Photoreceptor Level Research Articles

Figure-ground discrimination by relative movement in the visual system of the fly

The visual system of the fly performs various computations on photoreceptor outputs. The detection and measurement of movement is based on simple nonlinear multiplication-like interactions between adjacent pairs and groups of photoreceptors. The position of a small contrasted object against a uniform background is measured, at least in part, by (formally) 1-input nonlinear flicker detectors. A fly can also detect and discriminate a figure that moves relative to a ground texture. This computation of relative movement relies on a more complex algorithm, one which detects discontinuities in the movement field. The experiments described in this paper indicate that the outputs of neighbouring movement detectors interact in a multiplication-like fashion and then in turn inhibit locally the flicker detectors. The following main characteristic properties (partly a direct consequence of the algorithm's structure) have been established experimentally: a) Coherent motion of figure and ground inhibit the position detectors whereas incoherent motion fails to produce inhibition near the edges of the moving figure (provided the textures of figure and ground are similar). b) The movement detectors underlying this particular computation are direction-insensitive at input frequencies (at the photoreceptor level) above 2.3 Hz. They become increasingly direction-sensitive for lower input frequencies. c) At higher input frequencies the fly cannot discriminate an object against a texture oscillating at the same frequency and amplitude at 0° and 180° phase, whereas 90° or 270° phase shift between figure and ground oscillations yields maximum discrimination. d) Under conditions of coherent movement, strong spatial incoherence is detected by the same mechanism. The algorithm underlying the relative movement computation is further discussed as an example of a coherence measuring process, operating on the outputs of an array of movement detectors. Possible neural correlates are also mentioned.

The luminance-dependent nature of the visual abnormality in strabismic amblyopia

Three representative strabismic amblyopes whose visual abnormality has previously been shown to be of a sensory neural nature were tested over a wide range of luminance. The results indicated that there is a range of spatial frequencies for each subject for which contrast sensitivity is abnormal under photopic conditions and yet normal under mesopic or scotopic conditions. The form of this contrast sensitivity “normalization” is frequency dependent. These findings together with recent results concerning the relationship between movement and pattern thresholds in amblyopia suggest that the amblyopic abnormality is primarily visual-field specific and does not occur at a photoreceptor level.

Image degradation by an irregular retinal mosaic.

In order to understand the significance of a regular photoreceptor mosaic we quantify the effect of deviations from regularity. Such deviations are shown to cause a deterioration in the quality of the image at the photoreceptor level because of image demodulation and image distortion. Consequently, a regular receptor array is necessary for extracting all of the spatial information available in the retinal image.

Light regulation of adenosine 3′, 5′ cyclic monophosphate levels in vertebrate photoreceptors

The regulation of adenylyl cyclase activity (measured as the production of labeled cyclic-AMP (cAMP) from labeled ATP) in frog rod outer segment photoreceptor membranes has been studied. The activity was found to be more than 20 times higher in fully dark-adapted than in bleached material. A number of experimental parameters must be carefully regulated to obtain this large ratio of dark to light activities. The disrupted rod outer segments must not be washed, since a soluble factor is essential for inhibition of activity by light. The photoreceptor membranes should be assayed for adenylyl cyclase immediately after preparation since the specific activity and light-dark differences are substantially reduced on storage at 4°C. Mechanical trauma (homogenization or sonication) causes some decrease in dark activity and a lessening of sensitivity to illumination. Preparation and assay of the dark material must be carried out using infrared light since there is a marked disproportionality between photic bleaching of rhodopsin and loss of cyclase activity. This disproportionality was further studied by mixing dark and totally bleached material in various proportions. With as little as 2% of the material bleached the cyclase activity is the same as in fully bleached material. The inhibitory action of the bleached material is destroyed by heating but is not affected by dialysis or treatment with tryspin. If the photoreceptor material is fractionated by sedimentation at 60 000 × g and 200 000 × g, the rhodopsin and adenylyl cyclase are primarily in the 60 000 × g pellet. The inhibitor is present in both pellets but not in the high speed supernatant. However, the high speed supernatant is essential for action of the inhibitor on unbleached material. Bleached (but not dark) photoreceptor material was found to inhibit cyclase in brain, liver and melanoma. Recent work carried out after presentation of the above findings has indicated that although the activity of cAMP phosphodiesterase is not affected by light under the usual assay conditions, in the presence of ATP light produces a marked stimulation of this activity. This stimulation of phosphodiesterase exhibits many of the characteristics previously attributed to the inhibition of cyclase and accounts completely or in large part for the apparent light dark differences in cyclase activity. At present the evidence points to phosphodiesterase as the main locus of light regulation of cAMP concentrations in photoreceptors, though a role for the light regulation of cyclase activity is not excluded.

Responses to directional stimuli in retinal preganglionic units.

