Vision while the eyes move: get a full picture
Why do we perceive the world as stable when our eyes are constantly moving? This question has puzzled researchers for decades (1, 2). Solving the puzzle could advance a myriad of artificial visual systems such as those used in robotics or in visual display development.
When photons (packets of light) hit the retina, they initiate a cascade of neural processing, first locally in the retina itself, then in the brain, in the occipital cortex and further downstream in other visual areas. cortical. In this processing flow, photons are integrated into lines, patterns, textures, movements and, finally, into abstract concepts that make up our rich visual experience.
When the eyes move, photons from the same object are projected onto different parts of the retina. Likewise, when an object moves, photons are projected onto different parts of the retina. This creates an ambiguous situation: at some point, when movement is detected by the visual system, have the eyes moved or has something in the outside world moved?
To understand how the brain produces a stable visual world, previous studies have focused on what we can see shortly before and after eye movement, thereby ignoring any visual input generated by the moving eye itself. However, this approach fails to clarify the extent to which the movement stimulus created by the saccade or “motion streak” shapes visual processing and influences concurrent behavior, such as quickly locating and identifying things you see. in the corner of my eye. It is often assumed that during eye movements, as the world moves rapidly through the retina, a pause occurs in perceptual input. However, a new study by Schweitzer and Rolfs (3) demonstrates that movement streaks help the oculomotor system (the system responsible for producing eye movements) to correct for variability in eye movements and thus help build a stable view of the world (Fig. 1).
Schweitzer and Rolfs reveal that visual information is continually sampled in our surroundings, even when the eyes are moving. Although the quality of visual information received during eye movement may be poorer than when the eyes are stable, the visual system uses all the information available to build our perceptual experience. Since the eyes are moving much of the time (it is estimated that we make an average of three eye movements per second), this could be an effective strategy for the visual system to continuously process visual inputs for a period of time. eye movement. In this way, the visual system can optimally benefit from the information available.
In their study, Schweitzer and Rolfs used a new high-speed projector and manipulated a visual scene during eye movement. Participants saw six small patches, all with unique designs. One of the patches has been designated as the target for eye movements. The unique pattern ensured that each patch would generate a unique motion sequence. To test whether the participants were (subconsciously) processing the motion sequences, they quickly shifted the position of the target while the participants’ eyes were moving. As a result, the eyes landed on a location between the actual target and one of the other patches. After the eye movement was completed, all the patches were masked to make them indistinguishable from each other.
To complete the task of moving the eye to the target patch, participants had to perform corrective eye movement based on the sequence of movement generated by the target during eye movements. Using this clever manipulation, Schweitzer and Rolfs noticed that corrective eye movements were successfully performed towards the target, indicating that the participants were using motion sequences to resolve the scene. Going a step further, the researchers manipulated the patch patterns during eye movement (that is, changing them or removing them altogether). They noticed that this activity slows down participants and increases the likelihood of eyeing a non-target patch.
One of the unique features of the Schweitzer and Rolfs study is the use of a high speed projector. The eyes are in motion for about 50 ms. Meanwhile, high-end monitors can display around seven images. However, the high-speed projector generates around 70 images during this time and allows researchers to rearrange the patches in addition to masking them.
These results convincingly demonstrate that visual input during eye movements can solve part of the visual stability problem and provide a potential solution to stabilize visual characteristics very quickly through eye movements (4). It now appears that the continued presence of the target could accelerate the stability of visual features and that temporary removal of the target likely deteriorates the stability of visual features by deteriorating the retinal movement sequence.
Improve artificial visual systems
Beyond the science of vision, these findings have far-reaching implications. Advances in areas such as neurological / ophthalmic care, robotics, computer vision, and the development of visual display are critically dependent on a fundamental understanding of the human visual system. If we really want to understand and emulate the human visual system, we have to take into consideration all of its architecture (5). Currently, a major difference between most artificial networks and the human visual system lies at the very basis of the human visual system: eye movements. The entire visual system works against the background of eye movements. As such, one would expect it to use eye movement-induced quirks (like motion streaks) and be equipped with tricks to distinguish movement stimuli created by eye movements. movements of objects in the outside world: the world has moved or have the eyes moved?
Recent findings demonstrate that the alternating cycle of fixations and eye movements alters the spectral content of visual input in a way that appears highly adaptive in our natural environment (6, 7). The new results from Schweitzer and Rolfs now show that the brain adapts even specific parts of the spectral content of the retinal movement sequence generated by eye movement. Together, these data demonstrate that eye movements are a fundamental part of the human visual system – not a nuisance, but useful.
In order to understand, replicate, and improve our visual system, we must remember that processing begins in the eyes and their mobility adds unique and fundamental characteristics to the overall visual processing flow. It is in these characteristics that part of the effectiveness of the human visual system begins.