Lower-level visual processing and models of light adaptation

Light adaptation characteristics of melanopsin

Following photopigment bleaching, the rhodopsin and cone-opsins show a characteristic exponential regeneration in the dark with a photocycle dependent on the retinal pigment epithelium. Melanopsin pigment regeneration in animal models requires different pathways to rods and cones. To quantify melanopsin-mediated light adaptation in humans, we first estimated its photopigment regeneration kinetics through the photo-bleach recovery of the intrinsic melanopsin pupil light response (PLR). An intense broadband light (~120,000 Td) bleached 43% of melanopsin compared to 86% of the cone-opsins. Recovery from a 43% bleach was 3.4X slower for the melanopsin than cone-opsin. Post-bleach melanopsin regeneration followed an exponential growth with a 2.5 min time-constant (τ) that required 11.2 min for complete recovery; the half-bleaching level (I_p) was ~ 4.47 log melanopic Td (16.10 log melanopsin effective photons.cm^-2.s^-1; 8.25 log photoisomerisations.photoreceptor^-1.s^-1). The effect on the cone-directed PLR of the level of the melanopsin excitation during continuous light adaptation was then determined. We observed that cone-directed pupil constriction amplitudes increased by ~ 10% when adapting lights had a higher melanopic excitation but the same mean photometric luminance. Our findings suggest that melanopsin light adaptation enhances cone signalling along the non-visual retina-brain axis. Parameters τ and I_p will allow estimation of the level of melanopsin bleaching in any light units; the data have implications for quantifying the relative contributions of putative melanopsin pathways to regulate the post-bleach photopigment regeneration and adaptation.

Advances in Neuroscience, Not Devices, Will Determine the Effectiveness of Visual Prostheses

Background: Innovations in engineering and neuroscience have enabled the development of sophisticated visual prosthetic devices. In clinical trials, these devices have provided visual acuities as high as 20/460, enabled coarse navigation, and even allowed for reading of short words. However, long-term commercial viability arguably rests on attaining even better vision and more definitive improvements in tasks of daily living and quality of life. Purpose: Here we review technological and biological obstacles in the implementation of visual prosthetics. Conclusions: Research in the visual prosthetic field has tackled significant technical challenges, including biocompatibility, signal spread through neural tissue, and inadvertent activation of passing axons; however, significant gaps in knowledge remain in the realm of neuroscience, including the neural code of vision and visual plasticity. We assert that further optimization of prosthetic devices alone will not provide markedly improved visual outcomes without significant advances in our understanding of neuroscience.

Detection of occluding targets in natural backgrounds

Detection of target objects in the surrounding environment is a common visual task. There is a vast psychophysical and modeling literature concerning the detection of targets in artificial and natural backgrounds. Most studies involve detection of additive targets or of some form of image distortion. Although much has been learned from these studies, the targets that most often occur under natural conditions are neither additive nor distorting; rather, they are opaque targets that occlude the backgrounds behind them. Here, we describe our efforts to measure and model detection of occluding targets in natural backgrounds. To systematically vary the properties of the backgrounds, we used the constrained sampling approach of Sebastian, Abrams, and Geisler (2017). Specifically, millions of calibrated gray-scale natural-image patches were sorted into a 3D histogram along the dimensions of luminance, contrast, and phase-invariant similarity to the target. Eccentricity psychometric functions (accuracy as a function of retinal eccentricity) were measured for four different occluding targets and 15 different combinations of background luminance, contrast, and similarity, with a different randomly sampled background on each trial. The complex pattern of results was consistent across the three subjects, and was largely explained by a principled model observer (with only a single efficiency parameter) that combines three image cues (pattern, silhouette, and edge) and four well-known properties of the human visual system (optical blur, blurring and downsampling by the ganglion cells, divisive normalization, intrinsic position uncertainty). The model also explains the thresholds for additive foveal targets in natural backgrounds reported in Sebastian et al. (2017).

Local reliability weighting explains identification of partially masked objects in natural images

A fundamental natural visual task is the identification of specific target objects in the environments that surround us. It has long been known that some properties of the background have strong effects on target visibility. The most well-known properties are the luminance, contrast, and similarity of the background to the target. In previous studies, we found that these properties have highly lawful effects on detection in natural backgrounds. However, there is another important factor affecting detection in natural backgrounds that has received little or no attention in the masking literature, which has been concerned with detection in simpler backgrounds. Namely, in natural backgrounds the properties of the background often vary under the target, and hence some parts of the target are masked more than others. We began studying this factor, which we call the "partial masking factor," by measuring detection thresholds in backgrounds of contrast-modulated white noise that was constructed so that the standard template-matching (TM) observer performs equally well whether or not the noise contrast modulates in the target region. If noise contrast is uniform in the target region, then this TM observer is the Bayesian optimal observer. However, when the noise contrast modulates then the Bayesian optimal observer weights the template at each pixel location by the estimated reliability at that location. We find that human performance for modulated noise backgrounds is predicted by this reliability-weighted TM (RTM) observer. More surprisingly, we find that human performance for natural backgrounds is also predicted by the RTM observer.

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

The range of c. 10¹² ambient light levels to which we can be exposed massively exceeds the <10³ response range of neurons in the visual system, but we can see well in dim starlight and bright sunlight. This remarkable ability is achieved largely by a speeding up of the visual response as light levels increase, causing characteristic changes in our sensitivity to different rates of flicker. Here, we account for over 65 years of flicker-sensitivity measurements with an elegantly-simple, physiologically-relevant model built from first-order low-pass filters and subtractive inhibition. There are only two intensity-dependent model parameters: one adjusts the speed of the visual response by shortening the time constants of some of the filters in the direct cascade as well as those in the inhibitory stages; the other parameter adjusts the overall gain at higher light levels. After reviewing the physiological literature, we associate the variable gain and three of the variable-speed filters with biochemical processes in cone photoreceptors, and a further variable-speed filter with processes in ganglion cells. The variable-speed but fixed-strength subtractive inhibition is most likely associated with lateral connections in the retina. Additional fixed-speed filters may be more central. The model can explain the important characteristics of human flicker-sensitivity including the approximate dependences of low-frequency sensitivity on contrast (Weber’s law) and of high-frequency sensitivity on amplitude (“high-frequency linearity”), the exponential loss of high-frequency sensitivity with increasing frequency, and the logarithmic increase in temporal acuity with light level (Ferry-Porter law). In the time-domain, the model can account for several characteristics of flash sensitivity including changes in contrast sensitivity with light level (de Vries-Rose and Weber’s laws) and changes in temporal summation (Bloch’s law). The new model provides fundamental insights into the workings of the visual system and gives a simple account of many visual phenomena.

