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VISION PATHWAY
● Vision ○ Take up a third of our brain ○ Visual information 10^8 ~10^9 bits per second ○ Milner & Goodale ■ Carbon monoxide passing out ➜appreceptive agnosia (can see but can’t really see) ● Demonstrate the existence of two parallel neural pathways ○ how/where ○ What ➜the one that doesn’t work ● Perception ✖action ✔ ● Gap between past memory and what she perceives in real life ➜ disconnection between memory and perception
○ Two processing streams originate in primary visual cortex ■ Ventral stream ● Identifying objects (what) ● Damages cause problems in perceiving faces and objects ■ Dorsal stream ● Assessing the location of objects (where) ● Guiding our movement toward them ● Optic ataxia : difficulty using vision to reach and grasp objects ● Motion blindness ○ Disorder in which a patient cannot perceive motion in their visual field, despite being able to see stationary objects without issue
ANATOMY OF THE EYE
● Retina ○ Turns the light into neural signals ■ Through transduction (transformation of physical energy into neural signals) ■ LIGHT ➜ retinal ganglion cells ➜ bipolar cells ➜receptor cells ● Lateral communication ○ Horizontal cells ○ Amacrine cells ○ Accurate optical image focused on retina ➜good vision ○ Fovea ■ 0.1% of retina, taken up 8~10% of cortex ■ Densely packed cones in the center of visual field ● Acuity is the highest ● Cones have the smallest receptive field (elsewhere is larger) ■ Light reaches the cones directly ■ Rods are absent ■ Optic disc ● The nasal side of fovea ● Where blood vessels and ganglion cell leave the eye ● No photoreceptor ○ Blind spot
○ Nasal hemiretina ■ The half of the retina toward the nose ■ Cross over to the opposite side of the brain
● presbyopia ○ The eyesight lost of aging aka 老花眼 ● Extraocular muscles ○ Control movement of the eyes ○ Three pairs of muscles that extend from the outside of the eyeball to the eye socket of the skull ● Visual processing ○ Photoreceptors ■ The dark current : light hyperpolarizes photoreceptor ● Release glutamate ■ Visible light: 400~700 nm wavelength light ● Wavelength (frequency): perception of color ● Intensity (amplitude): perception of brightness ■ Wavelength of light ● Faster-vibrating photons (shorter-wavelength) ○ Blue ○ Green ● Slower-vibrating photons (longer wavelength) ○ Orange ○ Red ■ Capture photons of light and convert it to neural activity through phototransduction ● Triggers a cascade of chemical reactions that hyperpolarize the cell when light hits photopigment in the photoreceptor ○ Release less neurotransmitter onto bipolar cells ■ Sensory neurons that detect light ● Rods (100 million) ○ Scotopic (nighttime) vision ○ Only one type of rod ■ Lack detail and color information ○ Respond to light of almost any wavelength ○ Provide high sensitivity with limited acuity ○ Peripheral vision (peripheral part of our retina/visual field) ○ Scotopic system ■ Rod-based system ■ Very sensitive ➜work well in low light ■ Defect objects in dim light insensitive to color ■ A lot of convergence ● Information from many rods converges onto each ganglion cell ● Diminish acuity ○ Rhodopsin ■ Photopigement receptor ● Cones (4 millions)
○ Respond to light of varying wavelengths ➜color vision ○ Photopic (daytime) vision ○ Under ordinary conditions, almost any subjects, stimulates at least two kinds of cones ○ Three different types of cones ■ L ong-wavelength (red) ● Best: 560 nm ● Show response to light of almost any wavelength ■ M edium-wavelength (green) ● Best: 530 nm ● Show response to light of almost any wavelength ■ S hort-wavelength (blue) ● 420 nm ■ Receptor pigments exhibit broad sensitivities that substantially overlap ■ Genes for M and L cone photopigments occupy adjacent positions on the X chromosomes and are much more similar to each other than S pigment ➜ suggest they derive from a common ancestral gene ○ Provide high acuity with limited sensitivity ○ Photopic system ■ Uses cones ■ Enabling color vision ■ Less convergence ● Some ganglion cells reporting information from only one single cone
● Range fractionation ○ The handling of different intensities by different receptors ○ Rods (low threshold) & cones (high threshold) ○ Photoreceptor adaptation ■ Photoreceptor constantly adjust its sensitivity to match the average level of ambient illumination ■ Bright sunlight to dim room ● Photopigments on rods need time to regenerate ■ Dim room to bright sunlight
