Recent work has brought to light a number of stimulus families whose perception is shaped by strong idiosyncratic biases. These biases differ significantly from one observer to the next, yet remain quite stable within observers when measured over multiple points in time, sometimes over months or even years. Nevertheless, we have previously shown that at least some of these biases undergo small but systematic changes over time. Although these temporal changes are also idiosyncratic, they generally act as a kind of memory that accumulates small random steps. Other research has shown that when stimuli are shown at different points in the visual field, biases can vary idiosyncratically across the spatial field as well. Here we ask whether variations in biases across space follow any regular pattern and whether spatial and temporal variations are independent of one another. Measuring biases for surface orientation in structure-from-motion stimuli, sampled at numerous points in space and time, we find that variations both in time and space are positively autocorrelated: the closer two points are to each other in space or in time, the more similar the biases at those two points. We also found that spatial and temporal variations of bias are correlated, both between and within participants. Bias variations over space, time, and space-time are therefore not random but follow dynamics that may provide clues about the underlying mechanisms.

Precisely measuring the properties of low-level visual processing is of theoretical interest as such properties often generalize to other processing in the brain and have immediate applications for image and video processing. The initial stages of human visual processing split the signal into spatial frequency and orientation-tuned channels, which interact nonlinearly, are typically modeled as divisive normalization. While we know in detail which spatial frequency and orientation channels interact, we know much less about how far the interactions reach across space. In particular, we lack psychophysical data to constrain the spatial area of the normalization pool. To provide such constraints, we set up forced choice masking experiments varying the size of the mask while we keep the spatial frequency and orientation of the mask the same as the targets. The target grating was always a small (0.5 cycles standard deviation) circular Gabor centrally superimposed over 15% and 40% contrast masks. We tested eight different mask sizes for 1, 2, and 4 cycles per degree vertical stimuli in the fovea and at 5° and 10° eccentricities. In the periphery at 5° and 10° eccentricities, contrast thresholds increase with increasing mask size and then saturate as expected, which allows us to estimate the spatial extent of the normalization pool by fitting a divisive normalization model. The fitted Gaussian normalization pool has a standard deviation of 0.65° for 1 cycle per degree gratings and scaled inversely proportional to the spatial frequency of the gratings. Neither the eccentricity nor the mask contrast influenced the size of the normalization pool. In the fovea, however, the size of the mask had no influence on thresholds, even though the mask reliably elevated thresholds. Thus, we are left with an accurate estimate of normalization pool sizes in the periphery, along with the unresolved question of why masking in the fovea shows a different behavior.

Randolph Blake has had a profound influence on the field of vision science. In a career spanning over five decades, he has revealed time and again the magic of psychophysics, leveraging it to make major contributions to our understanding of basic vision, rivalry, biological motion perception, multisensory processing, synesthesia, and the neural bases of visual awareness, to name a few examples. He has also mentored countless graduate students and postdoctoral researchers, many of whom have gone on to shape vision science in their own right. In this retrospective, we honor Randolph's legacy as a scientist and as a mentor. Rather than aiming for an exhaustive survey of his corpus of work, we have instead tried to paint a portrait of what we consider an aspirational career in science: one that valued process over product and mentorship over metrics. A constant throughout Randolph's career was something hard to quantify: a mind characterized by a knack for creative experimental design and by a persistent curiosity that led him to pursue vision science's most important unanswered questions. We hope this piece captures not only his scientific contributions but also the personality he brought to his academic roles and how that shaped the people fortunate enough to work alongside him.

Two eyes are better than one for detecting both achromatic and chromatic contrast, an advantage termed binocular summation. It is widely believed that binocular summation is mediated by neurons that sum the signals from the two eyes. While binocular summation for luminance-defined stimuli has been measured for both threshold and increment-threshold contrasts, those for chromatic stimuli have only been measured at threshold contrasts. Here we measured binocular summation for both threshold and increment-threshold contrasts defined along the black-white, red-cyan, and violet-lime axes of cone-contrast color space. Stimuli were 1.0 cycles per degree Gabor patches at a range of pedestal contrasts, each presented to both eyes, added to which was an increment to the left, right or both eyes. Left eye and right eye data were combined, and binocular summation was calculated as the ratio of monocular to binocular increment thresholds. For all three color directions, binocular summation was observed throughout the range of pedestal contrasts, with greater summation for the red-cyan and violet-lime compared with the black-white color directions. Four models of binocular summation were tested: a probability and an additive summation model each with two free parameters, a three-free-parameter model of additive summation, and a four-free-parameter model based on the modelled responses of binocular adding and binocular differencing channels, termed Max(S + S -). The three-free-parameter additive and Max(S+S-) models fared best and equally well.

