Ultraviolet colour opponency in the turtle retina.

We have examined the functional architecture of the turtle Pseudemys scripta elegans retina with respect to colour processing, extending spectral stimulation into the ultraviolet, which has not been studied previously in the inner retina. We addressed two questions. (i) Is it possible to deduce the ultraviolet cone spectral sensitivity function through horizontal cell responses? (ii) Is there evidence for tetrachromatic neural mechanisms, i.e. UV/S response opponency? Using a constant response methodology we have isolated the ultraviolet cone input into the S/LM horizontal cell type and described it in fine detail. Monophasic (luminosity), biphasic L/M (red-green) and triphasic S/LM (yellow-blue) horizontal cells responded strongly to ultraviolet light. The blue-adapted spectral sensitivity function of a S/LM cell peaked in the ultraviolet and could be fitted to a porphyropsin cone template with a peak at 372 nm. In the inner retina eight different combinations of spectral opponency were found in the centre of the receptive field of ganglion cells. Among amacrine cells the only types found were UVSM-L+ and its reverse. One amacrine and four ganglion cells were also opponent in the receptive field surround. UV/S opponency, seen in three different types of ganglion cell, provides a neural basis for discrimination of ultraviolet colours. In conclusion, the results strongly suggest that there is an ultraviolet channel and a neural basis for tetrachromacy in the turtle retina.

Tetrachromatic input to turtle horizontal cells.

Recent physiological experiments support behavioral and morphological evidence for a fourth type of cone in the turtle retina, maximally sensitive in the ultraviolet (UV). This cone type has not yet been included in the models proposed for connectivity between cones and horizontal cells. In this study, we examined the inputs of UV, S, M, and L cones to horizontal cells. We used the high-resolution Dynamic Constant Response Method to measure the spectral sensitivity of horizontal cells without background light and after adaptation to UV, blue (B), green (G), and red (R) light. We concluded the following: (1) Tetrachromatic input to a Y/B horizontal cell was identified. The spectral-sensitivity curves of the cell in three of the adaptation conditions were well represented by L-, M-, and S-cone functions. Adaptation to blue light revealed a peak at 372 nm, the same wavelength location as that determined behaviorally in the turtle. A porphyropsin template could be closely fitted to the sensitivity band in that region, strong evidence for input from a UV cone. (2) The spectral-sensitivity functions of R/G horizontal cells were well represented by the L- and M-cone functions. There was no indication of UV- or S-cone inputs into these cells. (3) The spectral sensitivities of the monophasic horizontal cells were dominated by the L cone. However, the shape of the spectral-sensitivity function depended on the background wavelength, indicating secondary M-cone input. Connectivity models of the outer retina that predict input from all cone types are supported by the finding of tetrachromatic input into Y/B horizontal cells. In contrast, we did not find tetrachromatic input to R/G and monophasic horizontal cells. Chromatic adaptation revealed the spectral-sensitivity function of the turtle UV cone peaking at 372 nm.

UV responses in the retina of the turtle.

To study processing of UV stimuli in the retina of the turtle, Trachemys dorbignii, we recorded intracellular responses to spectral light from 89 cells: 54 horizontal (47 monophasic, five (R/G) biphasic and two (Y/B) triphasic), 14 bipolar, 12 amacrine, and nine ganglion cells. Spectral sensitivities were measured with monochromatic flashes or with the dynamic constant response method in dark or chromatic adapted states. Stray light and second-order harmonics were also measured. (1) All cells responded to UV stimuli, although none had maximum sensitivity in the UV. (2) Most horizontal, bipolar, and amacrine cells had red-peaked spectral sensitivities. (3) Red adaptation of all monophasic horizontal cells indicated a single red input, except one that had additional peaks in the blue and UV. (4) Responses of biphasic and triphasic horizontal cells to UV light were always hyperpolarizing. Opposition between hyperpolarizing and depolarizing responses at long wavelengths indicates that UV responses were not due to the beta band of red receptors. (5) An unstained spectrally opponent bipolar cell hyperpolarized in the center to green light and antagonistically depolarized in the surround to UV, blue, and green flashes, but hyperpolarized to red. (6) All dark-adapted amacrine cells were red-peaked monophasic cells, but red adaptation broadened their spectral-sensitivity curves or displaced their peaks. An A15, an A18, and an A24 wide-field amacrine cell were stained. (7) A G15 bistratified ganglion cell is shown here for the first time to be spectrally opponent. This UVB/RG cell depolarized to UV and blue and hyperpolarized to red and green. It differs from previously reported turtle ganglion cells in being color opponent in the entire field, not only in the surround, and in showing spatial opponency.