1. Extracellular recordings were made from directionally selective ganglion cell units in the isolated frog retina and decapitated Necturus preparation.2. Intracellular recordings were made from individual photoreceptor cells in the frog and Necturus retinae while stimuli which had evoked directionally selective responses at the ganglion cell level were presented. No evidence for inhibition of photoreceptors for any direction of movement of the light stimulus was found. This appeared to rule out a mechanism for directional selectivity involving inhibition of photoreceptor potentials.3. Intracellular recordings were made from the nuclear layer between photoreceptors and ganglion cells in Necturus. The responses were of two types: either transitory or sustained.4. The sustained type responses could be divided into two classes depending on their receptive field organization. One type of sustained potential had a large receptive field without any evidence for a centre-surround antagonism and corresponded to the luminosity type S-potential recorded in fish. The other type had a smaller receptive field and showed a difference in sign of response between centre and surround if the centre was flooded with a steady light. This is very similar to what has been described for a type of on-centre, off-surround ganglion cell.5. The transitory type of responses showed some centre-surround antagonistic organization. Some of these transitory units also appeared to show some discrimination in response as a function of the distribution of light on the retina.6. No specific directional selectivity was found from units at the inner nuclear layer. This further excluded any mechanism of directional sensitivity which involves selectivity at the photoreceptor level.7. It was concluded that although inner nuclear layer units may play a role in the mechanism of directional selectivity, no specific directionality was found at the first synaptic level of the retina.

Spectral response curves of single cones in the carp.

Single cone spectral response curves were obtained with the aid of specially designed equipment. Statistical analysis of 142 records selected from hundreds, based solely on the greatest signal-to-noise ratio, revealed three groups of cones; the red cones (74 per cent) with the peaking wavelength at 611±23mμ, green cones (10 per cent) at 529±14mμ , and blue cones (16 per cent at 462±15mμ). These peaking wavelengths are in close accord with those of single cone absorption spectra measured by Marks and MacNichol in the goldfish by a microspectrophotometer. Apparently, Young's trichromatic theory applies at the photoreceptor level in Cyprinidae. On a obtenu avec un appareillage spe´cialement con¸u des courbes de re´ponse spectrale de coˆnes isole´s. L'analyse statistique de 142 enregistrements choisis parmi des centaines, et fonde´e uniquement sur le maximum du rapport signal-bruit, re´vi´le trois groupes de coˆnes; les rouges (74%) avec une pointe en longueur d'ondea` 611±23mμ, les verts (10%)a` 529±14mμ , et les bleus (16%)a` 462±15mμ. Ces maxima sont en accorde´troit avec les spectres d'absorption de coˆnes isole´s mesure´s par Marks et MacNichol sur le poisson rouge par microspectrophotome´trie. La the´orie trichromatique de Young semble donc s'appliquer au niveau des photore´cepteurs chez les Cyprinide´s. Mit Hilfe einer extra entwickelten Apparatur wurden die spektralen Reaktionskurven einzelner Zapfen erhalten. Eine statistische Analyse von 142 Aufzeichnungen, die aus mehreren hundert wegen des gro¨sseren Signal: Rausch-Verha¨ltnisses ausgewa¨hlt wurden, ergab drei Gruppen von Zapfen; rote Zapfen (74%) mit einer Maximumswellenla¨nge von 611±23mμ , gru¨ne Zapfen (10%) bei 529 ±14mμ und blaue Zapfen (16%) bei462±15mμ. Diese Maximumswellenla¨ngen sind in guterU¨bereinstimmung mit den Absorptionsspektren einzelner Zapfen, die Marks und MacNichol beim Goldfisch mit dem Mikrospektrophotometer erhielten. Anscheinend gilt bei Cypriniden, auf der Rezeptorstufe, die Youngsche Dreifarbentheorie. Пpи пoмoщи cпeциaльHo cкoHcтpyиpoвaHHoй ycтaHoвки были пoлyхeHы кpивыe cпeктpaльHых peaкций oтдeльHхх кoлбoхeк. Cтaтиcтихecкий aHaлиз 142 зaпиceй, выбpaHHгх из coтeH зaпиceй, eдиHcтвeHHo тoлькo Ha ocHoвaHии Haибoльшeгo oтHoшeHия cигHaлa к шyмy, oбHapyзил тpи гpyппы кoлбoхeк; кpacHыe кoлбoхки (74%) c мaкcимyмoм хyвcтвитeльHocти пpи длиHe вoлHы611 ± 23Hм, зeлeHыe кoлбoхки (10%) —529 ± 25им, и cиHиe кoлбoхки (16%) — 462 ±15Hм . Эти мaкcимyмы Haхoдятcя в тecHoм coглacии co cпeктpaми пoглoщeHия eдиHихHых кoлбoхeк, oпpeдeлeHHыми Mapкcoм и Maк-Hикoлeм для зoлoтых pыбoк c пoмoщью микpocпeктpoфoтoмeтpa. Пoвидимoмy, тpихpoмaтихecкaя тeopия ЮHгa пpилoзимa к фoтopeцeптopHoмy ypoвHю y ceмeйcтвa кapпoвых.