Brightness In The Photopic Range: Psychophysical Modelling With Blue-sensitive Retinal Signals

The brightness perception of a large (41°) uniform visual field was investigated in a visual psychophysical experiment. Subjects assessed the brightness of 20 light source spectra of different chromaticities at two luminance levels, Lv=267.6 cd/m2 and Lv=24.8 cd/m2. The resulting mean subjective brightness scale values were modelled by a combination of the signals of retinal mechanisms: S-cones, rods, intrinsically photosensitive retinal ganglion cells (ipRGCs) and the difference of the L-cone signal and the M-cone signal. A new quantity, “relative spectral blue content”, was also considered for modelling. This quantity was defined as “the spectral radiance of the light stimulus integrated with the range (380–520) nm, relative to luminance”. The “relative spectral blue content” model could describe the subjective brightness perception of the observers with reasonable accuracy.

Rod-cone signal interference in the retina shapes perception in primates

Linking the activity of neurons, circuits and synapses to human behavior is a fundamental goal of neuroscience. Meeting this goal is challenging, in part because behavior, particularly perception, often masks the complexity of the underlying neural circuits, and in part because of the significant behavioral differences between primates and animals like mice and flies in which genetic manipulations are relatively common. Here we relate circuit-level processing of rod and cone signals in the non-human primate retina to a known break in the normal seamlessness of human vision – a surprising inability to see high contrast flickering lights under specific conditions. We use electrophysiological recordings and perceptual experiments to identify key mechanisms that shape the retinal integration of rod- and cone-generated retinal signals. We incorporate these mechanistic insights into a predictive model that accurately captures the cancellation of rod- and cone-mediated responses and can explain the perceptual insensitivity to flicker.

Strain variations in cone wavelength peaks in situ during zebrafish development

There are four cone morphologies in zebrafish, corresponding to UV (U), blue (B), green (G), and red (R)-sensing types; yet genetically, eight cone opsins are expressed. How eight opsins are physiologically siloed in four cone types is not well understood, and in larvae, cone physiological spectral peaks are unstudied. We use a spectral model to infer cone wavelength peaks, semisaturation irradiances, and saturation amplitudes from electroretinogram (ERG) datasets composed of multi-wavelength, multi-irradiance, aspartate-isolated, cone-PIII signals, as compiled from many 5- to 12-day larvae and 8- to 18-month-old adult eyes isolated from wild-type (WT) or roy orbison (roy) strains. Analysis suggests (in nm) a seven-cone, U-360/B1-427/B2-440/G1-460/G3-476/R1-575/R2-556, spectral physiology in WT larvae but a six-cone, U-349/B1-414/G3-483/G4-495/R1-572/R2-556, structure in WT adults. In roy larvae, there is a five-cone structure: U-373/B2-440/G1-460/R1-575/R2-556; in roy adults, there is a four-cone structure, B1-410/G3-482/R1-571/R2-556. Existence of multiple B, G, and R types is inferred from shifts in peaks with red or blue backgrounds. Cones were either high or low semisaturation types. The more sensitive, low semisaturation types included U, B1, and G1 cones [3.0–3.6 log(quanta·μm−2·s−1)]. The less sensitive, high semisaturation types were B2, G3, G4, R1, and R2 types [4.3-4.7 log(quanta·μm−2·s−1)]. In both WT and roy, U- and B- cone saturation amplitudes were greater in larvae than in adults, while G-cone saturation levels were greater in adults. R-cone saturation amplitudes were the largest (50–60% of maximal dataset amplitudes) and constant throughout development. WT and roy larvae differed in cone signal levels, with lesser UV- and greater G-cone amplitudes occurring in roy, indicating strain variation in physiological development of cone signals. These physiological measures of cone types suggest chromatic processing in zebrafish involves at least four to seven spectral signal processing pools.

Unexpectedly low UV-sensitivity in a bird, the budgerigar

Photoreceptor adaptation ensures appropriate visual responses during changing light conditions and contributes to colour constancy. We used behavioural tests to compare UV-sensitivity of budgerigars after adaptation to UV-rich and UV-poor backgrounds. In the latter case, we found lower UV-sensitivity than expected, which could be the result of photon-shot noise corrupting cone signal robustness or nonlinear background adaptation. We suggest that nonlinear adaptation may be necessary for allowing cones to discriminate UV-rich signals, such as bird plumage colours, against UV-poor natural backgrounds.

Endocannabinoids via CB 1 receptors act as neurogenic niche cues during cortical development

During brain development, neurogenesis is precisely regulated by the concerted action of intrinsic factors and extracellular signalling systems that provide the necessary niche information to proliferating and differentiating cells. A number of recent studies have revealed a previously unknown role for the endocannabinoid (ECB) system in the control of embryonic neuronal development and maturation. Thus, the CB 1 cannabinoid receptor in concert with locally produced ECBs regulates neural progenitor (NP) proliferation, pyramidal specification and axonal navigation. In addition, subcellularly restricted ECB production acts as an axonal growth cone signal to regulate interneuron morphogenesis. These findings provide the rationale for understanding better the consequences of prenatal cannabinoid exposure, and emphasize a novel role of ECBs as neurogenic instructive cues involved in cortical development. In this review the implications of altered CB 1 -receptor-mediated signalling in developmental disorders and particularly in epileptogenesis are briefly discussed.

Changes in induced hues at low luminance and following dark adaptation suggest rod-cone interactions may differ for luminance increments and decrements

Color contrast describes the influence of one color on the perception of colors in neighboring areas. This study addressed two issues: (1) the accurate representation of the color changes; (ii) the underlying visual mechanisms. Observers matched the hue that was induced in a neutral square when it was set in one of four standard colored surrounds: “red” (+L(−M) relative to neutral), “green” (−L(+M)), “purple” (+S), and “yellow” (−S). The standard and matching displays were viewed haploscopically. The standard neutral square was either a luminance increment, or decrement, both of which appeared the complementary color to the surrounds in which they were inset. In Experiment 1, the surround luminance in each eye's display was either equal, at 18 cd·m−2, or the match surround luminance was reduced to 2.5 cd·m−2. The matches with equal surround luminances could be represented as vector shifts in a logarithmic MacLeod–Boynton (r, b) chromaticity diagram, as described previously (Shepherd, 1997, 1999). The low luminance matches were, however, displaced further from neutral, as if larger chromatic differences were needed. The precise direction of the displacements differed for luminance increments and decrements: the red, green and yellow decrement matches were also displaced vertically downwards in the MacLeod-Boynton diagram. In Experiment 2, dark-adapting before setting repeat color matches displaced the decrement matches vertically, but did not affect the increment matches. Thus, rod intrusion in S-cone pathways may have boosted the S-cone signal for the lowest luminance decrement matches in Experiment 1 and account for the vertical shift in MacLeod-Boynton co-ordinates. The distinct pattern of displacements for low luminance increments and decrements may be explained if the match is set at a cone-opponent, rather than a cone contrast, site and if rod signals have an input only to S-cone decrement, perhaps S-OFF, pathways.