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M K Powers

Publications and source records attributed to M K Powers.

At least 37 records · Page 2Linked to original sources
PubMed1988

Lighting conditions and retinal development in goldfish: photoreceptor number and structure.

The retinas of 63 goldfish were examined after varying durations of exposure to one of three environmental lighting conditions beginning before hatching: constant light (340 lux), cyclic light (12 hr 320 lux, 12 hr dark) and constant dark. Up to 8 months, no effects of constant light or dark on photoreceptor numbers or structure were apparent. Densities of rod and cone nuclei were normal and all retinal layers appeared normal by light microscopy. Exposure to constant light for 12 months or longer resulted in a reduction in rod density by 37%. Cone numbers were unaffected by constant light, even with exposures of 3 yr, and rod and cone outer segments were normal in length at 11-20 months under all environmental conditions. Due to poor survival, only one animal was available for quantitative examination from the group reared in constant dark 12 months or longer. Photoreceptor size and number in this retina were similar to those in the constant light condition. The results suggest that the formation and maturation of rods and cones in goldfish retina is unaffected by rearing in constant light. However, long-term exposure (greater than or equal to 12 months) may disrupt maintenance of differentiated rods.

Aging↗
PubMed1988

Lighting conditions and retinal development in goldfish: absolute visual sensitivity.

Goldfish (Carassius auratus) were reared from hatching in constant light (340 lux), cyclic light (12 hr 320 lux, 12 hr dark) or constant dark. Absolute visual threshold was determined psychophysically in animals that still responded to visual stimuli after 1-3 years of exposure, by means of a classically conditioned respiration suppression technique wherein animals were presented with different intensities of large diffuse flashes of monochromatic light. Fish reared in constant light and fish reared in cyclic light responded reliably to stimuli above threshold, but fish reared in constant light were on average 0.58 log unit less sensitive at 532 nm, near the peak of the rod action spectrum. Two of the four fish reared in darkness did not respond to the stimuli, and thus could not be conditioned, and another fish reared in darkness responded only occasionally; threshold could not be measured in these three fish. The one fish reared in darkness that responded consistently enough to be conditioned was more than 5 log units less sensitive than normally reared fish on the first day of testing, and became progressively less sensitive over the next 2 days. Rearing under constant dark or constant light had no obvious effect on spectral sensitivity at absolute threshold. The effect of rearing in constant light on absolute threshold correlates with morphological changes in rod density, but the effect of rearing in constant darkness does not.

Animal Husbandry↗
PubMed1987

Circadian rhythm in goldfish visual sensitivity.

To determine whether rod-mediated vision in goldfish is regulated by a circadian clock, absolute threshold was measured psychophysically in animals maintained in constant darkness. Responses were recorded approximately every 4 hr to a diffuse 532 nm stimulus, 5 sec in duration, with no background light present. Visual threshold tended to be lowest at the time of transition from light to dark, as experienced by the fish before it was placed in constant darkness. Threshold tended to be highest at the time of transition from dark to light. The average peak-to-trough fluctuation for five fish was 0.5 log unit in amplitude, and its period was about 24 hr. The results show that the rhythm in visual threshold can still be detected after 7 days of darkness, and that it can be entrained to a new light-dark cycle. These properties are characteristic of regulation by a circadian oscillator.

Animals↗
PubMed1986

Daily fluctuations in the detectability of dim lights by humans.

Several aspects of retinal physiology are known to vary on daily or circadian cycles, suggesting that certain aspects of visual function might also vary. We measured absolute threshold--the dimmest visual stimulus that can be reliably detected--in seven human observers at mid-day (1200-1400) and mid-night (0000-0200), by means of standard psychophysical techniques. Five of the subjects were slightly more sensitive at night: they could detect stimuli that were on the average 20% dimmer at night than during the day, and this difference was not due to changes in the criterion for detection. Three of four subjects whose thresholds were measured hourly while they remained otherwise in total darkness for greater than 24 hours showed changes in absolute visual threshold that correlated with hourly changes in body temperature. These results suggest that the ability to detect dim lights varies systematically with time of day in many human observers.

Body Temperature↗
PubMed1985

A method for determining threshold from single-unit neural activity.

A computationally straightforward method is described for determining the latency, duration and magnitude of stimulus-evoked single-unit neural activity. A unique feature of the method is its ability to define the neural response without reference to stimulus parameters. First, the temporal component of the spike train that represents the response is located and then that component is analyzed to determine the magnitude of the response. Intensity-response functions can then be constructed, using the number of extra spikes above baseline activity as a measure of response magnitude. Threshold can be defined as any point on the intensity-response function.

Action Potentials↗
PubMed1984

Light transmittance by goldfish eyes of different sizes.

The transmittance of the optic media was measured in eyes from small (4.3-5.5 cm standard body length), medium (8.3-10.0 cm) and large (14.2-17.5 cm) goldfish. Few differences were observed over this size range, despite a doubling in diameter of the globe. Relative transmittance was constant (+/- 0.04 log unit) between 450-700 nm, and absolute transmittance through the lens was approximately constant as well. Changes did occur, however, in the shorter wavelengths: at 350 nm, transmission of large eyes was diminished by about 0.40 log unit relative to small eyes.

Animals↗
PubMed1984

Psychosocial findings in radial keratotomy patients two years after surgery.

In a psychosocial study of patients who participated in an evaluation of the visual, refractive and keratometric results of radial keratotomy, respondents stated that their primary reasons for electing the operation related to anticipated changes in vision. Improving appearance was not reported as a primary reason for seeking radial keratotomy by these patients. When asked two years after surgery if their vision had improved, remained the same, or worsened, 94.3% reported improvement over preoperative vision. About 40% still wear corrective lens, 26% full time and 14% only part of the time. Most patients (84.1%) reported overall satisfaction with the surgical outcome. Patient satisfaction was strongly related to perceived improvement of vision after surgery, and not to patient self-esteem, to changes in appearance or lifestyle brought about by the operation, or to having a particular physician perform the surgery.

