We have investigated the effects of a serotonin 5-HT2 antagonist and a 5-HTIA agonist on horizontal and ganglion cells in the rabbit retina. Simultaneous intracellular horizontal cell and extracellular ganglion cell recordings were obtained from a superfused in vitro rabbit eyecup preparation and the effects of bath applied drugs on these cells' light responses observed. Sinusoidally modulated current was also injected into horizontal cells while the extracellular spike activity of nearby, single-unit ganglion cells was monitored. Although the ON components of the light-evoked responses of ganglion cells were reduced by the 5-HT2 antagonist or the 5-HTIA agonist, the membrane potential and the light responses of horizontal cells and the 6-wave of the ERG were simultaneously unaffected. However, the drugs blocked current-driven ganglion cell spike activity induced by current injections into nearby horizontal cells. These results are discussed with respect to the site of action of these serotonin drugs and with respect to the circuitry of serotonergic neurons.

The photosensory pineal complex of anurans comprises an extracranial part, the frontal organ, and an intracranial part, the pineal organ proper. Although the pineal organ functions mainly as a luminosity detector, the frontal organ monitor the relative proportions of short and intermediate/long wavelengths in the ambient illumination. The major pathway of information processing in the pineal and frontal organs is the photoreceptor to ganglion cell synapse. It is not known whether interneurons form part of the neural circuitry. In the present study, we demonstrate GABA-immunoreactive (GABA-IR) neurons in the pineal and frontal organs of the frog, Rana esculenta. No GABA-IR axons were observed in the pineal nerve between the frontal and pineal organs, or in the pineal tract that connects the pineal complex with the brain. The GABA-IR neurons differed in morphology from centrally projecting neurons visualized by retrograde labeling with horseradish peroxidase. Thus, we suggest that the GABA-IR neurons in the pineal and frontal organs represent local interneurons.

Axons of central origin, immunoreactive with a sensitive antiserum against the tetrapeptide Phe-Met-Phe-Arg-NH2 (FMRFamide), were observed in the intracranial portion of the photosensory pineal organ. The immunoreactive axons enter the caudal pole of the pineal organ via the posterior commissure. The largest density of axons was observed in the caudal part, while fewer axons were detected in the rostral portion. The uneven distribution of the FMRFamide-immunoreactive axons may be related to the distribution of different types of intrapineal neurons. FMRFamide-immunoreactive varicose axons were observed in the extracranial frontal organ. A central innervation of the pineal organ, previously known exclusively from amniotes, is probably not per se linked with the evolutionary transition of the pineal organ from a directly photosensory organ to a neuroendocrine organ. It could rather represent a centrifugal input to a sensory system which has been retained when the directly sensory functions have changed, during phylogency, to neuroendocrine functions.

A method for constructing nonlinear models for light adaptation in the retina is introduced. The components of the models are linear filters and static (instantaneous) nonlinear elements configured in a feedback arrangement. The signals in the models are combined through algebraic addition or multiplication. We apply the method to model light adaptation measured in turtle horizontal cells. Given a particular wiring diagram for the components, the functional forms of the static nonlinearities and frequency responses of the linear filters are determined by constraining the model to give temporal frequency responses (linear regime behavior) consistent with a family of linear feedback models which has been shown to provide a good description of adaptation in these cells. Two particular models, quite different in structure, are presented. Each model responds to perturbations around a mean light level as a feedback circuit in which the gain (strength) of feedback is adjusted to be proportional to the mean light level, but neither model has a separate pathway for measuring the mean light level. Thus, each of these nonlinear models embeds an entire family of linear models parametric in mean light level. Harmonic distortion in the responses of these models to sinusoidal input is found to be qualitatively consistent with physiological data. An alternative class of nonlinear models in which feedback gain is set by a separate slow pathway which tracks the mean light level is ruled out on the basis of its incorrect steady-state input-output behavior. The methods presented can be used to develop specific physical models for light adaptation based on the chemical kinetics of phototransduction or on nonlinear neural feedback. The relevance of the nonlinear models and construction techniques to modeling phototransduction is discussed.

