Katherine V. Fite

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 Clarks nutcrackers (Nucifraga columbiana). The results show that Clarks nutcrackers have a larger hippocampal complex, relative to both body and total brain size, than the other three species. Clarks nutcrackers rely more extensively on stored food in the wild than the other three species. Clarks 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.

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.

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 μm 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) μm, and 17 (SEM ±1.5) μm, 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. J. Comp. Neurol. 414:469–484, 1999.

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.

Single-unit analysis of binocular neurons in the frog optic tectum

Abstract Responses of single, postsynaptic neurons in the deeper cellular layers of the frog optic tectum were recorded with glass-coated metal microelectrodes. Particular emphasis was placed upon the analysis of input and response properties of binocularly driven neurons. Section of the posterior and tectal commissures did not affect binocular response activity. However, removal of the contralateral tectal lobe or section of the deep tegmental commissures eliminated the ipsilateral component of binocular unit activity. The pathway of visual information from the ipsilateral retina is probably via the contralateral tectum, through the tegmental commissures to the ipsilateral tectum. The majority of tectal neurons observed could be activated by visual, tactile, and occasionally, auditory stimuli. Such multisensory units showed a spontaneous level of activity, whereas purely visual neurons did not. Visual receptive fields of binocular neurons were larger than those of monocular neurons in the intact preparation. Tectal neurons responded only to moving stimulus targets and often showed rapid adaptation to a repeated stimulus. However, some units showed little stimulus adaptation and the majority of units observed fell between those two extremes. Tectal neurons showed a greater response to vertical than to horizontal stimulus motion, which may be related to the predominance of vertical eye movements in the frog.

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 visual fields of the frog and toad: a comparative study.

Perimetric mappings and quantitative behavioral observations of orienting and feeding responses were made for four species of North American frog ( Rana clamitans, Rana catesbieana, Rana pipiens, Rana palustris ) and three species of toad ( Bufo americanus, Bufo terrestris, Bufo marinus ). Differences were found between frogs and toads with respect to the configuration of binocular and monocular portions of the visual field and in the total number of orientations which preceded capture of live prey presented at different locations in the field-of-view. Frogs have a larger superior and posterior binocular field, while in toads the anterior binocular field-of-view is more extensive. Toads showed significantly more orientation movements toward prey objects than did frogs, and always struck at prey from a frontal midline position. Frogs, however, frequently struck at prey located 45° or more from the frontal midline without prior orientation. Uniocular frogs and toads were capable of capturing prey at normal accuracy levels following a brief, transient period of reduced accuracy in prey-catching which occurred immediately following monocular blinding.

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.

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. J. Comp. Neurol. 440:31–42, 2001.

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.