Test 2 Flashcards
What is the retina and why is it important?
It’s the back surface of the eye that is lined with visual receptors and where incoming light hits. It detects photons of light and then fires impulses along optic nerve to the brain
In what way does image hit the retina and how do we see it right-side-up?
Light from the left side of the world hits the right side of the retina and vice versa. Light from below strikes the top half. Refraction through a convex lens flips images.
Cells in the retina - photoreceptors
Located in the back of the eye. Message bipolar cells.
Cells in the retina - bipolar cells
Located close to the center of the eye. Message ganglion cells.
Cells in the retina - amacrine cells
Helps ganglion cells respond to certain shapes, directions of movement or other visual features.
Cells in the retina - horizontal cells
If light hit one receptor but not the other ones, the horizontal cells inhibits the surrounding receptors for edge detection.
Cells in the retina - ganglion cells
Located even closer to the center of the eye.
Optic nerve
The bundled axons from ganglion cells that exit the retina.
Blind spot
Blind spot is where the optic exit and where blood vessels enter and leave. There are no receptors in this spot. We don’t notice it because 1) your brain fills in the gap, 2) anything in the blind spot in one eye is visible to the other eye.
Fovea
A tiny area specialized for acute, detailed vision.
Difference between foveal and peripheral vision
Foveal vision: useful in bright lights, but poor in dim lightning. Because there is one ganglion dedicated to each receptor, there is more acuity (better vision for details). Because many cones it’s best for color vision.
Peripheral vision: more rods than cones. One ganglion cell receives input from multiple receptors = lower acuity. Given higher number of rods = it’s good for dim lightning, not for bright light or color vision.
Difference between rods and cones
Rods: abundant in the periphery of retina. Responds to faint light but not useful in daylight because bright light bleaches them.
Cones: abundant in and near the fovea. Essential for color vision. Less active in dim light and more useful in bright light.
Trichromatic Theory (Young-Helmholtz Theory)
Hypothesized there to be 3 types of photoreceptors in the eye. Each type sensitive to a particular range of visual light. Found that people could match any color by mixing the appropriate amounts of just 3 wavelenghts. The 3 types of cone photoreceptors classified by what wavelenght they responded to. Short-preferring (blue), middle-preferring (green), and long-preferring (red). The relative strenghts of the signals detected by each type was interpreted by the brain as a visible color.
Color constancy
Our ability to recognize colors despite changes in lightning. Ex: if you wear green-tinted glasses, a banana will still be yellow for you.
Retinex Theory
Says both the eye and the brain are involved in color processing. The brain compares info from various parts of the retina to determine the brightness and color for each area. Our brain uses context to perceive color. A certain wavelenght of light can appear as different colors depending on the background or context. When we see something, we make an inference. Visual perception requires reasoning and inference, not just retinal stimulation.
What occurs in red-green color deficiency (protanopia)?
Long and medium wavelenght cones have the same photo-pigment instead of different ones. People with this can’t tell the difference between red and green. This gene is in the X chromosome so it affects more men than women.
Photopigments
Chemicals that release energy when struck by light.
What is 11-cis-retinal, and what does it convert to? What happens as a result of the conversion of this molecule?
It’s a derivative of vitamin A. Converts to all-trans-retinal. This conversion creates the electrical current that goes to our brain.
Opsins
Proteins that bind to 11-cis-retinal and modify its sensitivity to different wavelenghts of light.
When light strikes one photoreceptor, what happens to the bipolar cell to which it’s connected? What happens to surrounding bipolar cells? Mention horizontal cells.
When light hits one receptor, it will result in a exitation of its bipolar cell. It also excites a horizontal cell, which inhibits surrounding bipolar cells.
Receptive field
A point in visual space from which light strikes the cell and excites or inhibits it.
Describe the pathway from the retina to the brain. Discuss optic chiasm.
Ganglion cells’ axons form the optic nerves. The optic nerves from each eye meet at the optic chiasm. Half of the axons from each eye cross to the opposite side of the brain. From the optic chiasm, most of the ganglion cell axons go to the lateral geniculate nucleus.
Lateral geniculate nucleus (LGN)
Part of the thalamus. Receives major sensory input from retina and relays it to V1 (primary visual cortex).
Two layers of the LGN
Parvocellular: mostly in fovea. Small cell bodies and small receptive fields. Detect visual details and colors. “What” am I seeing?
Magnocellular: throughout retina. Larger cell bodies and receptive fields. Detect movement of large, overall patterns. “Where” am I seeing it?
Visual info from the right visual field is processed in the … occipital lobe, and visual info from the left visual field is processed in the … occipital lobe.
left.
right.
Blindsight
An ability to respond in limited ways to visual info without perceiving it consciously. Can occur among those with damage to V1. 2 possible explanations:
1) some healthy V1 tissue may remain but not enough fto provide conscious perception.
2) the thalamus sends visual input to other brain areas outside of the V1 to respond without conscious awareness of it.
Main point: V1 is crusial for conscious visual perception.
Cells in visual cortex - simple cells
Respond to exact location of a stimulus. Have receptive fields with fixed excitatory and inhibitory zones. The more light shines on excitatory zone, the more cell activity. The more light shines on inhibitory zone, the less activity.
Cells in visual cortex - complex cells
Larger receptive fields. Don’t respond to exact location of a stimulus. Will respond to a stimulus equally throughout a large area. Responds to patterns of light in a particular orientation anywhere within its large receptive fields. Responds most strongly to a moving stimulus.
Cells in visual cortex - hypercomplex (end-stopped) cells
Resemble complex cells, but have a strong inhibitory area at one end of its receptive field. Responds to a pattern of light anywhere in a broad field; provided the stimulus doesn’t extend beyond a certain point.
Ventral pathway
The “what” pathway. It’s going through the temporal cortex. Specialized for identifying and recognizing objects.
Visual agnosia
The inability to recognize objects despite otherwise satisfactory vision. Someone with this can point to the object and describe features, but can’t recognize what it is.
What type of object does the parahippocampal cortex most strongly respond to?
Places/landscapes
What type of object does the fusiform gyrus most strongly respond to?
Faces
What does the occipital face area respond most strongly to?
Facial features
Prosopagnosia
Impairment in ability to recognize faces. From damage or innate structural/functional differences. When people with this look at a face, they can describe a few features but can’t identify the person.
Dorsal pathway
The “how” pathway. It goes through the parietal cortex. Important for visually guided movements.
Damage to dorsal pathway
They can see objects but can’t integrate their vision with movements. Can read/recognize faces, describe objects in detail but can’t accurately reach out and grasp the object. May bump into objects. Can tell you what their furniture at home looks like, but can’t tell you how it’s spatially arranged.
Role of middle temporal cortex (MT) and medial superior temporal cortex (MST) play in motion perception
MT: detect acceleration and deceleration and speed in three dimentions. Responds to photographs that imply movement. Electrical stimulation of this area results in seeing movements that doesn’t exist.
MST: responds if something moves relative to background.