Spontaneous eye movements as well as visual, vestibular, and proprioceptive cervical reflexes which contribute to gaze stabilization were investigated in the chameleon using the magnetic search-coil technique. The oculomotor range of each eye was very large (180 deg horizontally × 80 deg vertically). Spontaneous ocular saccades were independent in the two eyes and could have very large amplitudes. The fast phases of nystagmus during the stabilization reflexes were also independent in the eyes. In the head-restrained condition, optokinetic nystagmus (OKN) had a low gain in both horizontal and vertical planes (0.35 at 5 deg/s) and showed little binocular interaction. The vestibulo-ocular reflex (VOR) exhibited a low gain (0.2–0.3 from 0.05–1 Hz) and a high-phase lead at low frequency (140 deg at 0.05 Hz). Rotation of the animal in the presence of a visible surround increased the overall gain of gaze stabilization to 0.4–0.5 (P < 0.01) and considerably reduced the phase lead (38 deg at 0.05 Hz). In the head-free condition, head and eye reflexes were active simultaneously during both optokinetic and vestibular stimulation, but nystagmic head movements appeared only occasionally with a rather loose eye-head coordination. During optokinetic stimulation, eye movements contributed more than head movements to gaze stabilization, whereas, during vestibular or visuo-vestibular stimulation, the relative contribution of eye and head responses varied with stimulus frequency. When the head was freed, overall gain for gaze stabilization increased from 0.35 to 0.45 (P < 0.05) for optokinetic stimulation at 5 deg/s and from 0.2–0.3 to 0.4–0.75 (P < 0.001) for vestibular stimulation at 0.05–1 Hz. Optimal gaze stabilization (gain of 0.8) was only obtained with combined visual and vestibular stimulation in the free-head condition. Cervical stimulation provoked a compensatory cervico-ocular reflex (COR) with a gain of 0.2–0.4 as well as ocular saccades, which were especially numerous in the presence of a visual surround. The direction of these saccades alternated between compensatory and anti-compensatory.

Visual cue integration strategies are known to depend on cue reliability and how rapidly the visual system processes incoming information. We investigated whether these strategies also depend on differences in the information demands for different natural tasks. Using two common goal-oriented tasks, prehension and object placement, we determined whether monocular and binocular information influence estimates of three-dimensional (3D) orientation differently depending on task demands. Both tasks rely on accurate 3D orientation estimates, but 3D position is potentially more important for grasping. Subjects placed an object on or picked up a disc in a virtual environment. On some trials, the monocular cues (aspect ratio and texture compression) and binocular cues (e.g., binocular disparity) suggested slightly different 3D orientations for the disc; these conflicts either were present upon initial stimulus presentation or were introduced after movement initiation, which allowed us to quantify how information from the cues accumulated over time. We analyzed the time-varying orientations of subjects’ fingers in the grasping task and those of the object in the object placement task to quantify how different visual cues influenced motor control. In the first experiment, different subjects performed each task, and those performing the grasping task relied on binocular information more when orienting their hands than those performing the object placement task. When subjects in the second experiment performed both tasks in interleaved sessions, binocular cues were still more influential during grasping than object placement, and the different cue integration strategies observed for each task in isolation were maintained. In both experiments, the temporal analyses showed that subjects processed binocular information faster than monocular information, but task demands did not affect the time course of cue processing. How one uses visual cues for motor control depends on the task being performed, although how quickly the information is processed appears to be task invariant.

