Visual Coding and the Retinal Receptors • Each of our senses has specialized receptors that are sensitive to a particular kind of energy. • Receptors for vision are sensitive to light. • Receptors “transduce” (convert) energy into electrochemical patterns.
Visual Coding and the Retinal Receptors • A receptor potential refers to a local depolarization or hyperpolarization of a receptor membrane. • The strength of the receptor potential determines how much excitation or inhibition is sent to the next neuron.
Visual Coding and the Retinal Receptors • Law of specific nerve energies states that activity by a particular nerve always conveys the same type of information to the brain. • Example: impulses in one neuron indicate light; impulses in another neuron indicate sound. • The brain does not duplicate what we see; sensory coding is determined by which neurons are active.
Visual Coding and the Retinal Receptors • Light enters the eye through an opening in the center of the eye called the pupil. • Light is focused by the lens and the cornea onto the rear surface of the eye known as the retina. • The retina is lined with visual receptors. • Light from the left side of the world strikes the right side of the retina and vice versa.
Visual Coding and the Retinal Receptors • Visual receptors send messages to neurons called bipolar cells, located closer to the center of the eye. • Bipolar cells send messages to ganglion cells that are even closer to the center of the eye. • The axons of ganglion cells join one another to form the optic nerve that travels to the brain.
Visual Coding and the Retinal Receptors • Amacrine cells are additional cells that receive information from bipolar cells and send it to other bipolar, ganglion or amacrine cells. • Amacrine cells control the ability of the ganglion cells to respond to shapes, movements, or other specific aspects of visual stimuli.
Visual Coding and the Retinal Receptors • The optic nerve consists of the axons of ganglion cells that band together and exit through the back of the eye and travel to the brain. • The point at which the optic nerve leaves the back of the eye is called the blind spot because it contains no receptors.
Visual Coding and the Retinal Receptors • The macula is the center of the human retina. • The central portion of the macula is the fovea and allows for acute and detailed vision. • Packed tight with receptors. • Nearly free of ganglion axons and blood vessels.
Visual Coding and the Retinal Receptors • Each receptor in the fovea attaches to a single bipolar cell and a single ganglion cell known as a midget ganglion cell. • Each cone in the fovea has a direct line to the brain which allows the registering of the exact location of input.
Visual Coding and the Retinal Receptors • In the periphery of the retina, a greater number of receptors converge into ganglion and bipolar cells. • Detailed vision is less in peripheral vision. • Allows for the greater perception of much fainter light in peripheral vision.
Visual Coding and the Retinal Receptors • The arrangement of visual receptors in the eye is highly adaptive. • Example: Predatory birds have a greater density of receptors on the top of the eye; rats have a greater density on the bottom of the eye.
Visual Coding and the Retinal Receptors • The vertebrate retina consist of two kind of receptors: • Rods - most abundant in the periphery of the eye and respond to faint light. (120 million per retina) • Cones - most abundant in and around the fovea. (6 million per retina) • Essential for color vision & more useful in bright light.
Visual Coding and the Retinal Receptors • Photopigments - chemicals contained by both rods and cones that release energy when struck by light. • Photopigments consist of 11-cis-retinal bound to proteins called opsins. • Light energy converts 11-cis-retinal quickly into all-trans-retinal. • Light is thus absorbed and energy is released in the process, controlling cell activities.
Visual Coding and the Retinal Receptors • The perception of color is dependent upon the wavelength of the light. • “Visible” wavelengths are dependent upon the species’ receptors. • The shortest wavelength humans can perceive is 400 nanometers (violet). • The longest wavelength that humans can perceive is 700 nanometers (red).
Visual Coding and the Retinal Receptors • Discrimination among colors depend upon the combination of responses by different neurons. • Two major interpretations of color vision include the following: • Trichromatic theory/Young-Helmholtz theory. • Opponent-process theory.
Visual Coding and the Retinal Receptors • Trichromatic theory - Color perception occurs through the relative rates of response by three kinds of cones. • Short wavelength, medium-wavelength, long-wavelength. • Each cone is maximally sensitive to a different set of wavelengths.
Visual Coding and the Retinal Receptors • Trichromatic theory (cont.) • The ratio of activity across the three types of cones determines the color. • More intense light increases the brightness of the color but does not change the ratio and thus does not change the perception of the color itself.
Visual Coding and the Retinal Receptors • The opponent-process theory suggests that we perceive color in terms of paired opposites. • The brain has a mechanism that perceives color on a continuum from red to green and another from yellow to blue. • A possible mechanism for the theory is that bipolar cells are excited by one set of wavelengths and inhibited by another.
