PSYC 370 VISION Dr. M B Hocaoglu Spring 201 4

1. PSYC 370 VISIONDr. M B HocaogluSpring 2014

2. 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.

3. 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. • The brain does not duplicate what we see. • Which neurons respond, the amount of response, and the timing of response influence what we perceive.

4. 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. Retina is lined with visual receptors. • Light from the left side of the world strikes the right side of the retina and vice versa.

7. Visual Coding and the Retinal Receptors Visual receptors Bipolar Cells Ganglion Cells Optic Nerve • 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.

8. 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.

10. 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.

12. 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. Macula is: • Packed tight with receptors. • Nearly free of ganglion axons and blood vessels.

14. 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.

15. 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.

16. 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. Why? • Rats have a greater density on the bottom of the eye. Why?

17. 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) 2. Cones - most abundant in and around the fovea. (6 million per retina) • Essential for color vision & more useful in bright light.

21. 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.

22. 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.

23. 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.

25. 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.

26. 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.

28. 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.

30. The Neural Basis of Visual Perception • Shape constancy is the ability to recognize an object’s shape despite changes in direction or size. • The inferior temporal neuron’s ability to ignore changes in size and direction contributes to our capacity for shape constancy. • Damage to the pattern pathways of the cortex can lead to deficits in object recognition.

31. 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.

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

33. The Neural Basis of Visual Perception • Motion blindness refers to the inability to determine the direction, speed and whether objects are moving. • Likely caused by damage in area MT. • Some people are blind except for the ability to detect which direction something is moving. • Area MT probably gets some visual input despite significant damage to area V1.

34. 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

35. 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.

37. 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.

38. 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

39. Development of Vision • Study of people born with cataracts but removed at age 2-6 months indicate that vision can be restored after early deprivation. • Subtle but lingering problems persist: • People with left eye cataracts show mild face recognition problems. • Early in life, each hemisphere of the brain gets input almost entirely from the contralateral eye; the fusiform gyrus is located in the right hemisphere.

40. Development of Vision • Research and case studies indicate that the visual cortex is plastic but much more so early in life. • Example: Early removal of cataracts leads to better improvement of various aspects of vision.