Signals from rods can alter chromatic discrimination. Here, chromatic discrimination ellipses were determined in the presence of rod incremental and decremental pedestals at mesopic light levels. The data were represented in a relative cone Troland space, normalized by discrimination thresholds measured along the cardinal axes without a rod pedestal. In the quadrant of cone space where L-cone relative to M-cone excitation increased, and S-cone excitation decreased, rod incremental pedestals degraded chromatic discrimination, and rod decremental pedestals improved chromatic discrimination. Discrimination in the other three quadrants of cone space was unaffected by the incremental or decremental rod pedestals. A second experiment measured chromatic discrimination under conditions where cone pedestals were matched to the appearances of the incremental and decremental rod pedestals. Based on the matching pedestal data, discrimination then could be measured independently along the cardinal axes using either chromatic [L/(L + M); S/(L + M)] or luminance (L + M) pedestal components. The discrimination data altered by the rod pedestals were similar to chromatic cone pedestals for L/M increment discrimination, but similar to luminance cone pedestals for S decrement discrimination. The results indicated that the rod and cone signals combined differently in determining chromatic discrimination for different post-receptoral pathways.

Visual spatial attention has been described as a process that favors the processing of sensory information that falls into the “spotlight of attention.” Recent studies have provided support for an ability to split this attentional focus to selectively process spatially separate locations. Using functional magnetic resonance imaging, the signature for the presence of multiple spotlights is the presence of multiple retinotopically specific foci of activation in striate and extrastriate visual areas. We used this approach to investigate the presence of such separable activations as a function of the eccentricity of the spatial foci of attention. Visual stimuli consisted of letters and digits displayed in rapid serial visual presentation (RSVP). Five RSVP streams were presented simultaneously, one in the center of the visual field and one in each visual field quadrant. Subjects had to deploy their attention either to a single peripheral location or two non-contiguous regions performing a match-mismatch judgment. The results show that dividing attention leads to multiple spotlights of attention for central as well as more peripheral locations of the visual field. However, depending on the exact location and width of the attentional spotlights, resulting activation maps might reveal merged activation patterns even in the presence of distinct attentional spotlights.

Effects of temperature on the spectral properties of visual pigments were measured in the physiological range (5–28°C) in photoreceptor cells of bullfrog (Rana catesbeiana) and crucian carp (Carassius carassius). Absorbance spectra recorded by microspectrophotometry (MSP) in single cells and sensitivity spectra recorded by electroretinography (ERG) across the isolated retina were combined to yield accurate composite spectra from ca. 400 nm to 800 nm. The four photoreceptor types selected for study allowed three comparisons illuminating the properties of pigments using the dehydroretinal (A2) chromophore: (1) the two members of an A1/A2 pigment pair with the same opsin (porphyropsin vs. rhodopsin in bullfrog “red” rods); (2) two A2 pigments with similar spectra (porphyropsin rods of bullfrog and crucian carp); and (3) two A2 pigments with different spectra (rods vs. long-wavelength-sensitive (L-) cones of crucian carp). Qualitatively, the temperature effects on A2 pigments were similar to those described previously for the A1 pigment of toad “red” rods. Warming caused an increase in relative sensitivities at very long wavelengths but additionally a small shift of λmax toward shorter wavelengths. The former effect was used for estimating the minimum energy required for photoactivation (Ea) of the pigment. Bullfrog rod opsin with A2 chromophore had Ea = 44.2 ± 0.9 kcal/mol, significantly lower (one-tailed P < 0.05) than the value Ea = 46.5 ± 0.8 kcal/mol for the same opsin coupled to A1. The A2 rod pigment of crucian carp had Ea = 42.3 ± 0.6 kcal/mol, which is significantly higher (one-tailed P < 0.01) than that of the L-cones in the same retina (Ea = 38.3 ± 0.4 kcal/mol), whereas the difference compared with the bullfrog A2 rod pigment is not statistically significant (two-tailed P = 0.13). No strict connection between λmax and Ea appears to exist among A2 pigments any more than among A1 pigments. Still, the A1 → A2 chromophore substitution in bullfrog opsin causes three changes correlated as originally hypothesized by Barlow (1957): a red-shift of λmax, a decrease in Ea, and an increase in thermal noise.