Retinal optic flow provides critical visual cues for locomotion and self-navigation. Forward self-motion, such as walking or driving, typically induces an expanding radial flow pattern on the retina. Despite the predominance of expanding optic flow in everyday self-motion, several studies have reported higher sensitivity for contracting flow that arises under relatively rare conditions, such as walking backward or falling onto one's back. The underlying basis of this anisotropy remains elusive. In this study, we compared discrimination sensitivity to speed for expanding versus contracting optic flow, given that speed is a fundamental attribute of motion and that sensitivity to it provides insight into underlying visual mechanisms. We found that sensitivity in speed discrimination was comparable between expansion and contraction at low and high speeds but was significantly higher for contraction at an intermediate speed corresponding to ecologically relevant self-motion, such as walking. This anisotropic pattern was observed for both two-dimensional planar (Experiment 1) and simulated three-dimensional (Experiment 2) optic flow stimuli. Together, these findings suggest that anisotropic speed discrimination sensitivity to expanding and contracting optic flow at ecologically typical egomotion speeds may reflect an adaptive visual mechanism that supports postural stability and balance during relatively rare backward self-motion.

This study examines visual processing in dyslexia using a psychophysical measure linked to reading speed, the visual span, defined as the number of letters reliably identified within a single fixation. We asked whether reduced visual span is associated with slower reading in dyslexia. In a laboratory study, adults with dyslexia and typical readers completed standardized assessments, reading speed, and visual span measurements. Individuals with dyslexia showed significantly slower reading speeds and narrower visual span profiles than typical readers. Visual span size was positively correlated with reading speed across participants. The paradigm was then adapted for online testing. In a validation study with typical readers, online visual span and reading speed test showed patterns consistent with laboratory patterns, including a strong positive relationship between visual span size and reading speed. Applying this paradigm to a larger online sample of individuals with self-reported dyslexia revealed markedly slower reading speeds, reduced visual span sizes, and a robust correlation between visual span and reading speed despite substantial individual variability. Together, these findings provide the first direct evidence that reduced visual span is a reliable characteristic of dyslexia and is closely linked to reading performance. Moreover, they demonstrate that a brief, psychophysically grounded visual span assessment can be implemented reliably online, enabling scalable investigation of visual factors in dyslexia across diverse populations.

Previous studies have reported diurnal variations in color perception and luminance contrast sensitivity, suggesting modulation of visual processing by the circadian pacemaker. To clarify potential time-of-day effects, we examined whether cone-mediated temporal contrast sensitivity is modulated by circadian timing. We disentangled circadian and homeostatic influences on human visual performance using a 40-hour forced desynchrony protocol. Twelve participants (five female participants, 24.4 ± 2.9 years, range 19-30) followed a repeating 3.75-hour cycle comprising 2 hours 30 minutes of wakefulness and 1 hour 15 minutes of sleep under constant environmental conditions. Circadian period was estimated from up to 46 salivary melatonin samples per participant. During wake periods, participants repeatedly completed temporal contrast sensitivity tasks at 2 Hz and 8 Hz, using tasks that probed both luminance (L+M+S) and chromatic (L-M, S-cone-directed) modulations. We tested for circadian modulation of visual performance by comparing hierarchical cosinor models with intercept-only null models using Bayes factors. All participants showed robust melatonin rhythms (mean τ = 24.18 ± 0.39 hours). Bayesian inference provided decisive evidence against circadian variation in luminance or chromatic contrast sensitivity. Within this controlled protocol, we found no detectable near-24-hour modulation of cone-mediated temporal contrast sensitivity. Previously reported time-of-day effects may instead reflect homeostatic influences, pupil-linked changes in retinal illuminance, or mechanisms outside the cone-mediated pathways probed here. By separating circadian phase from sleep-wake-related influences, this study clarifies when circadian modulation of visual performance can and cannot be inferred.