Electrophysiological measurements of spectral sensitivities: a review.

Spectral sensitivities of visual systems are specified as the reciprocals of the intensities of light (quantum fluxes) needed at each wavelength to elicit the same criterion amplitude of responses. The review primarily considers the methods that have been developed for electrophysiological determinations of criterion amplitudes of slow-wave responses from single retinal cells. Traditional flash methods can require tedious dark adaptations and may yield erroneous spectral sensitivity curves which are not seen in such modifications as ramp methods. Linear response methods involve interferometry, while constant response methods involve manual or automatic adjustments of continuous illumination to keep response amplitudes constant during spectral scans. In DC or AC computerized constant response methods, feedback to determine intensities at each wavelength is derived from the response amplitudes themselves. Although all but traditional flash methods have greater or lesser abilities to provide on-line determinations of spectral sensitivities, computerized constant response methods are the most satisfactory due to flexibility, speed and maintenance of a constant adaptation level.

Electrophysiological measurements of spectral sensitivities: a review

Spectral sensitivities of visual systems are specified as the reciprocals of the intensities of light (quantum fluxes) needed at each wavelength to elicit the same criterion amplitude of responses. This review primarily considers the methods that have been developed for electrophysiological determinations of criterion amplitudes of slow-wave responses from single retinal cells. Traditional flash methods can require tedious dark adaptations and may yield erroneous spectral sensitivity curves which are not seen in such modifications as ramp methods. Linear response methods involve interferometry, while constant response methods involve manual or automatic adjustments of continuous illumination to keep response amplitudes constant during spectral scans. In DC or AC computerized constant response methods, feedback to determine intensities at each walvelength is derived from the response amplitudes themselves. Although all but traditional flash methods have greater or lesser abilities to provide on-line determinations of spectral sensitivities, computerized constant response methods are the most satisfatory due to flexibility, speed and maintenance of a constant adaptation level.

An AC constant-response method for electrophysiological measurements of spectral sensitivity functions.

A number of methods have been used in the past to measure spectral sensitivity (S(lambda)) functions of electric responses in the visual system. We present here a microcomputer based, AC, constant-response method for automatic on-line measurement of S(lambda) in cells with or without a sustained tonic response. It is based on feedback adjustment of light intensity to obtain constant peak-to-peak amplitudes of response to a flickering stimulus as the spectrum is scanned between 300 and 700 nm in 4 nm steps. It combines the advantages of: (1) on-line presentation of S(lambda) curves; (2) constant light adaptation; (3) sampling of many points; and (4) fast data collection time. The system can be applied to sensitivity or threshold (e.g., S(lambda), dark adaptation, receptive field) measurements of any electrically recorded visual response.

Discrimination of visual motion from flicker by identified neurons in the medulla of the fleshfly Sarcophaga bullata.

1. Responses to moving contrast gratings and to flicker have been studied in cells in the medulla of the fleshfly Sarcophaga bullata using intracellular recordings and stainings. Medullary neurons responded periodically to flicker. Those which primarily discriminated motion had periodic responses or DC shifts in membrane potentials or increased noise. Intrinsic neurons included a T1a cell which was directionally selective (DS) and specific non-DS amacrine cells (6 types) arborizing either distal or proximal to the serpentine layer. Among the 12 types of output neurons recorded, 1 projected to the lobula plate, 6 to the lobula (Tm and T2 cells). 3 to both the lobula and lobula plate (Y cells), and 2 to the central brain. 2. Irrespective of their projection, medulla neurons which arborize in the stratum of the L2 terminals respond to flicker as does L2 and have the simplest, primarily periodic, responses to motion. The responses have significant power at the second harmonic of the stimulus temporal frequency suggesting that a non-linear operation, such as multiplication, may occur in the L2 stratum. Cells with arbors coinciding with either of the two levels of L1 terminals have much more complex responses to motion. All cells projecting to the lobula plate responded periodically to movement in some direction(s).

