Rods & Cones
There are two types of photoreceptors in the human retina, rods and cones.
Rods are responsible for vision at low light levels (scotopic vision). They do not mediate color vision, and have a low spatial acuity.
Cones are active at higher light levels (photopic vision), are capable of color vision and are responsible for high spatial acuity. The central fovea is populated exclusively by cones. There are 3 types of cones which we will refer to as the short-wavelength sensitive cones, the middle-wavelength sensitive cones and the long-wavelength sensitive cones or S-cone, M-cones, and L-cones for short.
The light levels where both are operational are called mesopic.
The bottom figure shows the distribution of rods and cones in the retina. This data was prepared from histological sections made on human eyes.
In the top figure, you can relate visual angle to the position on the retina in the eye.
Notice that the fovea is rod-free and has a very high density of cones. The density of cones falls of rapidly to a constant level at about 10-15 degrees from the fovea. Notice the blind spot which has no receptors.
At about 15°-20° from the fovea, the density of the rods reaches a maximum. (Remember where Hecht, Schlaer, and Pirenne presented their stimuli.) A longitudinal section would appear similar however there would be no blind spot. Remember this if you want to present peripheral stimuli and you want to avoid the blind spot.
|Here is a figure from the textbook that shows the changes in the size of the photoreceptors with eccentricity. The bottom graph shows individual variations in the density of cones.|
Here are schematic diagrams of the structure of the rods and cones:
This figure shows the variety in the shapes and sizes of receptors across and within species.
Here is a summary of the properties and the differences in properties between the rods and cones:
Properties of Rod and Cone Systems
|More photopigment||Less photopigment|
|Slow response: long integration time||Fast response: short integration time||Temporal integration|
|High amplification||Less amplification||Single quantum detection in rods (Hecht, Schlaer & Pirenne)|
|Saturating Response (by 6% bleached)||Non-saturating response (except S-cones)||The rods' response saturates when only a small amount of the pigment is bleached (the absorption of a photon by a pigment molecule is known as bleaching the pigment).|
|Not directionally selective||Directionally selective||Stiles-Crawford effect (see later this chapter)|
|Highly convergent retinal pathways||Less convergent retinal pathways||Spatial integration|
|High sensitivity||Lower absolute sensitivity|
|Low acuity||High acuity||Results from degree of spatial integration|
|Achromatic: one type of pigment||Chromatic: three types of pigment||Color vision results from comparisons between cone responses|
If you look above at the schematic diagram of the rods and cones, you will see that in the outer segments of rods the cell membrane folds in and creates disks. In the cones, the folds remain making multiple layers. The photopigment molecules reside in membranes of these disks and folds. They are embedded in the membranes as shown in the diagram below where the two horizontal lines represent a rod disk membrane (either the membrane on the top or bottom of the disk) and the circles represent the chain of amino acids that make up a rhodopsin molecule. Rhodopsin is the photopigment in rods.
Each amino acid, and the sequence of amino acids are encoded in the DNA. Each person possesses 23 pairs of chromosomes that encode the formation of proteins in sequences of DNA. The sequence for a particular protein is called a gene. In recent years, researchers have identified the location and chemical sequence of the genes that encode the photopigments in the rods and cones.
This figure shows the structure of the rhodopsin molecule. The molecule forms 7 columns that are embedded in the disk membrane. Although not shown in this schematic, the columns are arranged in a circle like the planks of a barrel. (Another molecule called a chromophore binds within this barrel.)
Each circle is an amino-acid which are the building blocks of proteins. Each amino acid is encoded by a sequence of three nucleic acids in the DNA.
Before identifying the genetic sequence of human rhodopsin, it was sequences in other animals. Here is shown the comparison between the bovine (cow) sequence and the human sequence. They are very similar with only a small number of differences (the dark circles). Even when there is a difference it may not be functionally significant.
The gene for human rhodopsin is located on chromosome 3.
This figure shows the sequence for the S-cone pigment compared to that of rhodopsin. The S-cone pigment gene is located on chromosome 7. Notice how different they are.
This figure shows the sequence of the L- and M-cone pigments compared to each other. These pigments are very similar. Only those differences within the cell membrane can contribute to the differences in their spectral sensitivity.
The M- and L- cone pigments are both encoded on the X chromosome in tandem. The 23rd pair of chromosomes determines gender. For females this pair is XX and for males this pair is XY.
We will return to this later on when we discuss color vision and color blindness.
The Receptor Mosaic
This figure shows how the three cone types are arranged in the fovea. Currently there is a great deal of research involving the determination of the ratios of cone types and their arrangement in the retina.
This diagram was produced based on histological sections from a human eye to determine the density of the cones. The diagram represents an area of about 1° of visual angle. The number of S-cones was set to 7% based on estimates from previous studies. The L-cone:M-cone ratio was set to 1.5. This is a reasonable number considering that recent studies have shown wide ranges of cone ratios in people with normal color vision. In the central fovea an area of approximately 0.34° is S-cone free. The S-cones are semi-regularly distributed and the M- and L-cones are randomly distributed.
Throughout the whole retina the ratio of L- and M- cones to S-cones is about 100:1.
Spatial Acuity Estimate From Mosaic
From the cone mosaic we can estimate spatial acuity or the ability to see fine detail.
In the central fovea, there are approximately 150,000 cones/ sq. mm. The distance between cone centers in the hexagonal packing of the cones is about 0.003 mm. To convert this to degrees of visual angle you need to know that there are 0.29 mm/deg so that the spacing is 0.003/0.29 = 0.013° between cone centers.
The Nyquist frequency, f, is the frequency at which aliasing begins. That is a grating pattern of cos(2*pi(N/2+f)) above the Nyquist frequency is indistinguishable from the signal cos(2*pi(N/2-f)) below the Nyquist frequency where N is the number of sample points per unit distance. The Nyquist frequency is f = 1/N. The value of N = 1/0.0102 = 97. Therefore f = 48 cycles per degree.
In actuality, the foveal Nyquist limit is more like 60 cycles per degree. This may be a result of the hexagonal rather than the rectangular packing of the cone mosaic. The optics of the eye blur the retinal image so that this aliasing is not produced. Using laser interferometry, the optics of the eye can be bypassed so we can reveal this aliasing. We will discuss this in more detail in the chapter on visual acuity.
The mosaic of the retina in addition to the processing in the visual system produces another ability to see fine resolution and ascertain alignment of object called hyperacuity. People have the ability to see misalignment of objects of 5 seconds of arc (which is 1/5 of a cone width). This corresponds to seeing the misalignment in headlights 39 miles away. Maybe you can try working this out to see if I am exaggerating.