (N-T) directions of pattern movement. T-N OKN functions were similar to those obtained with binocular viewing but N-T movement elicited OKN over a much narrower range of stimulus velocites. Lesions of the foveal area appeared to have little effect on N-T OKN functions, although an increase in N-T upper velocity thresholds was obtained from at least one eye in 4 of 5 animals. Thus, lesions of the foveal and parafoveal area do not impair, and may actually facilitate, OKN in the pigeon.

Optokinetic nystagmus and the accessory optic system of pigeon and turtle

Optokinetic nystagmus (OKN) response functions were obtained in pigeon (Columba livia) and turtle (Chrysemys picta) before and after electrolytic lesions of the accessory optic nuclei (AON). Postlesion retinal input to the AON was evaluated using standard autoradiographic techniques. Bilateral destruction of AON in both pigeon and turtle did not abolish OKN, but was correlated instead with a reduction in OKN frequencies at high pattern velocities. A difference was observed between species with respect to the effects of partial lesions. Incomplete destruction of AON produced no observable change in OKN response functions in pigeon, but correlated with reduced OKN response functions in turtle. These results suggest that the AON mediate a portion of OKN in both pigeon and turtle, particularly at high pattern velocities, but are not essential for its occurrence.

Wavelength discrimination in the leopard frog: a reexamination

A reexamination of wavelength discrimination using a 2-choice paradigm and both chromatic-chromatic and chromatic-achromatic stimulus pairs has reaffirmed the positive tendency of Rana pipiens to approach short wavelengths. In addition, a strong avoidance of long wavelengths was observed and an interaction of these two opposite response tendencies was demonstrated. Thus, the percent choice frequency for a short-wavelength stimulus depends upon the stimulus with which it is paired. A positive phototaxis appears to influence response to some short-wavelength stimuli when they differ in brightness from an achromatic stimulus, but not with wavelengths above 471 nm.