Spatial contrast sensitivity from star- to sunlight in healthy subjects and patients with glaucoma

Glaucoma is traditionally considered an asymptomatic disease until later stages. However, questionnaire studies revealed visual complaints related to various tasks, especially under extreme luminance conditions (such as outdoor at night on an unlit road or outside in the sun). We measured contrast sensitivity (CS) over a luminance range of 6 log units spanning the scotopic to photopic range and we aimed (1) to determine whether Weber's law also holds under extremely high luminance conditions and (2) to compare CS as a function of spatial frequency and luminance between glaucoma patients and healthy subjects. We included 22 glaucoma patients and 51 controls, all with normal visual acuity. For the second aim, we used a subgroup of 22 age-similar controls. Vertically oriented sine-wave gratings were generated with a projector-based setup (stimulus size 8x5 degrees). CS was measured monocularly at 1, 3, and 10 cycles per degree (cpd); mean luminance ranged from 0.0085 to 8500 cd/m². ANOVA was used to analyze the effect of glaucoma, luminance, and spatial frequency on logCS. In controls, Weber's law held for 3 and 10 cpd; for 1 cpd, CS dropped above 1000 cd/m² (P = 0.003). The logCS versus log luminance curves did not differ grossly between patients and controls (P = 0.14; typically 0-0.2 log units); the difference became larger with decreasing luminance (P = 0.003) but did not depend clearly on spatial frequency (P = 0.27). We conclude that differences between glaucoma and healthy were relatively modest for the spatially redundant, static stimulus as used in the current study.

Experimentally derived model shows that adaptation acts as a powerful spatiotemporal filter of visual responses in the rat collicular neurons

Adaptation of visual responses enhances visual information processing mainly by preserving the full dynamic range of neuronal responses during changing light conditions and is found throughout the whole visual system. Although adaptation in the primate superior colliculus neurons has received much attention little is known about quantitative properties of such adaptation in rodents, an increasingly important model in vision research. By employing single unit recordings, we demonstrate that in the rat collicular neurons visual responses are shaped by at least two forms of adaptation. When visual stimuli were repeatedly presented in the same location, visual responses were reduced in the majority of single units. However, when the adaptor stimulus was outside a small diameter receptive field (RF), responses to stimulus onset but not offset were enhanced in the majority of units. Responses to stimulus offset were reduced less and recovered faster than responses to stimulus onset and the effect was limited to a fraction of RF area. Simulations showed that such adaptation acted as a powerful spatiotemporal filter and could explain several tuning properties of collicular neurons. These results demonstrate that in rodents the adaption of visual responses has a complex spatiotemporal structure and can profoundly shape visual information processing.

Delayed S-cone sensitivity losses following the onset of intense yellow backgrounds linked to the lifetime of a photobleaching product?

Thirty years ago, Mollon, Stockman, & Polden (1987) reported that after the onset of intense yellow 581-nm backgrounds, S-cone threshold rose unexpectedly for several seconds before recovering to the light-adapted steady-state value-an effect they called: "transient-tritanopia of the second kind" (TT2). Given that 581-nm lights have little direct effect on S-cones, TT2 must arise indirectly from the backgrounds' effects on the L- and M-cones. We attribute the phenomenon to the action of an unknown L- and M-cone photobleaching product, X, which acts at their outputs like an "equivalent" background light that then inhibits S-cones at a cone-opponent, second-site. The time-course of TT2 is similar in form to the lifetime of X in a two-stage, first-order biochemical reaction A→X→C with successive best-fitting time-constants of 3.09 ± 0.35 and 7.73 ± 0.70 s. Alternatively, with an additional slowly recovering exponential "restoring-force" with a best-fitting time-constant 23.94 ± 1.42 s, the two-stage best-fitting time-constants become 4.15 ± 0.62 and 6.79 ± 1.00 s. Because the time-constants are roughly independent of the background illumination, and thus the rate of photoisomerization, A→X is likely to be a reaction subsidiary to the retinoid cycle, perhaps acting as a buffer when the bleaching rate is too high. X seems to be logarithmically related to S-cone threshold, which may result from the logarithmic cone-opponent, second-site response compression after multiplicative first-site adaptation. The restoring-force may be the same cone-opponent force that sets the rate of S-cone recovery following the unusual threshold increase following the offset of dimmer yellow backgrounds, an effect known as "transient-tritanopia" (TT1).

Effect of eccentricity and light level on the timing of light adaptation mechanisms

We explored the complexity of the light adaptation process, assessing adaptation recovery (Ar) at different eccentricities and light levels. Luminance thresholds were obtained with transient background fields at mesopic and photopic light levels for temporal retinal eccentricities (0°-15°) with test/background stimulus size of 0.5°/1° using a staircase procedure in a two-channel Maxwellian view optical system. Ar was obtained in comparison with steady data [Vis. Res.125, 12 (2016)VISRAM0042-698910.1016/j.visres.2016.04.008]. Light level proportionally affects Ar only at fovea. Photopic extrafoveal thresholds were one log unit higher for transient conditions. Adaptation was equally fast at low light levels for different retinal locations with variations mainly affected by noise. These results evidence different timing in the mechanisms of adaptation involved.

Spatiochromatic Interactions between Individual Cone Photoreceptors in the Human Retina

A remarkable feature of human vision is that the retina and brain have evolved circuitry to extract useful spatial and spectral information from signals originating in a photoreceptor mosaic with trichromatic constituents that vary widely in their relative numbers and local spatial configurations. A critical early transformation applied to cone signals is horizontal-cell-mediated lateral inhibition, which imparts a spatially antagonistic surround to individual cone receptive fields, a signature inherited by downstream neurons and implicated in color signaling. In the peripheral retina, the functional connectivity of cone inputs to the circuitry that mediates lateral inhibition is not cone-type specific, but whether these wiring schemes are maintained closer to the fovea remains unsettled, in part because central retinal anatomy is not easily amenable to direct physiological assessment. Here, we demonstrate how the precise topography of the long (L)-, middle (M)-, and short (S)-wavelength-sensitive cones in the human parafovea (1.5° eccentricity) shapes perceptual sensitivity. We used adaptive optics microstimulation to measure psychophysical detection thresholds from individual cones with spectral types that had been classified independently by absorptance imaging. Measured against chromatic adapting backgrounds, the sensitivities of L and M cones were, on average, receptor-type specific, but individual cone thresholds varied systematically with the number of preferentially activated cones in the immediate neighborhood. The spatial and spectral patterns of these interactions suggest that interneurons mediating lateral inhibition in the central retina, likely horizontal cells, establish functional connections with L and M cones indiscriminately, implying that the cone-selective circuitry supporting red-green color vision emerges after the first retinal synapse.SIGNIFICANCE STATEMENT We present evidence for spatially antagonistic interactions between individual, spectrally typed cones in the central retina of human observers using adaptive optics. Using chromatic adapting fields to modulate the relative steady-state activity of long (L)- and middle (M)-wavelength-sensitive cones, we found that single-cone detection thresholds varied predictably with the spectral demographics of the surrounding cones. The spatial scale and spectral pattern of these photoreceptor interactions were consistent with lateral inhibition mediated by retinal horizontal cells that receive nonselective input from L and M cones. These results demonstrate a clear link between the neural architecture of the visual system inputs-cone photoreceptors-and visual perception and have implications for the neural locus of the cone-specific circuitry supporting color vision.