It is believed that spatial summation in most simple cells is a linear process. If this were so, then the Fourier transform of a simple cell's line weighting function should predict the cell's spatial frequency tuning curve. We have compared such predictions with experimental measurements and have found a consistent discrepancy: the predicted tuning curve is much too broad. We show qualitatively that this kind of discrepancy is consistent with the well-known threshold nonlinearity shown by most cortical cells. We have tested quantitatively whether a response threshold could explain the observed disagreements between predictions and measurements: a least-squares minimization routine was used to fit the inverse Fourier Transform of the measured frequency tuning curve to the measured line weighting function. The fitting procedure permitted us to introduce a threshold to the reconstructed line weighting function. The results of the analysis show that, for all of the cells tested, the Fourier transforms produced better predictions when a response threshold was included in the model. For some cells, the actual magnitude of the response threshold was measured independently and found to be compatible with that suggested by the model. The effects of nonlinearities of spatial summation are considered.

Light-evoked synaptic currents were recorded from on-cone bipolar cells in the mouse retina. Fluctuations in the synaptic current observed during maintained light steps were analyzed in order to estimate the amplitude of the underlying unitary event. The maximal synaptic current variance was 5-fold larger than the maximum expected from fluctuations in the number of active postsynaptic channels. Due to uncertainty in the contribution from channel variance, we calculated a range of values for the unitary event amplitude. The observed variance could be accounted for if 30–39 synaptic sites randomly generated unitary events with a waveform identical to the flash-response, and an amplitude of −3.1 to −2.4 pA. The amplitude is consistent with gating about five mGluR6 channels. The shape of the variance–mean relation suggests that in bright light transmitter release approaches zero, while in darkness transmitter release saturates the postsynaptic response. Thus the on-cone bipolar cell synapse is operating over its entire possible range. If it is assumed that the postsynaptic response saturates when one unitary event occurs per integration time, then a lower bound for the unitary event rate is 18 events/s/synaptic site. If the unitary event is generated by a single synaptic vesicle, the results suggest the total vesicle cycling rate available for encoding the on-cone bipolar cell signal is about 540–700 s−1.

The retinotopic organization of striate and several extrastriate areas of ferret cortex has been established. Here we describe the representation of the visual field on the Suprasylvian visual area (Ssy). This cortical region runs mediolaterally along the posterior bank of the suprasylvian sulcus, and is distinct from adjoining areas in anatomical architecture. The Ssy lies immediately rostral to visual area 21, medial to lateral temporal areas, and lateral to posterior parietal areas. In electrophysiological experiments we made extracellular recordings in adult ferrets. We find that single and multiunit receptive fields range in size from 2 deg × 4 deg to 21 deg × 52 deg. The total visual field representation in Ssy spans over 70 deg in azimuth in the contralateral hemifield (with a small incursion into the ipsilateral hemifield), and from +36 deg to −30 deg in elevation. There are often two representations of the horizontal meridian. Furthermore, the location of the transition from upper to lower fields varies among animals. General features of topography are confirmed in anatomical experiments in which we made tracer injections into different locations in Ssy, and determined the location of retrograde label in area 17. Both isoelevation and isoazimuth lines can span substantial rostrocaudal and mediolateral distances in cortex, sometimes forming closed contours. This topography results in cortical magnifications averaging 0.07 mm/deg in elevation and 0.06 mm/deg in azimuth; however, some contours can run in such a way that it is possible to move a large distance on cortex without moving in the visual field. Because of these irregularities, Ssy contains a coarse representation of the contralateral visual field.

