Module 9 How Do We Sense, Perceive, and See the World? Flashcards
Migraines
- They were caused by the dilation of cerebral blood vessels that occurs during an aura
- Usually vary in severity, frequency, and duration (left untreated, some may last for hours or even days) and are often accompanied by nausea and vomiting
- Most common of all neurological disorders, affecting some 5 to 20% of the pi=opulation at some time in their lives
Auras
-May be auditory, tactile, or visual, and they may result in an inability to move or talk
Scotoma
-A small blind spot
Blindsight
- When a light blinked and where it appeared
- Which the brain knew more that they are aware of consciously
Selective Awareness
-An important working principle behind human sensation and perception
Sensory Receptors Neurons
- Are specialized to transduce (convert) environmental energy-light
- Are designed to respond only to a narrow band of energy analogous to particles of certain sizes-such as specific wavelengths of electromagnetic energy that form the basis of our vision
Each sensory system’s receptors are specialized to filter a different form of energy
-For vision
~The photoreceptors in the retina convert light energy into chemical energy, which is, in turn, converted into action potentials
-The auditory system
~Air pressure waves are first converted into mechanical energy, which activates the auditory receptors that produce action potentials in auditory receptor neurons
-The somatosensory system
~mechanical energy activates receptors sensitive to touch, pressure, or pain; these receptors, in turn, generate action potentials in somatosensory neurons
-Taste and olfaction
~Various chemical molecules in the air or in food fit themselves into receptors of various shapes to activate action potentials in the respective receptor neurons
Receptive Field
- Region of sensory space (example, skin surface) in which a stimulus modifies a receptor’s activity
- Not only to identify sensory information but also to contrast the information each receptor field is providing
- Not only sample sensory information but also help locate events in space, because adjacent receptive fields may overlap, the contrast between their responses to event help us localize sensations
- The spatial dimension of sensory information produces cortical patterns and maps that form each person’s sensory reality
Photoreceptor Cells
- About 120 million
- The eye points in a slightly different direction and so has a unique receptor field
Visual Receptors
-Are more numerous in the center of our visual field than toward the edges
Density of Receptors
- Is related to sensory sensitivity
- Our sensory systems used different types of receptors to enhance our perceptual experience
Color Photoreceptors
- Are small and densely packed to make sensitive color discriminations in bright light
- receptors for black-white vision are larger and more scattered, but their sensitivity to light-say, a lighted match as a distance of 2 miles on a dark night-is truly remarkable
Receptors Connect
- To the cortex through a sequence of intervening neurons
- The number of these neural relays varies across different sensory systems
Sensory Information
-Is modified at each stage in the relay, allowing each region to construct different aspects of the sensory experience
Visual system
-Each of our eyes has a separate view of the world; the information from the two views is combined in the thalamus such that the input from the left side and right side of each field is superimposed to produce two visual fields one from the left and one from the right
-The brain begins to separate different aspects if the visual input such as shape and color
~Also a second visual pathway that goes from the retina to the superior colliculus and then to the thalamus and cortex
*This pathway is involved in the perception of movement
Interaction Effect is Potent
-It highlights the fact that a speaker’s facial gestures influence our perception of speech sounds
-Synchrony of gestures and sounds is an important aspect of language acquisition
~The difficulty of learning a foreign language can relate to the difficulty of blending a speaker’s articulation movements with the sounds the speaker produce
All information from all sensory systems is encoded
- Action potentials that travel along nerves until they enter the spinal cord or brain
- From there action potentials that travel on nerve tracts within the CNS; every bundle carries the same kind of signal
Presence of Stimulus
-Can be encoded by an increase or decrease in a neuron’s discharge rate, and the amount of increase or decrease can encode stimulus intensity
Qualitave visual chagnes
-Such as from red to green, can be encoded by activity in different neurons or even by different levels of discharge in the same neuron
Synesthesia
- This mixing of the senses
- Some people hear in color or identify smells by how the smells sound to them
- Anyone who shivers when hearing a piece of music or cringed at the noise fingernails make when scraping a blackboard has “felt” sound
Topographic Map
-Spatially organized neural representation of the external world
-A neural-spatial representation of the body or of the areas of the sensory world perceived by a sensory organ
~All mammals have at least one primary cortical area for each sensory system areas are usually referred to as secondary because most of the information that reaches these areas is relayed through the primary area
-Each additional representation is probably dedicated to encoding one specific aspect of the sensory modality
Sensation
- Registration by the sensory organs pf physical stimuli from the environment
- Is far more than the simple registration of physical stimuli from the environment by the sensory organs
