Ch. 7 Flashcards
Chapter 7 of The Mind's Machine
Retina
The receptive surface inside the eye that contains photoreceptors and other neurons
Transduction
The conversion of one form of energy to another, such as from light to neuronal activity
Cornea
The transparent outer layer of the eye, whose curvature is fixed. The cornea bends light rays and is primarily responsible for forming the image on the retina
Refraction
The bending of light rays by a change in the density of a medium, such as the cornea and the lens of the eye
Lens
A structure in the eye that helps focus an image on the retina
Ciliary Muscles
One of the muscles that control the shape of the lens inside the eye, focusing an image on the retina
Accommodation
The process by which the ciliary muscles adjust the lens to bring nearby objects into focus
Farsightedness
The declining ability to bring nearby objects into focus due to lenses becoming less elastic with age
Myopia
Nearsightedness; the inability to focus the retinal image of objects that are far away. Develops if the eyeball is too long, causing the cornea and lens to focus images in front of the retina rather than on it.
Extraocular Muscles
Three pairs of muscles attached to the eyeball that control its position and movements
Photoreceptors
A neural cell in the retina that responds to light
Rods
A photoreceptor cell in the retina that is most active at low levels of light
Cones
Any several classes of photoreceptor cells in the retina that are responsible for color vision
Bipolar Cells
An interneuron in the retina that receives information from rods and cones and passes the information to retinal ganglion cells
Ganglion Cells
Any of a class of cells in the retina whose axons form the optic nerve
Optic Nerve
Cranial nerve II; the collection of ganglion cell axons that extends from the retina to the brain
Horizontal Cells
A specialized retinal cell that contacts both photoreceptors and bipolar cells
Amacrine Cells
A specialized retinal cell that contacts both bipolar cells and ganglion cells and is especially significant in inhibitory interactions within the retina
Do rods, cones, bipolar cells, and horizontal cells produce action potentials?
No, they only generate local potentials- these cells affect each other through the graded release of neurotransmitters in response to graded changes in electrical potentials
Scotopic System
A system in the retina that operates at low levels of light and involves the rods
Convergence
The phenomenon of neural connections in which many cells send signals to a single cell
How does the scotopic system use convergence?
Information from many rods converges onto each ganglion cell
Photopic System
A system in the retina that operates at high levels of light, shows sensitivity to color, and involves the cones. Has less convergence than the scotopic system
Rhodopsin
The photopigment in rods that responds to light
Photos
Light particles that strike the discs in rods and cones
What happens when light hits photopigment in the photoreceptor (rod)?
It triggers a cascade of chemical reactions that hyperpolarize the cell, causing the cell to release less neurotransmitter onto bipolar cells
For both rods and cones, the size of the hyperpolarizing photoreceptor potential determines what?
How much less transmitter will be released
Pupil
The opening, formed by the iris, that allows light to enter the eye
Iris
The circular structure of the eye that provides an opening to form the pupil
Dilation (opening) of the pupil is controlled by which sympathetic dividion?
autonomic system and that constriction is triggered b the parasympathetic division
Range Fractionation
The means by which sensory systems cover a wide range of intensity values, as each sensory receptor cell specializes in just one part of the overall range of intensities
Photoreceptor Adaptation
The tendency of rods and cones to adjust their light sensitivity to match current levels of illumination
Visual Field
The whole area that you can see without moving your head or eyes
Visual Acuity
Sharpness of vision; especially fine in the center of the visual field and falls off rapidly toward the periphery
Fovea
The central portion of the retina, which is packed with the highest density of cones and is the center of gaze
What is the difference in distribution of rods and cones?
Rods are absent in the fovea but more numerous than cones in the periphery of the retina
What is the sensitivity and acuity like with rods and cones?
Rods provide high sensitivity with limited acuity; cones provide high acuity with limited sensitivity.
Why is acuity greater in the fovea?
The focia is a region of the retina where light reaches the cones directly, without having to pass through other layers of cells and blood vessels
Optic Disc
The region of the retina that is devoid of photoreceptors because ganglion cell axons and blood vessels exit the eyeball there. Creates a blind spot
Blind Spot
The portion of the visual field from which light falls on the optic disc, where photoreceptors are not present
Occipital Cortex
Also called visual cortex. The cortex of the occipital lobe of the brain, corresponding to the visual area of the cortex
Optic Chiasm
The point at which parts of the two optic nerves cross the midline
How do the ganglion cells conduct information to the brain?
The ganglion cells in each eye produce action potentials that are conducted along their axons (make up the optic nerve) to send visual information to the brain-> reaches occipital cortex at the back of the brain
Optic Tract
The axons of retinal ganglion cells after they have passed the optic chiasm. Most of these axons terminate in the lateral geniculate nucleus
Lateral Geniculate Nucleus (LGN)
The part of the thalamus that receives information from the optic tract and sends it to visual areas in the occipital cortex
Optic Radiations
Axons from the lateral geniculate nucleus that terminate in the primary visual areas of the occipital cortex
Primary Visual Cortex (V1)
The region of the occipital cortex where most visual information first arrives
Binocular
Referring to two-eyed processes
What is the steps of the visual pathway?
