Light adaptation controls visual sensitivity by adjusting the speed and gain of the response to light

A reinterpretation of critical flicker-frequency (CFF) data reveals key details about light adaptation and normal and abnormal visual processing

Our ability to see flicker has an upper frequency limit above which flicker is invisible, known as the "critical flicker frequency" (CFF), that typically grows with light intensity (I). The relation between CFF and I, the focus of nearly 200 years of research, is roughly logarithmic, i.e., CFF ∝ log(I)-a relation called the Ferry-Porter law. However, why this law should occur, and how it relates to the underlying physiology, have never been adequately explained. Over the past two decades we have measured CFF in normal observers and in patients with retinal gene defects. Here, we reanalyse and model our data and historical CFF data. Remarkably, CFF-versus-I functions measured under a wide range of conditions in patients and in normal observers all have broadly similar shapes when plotted in double-logarithmic coordinates, i.e., log (CFF)-versus-log(I). Thus, the entire dataset can be characterised by horizontal and vertical logarithmic shifts of a fixed-shape template. Shape invariance can be predicted by a simple model of visual processing built from a sequence of low-pass filters, subtractive feedforward stages and gain adjustment (Rider, Henning & Stockman, 2019). It depends primarily on the numbers of visual processing stages that approach their power-law region at a given intensity and a frequency-independent gain reduction at higher light levels. Counter-intuitively, the CFF-versus-I relation depends primarily on the gain of the visual response rather than its speed-a conclusion that changes our understanding and interpretation of human flicker perception. The Ferry-Porter "law" is merely an approximation of the shape-invariant template.

Temporal vision: measures, mechanisms and meaning

Time is largely a hidden variable in vision. It is the condition for seeing interesting things such as spatial forms and patterns, colours and movements in the external world, and yet is not meant to be noticed in itself. Temporal aspects of visual processing have received comparatively little attention in research. Temporal properties have been made explicit mainly in measurements of resolution and integration in simple tasks such as detection of spatially homogeneous flicker or light pulses of varying duration. Only through a mechanistic understanding of their basis in retinal photoreceptors and circuits can such measures guide modelling of natural vision in different species and illuminate functional and evolutionary trade-offs. Temporal vision research would benefit from bridging traditions that speak different languages. Towards that goal, I here review studies from the fields of human psychophysics, retinal physiology and neuroethology, with a focus on fundamental constraints set by early vision.

Summary: Simple measures of temporal vision such as the critical flicker frequency can be useful for modelling natural vision only if their relationship to photoreceptor responses and retinal processing is understood.

Temporal filtering of luminance and chromaticity in macaque visual cortex

Contrast sensitivity peaks near 10 Hz for luminance modulations and at lower frequencies for modulations between equiluminant lights. This difference is rooted in retinal filtering, but additional filtering occurs in the cerebral cortex. To measure the cortical contributions to luminance and chromatic temporal contrast sensitivity, signals in the lateral geniculate nucleus (LGN) were compared to the behavioral contrast sensitivity of macaque monkeys. Long wavelength-sensitive (L) and medium wavelength-sensitive (M) cones were modulated in phase to produce a luminance modulation (L + M) or in counterphase to produce a chromatic modulation (L - M). The sensitivity of LGN neurons was well matched to behavioral sensitivity at low temporal frequencies but was approximately 7 times greater at high temporal frequencies. Similar results were obtained for L + M and L - M modulations. These results show that differences in the shapes of the luminance and chromatic temporal contrast sensitivity functions are due almost entirely to pre-cortical mechanisms.

Clinical vision and molecular loss: Integrating visual psychophysics with molecular genetics reveals key details of normal and abnormal visual processing

