This post describes how eye movements contribute to the perception of visual objects. Object identification is a central problem in visual perception and understanding how movement contributes to this system enables us to be critical thinkers when designing visual components of products.
Eye and head movements help to capture physical stimulus from the environment. Eyes use oculomotor muscles within the eye socket to rotate within the skull while head is held still in order to capture a large swath of environment. The head rotates to help the eyes capture an even wider range of visual stimulus. The combination of movement from these two body parts enables humans to see most of the environment in front and on the left and right sides of the body.
Visual perception starts with light waves entering the eyes. There are a couple of physical movements of the eye that help to facilitate the capture of stimuli. Humans have the ability to perceive objects in environments that vary greatly in illumination. The iris moves in response to high/low illumination environments to vary pupil size. The pupil changes by a factor of 2x which helps with allowing or preventing photons from entering the eye and to help with glare in high-illumination settings. In addition, the ciliary muscle of the eye moves in a contract/relax manner to help with the focus of distant vs. close stimuli.
Eye movement is also critical for object perception because different areas of the retina capture different types of stimuli. The center of the retina has a large dispersion of cone photoreceptors. These receptors work well in high-illumination and are responsible for detailed vision and color perception. The peripherals of the retina have a large dispersion of rod photoreceptors. These receptors work well in low-illumination and respond well to movement. The eye must move in order to bring in both types of stimuli for each region of physical space that will be viewed. Higher levels of visual processing take care of combining the different stimuli into a coherent object.
There is high practical applicability to using eye movement to capture different aspects of physical space with different regions of the retina. This method is complimentary to the processing methods of the striate cortex. Far more cerebral space in the striate cortex is allocated to stimuli that is captured from the center regions of the visual area than the peripherals. In general, the brain experiences a physical constraint of being confined to the skull. The skull size of humans cannot increase without structural changes to the rest of the body for physical support of a larger head. A larger skull would also require changes to the female physiology to enable childbirth of babies with larger heads. By using eye movement to combine different aspects of visual stimuli from different regions of the eye’s visual space, we have the benefit of conserving the cerebral area dedicated to vision.
Eye movement is constant and continuous during visual processing. This directly contributes to a continuous and consistent visual experience by enabling the brain to combine stimuli from different parts of the eye.
Quick systematic eye movement also helps mollify some of the limitations with visual processing. Visual acuity is decreased for vision in the peripherals of the eye. Quick eye movement allows the visual system to compensate by enabling the center of the eye to process stimuli in the peripheral of the physical space and then quickly return to the center of the physical space. The same concept alleviates problems with visual crowding in the peripherals. It is difficult for the visual system to correctly group objects in the peripheral when there is too much stimuli. This problem can be resolved with quick movements of the center of the eye to the peripheral. Even though cortical processing typically takes care of accidental viewpoints, they can also be quickly resolved by eye movement. An accidental viewpoint occurs when a viewing position causes a rare instance of regularity between two objects that are not present in the physical environment. For example, the edge of two objects may line up perfectly from one very specific angle. This occurrence may be able to trick the visual system into seeing the two objects as one because of the system’s dependence on edges, lines, colors, etc. to organize visual elements into groups. If the visual system moves to a different angle through physical eye movement, then the accidental viewpoint is highlighted and it is easy for the “committees” of the visual system to ignore.
To prevent adaptation to constant stimuli the eye needs to move. This enables specified cells, such as a ganglion ON-cell, to have a break from reactive or inhibitory firing. Eye movement leads to varying inputs for specialized cells which enables them to have time to re-calibrate. Without re-calibration, the cell will “get tired” and adapt to the stimuli. If this happens, it will no longer signal the stimulus for further object processing and the object will effectively “disappear” from the visual experience.
In summary, eye and head movement is essential to visual processing and object perception. Movement is used to capture stimulus in a varied but systematic manner. These stimuli are combined in a logical process that can be understood from a high level as a grouping of stimuli collected from different regions of the eye, and processed in different regions of the brain, that combine based on probability of occurrence in the world. Processing does not happen in a single direction. During object recognition and perception, later stages of processing may require additional follow-up information from earlier stages of processing and a back-forth interaction is used to facilitate processing. Therefore, eye and head movement enables the continuous flow of a variety of visual stimuli that is used throughout the full process from stimuli to object perception. It is quite interesting that the system that enables human’s perception of an uninterrupted continuous visual experience is comprised of a piece-wise set of varying stimuli that is collected from a cacophony of discontinuous movement.