Spatial suppression and perceptual stabilization are two distinct aspects of motion processing that reduce the influence of redundant motion signals to enhance behaviorally relevant motion. Spatial suppression reduces sensitivity to large, uniform background motion, whereas perceptual stabilization compensates for global retinal motion arising from involuntary eye movements during fixation. Although both mechanisms reduce the influence of redundant motion, it remains unclear whether they are behaviorally related. Comparing younger and older adults provides an opportunity to examine this relationship because spatial suppression is known to be weaker in older adults from previous work. If perceptual stabilization relies on similar motion processing as the spatial suppression of motion, it would be expected to be weaker in older adults. To test this prediction, 34 younger adults (aged 19-35 years) and 34 older adults (aged 60-80 years) performed psychophysical tasks measuring spatial suppression and perceptual stabilization. Spatial suppression strength was measured using a motion direction discrimination task, deriving a suppression index from the duration thresholds for a large stimulus presented at high and low contrast. The influence of perceptual stabilization on motion speed discrimination was assessed by comparing conditions that preserved and disrupted stabilization, with the difference quantified as a stabilization index. Across all participants, the suppression index was positively correlated with the stabilization index, demonstrating a relationship between spatial suppression and perceptual stabilization across both age groups. Older adults exhibited weaker spatial suppression and perceptual stabilization than younger adults. To determine whether the reduced perceptual stabilization reflected age differences in fixational eye movements, fixation stability was assessed and did not differ between the groups. Therefore, our findings provide behavioral evidence that the spatial suppression of motion and the perceptual stabilization of fixational eye movements are linked and may reflect shared motion-processing mechanisms that weaken in older adults.

Humans exhibit vertical asymmetry in color perception, with the lower visual field generally showing superior performance. We investigated whether this asymmetry extends to central vision by having participants judge the relative size of colored and grayscale regions within small, centrally presented stimuli (1.5° × 2.0°). Natural-scene images were partially desaturated so that either the upper or lower portion remained in color, and participants reported which region (colored or grayscale) appeared to be larger. To examine the role of scene structure, we also used pixel-shuffled versions of the same images, which preserved the vertical chromaticity distribution but removed scene recognizability. Two presentation durations were tested: 50 ms and 250 ms. With a brief presentation duration (50 ms), the colored region was overestimated in size relative to the grayscale region more strongly in the lower colored than in the upper colored condition for natural-scene images, t(19) = 5.34, p < 0.001, d = 1.20 in Experiment 1; \(F({1,14})\; = \;23.54,\;p\; < \;0.001,\;\hat \eta _p^2\; = \;0.627\) in Experiment 2-1, but this asymmetry was not observed for pixel-shuffled images, \(F({1,14}) = \;0.28,\; p\; = \;0.602, \hat \eta _p^2\; = \;0.020\). With a longer presentation duration (250 ms), the lower-half effect was significant across both natural-scene and pixel-shuffled conditions, \(F({1,15})\; = \;9.85,\;p\; = \;0.007,\;\hat \eta _p^2\; = \;0.396\), with no significant interaction between stimulus type and color position, F(1, 15) = 1.14, p = 0.303. These findings suggest that the lower visual space biases color evaluation even in central vision and that this bias emerges more readily when a natural-scene structure is available.

Everyday life requires successful navigation often through complex and dynamic environments. The ability to identify efficient and safe routes of egress through scenes relies on perception of the navigational affordances of scenes. Recent behavioral work identified that navigational affordances of indoor urban environments can emerge with as little as 33 to 66 ms of stimulus presentation but only become stable with 132 ms and beyond. Here, we extend this to investigate the temporal emergence of navigational affordances using a more heterogeneous stimulus set that includes both indoor urban and outdoor environments (both person-made and natural). We observed an almost identical pattern of results to prior work: navigational affordances were emergent rapidly (∼66 ms) and stabilized with as little as 132 ms of stimulus exposure. This pattern was also evident when considering the outdoor scenes in isolation. Taken together, these data suggest that the human visual system is capable of forming stable navigationally relevant representations for maximal behavioral relevance rapidly.

