Biomedical subjects

M K Powers

Publications and source records attributed to M K Powers.

At least 19 recordsLinked to original sources

PubMed1991

DL-2-amino-4-phosphonobutyric acid does not eliminate "ON" responses in the visual system of goldfish.

DL-2-Amino-4-phosphonobutyric acid (APB) suppresses activity in retinal ON pathways. It is generally assumed that loss of the ON pathway would result in loss of ON responses in the visual system. We tested this assumption by recording activity from the optic nerves of intact goldfish (Carassius auratus) before and after intraocular injection of APB. Whole-nerve responses to increments and decrements of light were compared to electroretinogram responses and to tectal evoked potentials. APB severely reduced the amplitude of the electroretinogram b-wave but left ON and OFF responses from the optic nerve and tectum intact, although decreased in sensitivity. We conclude that APB does not completely eliminate ON responses in the visual system, at least in goldfish. The selectivity and effectiveness of APB must be evaluated in other species before this agent can be relied upon as a useful tool in understanding the roles of ON and OFF pathways in visual function.

Aminobutyrates↗

PubMed1991

Spatial contrast sensitivity of goldfish: mean luminance, temporal frequency and a new psychophysical technique.

Behavioral contrast sensitivity in goldfish was examined at various mean luminances and stimulus drift rates. Goldfish were classically conditioned to suppress respiration upon presentation of a drifting sinusoidal grating. Contrast threshold at each spatial frequency was determined by means of a new two-alternative forced-choice procedure in which the observer's decision about the presence of the stimulus was based on the animal's respiration pattern. The results show that: (1) as mean luminance decreases, contrast sensitivity to high spatial frequencies decreases and peak sensitivity shifts to lower spatial frequencies; (2) as drift rate increases, contrast sensitivity to low spatial frequencies increases, but sensitivity to high spatial frequencies is relatively unaffected by stimulus drift rate. Both the mean luminance and temporal frequency of the stimulus clearly influence the behavioral contrast sensitivity of the goldfish in ways that would be predicted from behavioral results from other species. We conclude that the mechanisms that mediate contrast sensitivity in goldfish are similar to those that mediate contrast sensitivity in other vertebrates.

Animals↗

PubMed1991

Visual performance of horseshoe crabs day and night.

A circadian clock modulates the structure and function of the lateral eyes of Limulus polyphemus, greatly increasing their sensitivity at night. During the mating season, male Limulus are visually attracted both day and night to females and objects that resemble females. This paper asks how well Limulus can see day and night, and whether the circadian changes in retinal sensitivity might influence the ability of these animals to find mates. We recorded the visual behavior of male and female horseshoe crabs in the vicinity of an object--a cement hemisphere (29.5 cm diameter) similar in size and shape to a female horseshoe crab--placed in a mating area near Mashnee Dike, Bourne, Massachusetts. Males oriented toward this target from an average distance of 0.94 m during the day and 0.88 m at night; and females appeared to avoid the target. We conclude that males can see potential mates at night almost as well as they can during the day. Apparently the circadian changes in the retina help compensate for the daily changes in illumination in the animal's normal environment. This study provides the first evidence for a role of visual circadian rhythms in an animal's natural behavior.

Animals↗

PubMed1991

Spectral sensitivity of ON and OFF responses from the optic nerve of goldfish.

The vertebrate retina processes visual information in parallel neural pathways known as the ON and OFF pathways. These pathways encode increments and decrements of light independently as excitatory responses. We examined the photopic spectral response of ON and OFF mechanisms in goldfish by measuring the sensitivity of optic nerve responses to the onset and termination of stimuli of various wavelengths. Using various adapting backgrounds, we found that the ON and OFF responses have different spectral sensitivities. The weighting of the cone inputs to the responses was estimated by an algebraic summation model. This model suggests that for the ON response, input from S-cones is stronger and more independent than for the OFF response, and M- and L-cones show stronger antagonism in the ON response than in the OFF response. The OFF response probably receives input from all cone types, but spectral antagonism is weak and its dominant input is from L-cones.

Animals↗

PubMed1990

Shedding of rod outer segments is light-driven in goldfish.

Rod outer segment (ROS) shedding in goldfish was quantified by measuring the number of phagosomes in the retinas of goldfish that were maintained under natural or artificial cyclic light, constant light, or constant dark conditions. Fish maintained in cyclic light, whether natural or artificial, had robust daily rhythms of ROS shedding. The ROS tips were shed primarily during the light phase of the cycle, and maximum shedding occurred 2-4 hours into the light period. Fish maintained for 1, 3, or 7 days in constant light or constant dark had no daily rhythms in ROS shedding. In these fish, ROSs lengthened, on average, 2.3 microns/day in constant light and 1.5 microns/day in constant dark. After 3 days in constant light, shedding was induced by placing fish in darkness for 2 hours, then returning them to light. Placing fish in darkness for 0.5 hours did not induce shedding, nor did placing them in darkness for 3 hours without returning them to light. ROS shedding thus appears to be goldfish is completely dependent on changes in ambient illumination; no circadian or endogenous components were found. Previous observations of circadian changes in behavioral visual sensitivity therefore cannot be due to endogenous changes in ROS length.

Animals↗

PubMed1990

Rod outer segment length and visual sensitivity.