Constrained sampling experiments reveal principles of detection in natural scenes

A fundamental everyday visual task is to detect target objects within a background scene. Using relatively simple stimuli, vision science has identified several major factors that affect detection thresholds, including the luminance of the background, the contrast of the background, the spatial similarity of the background to the target, and uncertainty due to random variations in the properties of the background and in the amplitude of the target. Here we use an experimental approach based on constrained sampling from multidimensional histograms of natural stimuli, together with a theoretical analysis based on signal detection theory, to discover how these factors affect detection in natural scenes. We sorted a large collection of natural image backgrounds into multidimensional histograms, where each bin corresponds to a particular luminance, contrast, and similarity. Detection thresholds were measured for a subset of bins spanning the space, where a natural background was randomly sampled from a bin on each trial. In low-uncertainty conditions, both the background bin and the amplitude of the target were fixed, and, in high-uncertainty conditions, they varied randomly on each trial. We found that thresholds increase approximately linearly along all three dimensions and that detection accuracy is unaffected by background bin and target amplitude uncertainty. The results are predicted from first principles by a normalized matched-template detector, where the dynamic normalizing gain factor follows directly from the statistical properties of the natural backgrounds. The results provide an explanation for classic laws of psychophysics and their underlying neural mechanisms.

Event-Based Tone Mapping for Asynchronous Time-Based Image Sensor

The asynchronous time-based neuromorphic image sensor ATIS is an array of autonomously operating pixels able to encode luminance information with an exceptionally high dynamic range (>143 dB). This paper introduces an event-based methodology to display data from this type of event-based imagers, taking into account the large dynamic range and high temporal accuracy that go beyond available mainstream display technologies. We introduce an event-based tone mapping methodology for asynchronously acquired time encoded gray-level data. A global and a local tone mapping operator are proposed. Both are designed to operate on a stream of incoming events rather than on time frame windows. Experimental results on real outdoor scenes are presented to evaluate the performance of the tone mapping operators in terms of quality, temporal stability, adaptation capability, and computational time.

CVD2014-A Database for Evaluating No-Reference Video Quality Assessment Algorithms

In this paper, we present a new video database: CVD2014-Camera Video Database. In contrast to previous video databases, this database uses real cameras rather than introducing distortions via post-processing, which results in a complex distortion space in regard to the video acquisition process. CVD2014 contains a total of 234 videos that are recorded using 78 different cameras. Moreover, this database contains the observer-specific quality evaluation scores rather than only providing mean opinion scores. We have also collected open-ended quality descriptions that are provided by the observers. These descriptions were used to define the quality dimensions for the videos in CVD2014. The dimensions included sharpness, graininess, color balance, darkness, and jerkiness. At the end of this paper, a performance study of image and video quality algorithms for predicting the subjective video quality is reported. For this performance study, we proposed a new performance measure that accounts for observer variance. The performance study revealed that there is room for improvement regarding the video quality assessment algorithms. The CVD2014 video database has been made publicly available for the research community. All video sequences and corresponding subjective ratings can be obtained from the CVD2014 project page (http://www.helsinki.fi/psychology/groups/visualcognition/).

Habitual wearers of colored lenses adapt more rapidly to the color changes the lenses produce

The visual system continuously adapts to the environment, allowing it to perform optimally in a changing visual world. One large change occurs every time one takes off or puts on a pair of spectacles. It would be advantageous for the visual system to learn to adapt particularly rapidly to such large, commonly occurring events, but whether it can do so remains unknown. Here, we tested whether people who routinely wear spectacles with colored lenses increase how rapidly they adapt to the color shifts their lenses produce. Adaptation to a global color shift causes the appearance of a test color to change. We measured changes in the color that appeared "unique yellow", that is neither reddish nor greenish, as subjects donned and removed their spectacles. Nine habitual wearers and nine age-matched control subjects judged the color of a small monochromatic test light presented with a large, uniform, whitish surround every 5s. Red lenses shifted unique yellow to more reddish colors (longer wavelengths), and greenish lenses shifted it to more greenish colors (shorter wavelengths), consistent with adaptation "normalizing" the appearance of the world. In controls, the time course of this adaptation contained a large, rapid component and a smaller gradual one, in agreement with prior results. Critically, in habitual wearers the rapid component was significantly larger, and the gradual component significantly smaller than in controls. The total amount of adaptation was also larger in habitual wearers than in controls. These data suggest strongly that the visual system adapts with increasing rapidity and strength as environments are encountered repeatedly over time. An additional unexpected finding was that baseline unique yellow shifted in a direction opposite to that produced by the habitually worn lenses. Overall, our results represent one of the first formal reports that adjusting to putting on or taking off spectacles becomes easier over time, and may have important implications for clinical management.

Contrast adaptation to luminance and brightness modulations

Perceptual brightness and color contrast decrease after seeing a light temporally modulating along a certain direction in a color space, a phenomenon known as contrast adaptation. We investigated whether contrast adaptation along the luminance direction arises from modulation of luminance signals or apparent brightness signals. The stimulus consisted of two circles on a gray background presented on a CRT monitor. In the adaptation phase, the luminance and chromaticity of one circle were temporally modulated, while the other circle was kept at a constant luminance and color metameric with an equal-energy white. We employed two types of temporal modulations, namely, in luminance and brightness. Chromaticity was sinusoidally modulated along the L-M axis, leading to dissociation between luminance and brightness (the Helmholtz-Kohlrausch effect). In addition, luminance modulation was minimized in the brightness modulation, while brightness modulation was minimized in the luminance modulation. In the test phase, an asymmetric matching method was used to measure the magnitude of contrast adaptation for both modulations. Our results showed that, although contrast adaptation along the luminance direction occurred for both modulations, contrast adaptation for luminance modulation was significantly stronger than that for the brightness modulation regardless of the temporal frequency of the adaptation modulation. These results suggest that luminance modulation is more influential in contrast adaptation than brightness modulation.

On-Off asymmetry in the perception of blur

Natural images appear blurred when imperfect lens focus reduces contrast energy at higher spatial frequencies. Here, we present evidence that perceived blur also depends on asymmetries between On (positive contrast polarities) and Off (negative contrast polarities) image signals. Psychophysical matching experiments involving natural and artificial stimuli suggest that attenuating Off signals at high spatial frequencies results in increased perceptual blur relative to similar attenuations of On signals. Results support the notion that Off image signals play an important role in blur perception.

A computational model of afterimage rotation in the peripheral drift illusion based on retinal ON/OFF responses

Human observers perceive illusory rotations after the disappearance of circularly repeating patches containing dark-to-light luminance. This afterimage rotation is a very powerful phenomenon, but little is known about the mechanisms underlying it. Here, we use a computational model to show that the afterimage rotation can be explained by a combination of fast light adaptation and the physiological architecture of the early visual system, consisting of ON- and OFF-type visual pathways. In this retinal ON/OFF model, the afterimage rotation appeared as a rotation of focus lines of retinal ON/OFF responses. Focus lines rotated clockwise on a light background, but counterclockwise on a dark background. These findings were consistent with the results of psychophysical experiments, which were also performed by us. Additionally, the velocity of the afterimage rotation was comparable with that observed in our psychophysical experiments. These results suggest that the early visual system (including the retina) is responsible for the generation of the afterimage rotation, and that this illusory rotation may be systematically misinterpreted by our high-level visual system.