Visual Coding and the Retinal Receptors • Both the opponent-process and trichromatic theory have limitations. • Color constancy, the ability to recognize color despite changes in lighting, is not easily explained by these theories. • Retinex theory suggests the cortex compares information from various parts of the retina to determine the brightness and color for each area. • Better explains color constancy.
Visual Coding and the Retinal Receptors • Color vision deficiency is an impairment in perceiving color differences. • Occurs for genetic reasons and the gene is contained on the X chromosome. • Caused by either the lack of a type of cone or a cone has abnormal properties. • Most common form is difficulty distinguishing between red and green. • Results from the long- and medium- wavelength cones having the same photopigment.
The Neural Basis of Visual Perception • Structure and organization of the visual system is the same across individuals and species. • Quantitative differences in the eye itself can be substantial. • Example: Some individuals have two or three times as many axons in the optic nerve, allowing for greater ability to detect faint or brief visual stimuli.
The Neural Basis of Visual Perception • Rods and cones of the retina make synaptic contact with horizontal cells and bipolar cells. • Horizontal cells are cells in the eye that make inhibitory contact onto bipolar cells. • Bipolar cells are cells in the eye that make synapses onto amacrine cells and ganglion cells. • The different cells are specialized for different visual functions.
The Neural Basis of Visual Perception • Ganglion cell axons form the optic nerve. • The optic chiasm is the place where the two optic nerves leaving the eye meet. • In humans, half of the axons from each eye cross to the other side of the brain. • Most ganglion cell axons go to the lateral geniculate nucleus, a smaller amount to the superior colliculus and fewer going to other areas.
The Neural Basis of Visual Perception • The lateral geniculate nucleus is a nucleus in the thalamus specialized for visual perception. • Destination for most ganglion cell axons. • Sends axons to other parts of the thalamus and to the visual areas of the occipital cortex.
The Neural Basis of Visual Perception • Lateral inhibition is the reduction of activity in one neuron by activity in neighboring neurons. • The response of cells in the visual system depends upon the net result of excitatory and inhibitory messages it receives. • Lateral inhibition is responsible for heightening contrast in vision and an example of this principle.
The Neural Basis of Visual Perception • The receptive field refers to the part of the visual field that either excites or inhibits a cell in the visual system. • For a receptor, the receptive field is the point in space from which light strikes it. • For other visual cells, receptive fields are derived from the visual field of cells that either excite or inhibit. • Example: ganglion cells converge to form the receptive field of the next level of cells.
The Neural Basis of Visual Perception • Ganglion cells of primates generally fall into three categories: • Parvocellular neurons • Magnocellular neurons • Koniocellular neurons
The Neural Basis of Visual Perception • Parvocellular neurons: • are mostly located in or near the fovea. • have smaller cell bodies and small receptive fields. • connect only to the lateral geniculate nucleus • are highly sensitive to detect color and visual detail.
The Neural Basis of Visual Perception • Magnocellular neurons: • are distributed evenly throughout the retina. • have larger cell bodies and visual fields. • mostly connect to the lateral geniculate nucleus but also connect to other visual areas of the thalamus. • are highly sensitive to large overall pattern and moving stimuli.
The Neural Basis of Visual Perception • Koniocellular neurons: • have small cell bodies. • are found throughout the retina. • connect to the lateral geniculate nucleus, other parts of the thalamus, and the superior colliculus.
The Neural Basis of Visual Perception • Cells of the lateral geniculate have a receptive field similar to those of ganglion cells: • An excitatory or inhibitory central portion and a surrounding ring of the opposite effect. • Large or small receptive fields.
The Neural Basis of Visual Perception • The primary visual cortex (area V1) receives information from the lateral geniculate nucleus and is the area responsible for the first stage of visual processing. • Some people with damage to V1 show blindsight, an ability to respond to visual stimuli that they report not seeing.
The Neural Basis of Visual Perception • The secondary visual cortex (area V2) receives information from area V1, processes information further, and sends it to other areas. • Information is transferred between area V1 and V2 in a reciprocal nature.
The Neural Basis of Visual Perception • Three visual pathways in the cerebral cortex include: • A mostly parvocellularneuron pathway sensitive to details of shape. • A mostly magnocellular neuron pathway with a ventral branch sensitive to movement and a dorsal branch responsible for integration of vision with action. • A mixed pathway sensitive to brightness, color and shape.
The Neural Basis of Visual Perception • The ventral stream refers to the most magnocellular visual paths in the temporal cortex. • Specialized for identifying and recognizing objects. • The dorsal stream refers to the visual path in the parietal cortex. • Helps the motor system to find objects and move towards them.
The Neural Basis of Visual Perception • Hubel and Weisel (1959, 1998) distinguished various types of cells in the visual cortex: • Simple cells. • Complex cells. • End-stopped/hypercomplex cells.