Chapter 7 Flashcards
what is the anatomy of the eye?
- light passes through cornea and bends light (refraction)
- light focused (accommodation) onto retina by lens
- layer of photoreceptors and neurons that make up retina transducer light to neural signals
what are the two types of photoreceptors?
- rods: respond to visible light of any wavelength - most active at low levels of light but without color vision
- cones: come in varieties, which respond differently to varying wavelengths for color vision
what is the structure of photoreceptors?
contain stack of discs -> increase probability that one of them will capture light particles that make it to retina
what is the biochemistry of photoreceptors?
photons captured by special photopigment receptor molecules -> trigger cascade of chemical reactions that hyper polarize cell -> cause cell to release less neurotransmitter onto bipolar cells
what are the other cells in the retina besides photoreceptors?
- bipolar cells: receive input from photoreceptors and synapse on ganglion cells
- ganglion cells: send information to brain and their axons form optic nerve
- horizontal cells in retina contact photoreceptors and bipolar cells
- amacrine cells contact bipolar and ganglion cells
which cells of the retina create graded vs. action potentials?
rods, cones, bipolar cells, and horizontal cells generate graded local potentials
- ganglion cells conduct action potentials
what is the transmission of light to action potential?
light strikes photoreceptor to release less neurotransmitter -> hyperpolarize membrane of some bipolar cells which causes reduction that causes depolarization -> cause bipolar cells to release more neurotransmitter to excite ganglion cell
what are the different types of bipolar cells?
- bipolar cell (on-center): more light is more neurotransmitter released to ganglion cells
- photoreceptors release neurotransmitters that inhibit on-center bipolar cells - bipolar cells (off-center): less light is more neurotransmitter released
- photoreceptors release neurotransmitter that excite off-center bipolar cells
what is the receptive field?
sensory cell consists of stimulus features that excite or inhibit cell
- light will excite or inhibit bipolar cells
- bipolar and ganglion cells and neurons within LGN have concentric receptive fields
- cortical neurons have receptive fields that respond to orientation, motion, and spatial frequency
how does on-center and off-center bipolar cells respond to glutamate?
on-center bipolar cells: turning on light excite the cell because it receives less glutamate (glutamate is inhibitory)
off-center bipolar cells: turning off light excite cells because they receive more glutamate (glutamate is excitatory)
what is a concentric receptive field?
photoreceptors in the central area and those in ring surrounding tend to have opposite effects
- always antagonistic (on-center/off-surround or off-center/on-surround)
what is lateral inhibition?
interconnected neurons inhibit their neighbors (producing contrast at edges of regions)
how does the eye adjust to seeing different light intensities?
- adjustment of size of pupil (opening in iris)
- range fractionation use different photoreceptors to handle different intensities
- photoreceptor adaption is where each one adjusts its level of sensitive to match average ambient level of light (graded-responses)
what controls the size of pupil?
- dilation allows more light to come in (controlled by sympathetic)
- constriction allows less light in (controlled by parasympathetic)
what is range fractionation?
sensory systems cover wide range of intensity values by this
- each sensory receptor specializes in just one of overall range
- rods have low light intensity thresholds while cones have higher light intensity thresholds
what is visual field?
whole era that you can see without moving your head or eyes
what is visual acuity?
measure of how much detail we see
- sharpest in center of visual field
- highest in fovea (center region of retina has higher density of smaller, tightly packed cones
- rods are located outside of fovea
what are the number of receptor cells to ganglion in the eye?
100 million rods and 4 million cones
- 1 million ganglion cells
what is special about foveal vision?
less data compression which leads to better visual acuity
- ganglion cells in fovea receive input from smaller number of photoreceptors which leads to more acuity
- also has unobstructed view
what is a blind spot?
region on retina that is sightless due to lack of photoreceptors in optic disk (where blood vessels and ganglion cells leave eye)
what are simple and complex cortical neurons?
