Precise visuotopic organization of the blind spot representation in primate V1

Fine-scale measurement of the blind spot borders

The blind spot is both a necessity and a nuisance for seeing. It is the portion of the visual field projecting to where the optic nerve crosses the retina, a region devoid of photoreceptors and hence visual input. The precise way in which vision transitions into blindness at the blind spot border is to date unknown. A chief challenge to map this transition is the incessant movement of the eye, which unavoidably smears measurements across space. In this study, we used high-resolution eye-tracking and state-of-the-art retinal stabilization to finely map the blind spot borders. Participants reported the onset of tiny high-contrast probes that were briefly flashed at precise positions around the blind spot. This method has sufficient resolution to enable mapping of blood vessels from psychophysical measurements. Our data show that, even after accounting for eye movements, the transition zones at the edges of the blind spot are considerable. On the horizontal meridian, the regions with detection rates between 80% and 20% span approximately 25% of the overall width of the blind spot. These borders also vary considerably in size across different axes. These data show that the transition from full visibility to blindness at the blind spot border is not abrupt but occurs over a broad area.

Extrastriate activity reflects the absence of local retinal input

The physiological blind spot corresponds to the optic disc where the retina contains no light-detecting photoreceptor cells. Our perception seemingly fills in this gap in input. Here we suggest that rather than an active process, such perceptual filling-in could instead be a consequence of the integration of visual inputs at higher stages of processing discounting the local absence of retinal input. Using functional brain imaging, we resolved the retinotopic representation of the physiological blind spot in early human visual cortex and measured responses while participants perceived filling-in. Responses in early visual areas simply reflected the absence of visual input. In contrast, higher extrastriate regions responded more to stimuli in the eye containing the blind spot than the fellow eye. However, this signature was independent of filling-in. We argue that these findings agree with philosophical accounts that posit that the concept of filling-in of absent retinal input is unnecessary.

Cortical maps as a fundamental neural substrate for visual representation

Visual perception is the product of serial hierarchical processing, parallel processing, and remapping on a dynamic network involving several topographically organized cortical visual areas. Here, we will focus on the topographical organization of cortical areas and the different kinds of visual maps found in the primate brain. We will interpret our findings in light of a broader representational framework for perception. Based on neurophysiological data, our results do not support the notion that vision can be explained by a strict representational model, where the objective visual world is faithfully represented in our brain. On the contrary, we find strong evidence that vision is an active and constructive process from the very initial stages taking place in the eye and from the very initial stages of our development. A constructive interplay between perceptual and motor systems (e.g., during saccadic eye movements) is actively learnt from early infancy and ultimately provides our fluid stable visual perception of the world.

Did you see it? A Python tool for psychophysical assessment of the human blind spot

The blind spot is a region in the temporal monocular visual field in humans, which corresponds to a physiological scotoma within the nasal hemi-retina. This region has no photoreceptors, so is insensitive to visual stimulation. There is no corresponding perceptual scotoma because the visual stimulation is “filled-in” by the visual system. Investigations of visual perception in and around the blind spot allow us to investigate this filling-in process. However, because the location and size of the blind spot are individually variable, experimenters must first map the blind spot in every observer. We present an open-source tool, which runs in Psychopy software, to estimate the location and size of the blind spot psychophysically. The tool will ideally be used with an Eyelink eye-tracker (SR Research), but it can also run in standalone mode. Here, we explain the rationale for the tool and demonstrate its validity in normally-sighted observers. We develop a detailed map of the blind spot in one observer. Then, in a group of 12 observers, we propose a more efficient, pragmatic method to define a “safe zone” within the blind spot, for which the experimenter can be fully confident that visual stimuli will not be seen. Links are provided to this open-source tool and a user manual.

Filling-in rivalry: Perceptual alternations in the absence of retinal image conflict

During perceptual rivalry, an observer's perceptual experience alternates over time despite constant sensory stimulation. Perceptual alternations are thought to be driven by conflicting or ambiguous retinal image features at a particular spatial location and modulated by global context from surrounding locations. However, rivalry can also occur between two illusory stimuli-such as two filled-in stimuli within the retinal blind spot. In this "filling-in rivalry," what observers perceive in the blind spot changes in the absence of local stimulation. It remains unclear if filling-in rivalry shares common mechanisms with other types of rivalry. We measured the dynamics of rivalry between filled-in percepts in the blind spot, finding a high degree of exclusivity (perceptual dominance of one filled-in percept, rather than a perception of transparency), alternation rates that were highly consistent for individual observers, and dynamics that closely resembled other forms of perceptual rivalry. The results suggest that mechanisms common to a wide range of rivalry situations need not rely on conflicting retinal image signals.

Binocular vs. monocular hue perception

Hue perception has been shown to differ for the same stimulus when presented to the temporal and the nasal areas of the retina. The present study investigated perceptual differences in stimuli viewed binocularly or monocularly in the peripheral retina to determine how hue information combines across the two retinas for a stimulus falling on the temporal retina of one eye and the nasal retina of the other. A hue-scaling procedure was utilized to ascertain hue perception for three color- and binocular-normal observers viewing monochromatic stimuli (450-670nm, 20nm steps) ranging in size from 1.0° to 3.7°. Peripherally-presented binocular stimuli fell upon the nasal retina of one eye and the temporal retina of the other. Hue-scaling results indicated that peripheral binocular hue and saturation perceptions for smaller stimuli were more similar to those of stimuli falling on the temporal retina in the monocular condition. Hue-scaling data were also used to determine perceptive field sizes for the four elemental hues. Binocular perceptive field sizes were more similar to those obtained for stimuli falling on the temporal retina in the monocular conditions. Eye dominance did not appear to have an effect on hue perception. The results seem to indicate that visual information from the temporal retina is weighted more heavily when information from the two eyes is combined cortically. This finding may relate to differences in V1 cortical activation for stimuli presented to the nasal retina versus the temporal retina.