Improving visual skills: II-remote assessment via Internet

BACKGROUND

Even though poor readers often have poor visual skills, such as binocular coordination and oculomotor control, students' visual skills are rarely assessed. Computer assessments have the potential to assist in identifying students whose visual skills are deficient. This study compared assessments made by an Internet-based computer orthoptics program with those of an on-site vision therapist.

METHODS

Students (N = 41) in grades 1 through 8, reading at least 2 levels below grade, were assessed for visual skill dysfunction (including binocular fusion and tracking ability) by a vision therapist at their school in Wisconsin. The therapist determined whether the student had adequate visual skills based on clinical and behavioral observations. A "remote" investigator located in California determined the adequacy of accommodative facility, tracking, and vergence skills in the same students, based on quantitative progress through the modules of an Internet-based computer orthoptics training program during 3 assessment sessions.

RESULTS

The on-site therapist made 33 referrals for possible visual skills training (80%). The remote investigator made 25 referrals (61%), all of which were consistent with referrals made by the on-site therapist; thus, no false-positives occurred when using the remote assessment technique. The 8 additional referrals by the therapist were attributed to the ability to observe student behavior during assessment.

CONCLUSIONS

Remote assessment of visual skills via an Internet orthoptics program may provide a simple means to detect visual skill problems experienced by poor readers.

Visual behavior of adult goldfish with regenerating retina

To determine whether regenerating neural pathways can support visual behavior, adult goldfish (Carassius auratus) were injected intraocularly with ouabain and tested for the presence of reflexive visual behaviors (dorsal light reflex and optokinetic nystagmus) and the ability to respond to visual stimuli in a classical conditioning paradigm. All visual behaviors were absent or greatly diminished until 8 to 10 weeks, when retinal layering had returned. At 10 weeks post-ouabain, reflexive behaviors to supra-threshold stimuli were near normal; however the ability to detect supra-threshold stimuli in the conditioning paradigm did not recover until 13 weeks. Absolute dark-adapted threshold and light-adapted spectral sensitivity measured at 13 to 17 weeks were abnormal: Dark-adapted threshold was elevated by 1.5 log units and light-adapted spectral sensitivity was markedly narrower than normal. No responses to 50% contrast sinusoidal gratings could be obtained through ouabain-treated eyes using the classical conditioning technique, even though responses through the untreated eye remained. Results demonstrate that: (a) visually mediated behaviors return in goldfish with ouabain-treated retinas; (b) the time course of recovery of reflexive responses in luminance and spatial domains parallels return of ERG function and of tectal activity; and (c) visual function that is mediated by regenerating retina appears not to be as sensitive as vision via normally developed retinal pathways.

Visual function in regenerating teleost retina following surgical lesioning

Regeneration of the teleost retina following surgical extirpation of 25% to 100% of the neural retina was investigated in goldfish (Carrasius auratus) and sunfish (Lepomis cyanellus). The retina will regenerate following removal of up to 95% of the neural retina, however complete extirpation prevented regeneration. Visual sensitivity was assessed by examining components of the electroretinogram (ERG) and the dorsal light reflex (DLR) during regeneration. B-wave amplitudes in the experimental eyes increased throughout the study and central connections were reestablished as indicated by the progressive improvement in the dorsal light reflex. The recovery of visual function was closely correlated with retinal regeneration. Visual recovery progressed more slowly than following complete cytotoxic destruction of the mature retina (Mensinger & Powers, 1999) because the surgery removed a large number of the pluripotent cell population and restricted the number and distribution of regenerating foci.

Metabolic activity in optic tectum during regeneration of retina in adult goldfish

Retinal and visual function returns following retinal destruction by ouabain in adult goldfish (Carassius auratus). Although the precise cellular mechanisms are unclear, the ability to regenerate CNS neurons and connections that subsequently sustain visual behavior is remarkable, especially for an adult vertebrate. In this paper, we ask whether visual stimulation via new retinal cells can activate existing cells in the optic tectum, which normally receives the largest retinal projection in this species. The right eyes of adult goldfish were injected with ouabain. After 1-18 weeks the conscious, freely moving fish were exposed to spatially and temporally varying visual stimuli and the resulting tectal metabolic activity was determined with the autoradiographic deoxyglucose method. In normal controls without lesions, visual stimulation produced equally strong metabolic activity in both tectal hemispheres, peaking in the layer where most retinotectal projections terminate (N = 6). One week after ouabain injection, metabolic activity in the contralateral, deprived tectum was dramatically reduced (N = 5), closely resembling the effect of unilateral ocular enucleation (N = 5). However, 9-18 weeks after ouabain injection, metabolic activity in the deprived tectum recovered to a level that was statistically indistinguishable from normal controls (N = 6). These findings suggest that, after a comprehensive cytotoxic lesion of the retina, regenerated ganglion cells not only establish new connections with the preexisting optic tectum, but also effectively transmit visual information they receive from newly generated photoreceptors to the "old" tectum.