Vertebrate rod and cone photoreceptors require continuous supply of chromophore for regenerating their visual pigments after photoactivation. Cones, which mediate our daytime vision, demand a particularly rapid supply of 11-cis retinal chromophore in order to maintain their function in bright light. An important contribution to this process is thought to be the chromophore precursor 11-cis retinol, which is supplied to cones from Müller cells in the retina and subsequently oxidized to 11-cis retinal as part of the retina visual cycle. However, the molecular identity of the cis retinol oxidase in cones remains unclear. Here, as a first step in characterizing this enzymatic reaction, we sought to determine the subcellular localization of this activity in salamander red cones. We found that the onset of dark adaptation of isolated salamander red cones was substantially faster when exposing directly their outer vs. their inner segment to 9-cis retinol, an analogue of 11-cis retinol. In contrast, this difference was not observed when treating the outer vs. inner segment with 9-cis retinal, a chromophore analogue which can directly support pigment regeneration. These results suggest, surprisingly, that the cis-retinol oxidation occurs in the outer segments of cone photoreceptors. Confirming this notion, pigment regeneration with exogenously added 9-cis retinol was directly observed in the truncated outer segments of cones, but not in rods. We conclude that the enzymatic machinery required for the oxidation of recycled cis retinol as part of the retina visual cycle is present in the outer segments of cones.

Some neurochemical features of the neuronal circuitry regulating dopamine release were examined in the retina of the turtle, Pseudemys scripta elegans. Glutamate antagonists that block hyperpolarizing bipolar cells, such as 2,3 piperidine dicarboxylic acid (PDA), produced dose-dependent dopamine release. In contrast, the glutamate agonist 2-amino-4-phosphonobutyric acid (APB), which blocks depolarizing bipolar cell responses with high specificity, had no effect on the release of dopamine. The γ-aminobutyric acid (GABA) antagonist, bicuculline, also produced potent dose-dependent release of dopamine. The release of dopamine produced by PDA was blocked by exogenous GABA and muscimol, suggesting that the PDA-mediated release process was polysynaptic and involved a GABAergic synapse interposed between the bipolar and dopaminergic amacrine cells. The only other agents that produced dopamine release were chloride-free media and high extracellular K+; in particular, kainic acid and glutamate itself were ineffective. These results suggest that the primary neuronal chain mediating dopamine release in the turtle retina is: cone → hyperpolarizing bipolar cell → GABAergic amacrine cell → dopaminergic amacrine cell.

Fish have large eyes, with short optic nerves that are continually flexed by compensatory eye movements during swimming. Here, I review the tissue construction of the fish optic nerve, to see how the glia and axons are adapted to withstand these mechanical stresses, which are not normally encountered by CNS tissue within the skull.

As in other lower vertebrates, the optic nerve astrocytes are highly unusual: their intermediate filaments are composed of cytokeratins (Giordano et al., 1989), not GFAP. Their processes are linked together by desmosomes, forming thin transverse lace-like partitions, placed at quasi-regular intervals longitudinally (Maggs & Scholes, 1990). This accordion-like arrangement is interpreted as providing a flexible tissue-skeleton for the optic nerve.

A new observation is that the optic axons run in coherent parallel waves. This pattern, which is complementary to that of the astroglia, reversibly accommodates limited axial stretches. The waves are equivalent to those underlying the optical banding of Fontana (1781) in peripheral nerves, but wavelength (30 μm) and amplitude (5 μm) are about an order of magnitude less, reflecting the much smaller average size of the optic axons. The pattern also occurs in mammals, and may be restricted to the visual pathway: if present elsewhere in the CNS, nerve-fiber waves are inconspicuous at best.

In fish, the astroglial partitions occur in register with the waves, suggesting that steric interactions between developing axons and glia may help to establish, or stabilize, the regular longitudinal spacing. This may have functional as well as mechanical implications, since the astrocytes form perinodal associations and their pattern is one which strongly clusters the nodes of Ranvier.

