Biomedical subjects

O Packer

Publications and source records attributed to O Packer.

8 recordsLinked to original sources

PubMed2001

Characterization and use of a digital light projector for vision research.

For creating stimuli in the laboratory, digital light projection (DLP) technology has the potential to overcome the low output luminance, lack of pixel independence, and limited chromaticity gamut of the cathode ray tube (CRT). We built a DLP-based stimulator for projecting patterns on the in vitro primate retina. The DLP produces high light levels and has good contrast. Spatial performance was similar to that of a CRT. Temporal performance was limited by the refresh rate (63 Hz). The chromatic gamut was modestly larger than that of a CRT although the primary spectra varied to a small degree with light output and numerical aperture.

Analog-Digital Conversion↗

PubMed1992

Blurring by fixational eye movements.

A complete description of the loss of contrast sensitivity at high spatial frequencies requires an estimate of the role of eye movements, which could blur fine detail. We describe a new technique to isolate their effect. Observers viewed either a 100 c/deg interference fringe, which the cone mosaic aliased to a low frequency zebra stripe, or an artificial zebra stripe. The real and artificial zebra stripes have similar spatial patterns, but differ in the temporal modulation produced by eye movements. Contrast threshold was measured as a function of duration for both stimuli flashed in the dark. The ratio of the contrast thresholds for the real and artificial zebra stripes with long durations, when eye movements could have a differential effect, is always within a factor of two or so of the ratio for 1 msec flashes, when eye movements are eliminated. These results support the view that eye movements are only a minor source of image degradation even at very high spatial frequencies, and provide no support for the view that they improve high resolution tasks.

Contrast Sensitivity↗

PubMed1991

Nonlinear distortion of gratings at the foveal resolution limit.

Aliasing by the foveal cone mosaic causes high frequency interference fringes to look like bright and dark zebra stripes (primary zebra stripes) [Williams, Vision Research, 25, 195 (1985); Vision Research, 28, 433 (1988)]. Some observers report another type of zebra stripes defined by variations in chromaticity as well as brightness, which we call secondary zebra stripes. The conditions required to see the secondary zebra stripes are almost identical to those required to see the primary zebra stripes, except that they are seen at approximately half the spatial frequency. We consider the hypothesis that the secondary zebra stripes arise from aliasing by a particular packing arrangement of the M and L cone submosaics, but present evidence favoring an alternative hypothesis based on a known local nonlinearity in the visual system.

Color Vision Defects↗

PubMed1990

Development redistribution of photoreceptors across the Macaca nemestrina (pigtail macaque) retina.

Redistributions of monkey cones and rods during the first year after birth include a fivefold increase in peak foveal cone density from 43,000 to 210,000 cones/mm2, a decrease in the diameter of the rod-sparse area, and a two- to threefold decrease in peripheral photoreceptor density. Two weeks before birth, higher cone density is already apparent in the future fovea, as are the nasotemporal asymmetry in cone distribution, a higher density "cone streak" along the horizontal meridian, a large rod-sparse central fovea, and a ring of high rod density. Despite the early appearance of these basic patterns, photoreceptor distribution is not mature until 1 to 5 years postnatally. Total cones varied from 4 million at birth to 3.1 million in the average adult. The two oldest eyes had fewer cones, suggesting up to a 25% loss late in development. There were 60 to 70 million rods in the adult macaque retina and little evidence of postnatal changes in number. Neither of these small changes is sufficient to account for the reduction in peripheral photoreceptor density and both are in the wrong direction to explain increasing foveal density, ruling out a major role for either photoreceptor death or generation. Retinal area increased by a factor of 2.4 from 2 weeks before birth to adulthood. In contrast, the posterior pole of the retina was dimensionally stable throughout this period, with the distance between the fovea and optic disc varying nonsystematically from 3.37 to 4.05 mm. Retinal coverage of the globe was also stable at 48-60%. Thus postnatal growth can be ruled out as a factor in the density changes occurring in central retina. Adult retinas have a higher proportion of both cones and rods in midperiphery, whereas young retinas have a higher proportion of photoreceptors in far periphery. It appears that photoreceptors are radially redistributed from peripheral toward central retina during postnatal development, resulting in the marked increase in foveal cone density and the decrease in the eccentricity of the rod ring. Up to 13 weeks postnatally, midperipheral growth of the retina is substantial and increases with eccentricity. At later ages, expansion continues only in the very far periphery. Retinal growth appears sufficient to explain the decreases in peripheral rod and cone density with age. These and previous data strongly suggest that differentiated photoreceptors, with complex cytology and synaptic contacts, migrate toward the foveal center, explaining the increase in foveal photoreceptor density.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1989

Photoreceptor topography of the retina in the adult pigtail macaque (Macaca nemestrina).