● ALL photopigments briefly broken down (SO BRIGHT), cones recover quickly ● Visual field ○ The whole area we can see without moving our head or eyes ● Visual acuity ○ Sharpness of vision ○ Especially fine in the center of the visual field ■ Because of fovea ● Occipital cortex ○ Back in the brain ○ Optic nerve (bundle of retinal ganglion cells axons) ■ brings visual information into the brain ■ made up by ganglion cells’ axons ■ Many of its axons cross the opposite cerebral hemisphere ● Optic chiasm ○ Optic nerves that cross the midline ○ More in prey animals ■ Wider visual field (send most of their axon across the midline) ■ Cost: poor depth perception (small binocular field) ○ Optic tract ■ After the axons of the retinal ganglion cells pass the optic nerve ■ Minority of retinal ganglion cells send optic tract axons to the superior colliculus in the midbrain ■ Most axons terminate on cells in the lateral geniculate nucleus (LGN) (portion of thalamus) ● Axons of LGN form the optic radiation ➜terminate in the primary visual cortex (V 1 ) ( striate cortex , corresponding to binocular input) ➜important for depth perception ○ Extrastriate cortex ■ Working in different aspects of visual perception ● Form ● Color ● Location
■ Light turned on: depolarize/Excite on-center ganglion cells ○ Off-center bipolar cells (on surround) ■ Respond when light is off ■ Depolarize by darkness ■ Glutamate ➜excitatory ■ Light on its receptive field ➜inhibit this bipolar cells ■ Light turned off: depolarize/ excite off-center ganglion cells
● Contrast Enhancement ○ Amplification of lightness differences between 2 surfaces ○ Eye tracking edges
● Lateral inhibition ○ Sensory receptor cells inhibit the reporting of information from neighboring receptor cells ○ Bipolar cell receives direct excitatory input from the photoreceptors in the center and indirect inhibitory input from the photoreceptors in the surround ■ On center bipolar cell ● Cortical cells ○ Hubel and Wiesel
○ Simple cortical cells ■ Respond best to an edge or a bar that has a particular width and a particular orientation and location in the visual field ■ Have elongated receptive fields with excitatory and inhibitory subregions ■ Center-surround receptive fields can be configured at every orientation and orientation selective simple cells will respond maximally to its preferred orientation ■ Bar detectors ■ Edge detectors ■ Stationary edges in receptive fields
○ Complex cortical cells ■ Acquire movement of the stimulus ■ Detect the movement of an edge through their receptive field ■ Spatial and temporal summation
○ A restricted region of neurons that are activated by faces ○ A pathway mediating visual recognition extends from V1 through temporal cortex to the prefrontal cortex ● Area V ○ Neurons respond to moving visual stimuli ➜specialized for the perception of motion and its direction
● Three dimensions of color perception ○ Brightness ■ Varies from dark to light ○ Hue (color) ■ Varies continuously through blue, green, yellow, orange, and red ○ Saturation ■ Varies from rich, full colors to gray ■ Ex: rich red through pink to gray ➜ saturation decreases ● Trichromatic hypothesi s of color perception ○ H. von Helmholtz ○ Three different types of cones ■ Responds to a specific, different part of the spectrum ■ Has a separate pathway to the brain ○ Color recognition based on which receptors are activated
○ The response of cone depends on which wavelength of light its pigment absorbs ➜ pigments don’t have narrow spectral distribution as Helmholtz predicted
● Color mixtures ○ RGB lights yield achromatic white ■ The more light we mix, the more light reaches your eye ➜ the brighter the color ○ RGB paints yield achromatic black ■ The more paints you mix, the less light reaches your eye ➜ the darker the color ● Opponent-process hypothesis of color perception ○ E. Hering ○ Four unique hues ■ Blue ■ Green ■ Yellow ■ Red ○ Three opposed pairs of colors ■ Blue v.s. Yellow ■ Green v.s. Red ■ Black v.s. White ○ Three physiological process with opposed positive and negative value are the basis of color vision ○ Most ganglion cells and LGN cells are excited & fire in response to some wavelengths and inhibited by other wavelengths