Adaptation, Psychological↗
PubMed1983

Postembryonic growth of the optic tectum in goldfish. II. Modulation of cell proliferation by retinal fiber input.

The proliferation of cells in the germinal zone of the optic tectum of adult goldfish was studied following unilateral optic nerve crush or removal of one eye. Dividing germinal cells were labeled with [3H]thymidine, which was injected at various times (0 to 30 days) following surgery; fish were sacrificed after short (48 hr) survival times. The numbers of labeled nuclei in the tectal germinal zones were compared on the two sides (intact and denervated). We show that permanent removal of optic input (by enucleation) resulted in a sustained depression of [3H]thymidine incorporation in the tectal germinal zone on the denervated compared to the intact side. Temporary denervation (by optic nerve crush) initially had a similar effect; however, upon reinnervation of the tectum by regenerating optic fibers, proliferation was enhanced on the experimental side compared to the intact side. Because cells in the germinal zone are known to produce new tectal cells, neurons as well as glia, in the normal growing adult brain (Raymond, P. A. and S. S. Easter, Jr. (1983) J. Neurosci. 3: 1077-1091), some of the proliferating cells may have been generating neurons. This inference is supported by the observation that in two fish whose right eye had been removed more than 2 years earlier, there were fewer neurons in the denervated tectum than in the intact tectum. Thus, it is likely that the observed decrease in incorporation of [3H]thymidine by cells in the germinal zone of the denervated optic tectum resulted in a slower rate of addition of new tectal cells on the affected side. We conclude that cytogenesis in the germinal zone of the growing optic tectum of adult goldfish is regulated by optic fiber input. This mechanism may be important in matching the rates of growth of retina and tectum in the normal brain of the growing adult fish.

Aging↗
PubMed1982

Effect of focus on visual acuity of human infants.

Recent theoretical arguments (Green et al. (1980) Vision Res. 20, 827-835) predict that young human infants should have large depths of focus compared to adults. If so, optical blur should have relatively little effect on the resolving power of infant subjects. We tested this hypothesis by measuring the influence of optical blur on acuity thresholds in 6-week old infants and adults. Using the forced-choice preferential looking (FPL) technique, we obtained acuity thresholds for each subject with five different lens powers (plano, -14 D, -3 D, + 6 D and +14 D). Acuity was differentially affected by lens power in all subjects, with the best acuity produced by high power lenses, both plus and minus, was considerably less for infants than for adults. Although the effects of negative lenses are difficult to interpret without knowing the infants' exact accommodative state, the results with positive lenses support the theoretical predictions.

Accommodation, Ocular↗
PubMed1982

Mechanisms of light adaptation in rat retina.

Green, Dowling, Siegel and Ripps (1975) J. gen. Physiol. 65, 483-502 found that both receptors and post-receptoral elements regulate the process of light adaptation in skate retina. As a test of the generality of this conclusion, we repeated their experiments on the retina of the intact albino rat. Increment threshold and intensity-response functions of aspartate-isolated receptor potentials, ERG b-waves and single retinal ganglion cell axons were measured, and all were found to be similar to those described by Green et al. (1975). In particular, the adaptive properties of the b-wave and ganglion cells were similar to each other, but different from those of the receptor potential. We conclude that the retinal mechanisms of light adaptation are similar in rat and skate.

Adaptation, Ocular↗
PubMedDate not supplied

Limulus vision in the ocean day and night: effects of image size and contrast.

Male horseshoe crabs, Limulus polyphemus, use their eyes to locate mates day and night. We investigated their ability to detect targets of different size and contrast in a mating area of Buzzards Bay, Cape Cod, MA. We found that males can see large, high-contrast targets better than small, low-contrast ones. For targets of the same size, animals must be about 0.1 m closer to a low-contrast target to see it as well as a high-contrast one. For targets of the same contrast, animals must be approximately 0.2 m closer to a small target to see it as well as one twice as large. A decrease of 0.05 steradians in the size of the retinal image of a target can be compensated by a four-fold increase in contrast. About 60% of the animals detect black targets subtending 0.110 steradians (equivalent to an adult female viewed from about 0.56 m), while only 20% detect targets subtending 0.039 steradians. This study shows that horseshoe crabs maintain about constant contrast sensitivity under diurnal changes in light intensity in their natural environment. As a consequence of circadian and adaptive mechanisms in the retina, male horseshoe crabs can detect female-size objects about equally well day and night.

Adaptation, Ocular↗
PubMedDate not supplied

Morphology and visual pigment content of photoreceptors from injured goldfish retina.

Adult teleost fish retinas can regenerate neurons following either surgical or pharmacological injury. The cellular milieu of the damaged retina within which regenerated neurons are produced might be different in these two model systems of retinal injury, and thus the phenotypic attributes of regenerated neurons in the two model systems might also differ. To determine if the phenotypic attributes of photoreceptors, and by extension the recovery of vision, are different between these two model systems, we compared the visual pigment content and morphology of photoreceptors derived from goldfish retinas of both models with control retina. Visual pigments-which consist of a protein moiety (opsin) and a chromophore--were analyzed in single, isolated photoreceptors using microspectrophotometric techniques. We report that visual pigments and photoreceptor morphologies in the surgical model closely matched those of native retina. In contrast, neither photoreceptor morphology nor visual pigment content matched closely in the pharmacological model. The results indicate that phenotypic attributes of photoreceptors can differ significantly between the two model systems of retinal regeneration, but that in both systems, rod- and cone-mediated visual functions can potentially be reestablished.

Animals↗