Detection threshold for the rod system was measured psychophysically in adult goldfish before and after exposure to constant illumination of 340 lux (91 microW/cm2) for 7 days. As shown in the previous paper, rod outer segment (ROS) length increases an average of 60% under these conditions. The present work shows that visual sensitivity also increases, in approximate proportion to the additional optical density predicted by the longer ROSs. These results are the first to show that exposure to constant light can enhance visual sensitivity. They imply further that detection threshold is related to ROS length. Apparently, the photopigment in the ROS tips that is normally shed on a daily basis retains its photon-catching ability.

Animals↗

PubMed1989

Sensitivity of ERG components from dark-adapted goldfish retinas treated with APB.

The electroretinogram (ERG) of the dark-adapted goldfish was examined before and after intravitreal injection of DL-2-amino-4-phosphonobutyric acid (APB). APB abolished the b-wave and decreased absolute sensitivity of the remaining waveform, which was composed of a vitreal-negative component followed by a vitreal-positive component. The sensitivity, time course and amplitude of these components differed from ERGs obtained from animals treated with sodium aspartate. Spectral sensitivity of both post-APB components closely resembled that of the normal dark-adapted b-wave. The results suggest that APB does not act selectively on any particular class of photoreceptors or photoreceptor pathways in the dark-adapted goldfish retina.

Aminobutyrates↗

PubMed1989

APB selectively reduces visual responses in goldfish to high spatiotemporal frequencies.

Visual responses of goldfish to rotating square-wave gratings were recorded before and after intraocular injection of 2-amino-4-phosphonobutyric acid (APB). High doses of APB reduced the rate of optokinetic nystagmus (OKN) to a relatively high spatial frequency grating moving at a high temporal frequency. Responses to a low spatial frequency grating were not altered, nor were responses to the higher spatial frequency when it rotated slowly. The effects of APB were transient and lasted no longer than 3 d. We conclude that APB reduces OKN to high spatiotemporal frequencies in goldfish.

Aminobutyrates↗

PubMed1988

Visual detection by the rod system in goldfish of different sizes.

New rods are continually generated and inserted across the entire differentiated retina in juvenile and adult goldfish; no other retinal cells share this characteristic. How does the preferential addition of rods affect visual function? To examine the relation between continued rod addition and visual sensitivity, we measured absolute threshold in fish of different sizes. Twenty-nine fish were trained in a classical conditioning paradigm, and psychometric functions were obtained for each of them for detection of a 532 nm light 5 sec in duration, 140 deg in angular subtense, presented while the fish was fully dark adapted. We found that absolute threshold (expressed in terms of retinal photon density) was lower in larger fish, but by a very small amount; on average, large fish (15.4 +/- 0.5 cm standard body length) were 1.45 times more sensitive than small fish (4.3 +/- 0.3 cm). Morphometric analysis showed that the planimetric density of rods in goldfish retina increases at a similar rate between small and large fish, while the density of retinal ganglion cells declines between small and large fish (by a factor of 3.8). The ratio of rods to ganglion cells (a possible indicator of neural convergence) increased, but by a factor that is too large to reconcile with the psychophysical results (5.3 x). The results suggest that absolute visual threshold in the goldfish is closely related to the density of rods in the retina.

Animals↗

PubMed1988

Responsivity and absolute sensitivity of retinal ganglion cells in goldfish of different sizes, when measured under "psychophysical" conditions.

Retinal neurogenesis occurs in adult goldfish, and more rods are added to the retina than any other class of cell as the fish grows. To determine whether the disproportionate addition of rods affects the responsivity and sensitivity of dark adapted retinal ganglion cells, we recorded activity from optic tract fibers in goldfish of different sizes. Experimental conditions were as similar as possible to those used in a separate study in which psychophysical absolute thresholds were measured: large, dim, monochromatic spots 1 sec in duration were projected close to the right eye of alert, self-respiring goldfish. A total of 214 fibers were recorded in small (5.0-5.7 cm), medium (9.5-11.0 cm) and large (13.0-20.0 cm) fish. Neither maintained activity (mean and variance of the discharge rate in darkness) nor responsivity (quantum-to-spike ratios) nor absolute threshold (quantal irradiance required to produce a difference of 1 spike/trial from spontaneous rates) varied reliably with size of fish. However, some Off cells were more active in the dark than On and On/Off cells; these had low QSR's and absolute thresholds, and were found in all sizes of fish. Fifty percent (50%) of Off cells (compared to 8% of On cells) had thresholds comparable to or lower than psychophysical threshold, and Off cell thresholds (but not On cell thresholds) tended to be lower in larger fish. Because psychophysical threshold is closely related to the planimetric density of rods in goldfish, the similarity between Off cell threshold and psychophysical threshold suggests that Off cells may be influenced relatively more than On cells by the addition of new rods to the retina.

Action Potentials↗

PubMed1988

Spectral sensitivity of the electroretinogram b-wave in dark-adapted goldfish.

The action spectrum of the ERG b-wave was measured under dark-adapted conditions in intact goldfish (Carassius auratus). It is substantially broader than the absorption spectrum of goldfish rod porphyropsin. Neither prolonged dark adaptation nor removal of possible efferent neural activity affected its shape. Moreover, a 682-nm background did not produce a selective loss of sensitivity to long wavelengths. The results imply that the spectral sensitivity of the b-wave in dark-adapted goldfish reflects the influence of at least two photoreceptor types which act as a single univariant mechanism near absolute threshold.

Animals↗

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↗