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A M Granda

Publications and source records attributed to A M Granda.

At least 19 recordsLinked to original sources
PubMed2002

Multiplied functions unify shapes of ganglion-cell receptive fields in retina of turtle.

Retinal ganglion cells in the turtle were extracellularly recorded to define the shapes of their receptive fields by small moving light spots. To better define the geometries, spectral-light adaptations and vitreal injections of 2-amino-4-phosphonobutyric acid (APB) were used to disrupt balances in field organization along dimensions of wavelength, ON and OFF responses, and center/surround areas. Three-dimensional data plots were fit by Gaussian, Gabor, and cardioid functions to show that the shapes of receptive fields are predicted by combinations of these multiplied functions. Results indicate that Gaussian functions describe simple symmetrical receptive fields that are center-only; Gabor functions describe center/surround color-opponent receptive fields that have a ring of spike activity in the periphery; and directionally selective receptive fields, in contrast, which are asymmetrical, are described by cardioid functions adjoined to Gaussian or Gabor functions. The advantage of linking multiplied functions is that receptive fields are unified by a model that predicts progressively more complex field geometries derived from particular stimulating conditions.

Adaptation, Ocular↗
PubMed1994

Functional morphologies of retinal ganglion cells in the turtle.

Retinal ganglion cells in the turtle, Pseudemys scripta elegans, were examined by intracellular recording with a protocol of stationary and moving lights. Responses were apportioned among OFF, ON, and ON-OFF categories, and directional selectivity. Cells were injected with Neurobiotin, then later conjugated with avidin-horseradish peroxidase in standard procedure. Morphological analysis of the stained cells included measurements of soma and dendritic field sizes, dendritic stratification, number of cell processes, dendritic branchings, and dendritic symmetry ratios. ON and ON-OFF cells are at least bistratified, sometimes tristratified, in both sublaminae A and B whether directionally selective or not. OFF cells, in contrast, are monostratified, or at least confined to sublamina A. Morphological parameters of somal and dendritic field areas, branch point densities, and dendritic field asymmetries do not predict directional selectivity. Membrane polarization accompanying moving stimulation is discussed in terms of shunting inhibition and recording site.

Animals↗
PubMed1991

Refractive state, contrast sensitivity, and resolution in the freshwater turtle, Pseudemys scripta elegans, determined by tectal visual-evoked potentials.

Visual-evoked potentials (VEPs) were recorded from the surface of the optic tectum of the freshwater turtle, Pseudemys scripta elegans, in response to phase reversal of square-wave gratings of different spatial frequency and contrast. The refractive state of a group of 12 turtles in air was assessed from VEPs by placing trial lenses in front of the eye. The group mean refraction did not differ significantly from emmetropia, as compared to 4.8 diopters of hyperopia when refracted retinoscopically. The difference was explained by the retinoscopic reflex originating from the interface between vitreous humor and retina. Peak VEP amplitude was approximately linear with log grating contrast; extrapolation to zero VEP amplitude yielded contrast thresholds as low as 1%. High spatial-frequency cutoffs ranged from 4.4-9.9 cycle/deg in different animals, the highest values corresponding to the intercone spacing in the area centralis and to behavioral measures of acuity in a related species.

Adaptation, Ocular↗
PubMed1991

Ocular dimensions and schematic eyes of freshwater and sea turtles.

Measurements were made of the ocular dimensions from living and frozen eyes of one species of freshwater turtle, Pseudemys scripta elegans, and of three species of marine turtles, Chelonia mydas, Dermochelys cariacea, and Eretmochelys imbricata. Estimates of refractive error by retinoscopy were also obtained with eyes in air and under water. The results suggest that unaccommodated eyes of all four species are approximately emmetropic in air but strongly hyperopic in water. Schematic eyes were calculated for each species in both air and water.

Adaptation, Physiological↗
PubMed1990

Wavelength-dependent temporal properties of retinal horizontal cells in turtles.

