Vision, Neural Processing, and Color Vision

• These notes cover vision, neural processing, and color vision.

Receptors and Neural Processing

• Rod and cone receptors trigger visual processing by sending signals to horizontal cells.
• Horizontal cells then communicate to bipolar cells, which in turn pass information to amacrine cells.
• Amacrine cells transmit signals to ganglion cells.
• Ganglion cells' optic nerve fibers carry visual information to the brain.
• Light rays enter the eye and activate this process.

Visual Pathways

• There are two main pathways: dorsal and ventral streams.
• The primary visual cortex receives parallel channel projections.
• The parallel channels project to separate layers and then converge for a new and integrated output.

Dorsal Stream

• Concerned with spatial vision, visually guided actions, and determining "where and how".
• This is also referred to as the “how” pathway
• It includes low spatial resolution and high temporal resolution processing as well as processing contrast and motion.

Ventral Stream

• Dedicated to object vision and determining "what."
• High spatial resolution acuity is processed here.
• Color processing takes place here.

Visual Field Processing

• The left visual field is processed in the right hemisphere and vice versa.
• Visual information travels from the nasal and temporal retinas through the optic nerve.
• Impulses pass through the optic chiasm to the lateral geniculate nucleus (LGN) and then to the primary visual cortex.
• Some visual information leaves the LGN and travels to the tectum.
• The superior colliculus of the tectum receives visual signals from the retina.
• The tectospinal tract mediates reflexive head movements in response to changes in visual stimuli.
• Eye saccades occur to the same part of the visual field.
• There is efferent activation of the reticular formation and areas innervating the neck muscles.

Primary Visual Cortex (V1)

• Located in the occipital lobe.
• Receives information from the LGN.
• Projects to the extrastriate cortex.
• Also known as the Striate cortex.
• Activity increases in V1 when imagining a scene, similar to actually seeing it.
• Damage here can cause blindsight, which is responding to visual information without consciously perceiving it.
• Small islands of functional vision may exist in V1, but not enough to generate conscious sight.
• The lack of activity in V1, with fully functional visual pathways from the thalamus to other regions, may allow blindsight to occur.
• V1 is organized retinotopically, with hemispheres representing the contralateral visual field.

Retinotopic Map

• The primary visual cortex contains a retinotopic map, which is a neural representation of the retina on the cortex.
• A small area of the fovea is represented with greater magnification than the peripheral areas.

Dorsal Stream Details

• Motor system uses visual information to decide on actions.
• It largely uses "M" cell input.
• Akinetopsia can occur if there is damage.

Ventral Stream Details

• Part of the vision system allows for recognition of objects.
• Largely uses "P" cell input.
• Cerebral Achromatopsia results in color blindness.

Inferior Temporal (IT) Cortex

• The end point of the "what" ventral pathway.
• There are modules here that are sensitive to places, bodies, and faces.
• The fusiform gyrus is activated by faces, drawings of faces, and touch sensation of faces.
• Damage in this area results in visual agnosia.
• Prosopagnosia is impaired facial recognition.

Rods versus Cones Characteristics

• Rods are large and cylindrical, while cones are small and tapered.
• The fovea consists solely of cones.
• The peripheral retina has both rods and cones.
• There are more rods than cones in the periphery.
• Rods contain rhodopsin, while cones contain different forms of iodopsins.
• Rods specialize in motion and dim light.
• Cones specialize in color, detail, and bright light.

Spectral Sensitivity

• Color is perceived through the relative rates of response of opsins in three kinds of cones.
• The rod pigment absorbs best at 500 nm.
• Cone types are S (419nm), M (531nm), and L (558nm).
• Long and medium wavelength cones are more abundant than short wavelength cones.

Principle of Univariance

• Once a photon of light is absorbed by a visual pigment molecule, the identity of the light's wavelength is lost.
• The receptor doesn't know the wavelength of light it has absorbed.
• Lights of different wavelengths using only one receptor type only appear to have different intensities.
• This means that lights of different wavelengths can be perceived in the same way if projected with different intensities.

Dim Light Vision (Scotopic Vision)

• Rods are responsible for light in dim conditions; they are highly sensitive to light at low luminance levels.
• Luminance is considered to be the intensity of light per unit surface area.
• Rod response will saturate when only a small amount of pigment is bleached.

Comparison

• Rods demonstrate low spatial acuity because of spatial integration of neural signals.
• Rod vision is achromatic because it is one type of pigment.

Scotopic Vision in Other Species

• Nocturnal species are highly sensitive to light in dim-light conditions, which provides greater visual acuity and lower threshold sensitivity.
• Animals frequently have large concentration of rods in the retina.
• A tapetum lucidum is a retroreflector of light behind the retina that increases light levels falling on the retina.

Spectral Sensitivity in Animals

• Spectral sensitivity varies in other species, reflecting adaptations to their specific environments and lifestyles.

Dog Vision Details

• Dogs are dichromats and have photopigments similar to human's Short and Long cones.
• This results in similar color vision to human deuteranopia.
• Dogs have lower visual acuity as compared to humans.

Human Color Vision Deficiency

• Color Vision Deficiency affects how an individual perceives color; it may be dichromat, monochromat, or anomalous.
• Ishihara plates test use colored dots to test for color deficiency, where trichromats perceive a pattern and people with color deficiency cannot.

Types of Dichromatism

• The types include Protanopia, Deuteranopia and Tritanopia.
• Protanopia affects 1% of males and 0.02% of females.
• Those with Protanopia are missing the L-cone.
• They see short wavelength light appears blue and as wavelength increases, the blue becomes less and less saturated, becoming gray.
• Above gray, protanope perceives yellow which becomes less intense as it approaches the end of the spectrum.
• Deuteranopia affects about 1% of males and 0.01% of females.
• Those with Deuteranopia are missing the M-cone.
• They perceive blue at short wavelengths and yellow at long wavelengths.
• Tritanopia is very rare, affecting about 0.008% of the population.
• Those with Tritanopia are missing the S-cone.
• They see blue at short wavelengths and red at long wavelengths.

Anomalous Trichromacy

• The most common category of color deficient vision.
• It is an impairment, not a loss of color vision.
• Cone variants come in a variety of forms and may shift their function up/down the wavelength spectrum to varying degrees.
• Ranges in its severity from near dichromacy to near typical color vision (which may be corrected by cortical processes to closely resemble typical color vision).
• 3x more common than dichromacy
• Depending on the cone affected, the name for the condition will have the same prefix, but the suffix will be "-anomaly", like Protanomaly, Deuteranomaly and Tritanomaly.