● Dichromacy ○ Color blindness ○ Red-green dichromacies ■ Sex-linked ■ Most common types ■ Mostly male affected ■ No red-green color discrimination (confuses colors that rely on red-green discrimination) ■ Deuteranope ● Missing M cone ■ protanope ● Missing L cone ● Enchroma glasses ○ Developed to block a certain wavelength of laser ○ Do not provide true color vision for those with color deficits ➜ provide greater ability to discriminate color differences for those with red-green color blindness ● Gene therapy for monkey with color deficiencies ○ Male squirrel monkey are born with red-green color blind ○ Use subretinal injection to deliver the human L-opsin gene to the retinal photoreceptor layer of 2 monkeys missing the L-opsin gene ● Visual disorders ○ Motion blindness ■ Disorder in which a patient cannot perceive motion in their visual field, despite being able to see stationary objects without issue ○ Amblyopia ■ Acuity is poor in one eye even though the eye and retina are normal ■ If the two eye are not aligned properly during the first few years of life, the primary visual cortex of the child tend to suppress the information arriving from one eye and that eye becomes functionally blind ○ Muscular degeneration ■ Visual impairment caused by damage to the retina ■ “Dry” muscular degeneration ● Caused by atrophy of the retinal pigmented epithelium ● Resulting in death of overlying photoreceptor ■ “Wet” muscular degeneration ● Abnormal growth of retinal capillaries ➜detachment of the retina or death of photoreceptors ■ Mostly restricted to fovea ■ Supplemental vitamins and antioxidants could slow the disease slightly
AUDITION
● QUESTIONS
○ WHAT IS IT
○ WHERE IS IT
● Transduction ○ Through hair cell ● Outer ear ○ Collecting pressure waves ○ Directs sounds into the inner parts of the ear ○ Pinnae ■ Funnel sound waves into the second part of the external ear ➜ ear canal (auditory canal) ● amplify higher pitch sound ■ A distinctly mammalian characteristic ● Middle ear ○ Collecting mechanical waves ○ Consists of the taut tympanic membrane (eardrum) ■ Sound waves strike ➜ tympanic membrane starts to vibrate with the same frequency as the sound ➜ ossicles start moving ■ Sealing the end of the ear canal plus a chain of tiny bones ➜ ossicles ● Smallest bones in the body ● Malleus ● Incus ● Stapes ● Mechanically couple the tympanic membrane to the inner ear at a specialized patch of membrane ( oval window ) ● Concentrate and amplify the vibrations ➜ focusing the pressures collected from the relatively large tympanic membrane onto the small oval window ○ Crucial for converting vibrations in air into movements of fluid in the inner ear ○ Equipped with the equivalent of a volume control ■ Helps protect against the damaging forces of extremely loud noises ■ Two tiny muscles attach to the end of the chain of ossicles ● Tensor tympani ● Stapedius ● Brain signals the muscles to contract ➜ stiffens the chain of ossicles & reduce the effectiveness of sounds ● Inner ear ○ Collecting fluid waves ○ Semi circular canals ■ Part of vestibular canals ■ Concerned with balance ○ Cochlea
○ Auditory sensory cells ○ bridge s between the basilar membrane & the overlying tectorial membrane ○ Feature stereocilia on its upper surface ■ Form a mechanical bridge, spanning between the two membranes ■ Mohawk Bending ➜ produce a large and rapid depolarization of the hair cells ■ Operation of a special type of large and nonselective ion channel ➜ depolarization inrush of K+^ and Ca2+^ ➜ rapid influx of calcium at the base of the hair cell ➜ synaptic vesicles fuse & release neurotransmitters ➜ stimulating adjacent nerve fibers ■ Allow hair cells to accurately track the rapid oscillations of the basilar membrane with exquisite sensitivity ○ Inner hair cells (IHCs) ■ 3500 ■ Responsible for sound transduction ■ IHC afferents : convey to the brain the perception of sounds ➜ 95% of the fibers leading to the brain ■ IHC efferents : lead from the brain to the IHCs ➜ allow th brian to control the responsiveness of IHCs ○ Outer hair cells (OHCs) ■ 12000 ■ function as a cochlear amplifier, which increases the sensitivity and frequency
selectivity of the ear's response to sound ■ Help with the mobility of basilar membrane ■ OHC afferents : convey information to the brain about the mechanical state of the basilar membrane ■ OHC efferents : enable the brain to activate the properties of OHCs ➜ the ability to change their length almost instantaneously ○ Fibers of the vestibulocochlear nerve ■ Contact the bases of the hair cells ● An elaborate framework of supporting cells ● The auditory nerve terminals ○ Transmit neural signals to and from the brain
- Auditory nerve fibers terminate in the cochlear nuclei
- Output from the cochlear nuclei projects to superior olivary nuclei (receives from both right & left cochlear nuclei)
- Superior olivary nuclei pass information from both ears t the inferior colliculi (primary auditory centers of the midbrain)
- Output from the inferior colliculi go to the medial geniculate nuclei of the thalamus
- Output from the medial geniculate nuclei extend to auditory cortical areas ■ Basilar membrane ● Tapered ➜ much wider at the apex of the cochlea than at the base ○ Each successive location along the membrane shows its strongest response to a different frequency of sounds
