In visual neuroscience, spectral sensitivity is used to describe the different characteristics of the photopigments in the rod cells and cone cells in the retina of the eye. It is known that the rod cells are more suited to scotopic vision and cone cells to photopic vision, and that they differ by the incident energy necessary to transduce a signal. It has been established that the maximum spectral sensitivity of the human eye under daylight conditions is at a wavelength of 555nm, while at night the peak shifts to 507 nm.
In photography, film and sensors are often described in terms of their spectral sensitivity, to supplement their characteristic curves that describe their responsivity. For X-ray films, the spectral sensitivity is chosen to be appropriate to the phosphors that respond to X-rays, rather than being related to human vision.
In sensor systems, where the output is easily quantified, the responsivity can be extended to be wavelength dependent, incorporating the spectral sensitivity. When the sensor system is linear, its spectral sensitivity and spectral responsivity can both be decomposed with similar basis functions. When a system's responsivity is a fixed monotonic nonlinear function, that nonlinearity can be estimated and corrected for, to determine the spectral sensitivity from spectral input–output data via standard linear methods.
The responses of the rod and cone cells of the retina, however, have a very context-dependent (coupled) nonlinear response, which complicates the analysis of their spectral sensitivities from experimental data. In spite of these complexities, however, the conversion of light energy spectra to the effective stimulus, the excitation of the photopigment, is quite linear, and linear characterizations such as spectral sensitivity are therefore quite useful in describing many properties of color vision.
Spectral sensitivity is sometimes expressed as a quantum efficiency, that is, as probability of getting a quantum reaction, such as a captured electron, to a quantum of light, as a function of wavelength. In other contexts, the spectral sensitivity is expressed as the relative response per light energy, rather than per quantum, normalized to a peak value of 1, and a quantum efficiency is used to calibrate the sensitivity at that peak wavelength. In some linear applications, the spectral sensitivity may be expressed as a spectral responsivity, with units such as amperes per watt.
Color vision is the most predominant trait represented in all classes of chordata, shedding light on the relationship of survival and ability to distinguish deviations in light. Striking resemblances can be seen upon further analysis of phototransduction between classes, and even the mechanism photosynthetic pigments undergo. Throughout its’ progression, phototransduction evolved by subtle changes in amino acid chains that undergo isomerization, namely: rhodopsin. Recent studies have alluded to an ancestor of the primates having tetrachromatic capabilities, possession of four classes of cones. While sounding advantageous, evolution has outed the vast majority of tetrachromats in the mammalian class, although varieties of fish, birds, and insects still possess this trait.
Recent publications have suggested that tetrachromacy is not an extinct trait, and is still used in ~3% of women. The prevalence of a fourth cone class is not defined concretely in the vision community, and even its existence is not accepted by all. Of those who agree with tetrachromacies existence have concluded that there are three likely cases upon which a fourth cone class can be represented.
1) Yellow hybrid gene, manifesting within the other cone classes, whose mechanism of action is similar, but differs in the required energy of incident photon to initiate excitation. The peak absorption of this class is 548 nm, lying central of the surrounding M and L cone peaks.
2) Two alleles exist in the long wavelength respective cone. This occurrence is similar to the previous hypothesis with a larger emphasis on the reason of occurrence. The fourth cone class will have a peak sensitivity close to that of the L cone.
3) The less likely possibility is an extra receptor whose peak sensitivity is just within the M and S cone boundaries. When viewing a graph of sensitivity of cone class versus the wavelength, it is apparent that there is a gap in that area, which would be advantageous to fill. This class would peak at ~500 nm, increasing the differentiation capabilities of observers who possess this trait.
George Palmer, a stained-glass maker of England, published a piece Theory of Colors and Vision in 1777. In his article, Palmer discussed three classes of light and three corresponding sensors in the eye that can detect them, he believed that there were three mechanisms in the eye, particles, which collaborate to represent all the colors in the spectrum. Although we now know the wavelength spectrum is vast, Palmer’s initial ideas of three sensors in the eye being activated by its’ own ray is accurate. These same ideas were analyzed further by the work of Thomas Young and Hermann von Helmholtz.
Thomas Young is a well known physicist in the early 19th century. His research involved Light, physiology, solid mechanics, even spanning to music and Egyptology. His best known work is still used today, the double slit experiment. This experiment, gave more insight of the wave-like properties contained within light. It was in his vision and color theory that it was first hypothesized that each of the ‘particles’ spoken about in Palmer’s theory were photoreceptors, and that each are sensitive to a range of light. These ideas were developed even further by Hermann von Helmholtz, a German physicist in the mid 1800s, whose research involved energy conservation, electro and chemical thermodynamics, and optics. His addition to Young’s theory was that each cone photoreceptors corresponding wavelengths could be classified as short, medium, and long, within the visible range. He also delved into relative intensity changes as a function of frequency instead of quantity; since the energy of light is dependent on frequency.
James Clerk Maxwell (1831-1879), a mathematical physicist, found that no three combinations of any spectrum can represent the entirety using only additive color mixing techniques. He later found that the only way all perceivable hues could be represented with three primaries is by means of subtractive color mixing.