Over the past two decades we have developed techniques and models to investigate the ways in which known molecular defects affect visual performance. Because molecular defects in retinal signalling invariably alter the speed of visual processing, our strategy has been to measure the resulting changes in flicker sensitivity. Flicker measurements provide not only straightforward clinical assessments of visual performance but also reveal fundamental details about the functioning of both abnormal and normal visual systems. Here, we bring together our past measurements of patients with pathogenic variants in the GNAT2, RGS9, GUCA1A, RPE65, OPA1, KCNV2 and NR2E3 genes and analyse the results using a standard model of visual processing. The model treats flicker sensitivity as the result of the actions of a sequence of simple processing steps, one or more of which is altered by the genetic defect. Our analyses show that most defects slow down the visual response directly, but some speed it up. Crucially, however, other steps in the processing sequence can make compensatory adjustments to offset the abnormality. For example, if the abnormal step slows down the visual response, another step is likely to speed up or attenuate the response to rebalance system performance. Such compensatory adjustments are probably made by steps in the sequence that usually adapt to changing light levels. Our techniques and modelling also allow us to tease apart stationary and progressive effects, and the localised molecular losses help us to unravel and characterise individual steps in the normal and abnormal processing sequences.

Limited Spatial Spread Explains the Dependence of Visual Response Adaptation on Stimulus Size in Rat Superior Colliculus Neurons

Although adaptation to light occurs in the eye and mainly preserves the full dynamic range of neuronal responses during changing background illumination, it affects the entire visual system and helps to optimize visual information processing. We have shown recently that in rat superior colliculus (SC) neurons adaptation to light acts as a local low-pass filter because, in contrast to the primate SC, in rat collicular neurons adaptation to small stimuli is largely limited to the vicinity of the adaptor stimulus. However, it was unclear whether large visual stimuli would induce the same spatially limited adaptation. We addressed this question by evaluating the effects of 1.8°, 6.2° and 20.8° wide adaptor stimuli on test stimuli of variable size. Single unit recordings in the adult rat SC were employed to estimate the response amplitude. Small, 1.8° and 6.2° adaptors habituated visual responses only to stimuli smaller than the adaptive stimuli. However, the 20.8° adaptor dramatically reduced responses even to test stimuli >3 times wider than the adaptor (up to 70° wide). The latter result may be explained by a nearly complete occlusion by a large adaptor of the neuron's receptive field (RF). All these results are consistent with the idea of a limited spatial spread of adaptation in rat SC neurons that is the consequence of high convergence of retinal inputs, in which small RFs limit the spatial spread of adaptation. It is concluded that, in this limited spatial spread of adaptation, rodent SC resembles higher visual system areas in primates and indicates potential differences in visual information processing between rodents and primates.

Light adaptation in the chick retina: Dopamine, nitric oxide, and gap-junction coupling modulate spatiotemporal contrast sensitivity

Adaptation to changes in ambient light intensity, in retinal cells and circuits, optimizes visual functions. In the retina, light-adaptation results in changes in light-sensitivity and spatiotemporal tuning of ganglion cells. Under light-adapted conditions, contrast sensitivity (CS) of ganglion cells is a bandpass function of spatial frequency; in contrast, dark-adaptation reduces CS, especially at higher spatial frequencies. In this work, we aimed to understand intrinsic neuromodulatory mechanisms that underlie retinal adaptation to changes in ambient light level. Specifically, we investigated how CS is affected by dopamine (DA), nitric oxide (NO), and modifiers of electrical coupling through gap junctions, under different conditions of adapting illumination. Using the optokinetic response as a behavioral readout of direction-selective ganglion cell activity, we characterized the spatial CS of chicks under high- and low-photopic conditions and how it was regulated by DA, NO, and gap-junction uncouplers. We observed that: (1) DA D2R-family agonists and a donor of NO increased CS tested in low-photopic illumination, as if observed in the high-photopic light; whereas (2) removing their effects using either DA antagonists or NO- synthase inhibitors mimicked low-photopic CS; (3) simulation of high-photopic CS by DA agonists was abolished by NO-synthase inhibitors; and (4) selectively blocking coupling via connexin 35/36-containing gap junctions, using a "designer" mimetic peptide, increased CS, as does strong illumination. We conclude that, in the chicken retina: (1) DA and NO induce changes in spatiotemporal processing, similar to those driven by increasing illumination, (2) DA possibly acts through stimulating NO synthesis, and (3) blockade of coupling via gap junctions containing connexin 35/36 also drives a change in retinal CS functions. As a noninvasive method, the optokinetic response can provide rapid, conditional, and reversible assessment of retinal functions when pharmacological reagents are injected into the vitreous humor. Finally, the chick's large eyes, and the many similarities between their adaptational circuit functions and those in mammals such as the mouse, make them a promising model for future retinal research.