Fixation point modulates monocular size and distance perception

Every vision science experiment seems to begin with the same unassuming element: a small dot or cross at the center of a screen, telling participants where to look. These fixation points are so ubiquitous that researchers rarely give them a second thought. They are treated as plumbing—necessary infrastructure that stabilizes gaze and reduces noise in the data, but has no bearing on what participants actually perceive. A new study from La Trobe University and the University of Trento, published in Attention, Perception, & Psychophysics, shows that this assumption is wrong. Even a tiny, unremarkable white dot, never mentioned to participants and never made the object of attention, measurably alters how the brain scales the size and distance of what it sees. The work is the first systematic investigation of how fixation points influence size–distance scaling, and its findings carry uncomfortable implications for decades of psychophysical research built on the assumption that fixation markers are perceptually inert.

The question matters because human vision performs a remarkable computational trick every moment we are awake. As an object moves closer, the image it casts on the retina swells enormously; as it recedes, that image shrinks to a fraction of its former extent. Yet we do not experience people as becoming giants as they approach or shrinking into insignificance as they walk away. This stability, known as size constancy, is achieved by combining the retinal image size with depth information—cues from binocular vision, accommodation of the lens, vergence of the eyes, and contextual information in the scene. When those depth cues are degraded, for example when viewing with only one eye, size constancy falters and becomes vulnerable to whatever fragments of visual information remain in the environment. In precisely these reduced-cue conditions, a fixation point is no longer just a gaze stabilizer; it is a visual stimulus in its own right, injecting spatial structure, luminance contrast, and a reference location into a scene that the struggling visual system must somehow interpret.

The research team, led by Amy Siobhan Millard with colleagues Irene Sperandio and Philippe A. Chouinard, recognized that the field had never systematically asked what fixation points actually do to perception. A targeted audit of studies summarized in a major review of size constancy mechanisms revealed substantial variability in how fixation points are designed—differences in size, shape, luminance, and placement—with little justification offered for any of these choices. Prior work had already hinted at perceptual consequences: fixation markers have been shown to alter contrast detection and to heighten susceptibility to visual illusions. What remained unknown was whether they shape the most fundamental perceptual computation of all, the scaling of size against distance.

To find out, the researchers exploited a classic comparison between two kinds of visual stimuli that differ profoundly in their reliance on depth information. The first was a real object: a blue ring displayed on an OLED screen, physically scaled at each viewing distance so that it always subtended the same 4.28 degrees of visual angle on the retina. The second was an afterimage: a negative image burned onto the retina by a red ring-shaped inducing light, which then appeared as a blue ring projected onto the same black screen. Afterimages are peculiar objects. They have a fixed retinal size, no physical structure, and no intrinsic depth cues, so their perceived size and distance depend almost entirely on contextual information and the brain’s internal scaling mechanisms. This makes them exquisitely sensitive to Emmert’s law, the nineteenth-century observation that an afterimage’s perceived size grows in proportion to its perceived distance. Real objects follow a similar principle, formalized as the size–distance invariance hypothesis, but they bring far more of their own depth information to the task.

Twenty participants with carefully screened vision—tested for acuity, stereoacuity, color vision, and the abilities to accommodate and converge their eyes—viewed these stimuli with one eye while seated in a chin rest in an 8-meter testing room. The OLED screen sat on a sliding track that moved it to ten distances, spaced 44 centimeters apart, ranging from roughly 2 to 6 meters. On half the trials, a small white dot just 0.07 degrees of visual angle across appeared at the center of the screen, rescaled at each distance to maintain constant retinal size. Crucially, participants were never told the dot existed or that its influence was under investigation. Each trial unfolded in three phases: a ten-second fixation on the central marker on the blocking screen, which generated afterimages when illuminated; an observation phase in which the blocking screen dropped away to reveal the display, and participants estimated the stimulus’s apparent distance; and a matching phase in which they adjusted an on-screen ring of random size and color until it captured their perceptual experience. The design also captured timing measures—the moment of stimulus awareness and how long participants observed it—and perceived color in hue, saturation, and brightness, providing converging checks on whether fixation points exerted narrow or global effects.

The results revealed a striking asymmetry. For size judgments, the fixation point was a modest but genuine helper. Size estimates adhered more closely to Emmert’s law when the dot was present, for both real objects and afterimages alike. Regression analyses quantified the contribution: the fixation point accounted for roughly 4.4 percent of size-scaling accuracy for real objects and about 5.5 percent for afterimages, with the bulk of the work done by monocular visual information itself. The improvement was real but small—fixation points stabilized the retinal input, steadying the basis on which size judgments were made, without transforming performance. Statistical testing confirmed significant main effects of both the fixation point and stimulus type on size deviations, with afterimages consistently more error-prone than real objects.

Distance perception told a very different story. Real objects maintained stable, accurate distance judgments regardless of whether the fixation point was present. But for afterimages, the introduction of the fixation point was actively harmful. The regression slopes for afterimage distance judgments flattened under the fixation-present condition, and the relative-impact analysis produced a negative contribution of −8 percent—meaning the fixation point impaired distance accuracy rather than enhancing it. The interpretation offered by the researchers is that for stimuli lacking intrinsic depth cues, the fixation dot introduced a conflicting spatial reference rather than a helpful anchor. The central marker and the ring contour could not be foveated simultaneously, and the dot may have altered how observers sampled information relevant to depth, injecting inconsistency into a judgment that was already fragile.

Correlational analyses added a further layer of insight, this one involving the muscles of the eye itself. Accommodation—the lens’s ability to change shape to focus at different distances—turned out to be a significant predictor of size-judgment accuracy for real objects, but only when no fixation point was present. Participants with stronger accommodative ability, assessed with the Royal Air Force near-point rule, showed more accurate size constancy in the fixation-absent condition, with the correlation accounting for a substantial 50 percent of the variance. When a fixation point was introduced, this correlation weakened markedly. The researchers suggest that the dot provided an alternative stabilizing reference, reducing participants’ reliance on their own oculomotor cues. Intriguingly, accommodation showed no relationship to afterimage judgments at all—unsurprising, perhaps, since afterimages are retinally anchored and generate no true accommodative demand. Other measures behaved as expected: real objects reached awareness more than two seconds faster than afterimages, participants spent longer observing stimuli when the dot was present, and perceived color was essentially untouched by the fixation manipulation, confirming that the effects were specific to size–distance scaling rather than a global shift in visual experience.

The authors are careful to note the limits of their conclusions. Participants were still instructed to fixate centrally even in the fixation-absent condition, so the study does not fully replicate the free, wandering gaze of everyday vision, where saccades and microsaccades continually refresh the retinal image. Nor were eye movements recorded, leaving open the possibility that fixational eye movements—which make afterimages wobble and drift perceptually since they move in lockstep with the eyes—contribute to the differential effects observed. Anecdotal debriefing feedback revealed that some participants used the dot as a helpful reference while others barely noticed it, suggesting that individual differences in attentional strategy may explain some of the variability in its effects.

The broader message, however, is difficult to escape. A tool treated as neutral methodological equipment for generations turns out to shape the very perceptual outcomes it was meant merely to enable. The benefits for size judgments are real but modest, and the costs for distance perception of depth-ambiguous stimuli are genuine. As vision science continues to probe how the brain constructs a stable world from a jittering, two-dimensional retinal image, the humble fixation dot deserves to be treated as what it apparently is: not invisible scaffolding, but an active participant in perception. The researchers call for greater standardization of fixation point design and for researchers to critically weigh the trade-offs involved whenever a small dot quietly takes its place at the center of the visual world.