Object processing is central to visual perception, and its neural substrates inform cognitive and computational models of vision. Electroencephalographic (EEG) research has increasingly adopted multivariate pattern analysis (MVPA) to decode distributed neural representations, moving beyond average signal changes and predefined electrodes. Yet, most MVPA studies use passive viewing paradigms that constrain eye movements and evoke brain activity by external stimuli presentation, creating an artificial dissociation between perception and action. Here, we examine object category representations during active vision by combining MVPA with a deconvolution method for overlapping fixation-related responses. By decoding category information from single fixations, we compare the temporal dynamics of object processing between free viewing (active) and rapid serial visual presentation (passive) conditions. We find that categorical information, across multiple levels of abstraction, can be decoded from individual fixations during natural viewing, and that object representations generalize across passive and active conditions. Notably, representational dynamics time-locked to fixation onset emerge earlier than those time-locked to stimulus onset. We discuss the possibility that object processing begins parafoveally, aligned with saccade onset, rather than foveally at fixation. Thus, information about what and when to expect new information, available in free viewing, facilitates processing. These findings underscore the importance of studying vision in naturalistic settings and highlight key methodological considerations for combining MVPA with deconvolution in overlapping neural data. Object processing is fundamental to visual perception and to cognitive and computational models of vision. Although MVPA has advanced EEG research by enabling the decoding of distributed neural representations with high temporal precision, it has been applied primarily in passive viewing paradigms that constrain natural eye movements. The current study bridges the gap between passive and active vision research by combining MVPA with overlap deconvolution to decode object category information from EEG during natural fixations in free viewing. This approach reveals that category-specific representations emerge earlier in active viewing, likely reflecting processing that begins during saccade planning or execution. The findings underscore the importance of studying vision under ecologically valid conditions and introduce a novel methodological framework for capturing neural dynamics during active perception.

The relationship between perceived stimulus magnitude and physical stimulus intensity has traditionally been probed through two largely separate experimental traditions: near-threshold measures of sensitivity and suprathreshold measures of appearance. Reconciling both approaches has proven difficult because they rely on different theoretical assumptions and involve different experimental techniques. Here we argue that the empirical relationship between sensitivity and appearance cannot be resolved by experiment alone and propose a simulation-based approach, which requires explicitly specifying the transducer, the noise model, and the observer's decision rule. Within this framework, we evaluate whether maximum likelihood difference scaling (MLDS) can recover a known transducer from simulated data, especially for the critical cases in which different transducer and noise combinations produce identical discrimination performance. We simulate MLDS experiments for logarithmic and power-law transducers combined with additive and multiplicative noise at magnitudes representative of psychophysical data. MLDS recovers compressive, linear, and moderately expansive transducers accurately for both noise types. Systematic bias appears for strongly expansive transducers under large multiplicative noise, a case not often encountered in low-level vision. These results indicate that, for the transducer and noise models simulated here, MLDS recovers transducer shape. Our results illustrate how simulation can be used to evaluate and compare psychophysical methods by making their assumptions explicit.

It has been reported that the surface area of the human primary visual cortex predicts the susceptibility to several spatial contextual illusions. Yet, contrary to common belief, illusions correlate only weakly at the perceptual level, raising the question of how they can correlate with a neural measure like V1 surface area. In addition, mounting evidence suggests that there is substantial interindividual variability not only in neural function and anatomy but also in the mapping between the two, which further challenges the findings of a neural common factor for illusions. In light of these concerns, our first goal was to re-evaluate the previous imaging results. Seeking to investigate whether these findings generalize, we expanded our study to other illusions. We could not replicate previous findings and did not find any other significant correlations, either at the perceptual level or between illusion susceptibility and visual functional neuroanatomy.

Visual noise is widely used as a stimulus in vision science, yet the neural representation of noise itself remains poorly understood. We investigated how external visual noise is encoded in the primary visual cortex (V1) using a biologically inspired neural network model (VOneNet). We simulated V1 responses to noise patterns and analyzed both the response of individual model neurons and the correlations among them. As expected, simulated neurons with smaller receptive fields showed stronger responses. Importantly, spatially independent noise stimuli elicited correlations across the population. These correlations were critical for reconstructing the original noise pattern in a reconstruction analysis that served as a probe of perceptual representation: decorrelating modeled neural activity substantially degraded reconstruction quality. Correlations among simulated simple cells, rather than complex cells, played a dominant role in encoding external noise. Our findings suggest that the perception of visual noise depends not only on local neural activity but also on population-level correlations in V1. Implications for visual snow syndrome, a condition in which illusory external noise is perceived, are discussed.