The physics and psychophysics of microperimetry

PURPOSE

To assess the influences of stimulus parameters (physics) on measures of visual field sensitivity (psychophysics).

METHODS

Subjects' thresholds were measured on three different clinically available perimeters: the Humphrey Field Analyzer (HFA), the Nidek MP1 (MP1), and the Opko OCT/SLO (OSLO). On all machines, visual field testing was done with a 10-2 spatial distribution of test points, using Goldmann Size III and Size I stimuli, with a presentation time of 200 ms, and using a 4-2 threshold algorithm.

RESULTS

All the MP1 and OSLO data fell below the values for the corresponding points on the HFA. For the Goldmann Size III target, the HFA median threshold was 33 dB, whereas the MP1 median threshold was 19 dB and the OLSO, 18 dB. Using the increment intensity values at each dB level for each microperimeter, the data were converted to equivalent HFA dB. Using this conversion, the smallest increment displayed in the MP1 (1.27 cd/m) was equivalent to 34 HFA dB, and the brightest increment displayed by the MP1 was 14 HFA dB (127 cd/m). The smallest increment displayed in the OSLO (1.56 cd/m) was equivalent to 33.1 HFA dB, and the brightest increment displayed by the OSLO was 13.6 HFA dB (137 cd/m). There was good correspondence among these results when compared using equivalent increment threshold units. However, discrepancies in our findings made us acutely aware of the importance of evaluating the consequences of design choices made by the manufacturers.

CONCLUSIONS

The findings underscore the need for users to check their assumptions about what the equipment is doing and to always evaluate the psychophysical consequences of the stimuli that are used by a particular instrument.

Eye smarter than scientists believed: neural computations in circuits of the retina

We rely on our visual system to cope with the vast barrage of incoming light patterns and to extract features from the scene that are relevant to our well-being. The necessary reduction of visual information already begins in the eye. In this review, we summarize recent progress in understanding the computations performed in the vertebrate retina and how they are implemented by the neural circuitry. A new picture emerges from these findings that helps resolve a vexing paradox between the retina's structure and function. Whereas the conventional wisdom treats the eye as a simple prefilter for visual images, it now appears that the retina solves a diverse set of specific tasks and provides the results explicitly to downstream brain areas.

Responses to static visual images in macaque lateral geniculate nucleus: implications for adaptation, negative afterimages, and visual fading

Adaptation to static scenes is a familiar and fundamental aspect of visual perception that causes negative afterimages, fading, and many other visual illusions. To establish a foundation for understanding the neuronal bases of such phenomena and to constrain the contributions of retinal versus cortical processing, we studied the responses of neurons in the dorsal lateral geniculate nucleus during and after the presentation of prolonged static visual stimuli. We found that parvocellular (P) cells (the more numerous and color-sensitive pathway) showed response adaptation with a time constant on the order of tens of seconds and that their response after the removal of a visual stimulus lasting 1 min was similar in amplitude and time course to the response evoked by the photographic negative stimulus. Magnocellular (M) cells (the faster-conducting and achromatic pathway) had after responses that were substantially weaker than responses evoked by patterned visual stimuli. This difference points to the existence of an adaptive mechanism in the P-pathway that is absent or impaired in the M-pathway and is inconsistent with full adaptation of photoreceptors, which feed both pathways. Cells in both pathways often maintained a substantial tonic response throughout 1 min stimuli, suggesting that these major feedforward inputs to cortex adapt too slowly to account for visual fading. Our findings suggest that faster-adapting mechanisms in cortex are likely to be required to account for the dynamics of perception during and after the viewing of prolonged static images.

Spatial filtering versus anchoring accounts of brightness/lightness perception in staircase and simultaneous brightness/lightness contrast stimuli

J. Cataliotti and A. Gilchrist (1995) reported that, consistent with anchoring theory, the lightness of a black step in a reflectance staircase was not altered by moving a white step from a remote to an adjacent location. Recently, E. Economou, S. Zdravkovic, and A. Gilchrist (2007) reported data supporting three additional predictions of the anchoring model (A. Gilchrist et al., 1999): 1) equiluminant incremental targets in staircase simultaneous lightness contrast stimuli appeared equally light; 2) the simultaneous lightness contrast effect was due mainly to the lightening of the target on the black surround; and 3) the strength of lightness induction was greatest for darker targets. We investigated similar stimuli using brightness/lightness matching and found, contrary to these reports, that: 1) the relative position of the steps in a luminance staircase significantly influenced their brightness/lightness; 2) equiluminant incremental targets in staircase simultaneous brightness/lightness contrast stimuli did not all appear equally bright/light; 3) an asymmetry due to a greater brightening/lightening of the target on the black surround was not general; and 4) darker targets produced larger effects only when plotted on a log scale. In addition, the ODOG model (B. Blakeslee & M. E. McCourt, 1999) did an excellent job of accounting for brightness/lightness matching in these stimuli.

Very-long-term chromatic adaptation: test of gain theory and a new method

This research had two goals. First, a new method of very-long-term chromatic adaptation was compared to an older method of long-wavelength ambient illumination. In the new method, the observer viewed for 1 h per day for 12 or 14 days a CRT screen composed of oriented lines that appeared red. One observer also replicated a previous procedure (Neitz et al., 2002) in which she was exposed to long-wavelength room illumination for 4 h per day for 14 days. For both methods, equilibrium yellow was measured each day about 20 h after the end of the adaptation period. Both methods of very-long-term chromatic adaptation gave similar results. Second, shifts in equilibrium yellow were measured over a 30:1 range of light levels to determine if changes in color percepts were explained solely by a gain change in cone sensitivities (von Kries coefficient law). The magnitude of shift of equilibrium yellow depended on the level of the test light, which was not consistent with a gain theory of very-long-term chromatic adaptation.

A method for investigating the temporal dynamics of local neuroretinal responses

Visual sensitivity improves with prolonged exposure to light. Global neuroretinal responses increase, but little is known about the dynamics of local retinal responses over brief time intervals after changes in light level. This study applies the time-slice multifocal electroretinogram (TS mfERG) paradigm for the measurement of local electrical responses of the human eye over brief time intervals. Sixty-one, localised retinal areas were assessed over 25 degrees of the visual field. Cone-mediated contributions to the time-slice waveform were established. The time-slice mfERG waveforms were similar in shape and timing for pre- and post-photopigment bleach conditions after saturation of rod-mediated responses, suggesting there was no rod-mediated intrusion in the waveform. The temporal dynamics of the mfERG components show that N1P1 amplitudes decrease with each successive time-slice probe, with larger amplitude responses in the central retina compared to nasal and temporal retina. The time-slice mfERG waveform is a technique for assessing the temporal dynamics of cone-generated neural responses over time. The data are interpreted in terms of the vascular supplies and lower-level visual adaptation mechanisms.