Nitric oxide (NO) activates soluble guanylyl cyclase (sGC) and the resulting increase in cyclic guanosine monophosphate (cGMP) is an important intracellular signalling pathway in the vertebrate retina. Immunocytochemical detection of cGMP following exposure to NO donors has proven an effective method of identifying cells that express sGC. While such an approach has proven useful for the study of several vertebrate retinas, it has not been applied to the well-characterized teleost retina. Therefore, in the present study, we have applied this approach to the retina of the goldfish (Carassius auratus). In the presence of the phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), incubation of goldfish eyecups in Ringer's solution containing (±)-S-nitroso-N-acetylpenicillamine (SNAP) increased cGMP-like immunoreactivity (cG-ir) in bipolar, horizontal, amacrine, and ganglion cells and in ganglion cell axons and optic nerve. Weak labeling was observed in horizontal cells but no change in cG-ir was noted within photoreceptors. The NO donor-stimulated increases of cG-ir in horizontal, bipolar, amacrine, and ganglion cells are consistent with known physiological effects of NO on these neurons. The physiological significance of NO action at the level of optic nerve is not known. The lack of an effect of SNAP on cG-ir in photoreceptors was unexpected, as there are known physiological actions of NO, mediated by cGMP, on these neurons. Although this may be due to insufficient sensitivity of immunolabeling, this result may indicate a difference between isoforms of sGC or cGMP PDE in these neurons, compared to neurons where exogenous NO increased cG-ir.

We investigated the effects of group II metabotropic glutamate receptor (mGluR) activation on excitatory synaptic transmission in the salamander retinal slice preparation. The group II selective agonists DCG-IV and LY354740 reduced light-evoked excitatory postsynaptic currents (EPSCs) in ganglion cells. To determine the synaptic basis of this effect, we also recorded from bipolar cells and horizontal cells. In ON bipolar cells, DCG-IV increased the inward current in darkness but did not affect the peak current at light onset. In OFF bipolar cells and horizontal cells, DCG-IV had the opposite effect, reducing the inward current in darkness. Given the opposite polarities of these two classes of synapses, our results suggest that group II mGluRs act presynaptically to reduce glutamate release from photoreceptors. To determine whether DCG-IV affected rods or cones, we applied light stimuli that selectively activate each type of photoreceptor. In horizontal cells, most of which receive mixed synaptic input from rods and cones, DCG-IV reduced rod-driven EPSCs evoked by 470-nm stimuli and cone-driven EPSCs elicited by 700-nm stimuli in the presence of a rod-saturating background. Thus, activation of group II mGluRs reduced rod- and cone-mediated glutamate release. Our results suggest that group II mGluRs could mediate feedback by which extracellular glutamate inhibits glutamate release from photoreceptor terminals.

The passive electrical properties of 17 ON-OFF retinal ganglion cells were derived from electrophysiological recordings. The parameters for each cells' equivalent model were obtained from the transient current responses to small step changes in clamp potential. Thirteen of the cells could be adequately approximated by a spherical soma connected to an equivalent dendritic cable. Estimates for the cell input conductance (GN), membrane time constant (τm), the dendritic-to-soma conductance ratio (ρ), and the normalized electrotonic length (L) were obtained (mean ± standard deviation, n = 13): GN = 580 ± 530 pS, τm = 97 ±72 ms, ρ = 2.8 ± 2.8, and L = 0.34 ± 0.13. Series resistance averaged 32 ± 11 MΩ The mean of the derived soma diameters was 18 ± 6 μm and the mean diameter and length of the equivalent cables were 1.4 ± 0.6 and 470 ± 90 μm, respectively. The average of the specific membrane conductances, 1.67 ± 1.08 S/cm2, corresponded to a membrane resistivity of 60 kΩ-cm2. Computer simulations of synaptic inputs were performed on a representative model, with an electrode at the soma and using the worst-case configuration, in which all synaptic inputs were confined to the tips of the dendrites. We draw three conclusions from the modeling: (1) Under voltage clamp, fast, spontaneous EPSCs would be significantly attenuated and slowed while the time course of the slower, light-evoked non-NMDA and NMDA EPSCs would be minimally distorted by dendritic filtering. (2) Excitatory synaptic reversal potentials can be accurately determined under voltage clamp. (3) In the absence of GABAergic and glycinergic inhibition, the efficacy at the soma of excitatory conductance changes is essentially independent of their dendritic location. The specific membrane resistivity appears to represent a good compromise between having a small membrane time constant and minimal EPSP attenuation.