Koenig, a physicist, was also interested in the study of physiological aspects of optics. in the late 1800s he moved to Berlin and studied under Helmholtz. His research presented in Fundamental Sensations and their Relative Intensity Distribution in the Spectrum showed that cone response sensitivity was a function of wavelength, and that each of the cone classes have a corresponding peak sensitivity.
Review of essential anatomical and physiological structures is necessary to better understand where transduction takes place, giving the tools to analyze spectral sensitivity. Light from the environment first comes into contact with the cornea, a superficial clear film which covers the front of the eye. The cornea has the highest refractive power of any structure in the eye. Although the cornea possesses no marked differences in shape or composition from other structures in the eye, the large difference in indices of refraction from the environment to this surface create roughly 70% of the eye’s power.
After light passes through the cornea, it enters a fluid filled compartment called the aqueous humor which provides a stable medium, and adds protection from the environment along with properties that exhibit immunological responses under pathogenic conditions. The fluid has a notable density of proteins that decrease time needed to create an immunological response. The fluid that fills the aqueous humor is provided from the ciliary epithelium, which resides on the surface of the ciliary muscle, a smooth striated muscle extending circumferentially around the lens. This muscle is responsible for anchoring the lens in place with help from ligaments connected to the posterior aspect known as zonule fibers. The tension in the zonule fibers determines the curvature of the lens, therefore determining the power through the structure, standardly accepted as contributing 30% of power to the eye. The vitreous humor is a fluid filled space that contains less protein density creating a less viscous consistency than the aqueous, leading to a small magnitude index of refraction.
Once a photon traverses structures in the eye, it will come into contact with a photopigment filled, multi-layered tissue space known as the retina, which possesses the ability to transduce light. The photoreceptive properties of the retina are located farthest from the vitreous humor; meaning light will have to travel from the vitreous humor through the ganglion cells, plexiform, and nuclear layers in order to reach and stimulate the photoreceptor cells. Once stimulated, the cell performing transduction will carry a signal via graded potential to the outer nuclear and plexiform layer, connected by bipolar cells. The signal is further conveyed to ganglion cells, which are the only cells in the retina to convey signals via action potential. An action potential needs a certain magnitude of input stimuli, known as the threshold, in order to proceed. If the threshold conditions are met, a message will be conveyed to the optic nerve.
The human eye has two types of photoreceptive cells distributed across the retina, rods which account for night vision, and cones, which are responsible for color vision. While both transduce light, they differ in many regards such as shape, sensitivity, and method of signalling. The sheer numbers alone set the two receptors apart, most recent findings suggest that there are 120 million rod cells in the eye, while the cones have ~7 million. The density of each cell is imperative to their sensitivity, where rods have their highest density off center, at 20 degrees, cones are mostly located in the fovea.
|Sensitivity||High (broad absorbance spectrum)||Low (Narrow peak)|
|Resolving power||Low (Can't distinguish location of source)||High (One ganglion per cone in foveal region)|
|Location density||Retina extending circumferential to fovea||Fovea, steep drop off outside of fovea|
|Response||Slow response, stimuli summed||Fast|
|Saturation||Saturating response||non-saturating response|
|Type of vision||achromatic||chromatic|
Rods are used in low light scotopic conditions, and can be distinguished from cones by their cylindrical shape. Rods are known to be extremely sensitive due to the manner in which they are connected. There will not always be a response travelling through the optic nerve if a photon is transduced by a photoreceptor, a signal needs a viable signal to distinguish it from noise when travelling through bipolar cells, and will need to have an adequate magnitude to surpass the threshold barrier. About 15 rods in proximity will connect at the same ganglion cell, and all their inputs will have a cumulative stimulus effect. This enables rods high sensitivity to less energetic photons, but trades the resolution in turn. Due to multiple receptors attaching to one ganglion cell, and multiple amacrine cells attaching to bipolar cells, the rod system can’t identify information about the location of the source.
Rhodopsin is a macromolecule ‘stack of discs’ contained within the rod photoreceptor. It is responsible for absorbing photons and undergoing metabolic processes that allow for signalling through the optic nerve, which is perceived as light. Each rod cell contains ~109 rhodopsin species, which are renewed 3 times a month. Rods are extremely sensitive to stimulation, only a small level of illuminance is required to start a signal cascade; in fact the least amount of light required to get a response was found experimentally found by Hecht et. al in 1942. The purpose was to determine the least amount of photons required to actively observe a response, known as the absolute threshold. The participants were dark adapted for forty minutes. The stimuli was a red light, held twenty degrees off axis for peak rod sensitivity. Due to subjects dark noise interference, the reaction only needed to be accurately detected by observer fifty percent of the time It was found that only nine photons are necessary to yield a reproducible response, and because of the lack of likelihood of rods being stimulated more than once, it was determined that only one photon of appropriate energy is necessary to excite a rod cell.
|Peak Absorbance||440 nm||545 nm||565 nm|
|Homologous||Not homologous||Yes, L cones||Yes, M cones|
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