Light adaptation in cone vision involves switching between receptor and post-receptor sites

We see over an enormous range of mean light levels, greater than the range of output signals retinal neurons can produce. Even highlights and shadows within a single visual scene can differ approximately 10,000-fold in intensity-exceeding the range of distinct neural signals by a factor of approximately 100. The effectiveness of daylight vision under these conditions relies on at least two retinal mechanisms that adjust sensitivity in the approximately 200 ms intervals between saccades. One mechanism is in the cone photoreceptors (receptor adaptation) and the other is at a previously unknown location within the retinal circuitry that benefits from convergence of signals from multiple cones (post-receptor adaptation). Here we find that post-receptor adaptation occurs as signals are relayed from cone bipolar cells to ganglion cells. Furthermore, we find that the two adaptive mechanisms are essentially mutually exclusive: as light levels increase the main site of adaptation switches from the circuitry to the cones. These findings help explain how human cone vision encodes everyday scenes, and, more generally, how sensory systems handle the challenges posed by a diverse physical environment.

Combining local and global contributions to perceived colour: An analysis of the variability in symmetric and asymmetric colour matching

Are surfaces' colours judged from weighted averages of the light that they reflect to the eyes and the colour contrast at their borders? To find out we asked subjects to set the colour and luminance of test disks to match reference disks, on various backgrounds, and analysed the variability in their settings. Most of the variability between repeated settings was in luminance. The standard deviations in the set colour were smallest when the disk and background were the same colour, irrespective of the colour itself. Matches were equally precise for greenish or reddish disks on a grey background, as for grey disks on a greenish or reddish background. The precision was less dependent on the colour contrast at the disks' borders when the backgrounds were more complex and when there was a large luminance contrast at the disks' borders. Subjects were less precise when different colours surrounded the two disks. These findings are consistent with the perceived colour at any position being a weighted average of the local cone excitation ratio and the change in the cone excitation ratio at the borders of the surface in question. However, the involved weights must be variable and depend systematically on parameters such as the luminance contrast at the surface's borders and other chromatic contrasts within the scene.

Local luminance and contrast in natural images

Within natural images there is substantial spatial variation in both local contrast and local luminance. Understanding the statistics of these variations is important for understanding the dynamics of receptive field stimulation that occur under natural viewing conditions and for understanding the requirements for effective luminance and contrast gain control. Local luminance and contrast were measured in a large set of calibrated 12-bit gray-scale natural images, for a number of analysis patch sizes. For each image and patch size we measured the range of contrast, the range of luminance, the correlation in contrast and luminance as a function of the distance between patches, and the correlation between contrast and luminance within patches. The same analyses were also performed on hand segmented regions containing only "sky", "ground", "foliage", or "backlit foliage". Within the typical image, the 95% range (2.5-97.5 percentile) for both local luminance and local contrast is somewhat greater than a factor of 10. The correlation in contrast and the correlation in luminance diminish rapidly with distance, and the typical correlation between luminance and contrast within patches is small (e.g., -0.2 compared to -0.8 for 1/f noise). We show that eye movements are frequently large enough that there will be little correlation in the contrast or luminance on a receptive field from one fixation to the next, and thus rapid contrast and luminance gain control are essential. The low correlation between local luminance and contrast implies that efficient contrast gain control mechanisms can operate largely independently of luminance gain control mechanisms.

An application of threshold-versus-intensity functions in automated static perimetry

Increment thresholds were measured over a range of adapting illuminances using a modified automated static perimeter. The data were fitted to a threshold versus intensity model (logT - logT(0) + log((A + A(0))/A(0))n) and the values logT(0) and logA(0) estimated. The effect of eccentricity and age on logT(0) and logA(0) was examined in normal subjects. A small group of patients with ocular disease were then assessed. Macular degeneration appeared to act as disease processes acting near the photoreceptor (d1 model). Glaucoma seemed to act near the site of retinal gain (d3 model). This analysis method may be of value in developing light adaptation strategies in people with ocular disease.

Visual sensitivity across the menstrual cycle

This study was designed to evaluate the hypothesis that hormonal change can affect lower level light-adaptation processes, which are likely to be retinally based. Foveal visual sensitivities were measured across several menstrual cycles of four women not using hormonally acting medication and across several menstrual cycles of three women using a triphasic oral contraceptive. One woman, diagnosed with premenstrual syndrome (PMS), was a subject for both groups. Sensitivities were measured for a series of test wavelengths for 580-nm backgrounds of 2.0 and 4.0 log td. Of the six individuals tested, one had clear evidence of visual-adaptation changes occurring in phase with the menstrual cycle. Prior to using the oral contraceptive, this individual (the PMS subject) experienced changes of short-wavelength-sensitive (SWS)-cone-mediated sensitivities of up to about 1.4 log unit on the 4.0 log td background. Her SWS-cone-mediated sensitivities tended to be highest near ovulation and lowest premenstrually. Threshold-versus-illuminance (TVI) curves confirmed that the rate of sensitivity decrease with increasing background illuminance (i.e. the TVI slope) was greater premenstrually. The degree of background-induced desensitization within her middle-wavelength-sensitive (MWS)/long-wavelength-sensitive (LWS) cone pathways also appeared to vary cyclically, but the magnitude of the variation was smaller and the time course appeared to be different. When this subject began oral contraceptive use, the patterns of sensitivity change were all altered. None of the other five subjects experienced changes of SWS-cone-mediated vision that were cyclic and significantly adaptation-state dependent. However, there was evidence for a limited degree of cyclic adaptation change within the MWS/LWS cone pathways of at least one additional subject. We conclude that hormonal change can--for some unknown proportion of women--be linked to alterations of retinal function. However, the alterations are not the same for all visual pathways, and there are pronounced individual differences. The data also demonstrate that individuals' visual adaptation capabilities can vary substantially over periods of weeks.

Visual cortex neurons of monkeys and cats: temporal dynamics of the spatial frequency response function

We measured the responses of striate cortex neurons as a function of spatial frequency on a fine time scale, over the course of an interval that is comparable to the duration of a single fixation (200 ms). Stationary gratings were flashed on for 200 ms and then off for 300 ms; the responses were analyzed at sequential 1-ms intervals. We found that 1) the preferred spatial frequency shifts through time from low frequencies to high frequencies, 2) the latency of the response increases as a function of spatial frequency, and 3) the poststimulus time histograms (PSTHs) are relatively shape-invariant across spatial frequency. The dynamic shifts in preferred spatial frequency appear to be a simple consequence of the latency shifts and the transient nature of the PSTH. The effects of these dynamic shifts on the coding of spatial frequency information are examined within the context of several different temporal integration strategies, and pattern-detection performance is determined as a function of the interval of integration, following response onset. The findings are considered within the context of related investigations as well as a number of functional issues: motion selectivity in depth, "coarse-to-fine" processing, direction selectivity, latency as a code for stimulus attributes, and behavioral response latency. Finally, we demonstrate that the results are qualitatively consistent with a simple feedforward model, similar to the one originally proposed in 1962 by Hubel and Wiesel, that incorporates measured differences in the response latencies and the receptive field sizes of different lateral geniculate nucleus inputs.