Comparisons of directionally selective with other ganglion cells of the turtle retina: intracellular recording and staining.

Directionally selective (DS) and other ganglion cells of the turtle retina were studied in in vitro eyecup preparations by using intracellular recording and staining techniques. DS ganglion cells responded to moving gratings with periodic, depolarizing synaptic potentials in both preferred and null directions. Although depolarizations were usually larger in the preferred than in the null directions, in a few cells spike discharges were directional but depolarizations were not. Directionality could disappear if inappropriate field sizes or grating spatial frequencies were used. Unlike non-DS cells, one-half of the DS cells penetrated showed two sizes of action potentials upon photostimulation. It is proposed that the smaller spikes originated at axonal initial segments and failed to invade the somas actively. DS cells also exhibited postspike depolarizations (PSDs). Three types of DS ganglion cells (ON-OFF, OFF center, and ON center) were identified morphologically with Lucifer yellow injection. Other ganglion cells, which were also recorded from and stained intracellularly, are compared to the DS cells.

Action spectra and chromatic mechanisms of cells in the median ocelli of dragonflies.

Spectral sensitivities were recorded intracellulary in median ocelli of Anax junius, Aeschnatuberculifera, and Libellulapulcella. All cells had peak sensitivities at 360 and 500 nm while UV-blue+green cells found only in Anax had a third peak sensitivity at 440 nm. Ratios of UV-to-green sensitivities varied from cell to cell in each ocellus, but no UV-only or green-only cells were recorded. Half of the cells tested had a reverse Purkinje shift: They were more sensitive in the green at low illuminations but more sensitive in the UV at high illuminations; their intensity-response curves at 370 and 520 nm crossed but became parallel for large responses. Wave-lengths 420 nm and shorter elicited a family of low intensity-response curves with one slope; wavelengths 440 nm and longer elicities a family of curves with another slope. Orange-adapting lights selectively adapted sensitivity in the green, but UV-adapting lights had little selective effect. Amounts of log-selective adaptation were proportional to log orange-adapting intensity. It is concluded that two spectral mechanisms can be recorded from each cell, possibly by coupling of UV and green cells or possibly because each cell contains two visual pigments. Selective chromatic adaptations may provide the ocellus with a kind of "authomatic color control," while the reverse Purkinje shift could extend the ocellus' sensitivity to prevailing skylight.

Adaptation in the ventral eye of Limulus is functionally independent of the photochemical cycle, membrane potential, and membrane resistance.

The early receptor potential (ERP), membrane potential, membrane resistance, and sensitivity were measured during light and/or dark adaptation in the ventral eye of Limulus. After a bright flash, the ERP amplitude recovered with a time constant of 100 ms, whereas the sensitivity recovered with an initial time constant of 20 s. When a strong adapting light was turned off, the recovery of membrane potential and of membrane resistance had time-courses similar to each other, and both recovered more rapidly than the sensitivity. The receptor depolarization was compared during dark adaptation after strong illumination and during light adaptation with weaker illumination; at equal sensitivities the cell was more depolarized during light adaptation than during dark adaptation. Finally, the waveforms of responses to flashes were compared during dark adaptation after strong illumination and during light adaptation with weaker illumination. At equal sensitivities (equal amplitude responses for identical flashes), the responses during light adaptation had faster time-courses than the responses during dark adaptation. Thus neither the photochemical cycle nor the membrane potential nor the membrane resistance is related to sensitivity changes during dark adaptation in the photoreceptors of the ventral eye. By elimination, these results imply that there are (unknown) intermediate process(es) responsible for adaptation interposed between the photochemical cycle and the electrical properties of the photoreceptor.

Dual sensitivities of cells in wolf spider eyes at ultraviolet and visible wavelengths of light.