Electrical responses of luminosity horizontal cells (L cells) to monochromatic stimuli were analyzed by intracellular recordings in the retinas of the freshwater turtle (Pseudemys scripta elegans) and of the sea turtle (Chelonia mydas mydas). Light intensity, duration, and wavelength were varied to assess temporal effects. For a given intensity of monochromatic light, response amplitude increased with stimulus duration until maximum amplitude occurred at a specific duration. This suprathreshold metric of temporal integration is called here summation time, and it is wavelength-dependent. L cells always had some level of red-sensitive cone input, although in some cells inputs from green- and blue-sensitive cones were also observed. For these latter cells, summation times were shorter for 640-nm than for 540-nm or 450-nm lights. These results were most evident in cells that received dominant inputs from blue- or green-sensitive cones. Responses of some other L cells were almost completely dominated by inputs from red-sensitive cones. Summation times of these cells were not wavelength-dependent. But when these inputs also included green-sensitive cones, shorter summation times were obtained to 640-nm light than to 540-nm light, even though dominant inputs were still from red-sensitive cones. These results, obtained from both retinal and 3,4-dehydroretinal photopigment systems, are consistent with reported observations in Pseudemys scripta elegans that show linear responses of red-sensitive cones to have shorter integration times and times-to-peak than green-sensitive cones. Responses from horizontal cells dominated by blue-sensitive cone inputs were the most sensitive of all; they also had the longest summation times.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Psychophysically derived visual mechanisms in turtle. I--Spectral properties.

Two-color increment thresholds were measured in turtle using a behavioral avoidance paradigm. Four visual mechanisms were isolated, and their spectral sensitivities were compared to action spectra of photoreceptors known to occur in this animal. One visual mechanism appears to be mediated by rods and red-sensitive cones at low background intensities, a second mechanism by coupled red- and green-sensitive cones at intermediate levels, a third and fourth mechanism by red-sensitive, and green-sensitive single cones, respectively, acting alone at high background intensities.

Adaptation, Ocular↗
PubMed1989

Psychophysically derived visual mechanisms in turtle. II--Spatial properties.

Visual mechanisms isolated in Pseudemys by the two-color threshold technique of Stiles show peak wavelength sensitivities at 650 nm (red light) and 540 nm (green light). Ricco critical areas were measured for the two test wavelengths under three conditions: dark, moderate and intense backgrounds. As expected, critical spatial areas decreased with light adaptation. Under dark adaptation only rods and red-sensitive cones were operative, and one photon per 12 rods was sufficient for green-light threshold, as was one photon per four red-sensitive cones for red-light threshold. Rods apparently pool their information among the several receptors within the threshold area. Under light adaptation, rods were not functional and thresholds were determined by red-sensitive and green-sensitive cones alone. Cones did not share information over many receptors, requiring close to one photon per receptor to function at threshold.

Adaptation, Ocular↗
PubMed1989

Classification of turtle retinal ganglion cells.

1. Receptive fields of 78 retinal ganglion cells were analyzed for their responses to moving and stationary lights that were presented under a variety of stimulus conditions. All cells were sensitive to moving stimuli, and their receptive fields often comprised excitatory and inhibitory sub-regions. 2. Properties used in the classification included responses to stationary flashed stimuli, receptive-field organization, changes in stimulus wavelength and adaptation, movement velocity, and direction of stimulus movement. Eight functional cell classes were derived: simple, ON-sustained, annular, wavelength-sensitive, directionally selective, bar-shaped, large-field, and velocity. 3. Simple cells, representing 21% of the sample, had circular or oval receptive fields of 3-22 degrees that gave transient responses to stationary, flashed lights. Many of these cells, but not all, showed antagonistic center-surround organizations. ON-sustained cells responded for the duration of the stimulus flash or for the duration of a light flash moving through the receptive field. These units comprised 8% of the sample; they had small, circular, non-directional receptive fields and they were most sensitive to red light. Their field sizes did not vary with changes in adaptation level. 4. Annular cells (4% of the sample) gave no responses to any stimulation in the field center, but they responded strongly to stimulation in the surround area, especially to stimuli that moved very slowly through the region. Annular cells were nondirectional, with circular centers of 5-6 degrees diam and annular surround widths of 2-4 degrees. They responded best in light adaptation. 5. Wavelength-sensitive cells, similar to most of the cells sampled, were sensitive to red light when light-adapted. Some cells in addition showed input from rods under dark adaptation. Intensity-response curves for these latter cells showed clear changes from one input to the other as the cells' functional ranges were explored. Some cells responded best to short- or middle-wavelength light, but these were more rarely met. Where multiple receptor inputs could be identified, long-wavelength stimuli evoked transient responses, whereas short-wavelength stimuli favored more sustained spike trains. Wavelength-sensitive cells in this category comprised 5% of the sample.