Spatio-temporal luminance contrast sensitivity and visual backward masking in schizophrenia

The aim of two experiments was to investigate the relationship between spatio-temporal contrast sensitivity and visual backward masking in normal observers and in subgroups with positive or negative symptoms in schizophrenia. Experiment 1 measured contrast sensitivity for stationary and counterphase-modulated sinusoidal gratings at four spatial (0.5, 2.0, 4.0, 8.0 cycles/degree) and four temporal frequencies (0, 4.0, 8.0, 16.0 Hz). The results showed that there were no differences in spatio-temporal contrast sensitivity between the control and positive-symptom group, and in comparison with these groups, contrast sensitivity was significantly lower at all spatial and temporal frequencies in the negative-symptom group. Experiment 2 measured the visibility of a Landolt C target with a constant target stimulus duration of 4.0 ms followed by a 150-ms backward mask, which was presented at 12 stimulus onset asynchronies from 0 to 110 ms in the same groups of observers. Consistent with the findings of the previous experiment, there were no significant differences in backward masking between the control and positive-symptom group, and in comparison with these groups, visual backward masking was significantly higher at all stimulus onset asynchronies from 40 to 110 ms in the negative-symptom group. The present findings show that there were no significant differences in contrast sensitivity and in backward masking between normal observers and a group with positive symptoms in schizophrenia. It was concluded that the reduction in contrast sensitivity for low spatial frequency counterphase flicker in the negative-symptom group is consistent with a reduction in the 'contrast gain control' mechanism of magnocellular channels, and that the reduction in contrast sensitivity for medium and high stationary gratings is consistent with a disorder in parvocellular channels. It was proposed that a disorder in magnocellular channels in the negative-symptom group may enforce a reliance on parvocellular channels that results in longer temporal summation and visible persistence, slower visual processing of single target stimuli at threshold and higher levels of sensory integration, and backward masking when multiple stimuli are presented rapidly in time.

Time course of adaptation to stimuli presented along cardinal lines in color space

Visual sensitivity is a process that allows the visual system to maintain optimal response over a wide range of ambient light levels and chromaticities. Several studies have used variants of the probe-flash paradigm to show that the time course of adaptation to abrupt changes in ambient luminance depends on both receptoral and postreceptoral mechanisms. Though a few studies have explored how these processes govern adaptation to color changes, most of this effort has targeted the L-M-cone pathway. The purpose of our work was to use the probe-flash paradigm to more fully explore light adaptation in both the L-M- and the S-cone pathways. We measured sensitivity to chromatic probes presented after the onset of a 2-s chromatic flash. Test and flash stimuli were spatially coextensive 2 degrees fields presented in Maxwellian view. Flash stimuli were presented as excursions from white and could extended in one of two directions along an equiluminant L-M-cone or S-cone line. Probes were presented as excursions from the adapting flash chromaticity and could extend either toward the spectrum locus or toward white. For both color lines, the data show a fast and slow adaptation component, although this was less evident in the S-cone data. The fast and slow components were modeled as first- and second-site adaptive processes, respectively. We find that the time course of adaptation is different for the two cardinal pathways. In addition, the time course for S-cone stimulation is polarity dependent. Our results characterize the rapid time course of adaptation in the chromatic pathways and reveal that the mechanics of adaptation within the S-cone pathway are distinct from those in the L-M-cone pathways.

High blood pressure and visual sensitivity

The study had two main purposes: (1) to determine whether the foveal visual sensitivities of people treated for high blood pressure (vascular hypertension) differ from the sensitivities of people who have not been diagnosed with high blood pressure and (2) to understand how visual adaptation is related to standard measures of systemic cardiovascular function. Two groups of middle-aged subjects--hypertensive and normotensive--were examined with a series of test/background stimulus combinations. All subjects met rigorous inclusion criteria for excellent ocular health. Although the visual sensitivities of the two subject groups overlapped extensively, the age-related rate of sensitivity loss was, for some measures, greater for the hypertensive subjects, possibly because of adaptation differences between the two groups. Overall, the degree of steady-state sensitivity loss resulting from an increase of background illuminance (for 580-nm backgrounds) was slightly less for the hypertensive subjects. Among normotensive subjects, the ability of a bright (3.8-log-td), long-wavelength (640-nm) adapting background to selectively suppress the flicker response of long-wavelength-sensitive (LWS) cones was related inversely to the ratio of mean arterial blood pressure to heart rate. The degree of selective suppression was also related to heart rate alone, and there was evidence that short-term changes of cardiovascular response were important. The results suggest that (1) vascular hypertension, or possibly its treatment, subtly affects visual function even in the absence of eye disease and (2) changes in blood flow affect retinal light-adaptation processes involved in the selective suppression of the flicker response from LWS cones caused by bright, long-wavelength backgrounds.

Chromatic light adaptation measured using functional magnetic resonance imaging

Sensitivity changes, beginning at the first stages of visual transduction, permit neurons with modest dynamic range to respond to contrast variations across an enormous range of mean illumination. We have used functional magnetic resonance imaging (fMRI) to investigate how these sensitivity changes are controlled within the visual pathways. We measured responses in human visual area V1 to a constant-amplitude, contrast-reversing probe presented on a range of mean backgrounds. We found that signals from probes initiated in the L and M cones were affected by backgrounds that changed the mean absorption rates in the L and M cones, but not by background changes seen only by the S cones. Similarly, signals from S cone-initiated probes were altered by background changes in the S cones, but not by background changes in the L and M cones. Performance in psychophysical tests under similar conditions closely mirrored the changes in V1 fMRI signals. We compare our data with simulations of the visual pathway from photon catch rates to cortical blood-oxygen level-dependent signals and show that the quantitative fMRI signals are consistent with a simple model of mean-field adaptation based on Naka-Rushton (Naka and Rushton, 1966) adaptation mechanisms within cone photoreceptor classes.

The influence of chromatic and achromatic variability on chromatic induction and perceived colour

Judgments of the colour of a surface are influenced by the colour of the surrounding. To determine whether only the average colour of the surrounding matters, or also the chromatic variability, judgments in colourful scenes are often compared with ones in which a target is surrounded by a plain background that provides the same average physical illumination of the retina as the colourful scene. The variability sometimes makes a difference (eg Shevell and Wei, 1998 Vision Research 38 1561-1566), and sometimes it does not (eg Brenner and Cornelissen, 1998 Vision Research 38 1789-1793). Is this because of the nonlinearity in cone responses? We designed scenes that stimulated the cones in an equivalent manner, both on average and in terms of variability, and yet differed markedly in chromatic variability. The more colourful surroundings had considerably less influence on subjects' colour judgments. We conclude that early cone-specific regulation of sensitivity cannot be responsible for the change in perceived colour, and deduce that chromatic induction takes place after contrast gain control.

