Search PubMed⌕ Search

Biomedical subjects

S S Easter

Publications and source records attributed to S S Easter.

68 records · Page 4Linked to original sources
PubMed1978

The paths and destinations of the induced ipsilateral retinal projection in goldfish.

Adult goldfish had one tectal lobe removed surgically, and several months later, the eye contralateral to the missing tectum was injected with radioactive proline. Radioautographs of the brains were studied to trace the paths and termination sites of the optic fibers. The optic tract decussated at the chiasm, as normally, but then ran caudally in a large neuroma on the tectum-less side of the brain. Substantial numbers of fibers left this neuroma to enter two or more of five commissures, through which they recrossed the midline. These commissures: transverse, minor, horizontal, posterior and ansate, ordinarily contain few or no optic fibers. All are normally linked with the tectum. Negligible numbers of aberrant optic fibers recrossed the midline elsewhere. On the intact side of the brain, ipsilateral to the injected eye, the optic fibers innervated some or all of the nuclei and areas normally served by contralateral retinal fibers. An earlier behavioral study of these same fish had shown that some of them made reversed optokinetic nystagmus in response to stripe movement seen by the eye projecting ipsilaterally; others failed to respond to stimuli through this eye. In all the reversed responders, a caudal group of retinal projection sites was labeled ipsilaterally. This included the basal optic nucleus and the caudal portions of nucleus dorsolateralis thalami and area pretectalis. In the non-responders, these targets were not labeled ipsilaterally. Together, these results suggest that one or more of these three sites is or are responsible for optokinetic nystagmus in normal goldfish.

Animals↗
PubMed1977

Growth of the adult goldfish eye. II. Increase in retinal cell number.

The retinas of adult goldfish, one to four years of age, 4-23 cm in length, were examined with standard paraffin histology to determine if new cells were being added with growth. Retinal cell nuclei were counted and the area of the retina was measured. An analysis of cell densities in various regions throughout the retina showed that the cells are distributed nearly homogeneously. The density (No./mm2 of retinal surface) of ganglion cells, inner nuclear layer cells and cones decreases with growth, but the density of rods remains constant. Thus the rods account for a larger proportion of the cells in larger retinas; The total number of cells per retina increases: the ganglion cells from 60,000 to 350,000; the inner nuclear layer cells from 1,500,000 to 4,000,000; the cones from 250,000 to 1,400,000; the rods from 1,500,000 to 15,000,000. This increase in the number of retinal neurons implies the formation of even more new synapses, and suggests the adult goldfish retina as a model for both neuro- and synaptogenesis.

Animals↗
PubMed1977

The role of the optic tectum in various visually mediated behaviors of goldfish.

Five visually mediated behaviors were assessed following ablation of one or both lobes of the optic tectum in goldfish. Three of the behaviors disappeared following tectal ablations: optomotor response (swimming with the stripes in a rotating striped drum), food pellet localization and shadow-induced deceleration of respiration. Two of the behaviors persisted following tectal ablation: optokinetic nystagmus (movement of the eyes with the stripes in a rotating striped drum) and dorsal light reflex (tilting of the vertical axis toward the brighter of two laterally placed lights). The unexpected result that lesioned fish tracked the stripes with their eyes, but did not swim after them as normal fish did, suggests that the tectum serves a pre-motor function in addition to its sensory role. In addition, the results demonstrate that selected behaviors can be used to establish whether functional tectal or non-tectal connections are made by regenerating goldfish optic nerves.

Animals↗
PubMed1977

Reversed visuomotor behavior mediated by induced ipsilateral retinal projections in goldfish.

1. When one optic lobe of a goldfish is removed and the optic nerve from the contralateral eye is deflected ipsilaterally, the nerve innervates the intact, previously foreign side. When the control eye is left intact, the animal shows normal swimming and coordination of head and eyes. In some cases, however, optokinetic nystagmus evoked through the experimental eye is reversed, indicating a functional ipsilateral projection which was apparently outweighed by the normal projection during swimming. 2. When the same surgical procedure is accompanied by partial or complete destruction of the control eye, the animals show overt reversal of visuomotor behavior soon after the ipsilateral projection is established. They make a spontaneous nystagmus and circle continuously. The nystagmus and the circling persist in total darkness for days before they are extinguished. They resume as soon as the animal returns to light. 3. The vestibuloocular reflex is biased differently from normal, but the sign of the gain is normal. 4. We conclude that the neurological reversal of vision triggered the nystagmus and the circling, and that the many months with reversed vision resulted in some longer lasting change in the nervous system such that the behaviors persisted even in the absence of moment-to-moment visual stimulation.