Evidence for two distinct mechanisms of neurogenesis and cellular pattern formation in regenerated goldfish retinas

After its destruction by intraocular injection of ouabain, the goldfish retina regenerates, but little is known about the histogenesis of the new tissue, including the structure and formation of regenerated cell mosaic patterns. In an effort to determine how retinal cells are generated and spatially organized within retina regenerated after ouabain injection, in situ hybridization and immunocytochemical techniques were combined with computational analyses of two-dimensional spatial patterns of identified neurons. Labeling with specific opsin riboprobes revealed two distinct cone patterns in the ouabain-injected eyes, each of which was different from the relatively orderly cone patterns of native retina. Central, regenerated regions had sparse aggregates of cones, and a relatively lower density of each cone type. Peripheral regions of experimental retina, likely derived from the circumferential germinal zone, had high densities of all cone types, each of which tended to be distributed randomly. The spatial patterns of inner retinal neurons in experimental eyes were also disorganized with respect to native retina. These results indicate that although some aspects of retinal regeneration resemble normal retinal development and growth, ouabain-induced regeneration does not produce well-organized mosaics of neurons, indicating a failure of the developmental interactions needed for proper pattern formation, which in turn could compromise visual recovery. Furthermore, the distinct cone patterns in different regions of experimental retina support the hypothesis that new goldfish retina arises via two spatially and cellularly distinct mechanisms after exposure to ouabain.

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.

Visual function in regenerating teleost retina following cytotoxic lesioning

Teleost fish retinas can regenerate in vivo in adulthood. Retinal and visual function was assessed in adult goldfish following comprehensive retinal destruction by intraocular injection of ouabain. Electroretinograms (ERGs) and the dorsal light reflex (DLR) were used to evaluate the return of visual function. ERGs were detectable in regenerating eyes 50 to 70 days following ouabain injection. Amplitudes of both a- and b-waves increased steadily through day 210 following ouabain treatment, at which time a-wave amplitude was 90% and b-wave amplitude approached 50% of the contralateral control eye. The progressive gain observed in the a-wave was attributed to photoreceptor regeneration. The increase in b-wave amplitude was attributed to an increase in the number of inner nuclear layer cells and the number and efficacy of neuronal connections to or within the inner retina. The photopic spectral sensitivity of the b-wave in regenerating retina closely matched the intrafish control retina, suggesting that the relative numbers of cone photoreceptors was normal in regeneration. The recovery of the DLR (indicated by improved postural balance during regeneration) paralleled electrophysiological gains during retinal regeneration. Fish displayed a marked longitudinal body imbalance toward the control eye following retinal destruction. Improvement in equilibrium was correlated with increasing b-wave amplitudes. When the b-wave reached 50% of control amplitude (30 weeks), normal posture was restored. The return of the ERG indicates that photoreceptors and their synaptic connections must be functional in regenerating retina. Failure of the retina to regenerate produced an abnormal DLR that persisted through 30 weeks and ERGs were not measurable. The return of normal equilibrium indicates that the regenerating retina can establish central connections to the brain, and that the regenerated connections can mediate functional visual behavior.

Effects of mean luminance on goldfish temporal contrast sensitivity

The goldfish is an important animal model for retinal processing and for understanding the relationship between retinal structure and function. The purpose of this study was to examine the temporal processing of the visual system of this species. Goldfish were classically conditioned to suppress respiration upon presentation of a sinusoidally flickering stimulus. Temporal contrast sensitivity functions (T-CSFs) were determined by measuring contrast threshold at a variety of temporal frequencies across different mean luminances. Goldfish T-CSFs were found to be similar in shape to those of humans. In addition, as the mean luminance of the stimulus decreased, temporal resolution decreased. This implies that the animal's ability to detect high flicker frequencies decreases as the level of light adaptation decreases, as does that of humans. The results support the notion that temporal processing is similar across vertebrate species, and therefore that the goldfish is a useful model for studying temporal processing in the vertebrate retina.

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.

The contributions of ON- and OFF-pathways to contrast sensitivity and spatial resolution in goldfish

DL-2-amino-4-phosphonobutyric acid (APB) reduces the sensitivity of ON- and OFF-responses in goldfish retina, although the ON-responses are reduced significantly more than the OFF-responses. This paper describes the effects of APB on behavioral sensitivity of goldfish to spatial sinusoidal gratings. Fish were classically conditioned to suppress respiration upon presentation of gratings drifting at 1 Hz; contrast thresholds were measured by an observer-based two-alternative forced-choice procedure. Thresholds were repeated following intraocular injections of APB or physiological saline. Saline had no effect, but APB dramatically reduced contrast sensitivity and shifted contrast sensitivity functions to lower spatial frequencies. The results suggest that both ON- and OFF-pathways are necessary for normal spatial vision and that the effects of APB are consistent with the disruption of both ON- and OFF-pathways.

APB alters photopic spectral sensitivity of the goldfish retina

Because the glutamate analog 2-amino-4-phosphonobutyric acid (APB) alters synaptic transmission at the outer plexiform layer in goldfish we asked whether intraocular injection of ABP would alter the spectral sensitivity of the retina. The spectral sensitivity of the ON and OFF components of the optic nerve response (ONR) in goldfish was measured in the presence and absence of APB, under four chromatic adaptation condition. APB decreased absolute sensitivity and altered spectral sensitivity for both ON and OFF responses under each adaptation condition. The spectral sensitivity of the OFF response was altered most at short wavelengths, in a manner consistent with a change in the balance of additive cone inputs. For the ON response, the effects of APB were consistent with a change in spectral antagonism, particularly between M- and L-cones. These results suggest that the activity in the retinal cone pathways in goldfish can be influenced by a mechanism incorporating an APB-sensitive receptor, and that this receptor may be intimately involved with setting the balance of cone inputs to spectrally-opponent neurons.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.