The unseen wonders of vision
Vision is responsible for up to 80 percent of the information we receive about the external world, and about a quarter of our brain is devoted to it – making it not only the most important of our perceptual systems, but also the most heavily studied. The performance of this system continues to amaze researchers.
Scientific support: Prof. Dr. Solon Thanos
- The visual system constructs a consistent image of the world from the huge amount of information that strikes the eyes.
- The eye itself has a complex anatomy that includes 125 million photoreceptors and comprises the first station in visual processing; initial steps are carried out in the retina.
- Information from the eyes passes into the brain along a route that runs through the CGL and on to the primary visual cortex. From there it is directed to the visual cortex.
- Along the way, visual information is continually processed by participating brain regions which filter, analyze, and sort it many times, then evaluate it.
Human vision gives us the ability to perceive objects as gigantic and remote as stars in distant galaxies, and as close at hand as a tiny ant crawling across the skin. Our eyes can discriminate ten million shades of color and respond to stimuli as weak as a single photon, the smallest unit of light. These features alone make the visual system remarkable – yet even more astonishing is the way our brains transform the incredible amount of information that strikes our eyes into a sensible representation of the world. Accomplishing this requires a collaborative effort between the organs that collect sensory impressions and the parts of the brain that sort it, filter it, evaluate it, and link it to memories and experiences in a way that permits us to navigate the world with seeming ease.
We normally take the wonder of sight for granted, yet even children notice the crucial role of this sense – through games like "blind man's bluff." Even so, in the absence of an accident or disease that strikes the visual system, we normally fail to fully appreciate its many functions in our lives until 40 or 50 years later. At that point light seems dimmer, the letters blur on a page, and it's time to visit the optometrist. It's only then that many people begin to reflect on the tremendous role sight has played in bringing the world into their minds.
Scientists may never achieve a fully satisfying understanding of vision. More studies have been devoted to this sense than any other, producing enough knowledge on the topic to fill entire libraries, yet there are still large gaps. Efforts to fill them are being pursued by brain researchers, anatomists, physiologists and psychologists in collaboration with computer specialists, philosophers, and experts from many other disciplines. Any of them will tell you that unraveling the mysteries of vision will occupy scientists for a long time to come.
Distilling meaning from beams of light
The huge role that vision plays in our lives is evident from the fact that overall, about a quarter of the brain is devoted to it. And a staggering 60% of the cerebral cortex – the domain of higher brain functions – participates in processing visual information. These components of the central nervous system are linked together in what scientists call the "visual system," where they collaborate to support sight and carry out specialized tasks in a highly coordinated way.
Merely opening our eyes exposes this system to a continual flood of information. It's more than our brains can actually process, so a selection takes place that has been compared to snatching a drink from a waterfall. The visual system filters particular types of information from the flow, sorts them, processes them, and assigns them meaning. The result is our ability to identify shapes and outlines, perceive gradiations in light, movement, objects, and people, and identify up to 10 million shades of color. Scientists have been able to work out many of the details of the processes on which these capacities are based.
More astounding, and far more difficult to understand, is the process by which the system connects visual information within our minds. Why does a faded photograph of an old lover evoke painful memories of a failed relationship? And how does a flash of movement at the edge of our visual field trigger a behavioral response at lightning speed, before the conscious mind recognizes any danger? Alongside such questions are even deeper philosophical issues raised by vision research: what we know of the world is so heavily dependent on our eyes, but they can be fooled by the simplest optical illusions. If that is the case, what do we truly know about ourselves and the world?
Rods, cones, bundles of nerves and switches
The first station along the visual highway is the eye – Das Auge – Digicam mit raffinierten Funktionen. The cornea and lens diffract light and focus it on the retina, upside-down, the way an image is inverted in a camera. Within the retina, light stimuli are translated into nerve impulses, the step that lies at the heart of the entire process of seeing. This is accomplished by cells that transform light energy into neuronal activity. There are 125 million of these photoreceptors: six million are cones that detect color, and the nearly 120 million that remain represent rods, which are much more sensitive to levels of light – Von Stäbchen und Zapfen.
These components form distinct regions of the retina and create two systems that partially overlap: one type of vision for dim light and the night, another for the colorful world of daylight – Bei Tag wie bei Nacht. Even more occurs in this tissue, which is considered part of the brain; the retina carries out an initial round of image processing before visal signals are passed deeper into the brain – Hauchdünner Hochleistungsrechner.
The conduit for this information is a bundle of about a million extensions of nerve cells. Together they create the optic nerve – the data highway that runs from the eyes into the brain – Die Sehbahn – Hochgeschwindigkeitsleitung ins Gehirn. Behind the eyes, the optic nerves from the left and right cross at a structure called the Chiasm opticum. It is structured in a way that leads signals from each eye to both halves of the brain. This is crucial for depth perception. Another structure that lies between the retinas and the cerebral cortex is the Corpus geniculatum laterale (CGL), which serves as a major switching station. It, too, influences the form of visual information, as shown by the fact that it is the target of a large number of nerves stretching from the brainstem and centers of higher brain function. The CGL's role seems to be to tune signals up or down, depending on the strength of signals from other perceptual systems or the body's momentary needs.
Die Zapfen sind eine Art von Fotorezeptoren der Netzhaut. Die drei unterschiedlichen S-, M– und L-Zapfen sind jeweils durch kurz-, mittel und langwellige Frequenzen des sichtbaren Lichts erregbar und ermöglichen so Farbsehen.
Corpus geniculatum laterale
Seitlicher Kniehöcker/Corpus geniculatum laterale/lateral geniculate body
Das Corpus geniculatum laterale (seitlicher Kniehöcker) ist derjenige Abschnitt des Thalamus (größter Teil des Zwischenhirns), in dem rund 90% der Axone des Sehnervs enden. Es zeigt eine charakteristische Schichtung in sechs Zelllagen, getrennt von den eingehenden Fasern der Sehnerven. Die Nervenzellen des Corpus geniculatum laterale senden ihre Fortsätze zur Sehrinde. Gemeinsam mit dem Corpus geniculatum mediale bildet es den Metathalamus.
Shape + color + (movement and depth) = meaning
Visual data from the environment undergo several steps of processing during their passage into the brain: they are transformed, filtered, analyzed, repeatedly sorted, and evaluated. After the first steps are accomplished in the retina and CGL, the process is continued in the primary visual cortex, the next station along the route. Specialized nerves devoted to features such as color, shape, contrast and movement analyze the incoming information in parallel – Ball oder Backstein? Erkennen von Form und Kontur. Next the primary visual cortex passes the information along to cells arranged in layers, called visual cortices II through V, and so-called "visual association centers." It is here that the brain generates what we perceive as visual impressions from all of the information.
The result is far more than all the diverse components of sight could ever produce on their own. Visual data only acquires meaning when it is combined with other types of information and preexisting knowledge. If the association centers are damaged, a coffee cup will appear as a round white form, but the viewer will not be able to link it to coffee or the behavior associated with drinking it. Experts call these disruptions visual agnosia. The German scientist who discovered the phenomena called it Seelenblindheit, or "soul blindness," which rather poetically captures the effects of the phenomena. People affected by this condition can precisely describe the shape and color of a face, for example, but are unable to recognize that they are looking at their spouse. While such symptoms seem simply bizarre to those of us with functioning visual association centers, they also reveal how utterly the world that we see and know depends on an intact visual system and its many impressive functions.
Veröffentlichung: am 11.11.2010
Aktualisierung: am 08.12.2016