Animals↗
PubMed1968

Excitation in the goldfish retina: evidence for a non-linear intensity code.

1. Experiments were done on isolated photopic goldfish retinas. They were stimulated by brief flashes of red light, and the spike activity of single ganglion cells was monitored by micro-electrodes. Red-ON-units were used exclusively.2. The spatial integration of intensity was investigated using concentric disks of various diameters. Under these conditions, Ricco's relation (1877) was obtained.3. Two small spots of light were positioned on two equisensitive sites in the receptive field; the (equal) intensities of both were varied in unison, and the responses recorded. An identical response was evoked by simultaneous illumination of both sites with an intensity, I, or by illumination of a single one of the sites with an intensity, KI. K always exceeded 2 (it averaged about 4) and it was constant in any one experiment.4. The analysis of these results employed the assumption that an hypothetical quantity, the excitation, intervenes between the stimulus (light intensity) and the response (spike train). The excitation is a function of intensity, and it determines the response. The excitation from two spots is assumed to be twice that from one.5. It was inferred that the excitation (E) was a power function of the intensity (I): E = CI(n), in which C and n are constants. The exponent, n, was always less than unity.6. Two other experiments tested the predictive value of this inference. It accurately predicted the responses to a single spot anywhere in the field, and to two unequal intensities simultaneously illuminating two equisensitive sites.

Action Potentials↗
PubMed1968

Adaptation in the goldfish retina.

1. Isolated photopic goldfish retinas were stimulated and adapted by red light, and the spike activity of single ganglion cells was monitored by micro-electrodes.2. A unit's threshold to illumination of one part of its receptive field was often raised by adapting spots which fell elsewhere. An adapting spot of fixed area and intensity always desensitized the unit most when focused at the test position.3. When adapting and testing spots were given concentrically, the large adapting spots raised the test threshold more than the smaller adapting spots, even though all completely covered the test spot. Quantitative considerations rule out scattered light as an explanation; there must be a neural mechanism, an adaptation pool, which sets the sensitivity on the basis of quanta received over a field hundreds of microns in diameter.4. The desensitization is a non-linear function of area and intensity.5. The adaptation pool's field is not coincident with the ganglion cell's receptive field.

Action Potentials↗
PubMedDate not supplied

The cone photoreceptor mosaic of the green sunfish, Lepomis cyanellus.

Recent empirical and theoretical evidence has implicated the geometrical birefringence of the double cones of the green sunfish (Lepomis cyanellus) as the biophysical basis of this vertebrate's sensitivity to polarized light. Because of the intimate link between the organization of the cone-photoreceptor mosaic and the psychophysical details of polarization sensitivity, we have examined the structural features of the green sunfish cone-photoreceptor mosaic, in particular the orientation of the elliptical cross sections of the double cones. Our primary observations are that (1) the arrangement of the cone-photoreceptor mosaic is constant across the retina (with two regional exceptions), with double cones arranged in a rhombic mosaic and aligned roughly +/- 45 deg to the nearest retinal margin; (2) the double-cone/single-cone ratio is everywhere the same; (3) cone density is inhomogeneous across the retina, with the highest densities in the temporal hemiretina. These results are discussed as they relate to the animal's retinal growth and visual mechanisms, particularly the sensitivity to polarized light.

Animals↗
PubMedDate not supplied

Neurogenesis in the visual system of embryonic and adult zebrafish (Danio rerio). off.

The zebrafish has recently assumed a central position in the study of vertebrate development. Numerous studies of other fish have shown that their central nervous systems, and especially their visual systems, continue to add new neurons throughout life, which is probably related to their abilities to regenerate axons and whole nervous tissue. Retinal neurogenesis had not been examined in adult zebrafish, and two reports concluded that the optic tectum ceased neurogenesis early in life, so the question arose whether the zebrafish was anomalous in this regard. We labeled embryonic (24- and 48-h postfertilization) and adult zebrafish with the thymidine analog, bromo-deoxyuridine, and, after short and long survivals, examined the retina and brain for labeled cells. They were abundant in both the optic tectum and the retina. Although the rate of retinal growth slows considerably between embryonic and adult stages, the patterns of neurogenesis in both the embryo and the adult are similar to those described in other fish, so these "fish-specific" features of general interest can justifiably be studied in zebrafish.

Animals↗