In spite of the crucial role retinal photoreceptors play in mapping optical images into a pattern of neural excitation, there are no complete studies of photoreceptor topography in any primate retina. We have measured the spatial density and inner segment areas of cones and rods across the whole mounted retinas of three adult pigtail macaques (Macaca nemestrina) and constructed maps of photoreceptor density and inner segment diameter. These retinas contain an average of 3.1 million cones (2.8-3.3 million), with an average peak foveal cone density of 210,000 cones/mm2 (190,000-260,000 cones/mm2). Cone density falls steeply with increasing eccentricity, to 100,000 cones/mm2 at 200 microns from the fovea, and to 50,000 cones/mm2 at 750 microns. Imposed on this gradient is a "streak" of higher cone density along the horizontal meridian. At equivalent eccentricities, cone density is higher in nasal and inferior retina. Cone inner segments increase in diameter from 2.3 microns at the foveal center to 11 microns in far temporal retina and 10 microns in far nasal retina. These retinas contain an average of 60.1 million rods (44.9-75.3 million). Rod density is zero within 20 microns of the foveal center, increases to the crest of a "rod ring" at the eccentricity of the optic disk, and then declines. Central rod topography is asymmetric, with higher densities in superior retina. Density along the crest of the rod ring peaks in superior retina at 177,000 rods/mm2, dips as low as 120,000 rods/mm2 along the horizontal meridian, and increases to about 150,000 rods/mm2 in inferior retina. Far peripheral rod topography is relatively symmetric around the fovea. Rod inner segment diameter ranged from 1.5 microns in the fovea to 4 microns at the temporal edge and 3.4 microns at the nasal edge of the retina. At eccentricities exceeding 6 mm, rod inner segment diameter was greater temporally than nasally. Cone inner segments cover 85-90% of the central fovea, with extrareceptor space accounting for the remainder. Cone coverage declines with increasing eccentricity to 20% at the temporal edge and 35% at the nasal edge of the retina. In contrast, rod coverage increases from zero at the foveal center to a maximum of 65% in temporal retina and 50% in nasal retina. The photoreceptor topography of the pigtail macaque is qualitatively similar to that of other macaques and to humans. Photoreceptor topography is formed by a complex interaction between regional changes in cone and rod density and inner segment diameter.

Animals↗

PubMed1987

Distribution of cones in human and monkey retina: individual variability and radial asymmetry.

The distribution of photoreceptors is known for only one complete human retina and for the cardinal meridians only in the macaque monkey retina. Cones can be mapped in computer-reconstructed whole mounts of human and monkey retina. A 2.9-fold range in maximum cone density in the foveas of young adult human eyes may contribute to individual differences in acuity. Cone distribution is radially asymmetrical about the fovea in both species, as previously described for the distribution of retinal ganglion cells and for lines of visual isosensitivity. Cone density was greater in the nasal than in the temporal peripheral retina, and this nasotemporal asymmetry was more pronounced in monkey than in human retina.

Animals↗

PubMed1987

A whole mount method for sequential analysis of photoreceptor and ganglion cell topography in a single retina.

Photoreceptors (PR) in human and monkey retina are visible in whole mounts cleared with glycerol or dimethyl sulfoxide and viewed with Nomarski differential interference contrast microscopy. These preparations substantially decrease the large tissue volume changes associated with dehydration and sectioning and reveal many details of PR organization and cytology with great clarity. Tissue may be subsequently stained to reveal ganglion cells so that the topography of both cell types may be studied in the same retina.

Animals↗

PubMed1984

Infant color vision: the effect of test field size on Rayleigh discriminations.

The capacity of 1- and 3-month-old infants to discriminate 589 nm and 650 nm test fields from a 589 nm surround was tested using the forced-choice preferential looking (FPL) technique. The size of the test field ranged from 1 to 8 degrees. Test field size strongly influenced the infants' performance. One-month-olds discriminated 8 and 4 degrees (but not 2 degrees) 650 nm fields from the 589 nm surround; 3-month-olds discriminated 4 and 2 degrees (but not 1 degree) 650 nm fields from the 589 nm surround. The dependence of performance on field size suggests that infants' discrimination failures with small fields are due to immaturities of spatial processing or postreceptoral chromatic mechanisms, rather than to any absence or anomaly of receptor types. In addition, adult subjects rated the hue, brightness, and salience of the test stimuli at 0, 26, 52, and 78 degrees eccentricity. The analogy often made between infant vision and adult peripheral vision is discussed in relation to these data.

Adult↗