Contents and links:

Helga's retina
book:

Ron Douglas:

Vision in deep
sea fish

Contents and links:

ON-OFF and
center-surround

Helga's retina
book:

From inside the US

From Europe:

Ron Douglas:

Vision in deep
sea fish

May 9, 1998 Fundamentals of Vision Michael Schütte, Department of Ophthalmology, Mt. Sinai School of Medicine, New York, NY 10029 Humans are visually oriented and without doubt, our eyes are considered by most of us as our primary source of information. Still, it is quite astounding that no less than about 60 % of our entire cortical brain structures are devoted solely to vision. The input for this major portion of the brain comes from the retina, a thin layer of neural tissue in the back of our eyes. An even more amazing feature of the human visual system is the fact that a small area within the retina, the fovea centralis, which is the part responsible for the highest visual acuity occupies a total of approximately 50% of the visual brain. In other words, 30 % of our entire brain have nothing else to do than to decipher the signals coming from two small spots of about 1 mm diameter (and 0.2 mm thick) in our eyes. This over-representation of the central visual field in the brain has been known since 1961 when it was first reported by Daniel and Whitteridge (*) who described it as so called “cortical magnification factor”. This term originated in the belief that only the brain is capable to perform higher integrative functions and that the retina, which itself is a protrusion of the diencephalon or midbrain, has not evolved in the same manner as the rest of the modern brain, commonly known as the neocortex. Thus, it was found hard to believe that the retina itself could perform such complicated tasks as reformatting and convoluting the physical images generated by rays of light on its light sensitive background layer, the photoreceptors. For about 25 years, neuroscientists struggled to detect the neuroanatomic and physiologic correlates of this magnification factor in the brain, unfortunately without much success. Interestingly, though, there were some studies published already in the early seventies, showing that the magnification factor was already manifest in the optic nerve (**) (which connects the eyes to the brain) as well as in the earliest central stations of the visual pathway, e.g. in the lateral geniculate nucleus, the primary projection of nerve fibers originating in the retinal ganglion cells (see below) (***). However, not too much attention was paid to these reports.

About 60% of the human brain are devoted to vision. Half of this only compiles signals coming from the central fovea, a small pit of about 1 mm diameter which is the area of highest visual acuity.

In the first few chapters of this essay, I will try to show that, indeed, the retina is capable of performing the above mentioned tasks and, actually quite a bit more which will be the subject of the consecutive pages.

I. Gross Anatomy:

Let’s start with some gross anatomical findings. There is one thing that has to be mentioned before going any further. In vertebrates, the photoreceptors stick out from the back of the retina, and thus, all the light that hits them, has to pass through the entire thickness of the retina. At first glance, this doesn’t seem to make too much sense but it is important and I will get back to that subject later.
Above, I mentioned the term fovea centralis which means “central pit”. This pit can be seen with the naked eye but it is more obvious in the examination of histological sections.

Histologic section of the human eye. Note that the retina itself is only a very thin layer which rests against the thick supporting structures as choroid and sclera. Click here to see a better image of the fovea of a monkey retina

The relevance of this pit was believed longtime to provide the easiest possible route for the rays of light captured by the photoreceptors without any interference from overlaying neuronal tissue. This point is certainly valid up to now but in 1941, Snyder and Miller made the interesting discovery that in birds of prey such as eagles and falcons, also known for their exceptional visual capabilities, this fovea is expressed to a degree which doesn’t make sense if its only purpose were to push other nerve cells out of the lightpath. Particularly, the small spot in the middle is flanked by the pit’s walls which rise at a steep slope making the pit almost paraboloid in shape. The reasoning is actually quite simple. If the retinal tissue could provide enough interference to severely distort the optical image projected onto the photoreceptors, then, increasing angles of the light rays hitting the interface between the retina and the vitreous body (a jelly-like compartment in the posterior chamber of the eye) would also increase the refraction. Thus the progressive distortion of the image resulting from an increasing angle of the pit’s walls would severely undermine the benefits obtained from undisturbed passing through of a few rays in the center.
So, what else could be the benefit obtained from such a design? The answer is not so difficult, all one has to do is to measure the angles between the light beams and the retinovitreal interface and to know the refractive index. Then one can apply the laws of physical optics and calculate the further path a ray of light takes until it hits the sensory cell which absorbs it, the photoreceptor.

Schematic drawing of the refraction of a beam of light falling onto the fovea which results in a magnification of the optical image

As one can see, the fovea centralis does something very important, it acts like a magnifying lens in that it spreads a bundle of light in a fashion that enlarges its diameter to about 150 % of the original size. Thus, within the retina, there exists another tiny structure, which performs the task of selectively magnifying the images which deserve the highest attention. Still, magnification by a factor of 1.5 does, by no means, account for the magnification found in the measurements of Daniel and Whitteridge. So what else could play a role?