Human Visual Development
Review these brief notes on visual development before continuing: Click (opens new page).
Elements of Vision
Vision sits atop the developmental heap as the leader in efficiency in acquisition of information from the environment. We all share this ability: To scan the environment (provided our visual system is healthy) and make sense of what we see. In our case as humans, we can also anticipate what we are looking for and have the necessary eye movements calculated steps in advance. The data we collect through our eyes(visual signals and eye position) is then processed in our brains for information relating to spatial positioning, balance, movement, audition (understanding sounds such as words), and object and facial recognition.
The eyes are the doorways to vision, but just like any other doorway, it is only the start, the opening. Even without a door, the house still stands, and all the rooms are there. The eyes are related to vision in this way – they are only one part of the whole system, and while vision requires sight to develop, once developed, many aspects of vision are still present even if the eyes are closed, or damaged. Vision can also be compared to a news photographer with a camera in-hand. The camera (the eyes, to be sure) has mechanisms to focus light and capture the focused light information. What the camera cannot do is point itself to objects of interest, or determine what those objects might be, or choose which objects to pay attention to. The camera also has limited computer processing power for anything but making the image appear, vision on the other hand can take the ‘image’ of what is seen and put it to good use in our minds, whether that means recognizing faces, or symbols on a page, or calculating that next stride for a hockey player in full flight.
Vision can be divided into three general categories of behaviours:
- Skills required for targeting visual signals of interest (that is, something we want to see). These are referred to as Visual Signal Acquisition (VSA) skills and consist of the mechanical side of vision.
- Skills required for making sense of what it seen. These are mental behaviours best described as the computer software side to vision, and categorized as Visual Signal Processing (VSP)
- Advanced behaviours that rely in large part upon visual input, such as balance, visual motor coordination, visualization, and reading.
These are reviewed in the next sections.
Visual Signal Acquisition – Finding What Is Important
Visual Signal Acquisition (VSA) is the mechanical side to vision, much like the camera in the hands of the photographer. The photographer will hold the camera just so, point it at the object of interest, compose and frame the view (that is, start to emphasize what is truly important in a scene), adjust the focal length of the lens and the amount of light required for good exposure, focus, and finally capture the image by releasing the shutter with a click!
The eyes function is a somewhat similar fashion in that they must be guided to the target of interest in a coordinated fashion (like having to handle two cameras simultaneously), the alignment of the cameras (eyes) must occur, then cropping and focusing. This requires action of the muscles on the outside of the eyes (the extraocular muscles) to point the eyes, but also of very small muscles in the eyes to focus the light. While we cannot control the focal length of the eyes, we can still adjust our attention so that we take in one specific element (‘zoom in’) or consider the entire scene as a whole (‘zoom out’).
The entire process of signal acquisition begins with a directive from the brain to capture some specific information. VSA, then, is that directive put into action by the visual system through control of eyes to target a visual signal, and make it as clear as possible to both eyes. Perhaps vision could be more accurately described as a video camera in that information is acquired as a continuous stream, but the still image of a photograph is also appropriate in that it emphasizes the fact that our vision is typically directed to highly some particular aspect of the scene.
Elements of Visual Signal Acquisition include:
- Pursuits – smooth eye movements needed for tracking moving targets or balancing spatial awareness when the body moves or when objects move around the body.
- Saccades – (sah-‘kawds or suh-‘kādes). ‘Jump’ eye movements. These can range from single long jumps, like from one scene to another on the complete opposite side of the visual field, to the rapid and automated very fine movements of fractions of millimeters such as are required for reading.
- Vergence – The movement in opposing directions of the eyes such as occurs when looking near to far and back again – the eyes will go from a crossed position to a parallel position (divergence), then re-cross in order to see the near object (convergence).
- Versions – These are movements of the eyes in the same direction, such as when then look up, down, left, right.
- Fixation – The ability to maintain visual focus on a single object, keeping the eyes steady and on target.
- Posture –When an eye is covered, it will move to its resting position, this is its posture. Given a target, most people’s eyes will tend to want to naturally drift outwardly from a target (exophoria), or inwardly (esophoria).
- Alignment – The eyes should be relatively balanced and symmetrical in their positioning. In the case of strabismus, one eye is misaligned when the other is on target.
- Focus – The eyes will always try to maintain maximum clarity of an image.
- Central vs Peripheral Awareness – These are the result of both internal brain functions, such as our internal mapping of external space, and the input of light from two parallel pathways that start in the retina: Central vision, involving the macula (the physical centre of the sensory tissue where we perceive the greatest detail), and peripheral retina which comprises most of the light sensitive tissue by area and produces very poor visual acuity. Our attention switches from central to peripheral awareness both automatically, and when we tell it to.
- Spatial Awareness – Our internal mapping and sense of where we are in the world, and where objects are positioned in space relative to ourselves.
- Visualization – Somewhat related to central, peripheral, and spatial awareness, this is the ability to see in one’s mind the world as it actually exists. This enables motor planning and anticipation.
- Eyesight – the refractive, or resolving power of the eye to focus images. This is a matter of ‘bending’ and ‘straightening’ light so it can be focused onto the retina. The focusing system contributes only part of the focusing ability of the eye, the rest is static; it is the static part of the focusing power that is the source of most focusing problems. There are a great variety of refractive combinations that exist, and each has its own effect on vision and behaviour; for example, nearsightedness and farsightedness have decidedly different effects on near work. See the section on behavioural effects of vision to learn more.