Raised visual detection thresholds depend on the level of complexity of cognitive foveal loading

The objective of the study was to measure the interactions between visual thresholds for a simple light (the secondary task) presented peripherally and a simultaneously performed cognitive task (the primary task) presented foveally The primary task was highly visible but varied according to its cognitive complexity. Interactions between the tasks were determined by measuring detection thresholds for the peripheral task and accuracy of performance of the foveal task. Effects were measured for 5, 10, 20, and 30 deg eccentricity of the peripherally presented light and for three levels of cognitive complexity. Mesopic conditions (0.5 lx) were used. As expected, the concurrent presentation of the foveal cognitive task reduced peripheral sensitivity. Moreover, performance of the foveal task was adversely affected when conducting the peripheral task. Performance on both tasks was reduced as the level of complexity of the cognitive task increased. There were qualitative differences in task interactions between the central 10 deg and at greater eccentricities. Within 10 deg there was a disproportionate effect of eccentricity, previously interpreted as the 'tunnel-vision' model of visual field narrowing. Interactions outside 10 deg were less affected by eccentricity. These results are discussed in terms of the known neurophysiological characteristics of the primary visual pathway.

A method for comparing psychophysical and multifocal electroretinographic increment thresholds

The multifocal electroretinogram (mfERG) has been commonly used as a method for obtaining objective visual fields. Although qualitative comparisons have been good, quantitative comparisons between the results from mfERG and the results from Humphrey Visual Field Analyser (HVFA) have found variable degrees of agreement depending upon the mfERG response parameter examined and/or the disease studied. Lack of agreement may be due to differences in methodology, differences in the sites of response generation, and/or differences derived from comparing suprathreshold versus threshold responses. In addition, the two procedures are performed at different levels of adaptation. We developed an approach for matching stimulus parameters and compared mfERG and psychophysical thresholds to assess the effects of technique and level of adaptation on the two responses. Psychophysical and mfERG thresholds were obtained as a function of the adaptation level (1.5-4.0 log td) and retinal location. The derived increment threshold-versus-intensity functions for both measures were fitted using the equation logT=logT(0)+log((A+A(0))/A(0))(n). We found that the values of A(0) for the mfERG data were one log unit higher than those for the psychophysical data. In addition, the value of the slope (n) for the mfERG data was shallower (0.8) than that of the psychophysical data (1.0). Predictions were made about comparisons of HVFA threshold and mfERG amplitude data in patients with retinal disease based upon a two-site model of adaptation. The data for some groups of patients could be best-fitted with a model of a disease acting at a site distal to all gain changes, whereas data from other patients were best fitted with a model of a disease acting at a site proximal to all retinal gain. The relationship between the Humphrey visual field threshold losses and mfERG amplitude reductions depends upon the site and mechanism of a particular disease process and the model of retinal gain assumed. In no case is a one-to-one relationship between the losses in the two measures predicted.

Comparing increment and decrement probes in the probed-sinewave paradigm

Using the probed-sinewave paradigm, we explore the differences between increment and decrement probes across a range of frequencies (approx. 1-19 Hz). In this paradigm, detection threshold is measured for a small test probe presented on a large sinusoidally flickering background (at eight different phases). Probe thresholds are very similar for increment and decrement probes, but there is a very small (and systematic) difference: increment thresholds are usually slightly higher relative to decrement thresholds during the part of the cycle when the background's intensity is increasing. Although Wilson's (1997, Vis. Neuro., 14, 403-423) model substantially underestimates the size of this difference, it predicts some phase dependency. However, the existence of On- and Off-pathways in Wilson's model is not important for these predictions. A recent model by Snippe, Poot, and van Hateren (2000, Vis. Neuro., 17, 449-462) may be able to predict this result by using explicit contrast-gain control rather than separate On- and Off-pathways. Auxiliary experiments measuring the perceived polarity of the probe provide a further argument suggesting that separate On- and Off-pathways are not useful in explaining increment and decrement probe thresholds.

Spatial and temporal properties of light adaptation in the rod system

Little is known about the mechanism that regulates the sensitivity of rod system at its normal operating light levels. Two experiments are reported in this paper. First, we searched for nonlinear distortion products in rod vision that could be generated from any local adaptation process, using a sensitive experimental procedure that has demonstrated local adaptation in cone vision. No local adaptation was evident in the rod system, even at near saturating light levels. Second, to investigate the dynamics of light adaptation in the rod system we presented a uniform flickering background, sinusoidally modulated in time, and measured increment thresholds for brief test flashes that were superimposed on this background at different times during the sinusoidal flicker cycle. At frequencies less than 5-6 Hz, the rod increment threshold follows the background modulation, with a slight phase advance. When the background is modulated faster than 5-6 Hz, the increment threshold remains the same regardless of when the test flash occurred during the background cycle. Thus the rod system sensitivity, unlike that of the cone system, can only change slowly, and is set by a space-integrated signal rather than independently for different rods.

Calculating the contrasts that retinal ganglion cells and LGN neurones encounter in natural scenes

Visual responses are known to depend on stimulus contrast and not simply on the absolute levels of retinal illumination. Here, we have determined the contrasts that mammalian retinal ganglion cells and lateral geniculate neurones (LGN) are likely to encounter in real world scenes. Local contrasts were calculated in 135 calibrated images of a variety of real world scenes using contrast operators that closely mirror the characteristic receptive-field organisation of mammalian retinal ganglion cells and LGN neurones. We have found that the frequency distribution of the calculated local contrasts has a pronounced peak at zero contrast and that it tails off roughly exponentially with increasing positive and negative contrasts; about 90% of the contrasts in the images were within the equivalent range of +/-0.5 Michelson and Weber contrasts. Further analysis suggests that the characteristic forms of the contrast-response functions of mammalian retinal and LGN neurones are matched to the range of contrasts that they experience when viewing real world images.

Interactions between chromatic adaptation and contrast adaptation in color appearance

Color appearance depends on adaptation processes that adjust sensitivity both to the average color in the stimulus (through light or chromatic adaptation) and to the variations in color (through contrast adaptation). We explored how these different forms of adaptation interact, by examining how the state of chromatic adaptation depends on the time-varying color contrasts in the stimulus, and conversely, how adaptation to the mean determines the stimulus contrasts underlying contrast adaptation. Light adaptation levels remain very similar whether observers adapt to a static chromaticity or to large temporal modulations in cone excitation that vary at rates of 0.5 Hz or higher. This suggests that up to the sites of light adaptation, the response to moderate contrasts is effectively linear and that the adaptation effectively averages over several seconds of the stimulus. For slower flicker rates color is differentially biased by the last half-cycle of the flicker, and perceived contrast may be altered by response polarization. This polarization selectively saturates responses to moderate (but not low) contrasts along the color direction complementary to the mean color bias, implying that the response changes occur within multiple mechanisms tuned to different chromatic axes. Chromatic adaptation often adjusts only partially to the mean color of the stimulus, and thus leaves a residual bias in the color appearance of the field. Contrast adaptation reduces perceived contrast relative to this residual color, and not relative to the stimulus that appears achromatic. Similarly, contrast discrimination thresholds appear lower around the residual color than around the achromatic point. Thus under biased states of chromatic adaptation alternative measures of 'zero contrast' can be dissociated, suggesting that they do not depend on a common null point within the channels encoding chromatic contrast.