- simple (bar detectors or edge detectors): respond to edge or bar of particular width, orientation, and location in visual field
- complex: respond best to bar of particular width and orientation that is in motion anywhere in visual field
what are the different sized receptive fields in neurons?
neurons with small receptive fields receive information from fovea to allow them to register high spatial frequencies
- neurons with large receptive fields receive from periphery to register low spatial frequencies
what is the spatial-frequency model?
visual system analyzes number of light-dark (or color) cycles in any stimulus
what is macular degeneration?
visual impairments caused by damage to retina especially fovea -> lack high spatial-frequency components
what is the pathway from the eye to the brain?
- ganglion cells conduct AP that make up optic nerve
- parts of two optic nerves cross midline at optic chiasma
- most axons of optic tract terminate on cells in lateral geniculate nucleus (LGN) of thalamus
- axons from LGN project to primary visual cortex
why do we have left and right visual fields?
result of some optic nerve crossing midline at optic chiasma
- axons from medial retina cross to opposite side of brain
- axons from lateral retina project to same side of brain
what is scotoma?
region of blindness within visual field caused by injury to visual pathway or brain
- if we know site of injury, we can predict location of perceptual gap (scotoma) in visual field
what is the primary visual cortex (V1) also called?
striate cortex (each striation corresponds to converging binocular input)
- outside of V1, those are called extrastriate cortex
what is V1 job?
primary visual cortex that receives information directly from LGN
what is V2 job?
responds to illusory boundaries and complex relations among parts of their receptive field
what is V4 job?
have strong response to frequency grating
- produce strong responses to concentric, radial, or color stimuli
what is V5 job?
respond to moving stimuli
- damage to V5 produces inability to perceive movement (akinetopsia)
what factors affect color perception?
- intensity of illumination
- prior exposure to different stimulus
- surrounding field
what are the different types of color blindness?
- protanopia: difficulty seeing red
- deuteranopia: difficulty seeing green
- tritanopia: difficulty seeing blue
what are the two early color hypotheses?
- trichromatic hypothesis: there’s three different types of cones excited by different region of spectrum but not good b/c cones do not correspond to colors
- opponent-process hypothesis: different systems produce opposite responses to light of different wavelengths but not good b/c perception of 3 opposite pairs of color occur in V4 neurons not ganglion cells
what are the different levels of color processing?
- different types of cone photodetectors
- spectrally opponent ganglion and LGN neurons
- V4 cortical neurons that contain color-sensitive neurons that match opponent-process hypothesis
- V8 cortical neurons that are key for color perception
what is level 1 for color processing?
cones named for peak area of wavelength sensitivity
- most objects stimulate at least two types of cones
- color recognized in brain by comparing responses from different cones
what is level 2 for color processing?
spectral opponent cell: visual system neuron that has opposite firing responses to different regions of spectrum (activated and inhibited by some wavelength)
- ganglion cells receive input from two or three different types of cones through bipolar cells -> activity of ganglion cell is difference in stimulation from different types of cones
why can ganglion and LGN spectrally opponent cell not be called color cells?
- also send outputs to higher circuits for detective of form, depth, and motion
- peak wavelength sensitivities don’t correspond to wavelength we see in principal colors
- other spectrally opponent ganglion cells stimulated or inhibited by M and L cones and detect brightness or darkness
what is level 3 for color processing?
V4 rich in color-sensitive cell -> respond best if color outside receptive field is different from color preferred inside receptive field
- demonstrate opponent-process hypothesis
what is level 4 for color processing?
damage to V8 leads to achromatopsia (condition characterized by partial or total absence of color vision)
- cannot perceive any color
what are the two types of visual streams?
- ventral stream is for identifying objects
- called what stream
- damage causes problems in perceiving faces and objects - dorsal stream for location of objects and guiding our movement toward them
- called where stream
- has condition of optic ataxia (difficulty using vision to reach for and grasp objects)