Intracellular recordings have been made from visual cells in principal and secondary eyes of in vitro wolf spider preparations. The responses of all cells to all wavelengths of light were graded depolarizations; no hyperpolarizations or nerve discharges were seen. Cells in a secondary eye, the anterior lateral eye, had a maximum sensitivity in the visible at 510 nm and a secondary maximum, or shoulder, of sensitivity in the near ultraviolet at 380 nm. Cells in principal eyes, the anterior median eyes, all responded maximally both in the visible at 510 nm and in the ultraviolet at 360-370 nm or less. However, there was no typical ratio of ultraviolet to visible sensitivities; the differences in log sensitivities (log UV/VIS) varied from 3.3 to -0.5. Each principal eye had a population of cells with different ratios. These populations varied with the time of the year, possibly due to changes in light upon the animals. Chromatic adaptations of cells in anterior median (but not anterior lateral) eyes resulted in small, selective changes in spectral sensitivities, and there was some facilitation of responses from cells repeatedly stimulated. It is concluded that cells of secondary eyes contain only a visual pigment absorbing maximally in the visible, while cells of principal eyes probably contain variable amounts of both this pigment and one absorbing in the ultraviolet as well.

Spectral sensitivities of wolf spider eyes.

ERG's to spectral lights were recorded from all eyes of intact wolf spiders. Secondary eyes have maximum relative sensitivities at 505-510 nm which are unchanged by chromatic adaptations. Principal eyes have ultraviolet sensitivities which are 10 to 100 times greater at 380 nm than at 505 nm. However, two animals' eyes initially had greater blue-green sensitivities, then in 7 to 10 wk dropped 4 to 6 log units in absolute sensitivity in the visible, less in the ultraviolet. Chromatic adaptations of both types of principal eyes hardly changed relative spectral sensitivities. Small decreases in relative sensitivity in the visible with orange adaptations were possibly retinomotor in origin. Second peaks in ERG waveforms were elicited from ultraviolet-adapted principal eyes by wavelengths 400 nm and longer, and from blue-, yellow-, and orange-adapted secondary eyes by wavelengths 580 nm and longer. The second peaks in waveforms were most likely responses of unilluminated eyes to scattered light. It is concluded that both principal and secondary eyes contain cells with a visual pigment absorbing maximally at 505-510 nm. The variable absolute and ultraviolet sensitivities of principal eyes may be due to a second pigment in the same cells or to an ultraviolet-absorbing accessory pigment which excites the 505 nm absorbing visual pigment by radiationless energy transfer.

Nonlinear transient responses from light-adapted wolf spider eyes to changes in background illumination.

Retinal action potentials were recorded at the corneas of light-adapted wolf spider eyes in response to large positive and negative step changes in background illumination. These incremental responses were superimposed upon the steady-state DC responses to the background illumination. Both positive and negative step responses had peaks which overshot the DC levels to which they decayed. The overshoot was greater for positive than for negative steps. Short term DC responses measured after one-half sec were larger for negative than for positive steps; these short-term DC responses were thus asymmetrical. However, responses to short positive and negative flashes were not asymmetrical; rather, they varied linearly with flash amplitude. Asymmetries were thus delayed in onset. The short-term DC responses were found to be different from the steady-state DC responses to maintained changes in background illumination. There was an approximately exponential decay or creep from the short-term to the steady-state DC responses. It is proposed that the dynamics of delayed asymmetries can explain the waveforms of the short-term transient responses.

A nonlinear model for transient responses from light-adapted wolf spider eyes.

A quantitative model is proposed to test the hypothesis that the dynamics of nonlinearities in retinal action potentials from light-adapted wolf spider eyes may be due to delayed asymmetries in responses of the visual cells. For purposes of calculation, these delayed asymmetries are generated in an analogue by a time-variant resistance. It is first shown that for small incremental stimuli, the linear behavior of such a resistance describes peaking and low frequency phase lead in frequency responses of the eye to sinusoidal modulations of background illumination. It also describes the overshoots in linear step responses. It is next shown that the analogue accounts for nonlinear transient and short term DC responses to large positive and negative step stimuli and for the variations in these responses with changes in degree of light adaptation. Finally, a physiological model is proposed in which the delayed asymmetries in response are attributed to delayed rectification by the visual cell membrane. In this model, cascaded chemical reactions may serve to transduce visual stimuli into membrane resistance changes.