Adaptation, Physiological↗
PubMed1986

Photoreceptor input and temporal summation in turtle retinal ganglion cells.

The relation between photoreceptor input and temporal summation in ganglion cells was investigated in the optical nerve of fresh-water turtle, Pseudemys. Action spectra derived for constant, high-criterion responses showed most cells to be maximally sensitive to 620 nm light, indicating dominant input from red-sensitive cones. At a lower criterion, sensitivity to 520 nm light greatly increased indicating an increase in rod input. Temporal summation curves for these intensity-dependent cells showed reciprocity between flash intensity and duration up to 128 msec for high-criterion summation curves. Low-criterion summation curves showed reciprocity up to the critical duration of 285 msec. Cells maximally sensitive to 620 nm light at all intensities often showed secondary sensitivity to 560 nm light, and were fitted by action spectra taken from red- and green-sensitive cones. Temporal summation curves for these cells had critical durations near 100 msec. Some cells responded best to light near 520 nm at all intensities and demonstrated little or no input from red-sensitive cones. These cells were fitted well by the action spectrum of rods and showed the longest critical durations of all at 375 msec. Chromatic input and temporal characteristics are intimately related: red-cone-dominated ganglion cells have the shortest critical durations while rod-dominated cells shows much longer ones. These findings are in keeping with psychophysical determinations of critical durations in this same animal.

Action Potentials↗
PubMed1981

Visual properties of cells in anterior dorsal ventricular ridge of turtle.

Single units in the anterior dorsal ventricular ridge (ADVR), a structure in the major afferent visual pathway of turtle, were investigated electrophysiologically for response properties to varied light patterns. The majority of cells responded to a broad range of spatially, temporally, and chromatically varied stimuli over most of the monocular visual field. One category of cells, seemingly specialized for 'novelty' detection, indicates one possible role for ADVR in visual sensory processing.

Animals↗
PubMed1980

Photoreceptor signals at visual threshold.

Electrical responses of cone photoreceptors in the retina of the freshwater turtle have been characterised for flashes and steps of light in darkness and in the presence of background light. These intracellular measurements have been combined with the behavioural increment threshold curve to yield an estimate of 5-10 muV for the signal developed in a cone when the turtle can just detect an increment flash. The signal developed when the cones under the stimulus image are dark-adapted is of interest, for its measurement would help to explain how known physiological processes subserve visual detection for a variety of photic conditions. The effective quantal absorption of dark-adapted, red-sensitive cones of Pseudemys scripta elegans for a stimulus that the turtle can just detect is reported here. By combining this result with previous electrical measurements on red-sensitive cones of this species, an estimate of 35-70 muV is obtained for the signal developed in a dark-adapted cone at behavioural threshold. This larger signal required for detection of a flash in darkness is of particular interest in view of the recent observation that the intrinsic noise of turtle cones in darkness is larger than that of illuminated cones.

Animals↗
PubMed1979

Seaward orientation of hatchling turtles: turning systems in the optic tectum.

This paper reports studies on the mechanisms underlying seaward orientation in hatchling turtles. The particular aim was to investigate whether activity in different regions of the retina and associated tectal areas, as assessed by some comparator mechanism, results in turning in different directions. Hatchling green turtles (Chelonia mydas) were tested for sea-finding ability in a natural situation on the beach following lesions of the optic tectum. Asymmetrical bilateral lesions resulted in a number of turtles making circles in the direction of the tectum with the posterior lesion and in other turtles deviated in this direction. Bilaterally lesioned turtles were also slower and less consistent in their sea-finding behaviour. No major disruptions of sea finding were detected in animals with unilateral tectal lesions. By suspending lights in the nasal visual field of unilaterally blindfolded green and leatherback turtles (Dermochelys coriacea) it was possible to produce circling in the direction of the covered eye; in contrast, with the light suspended in the temporal field, turning was always in the direction of the uncovered eye. The results are consistent with the view that sea finding depends on a complex phototropotactic system with stimulation in different parts of a single retina being associated with turning in opposite directions.

Animals↗