Immunocytochemical and autoradiographic methods were used to identify neurons in the pure cone retina of the lizard (Anolis carolinensis) that are likely to employ glutamate (GLU) or aspartate (ASP) as a neurotransmitter.

GLU immunocytochemistry demonstrated high levels of endogenous GLU in all cone types and numerous bipolar cells. Moderate GLU levels were found in horizontal and ganglion cells. Müller cells and most amacrine cells had very low GLU levels. GLU immunoreactivity (GLU-IR) in the cones was present from the inner segment to the synaptic pedicle. A large spherical cell type with moderate GLU-IR was identified in the proximal inner plexiform layer (IPL). These cells also contain ASP and have been tentatively identified as amacrine cells. Uptake of [3H]-L-GLU labeled all retinal layers. All cone types and Müller cells sequestered [3H]-D-ASP, a substrate specific for the GLU transporter.

Anti-ASP labeling was observed in cones, horizontal cells, amacrine cells, and cells in the ganglion cell layer. ASP immunoreactivity (ASP-IR) in the cones was confined to the inner segment. One ASP-containing pyriform amacrine cell subtype ramifying in IPL sublamina b was identified.

Analysis of GLU-IR, ASP-IR, and GABA-IR on serial sections indicated that there were two distinct populations of horizontal cells in the Anolis retina: one containing GABA-IR, GLU-IR, and ASP-IR; and another type containing only GLU-IR and ASP-IR. Light GLU-IR was frequently found in GABA-containing amacrine cells but ASP-IR was not.

The distinct distributions of GLU and ASP may indicate distinctly different roles for these amino acids. GLU, not ASP, is probably the major neurotransmitter in the cone-biploar-ganglion cell pathway of the Anolis retina. Both GLU and ASP are present in horizontal cells and specific subpopulations of amacrine cells, but it is unclear if GLU or ASP have a neurotransmitter role in these cells.

In previous electrophysiological experiments from hypopigmented animals (mice, rats, rabbits), single-unit recordings from both retinal ganglion axons and cells in the superior colliculus have demonstrated an increase in threshold in the dark-adapted state which is roughly proportional to the animal's ocular melanin concentration. We have examined the thresholds in hypopigmented mice by using a behavioral water maze screening test and found similar threshold elevations to the electrophysiology. In the present study, we investigated the contribution of retinal light damage to the threshold elevation in an albino mouse strain which is relatively resistant to light damage (C57BL/6J c2J/c2J) and mice with profound retinal degeneration (C57BL/6J rd/rd).

Black or albino littermates (C57BL/6J + / c2J or c2J / c2J) were placed in either constant light (350 cd/m2) or dim cycling light (0.001 cd/m2) for 21 days before testing. The normally pigmented animals had thresholds of 1.00 × 10−5 cd/m2 regardless of their light history. The albino mice (c2J/c2J) maintained in constant light had a slight 0.30 log unit elevation compared to their controls that were maintained in dim cycling light 6.3 × 10−4 cd/m2 (similar to previously published reports).

We examined the retinal morphology of representative animals in semi-thin plastic sections. We could not detect any light damage (overall morphology or cell counts in the outer-nuclear layer) in either the normally pigmented animals or the albino mice (c2J/c2J) maintained in dim cycling light. We found extensive light damage in the albino mice (c2J/c2J) maintained in constant light (virtual absence of photoreceptor outersegments) that corresponded to the slight elevation in threshold. We conclude that the elevation in threshold found in albino mice (c2J/c2J) maintained in dim cycling light is not the result of light damage. These results support our previous findings that the sensitivity defect in hypopigmented animals is proportional to the degree of ocular hypopigmentation.