Chapter 6 Vision

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 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. Which neurons respond, the amount of response, and the timing of response influence what we perceive.

Visual Coding and the Retinal Receptors Light enters the eye through an opening in the center of the iris 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 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: 1.Rods - most abundant in the periphery of the eye and respond to faint light. (120 million per retina) 2.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 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: 1.Trichromatic theory/Young-Helmholtz theory. 2.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. Incomplete theory of color vision. –Example: negative color afterimage

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 Color vision deficiency is an impairment in perceiving color differences. Gene responsible 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 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 part of 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. –Cortex and thalamus feed information back and forth to each other.

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 the retina’s way responsible of sharpening contrasts to emphasize the borders of objects.

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 of the brain. 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 Pattern recognition in the cerebral cortex occurs in a few places 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 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 Visual agnosia is the inability to recognize objects despite satisfactory vision. –Caused by damage to the pattern pathway usually in the temporal cortex. Prosopagnosia is the inability to recognize faces. –Occurs after damage to the fusiform gyrus of the inferior temporal cortex.

The Neural Basis of Visual Perception Several mechanisms prevent confusion or blurring of images during eye movements. 1.Saccades are a decrease in the activity of the visual cortex during quick eye movements. 2.Neural activity and blood flow decrease shortly before and during eye movements.

Development of Vision Vision in newborns is functional but poorly developed at birth: –Face recognition occurs relatively soon after birth (2 days) –Show strong preference for a right-side-up face and support idea of a built-in face recognition system

Development of Vision Animal studies have greatly contributed to the understanding of the development of vision. Early lack of stimulation of one eye leads to synapses in the visual cortex becoming gradually unresponsive to input from that eye. Early lack of stimulation of both eyes, cortical responses become sluggish but do not cause blindness.

Development of Vision Sensitive/critical periods are periods of time during the lifespan when experiences have a particularly strong and enduring effect. Critical period begins when GABA becomes widely available in the brain. Critical period ends with the onset of chemicals that inhibit axonal sprouting. Changes that occur during critical period require both excitation and inhibition of some neurons.

Development of Vision Stereoscopic depth perception is a method of perceiving distance in which the brain compares slightly different inputs from the two eyes. Relies on retinal disparity or the discrepancy between what the left and the right eye sees. The ability of cortical neurons to adjust their connections to detect retinal disparity is shaped through experience.

Development of Vision Strabismus is a condition in which the eyes do not point in the same direction. –Usually develops in childhood. Also known as “lazy eye”. If two eyes carry unrelated messages, cortical cell strengthens connections with only one eye. Development of stereoscopic depth perception is impaired.

Development of Vision Early exposure to a limited array of patterns leads to nearly all of the visual cortex cells becoming responsive to only that pattern. Astigmatism refers to a blurring of vision for lines in one direction caused by an asymmetric curvature of the eyes. –70 % of infants

Copyright © 2009 Allyn & Bacon Cortical Mechanisms of Vision and Conscious Awareness Flow of visual information: –Thalamic relay neurons, to –1˚ visual cortex (striate), to –2˚ visual cortex (prestriate), to –Visual association cortex As visual information flows through hierarchy, receptive fields –become larger –respond to more complex and specific stimuli Visual areas of the human cerebral cortex

Copyright © 2009 Allyn & Bacon Damage to Primary Visual Cortex Scotomas –Areas of blindness in contralateral visual field due to damage to primary visual cortex –Detected by perimetry test Completion –Patients may be unaware of scotoma – missing details supplied by “completion”

Copyright © 2009 Allyn & Bacon Damage to Primary Visual Cortex (continued) Blindsight –Response to visual stimuli without conscious awareness of “seeing” –Possible explanations of blindsight Islands of functional cells within scotoma Direct connections between subcortical structures and secondary visual cortex, not available to conscious awareness

Copyright © 2009 Allyn & Bacon Functional Areas of Second and Association Visual Cortex Neurons in each area respond to different visual cues, such as color, movement, or shape Lesions of each area results in specific deficits Anatomically distinct Retinotopically organized

Copyright © 2009 Allyn & Bacon Dorsal and Ventral Streams Dorsal stream: pathway from primary visual cortex to dorsal prestriate cortex to posterior parietal cortex –The “where” pathway (location and movement), or –Pathway for control of behavior (e.g. reaching) Ventral stream: pathway from primary visual cortex to ventral prestriate cortex to inferotemporal cortex –The “what” pathway (color and shape), or –Pathway for conscious perception of objects

Copyright © 2009 Allyn & Bacon Prosopagnosia Inability to distinguish among faces Most prosopagnosic’s recognition deficits are not limited to faces Often associated with damage to the ventral stream Prosopagnosics have different skin conductance responses to familiar faces compared to unfamiliar faces, even though they reported not recognizing any of the faces

Copyright © 2009 Allyn & Bacon Retinal Diseases Macular Degeneration-destruction of photoreceptors –Wet (blood vessels) and Dry (drusen)

Copyright © 2009 Allyn & Bacon Retinitis Pigmentosa Progressive degeneration of photoreceptors

Copyright © 2009 Allyn & Bacon Prostethic Retina Bioelectronic implant Images collected by camera hidden in glasses  data sent to the unharmed retinal cells then onto optic nerve 60 pixels (distinguish btwn light and dark) Artifical Retina Project Video