Infant light adaptation shows Weber's law at photopic illuminances

In the present study we investigate the dependence of photopic contrast thresholds on retinal illuminance in infants and adults. Contrast thresholds were measured at five retinal illuminances between about 6 and about 20,000 Td in subjects in both age groups. The forced-choice preferential looking technique was used in 3-month-old infants, and standard forced-choice techniques were used in adults. The stimulus was a 0.25 cy/deg squarewave grating phase alternated at 6 Hz. Infants' contrast thresholds were more than two log units higher than those of adults at all retinal illuminances. Contrast thresholds had a similar dependence on retinal illuminance in both infants and adults. For both age groups, contrast thresholds initially decreased with increasing retinal illuminance. However, at both ages, above a critical illuminance of about 200 Td, contrast thresholds remained constant, following Weber's law. Thus a vertical shift was sufficient to bring the two data sets into correspondence. In the context of a two-site model of light adaptation, our results imply that infants' elevated contrast thresholds cannot be explained solely on the basis of photoreceptoral immaturities. Later physiological immaturities must also limit infants' photopic contrast thresholds.

Normalization: contrast-gain control in simple (Fourier) and complex (non-Fourier) pathways of pattern vision

Results from two types of texture-segregation experiments considered jointly demonstrate that the heavily-compressive intensive nonlinearity acting in static pattern vision is not a relatively early, local gain control like light adaptation in the retina or LGN. Nor can it be a late, within-channel contrast-gain control. All the results suggest that it is inhibition among channels as in a normalization network. The normalization pool affects the complex-channel (second-order, non-Fourier) pathway in the same manner in which it affects the simple-channel (first-order, Fourier) pathway, but it is not yet known whether complex channels' outputs are part of the normalization pool.

Exploring the dynamics of light adaptation: the effects of varying the flickering background's duration in the probed-sinewave paradigm

In the probed-sinewave paradigm, threshold for detecting a probe is measured at various phases with respect to a sinusoidally-flickering background. Here we vary the duration of the flickering background before (and after) the test probe is presented. The adaptation is rapid; after approximately 10-30 ms of the flickering background, probe threshold is the same as that on a continually-flickering background. It is interesting that this result holds at both low (1. 2 Hz) and middle (9.4 Hz) frequencies because at middle frequencies (but not at low) there is a dc-shift, i.e. probe threshold is elevated at all phases relative to that on a steady background (of the same mean luminance). We compare our results to predictions from Wilson's model [Wilson (1997), Visual Neuroscience, 14, 403-423; Hood & Graham (1998), Visual Neuroscience, 15, 957-967] of light adaptation. The model predicts the rapid adaptation, and the dc-shift, but not the detailed shape of the probe-threshold-versus-phase curve at middle frequencies.

Mesopic state: cellular mechanisms involved in pre- and post-synaptic mixing of rod and cone signals

At least twice daily our retinas move between a light adapted, cone-dominated (photopic) state and a dark-adapted, color-blind and highly light-sensitive rod-dominated (scotopic) state. In between is a rather ill-defined transitional state called the mesopic state in which retinal circuits express both rod and cone signals. The mesopic state is characterized by its dynamic and fluid nature: the rod and cone signals flowing through retinal networks are continually changing. Consequently, in the mesopic state the retinal output to the brain contained in the firing patterns of the ganglion cells consists of information derived from both rod and cone signals. Morphology, physiology, and psychophysics all contributed to an understanding that the two systems are not independent but interact extensively via both pooling and mutual inhibition. This review lays down a rationale for such rod-cone interactions in the vertebrate retinas. It suggests that the important functional role of rod-cone interactions is that they shorten the duration of the mesopic state. As a result, the retina is maintained in either in the (rod-dominated) high sensitivity photon counting mode or in the second mode, which emphasizes temporal transients and spatial resolution (the cone-dominated photopic state). Experimental evidence for pre- and postsynaptic mixing of rod and cone signals in the retina of the clawed frog, Xenopus, is shown together with the preeminent neuromodulatory role of both light and dopamine in controlling interactions between rod and cone signals. Dopamine is shown to be both necessary and sufficient to mediate light adaptation in the amphibian retina.

Assessing retinal function with the multifocal technique

With the multifocal technique, as developed by Erich Sutter and colleagues, scores of focal electroretinogram (ERG) responses can be obtained in a matter of minutes. Although this technique is relatively new, it has already provided insights into the mechanisms of retinal disease. However, because it is new, there also remain questions about how it works and what it measures. This chapter considers some of these insights and some of these questions. The first part (Section 2) describes how the multifocal ERG (mERG) is recorded and considers its relationship to the full-field ERG. The mERG responses are shown to be from relatively local regions of the retina and are comprised of the same components as the full-field ERG. The diagnostic advantage of the mERG as compared to the full-field ERG is also illustrated. In Section 3, the effects of damage to different cell layers of the retina are shown to affect the mERG differently, and these changes are summarized within a conceptual framework. It is argued, for example, that when diseases of the receptor outer segment, like retinitis pigmentosa, result in small, depressed mERG responses, then the damage is, as expected, at the outer segment. However, when these diseases result in mERG responses that are reasonably large but very delayed, then the damage is beyond the outer segment, probably in the outer plexiform layer. The implicit time of the mERG, not amplitude, is the more sensitive measure of damage in degenerative diseases of the receptors. On the other hand, diseases, like glaucoma, which act on the ganglion axon, do not result in easily identified changes to the mERG unless inner retinal damage is involved as well. Inner retinal damage changes the waveform of the mERG and decreases the naso-temporal variation normally observed. Finally, diseases, like diabetes, that act on more than one layer of the retina can have a range of effects. In Section 4, recent work with the monkey mERG is reviewed, with emphasis on the relevance to human diseases. For example, blocking the sodium-based action potentials produced by ganglion and amacrine cells eliminates the naso-temporal variation in the monkey mERG and these altered mERG responses resemble those from some patients with diabetes or glaucoma. Finally, in Section 5 the second-order kernel is described. The presence of a second-order kernel has important implications for understanding the shape of the mERG response (first-order kernel). Full-field simulations of the mERG paradigm illustrate that the first-order kernel is comprised of responses with different waveforms. Further, it is argued that the nonlinear, adaptive mechanisms that produce the second-order kernel are involved in shaping the time course of the response. Patients with large, but abnormally delayed mERG responses (first-order kernel), do not have a detectable second-order kernel. It is speculated that a markedly diminished second-order kernel is diagnostic of outer plexiform layer damage, not inner plexiform layer damage as is commonly assumed.