- The retinal image is inverted and reversed right to left compared with the visual field
- The axons of retinal ganglion cells form the optic nerves
- At the optic chiasm, axons from the temporal halves of each retina continue into the optic tract on the same side. Axons from the nasal halves cross the optic tracts on the opposite side.
4a. Some axons in the optic tract terminate in the superior colliculus
4b. Most axons in the optic tract terminate in the lateral geniculate nucleus (LGN) of the thalamus - Axons project back and forth between the LGN and the striate cortex via that optic radiations
6a. Most of the primary visual cortex is on the medial surface of the human brain
6b. An especially large proportion of primary visual cortex represents the foveal region - The left primary visual cortex gets input from both eyes, but only from the right visual field
Extrastriate Cortex
Visual cortex outside the primary visual (striate cortex).
Topographic Projection
A mapping that preserves the point-to-point correspondence between neighboring parts of space. For example, a topographic projection extends from the retina to the cortes. Much of this topographic projection of visual space is devoted to the foveal region
Scotoma
A region of blindness within the visual fields, caused by injury to the visual pathway or brain
Blindsight
The paradoxical phenomenon whereby, within a scotoma, a person cannot consciously perceive visual cues but may still be able to make some visual discrimination
Receptive Field
The stimulus region and features that affect the activity of a cell in a sensory system
What is the response of photoreceptors to light?
Both rod and cone photoreceptors steadily release the synaptic neurotransmitter glutamate. Light always hyperpolarizes the photoreceptors, causing them to release less glutamate.
On-Center Bipolar Cells
A retinal bipolar cell that is excited by light in the center of its receptive field. Glutamate is inhibitory to this type of cell, so light on the on-center bipolar cell’s receptive field (release less glutamate) would excite this bipolar cell
Off-Center Bipolar Cells
a retinal bipolar cell that is inhibited by light in the center of its receptive field. Glutamate is excitatory to off-center bipolar cells, so shining light on this cell’s receptive field (photoreceptor releases glutamate) would inhibit this bipolar cell
On-center Ganglion Cells
A retinal ganglion cell that is activated when light is presented to the center, rather than the periphery, of the cell’s receptive field
Off-center Ganglion Cells
A retinal ganglion cell that is activated when light is presented to the periphery, rather than the center, of the cell’s receptive field
The entire receptive field of a bipolar cell is what?
Concentric (consisting of a roughly circular central area and the ringlike area surrounding it
Both bipolar cells and ganglion cells have two basic types of retinal receptive fields:
on-center/off-surround and off-center/on-surround
On-center/Off-surround
Referring to a concentric receptive field in which stimulation of the center excites the cell of interest while stimulation of the surround inhibits it
Off-center/On-surround
Referring to a concentric receptive field in which stimulation of the center inhibits the cell of interest while stimulation of the surround excites it
Lateral Inhibition
The phenomenon by which interconnected neurons inhibit their neighbors, producing contrast at the edges of regions
Simple Cortical Cells
A cell in the visual cortex that responds best to an edge or a bar that has a particular width, as well as a particular orientation and location in the visual field.
Complex Cortical Cells
A cell in the visual cortex that responds best to a bar of a particular size and orientation anywhere within a particular area of the visual field that needs movement to make it respond actively
Spatial-Frequency Model
A model of visual perception that emphasizes the analysis of the different spatial frequencies present in various orientations and in various parts of a visual scene
Wavelenght
The length between two peaks in a repeated stimulus such as a wave, light, or sound
The human visual system responds only to photons whose wavelengths lie between what?
400 to 700 nanometers
Brightness
varies from dark to light
Hue
Varies continuously through blue, green, yellow, orange, and red
Trichromatic Hypothesis
A hypothesis of color perception stating that there are three different types of cones, each excited by a different region of the spectrum and each having a separate pathway to the brain
Saturation
Varies from rich, full colors to gray
Opponent-Process Hypothesis
A hypothesis of color perception stating that different systems produce opposite responses to light of different wavelengths
Spectrally Opponent Cell
Also called color-opponent cell. A visual system neuron that has opposite firing responses to different regions of the spectrum
What is the ventral processing stream responsible for?
identifying objects (what system)
What is the dorsal stream responsible for?
appreciating the spatial location of objects and for visually guiding out interactions with them (where system)
Optic Ataxia
Spatial disorientation in which the person is unable to accurately reach for objects using visual guidance
What seems to be a cause of myopia?
indoor lighting/not getting enough sunlight
Amblyopia
Reduced visual acuity of one eye that is not caused by optical or retinal impairments. Could be because the eyes are not aligned properly during the first few years of life.
Macular Degeneration
Degeneration of the retinal fovea, impairing vision in the center of the visual field