- Our sensory impressions are affected by the cortex in which they take place, by our emotional state
Perception
- Subjective interpretation of sensations by the brain
- How we interpret what we sense
- Is more than sensation lies in the fact that different people transform the same sensory stimulation into totally different perceptions
Retina
- Light-sensitive surface at the back of the eye consisting of neurons and photoreceptors
- Light energy travels from the outside world through the pupil and into the eye, where it strikes a light-sensitive surface
- Unevenly distributed between cones and rods
Photoreceptors
-Specialized retinal neuron that transduces light into neural activity
-Stimulation of cells on the retina, we begin to construct a visual world
-The neurons lie in front of the photoreceptors beneath a layer of neurons connected to them; the neurons lie in front of the photoreceptors because they do not prevent incoming light from being absorbed by those receptors because the neurons are transparent and the photoreceptors are extremely sensitive to light
-The photoreceptors and the retinal neurons perform some amazing functions
~translate light into action potentials, discriminate wavelengths so that we can distinguish colors, and work in a range of light intensities from bright to dim; these cells afford visual precision sufficient for us to see a human hair lying on the page of this book from a distance of 18 inches
As in a camera
-The image of objects project onto the retina is upside down and backward
-This flip-flopped orientation poses no problem for the brain
~Remember that the brain is constructing the outside world, so it does not really care how the image is oriented initially
-The brain can make adjustments regardless of the orientation of the images that it receives
For several days
- You were to wear your glasses that invert visual images, the world would first appear upside down but then would suddenly appear right side up again because your brain would correct the distortion
- Upon removing the glasses, the world would temporarily seem upside down once again because your brain at first would be unaware that you had tricked it again; eventually, your brain would solve this puzzle, and the world would flip back to the correct orientation
Periphery
-Letters at the periphery must be much larger than those in the center for us to see them well
Fovea
-Central region of the retina is specialized for high visual acuity; its receptive fields are at the center of the eye’s visual field
-The difference is partly due to the fact that photoreceptors are more densely packed at the center of the retina
-This depression is formed because many optic nerve fibers skirt the fovea to facilitate light access to its receptors
-Only has cones, but their density drops dramatically outside this area
~For this reason , our vision is not so sharp at the edges of the visual field
Blind Spot
- The retinal region where axons forming the optic nerve leave the eye and where blood vessels enter and leave; has no photoreceptors and is thus said to be blind
- A small area of the retina known as the optic disc
- This is the area where blood vessels enter and exit the eye and where fibers leading from retinal neurons from the optic nerve, which goes to the brain
- Therefore are no photoreceptors in this part of the retina
Visual System solves the blind spot problem
-Your optic disc is in a different location in each eye; the optic disc is lateral to the fovea in the left eye and to the right of the fovea in the right eye; because the two eyes’ visual fields overlap, the right eye can see the left eye’s blind spot and vice versa
Blind Spot is important in neurology
- Is allows neurologists to indirectly view the condition of the optic nerve while providing a window on events in the brain
- If intracranial pressure increases, as occurs with a tumor or brain abscess (an infection), the optic disc swells, leading to papilledema (swollen disc)
Papilledema
-the swelling occurs in part because, like all other neural tissue, the optic nerve is surrounded by cerebrospinal fluid (CSF)
-Pressure inside the cranium can displace CSF around the optic nerve, causing swelling at the optic nerve
-Is inflammation of the optic nerve itself, a condition known as optic neuritis
-Whatever the cause, a person with a swollen optic disc usually loses vision due to pressure on the optic nerve
~If the swelling is a result of optic neuritis, the prognosis for recovery is good
Rods
- Photoreceptors specialized for functioning at low light levels
- Are longer than cones and cylindrical at one end
- Are more numerous than cones; are sensitive to low levels of brightness (luminance), especially in dim light; and function mainly for night vision
- All rods have the same pigment
Cones
-Photoreceptors specialized for color and high visual acuity
-Have a tapered end
-Do not respond to dim light, but they are highly responsive to bright lights
-Mediate both color vision and our ability to see fine detail (visual activity)
-Three pigments in the cones
~Absorb light across a range of visible frequencies, but each is most responsive to a small range of wavelenghts- short (bluish light), medium (greenish light), and long (redish light)
-That you are looking at the lights with all three of the cone tyoes and that each cone pigment responds to light across a range of frequencies, not just to its frequency of maximum absorption
-Both the present of three cone receptor types and their relative numbers and distribution across the retina contribute to our perception of color
Visual Illuminance
-The eye works correctly only when sufficient light passes through the lens and is focused on the receptor surface- the retina of the eye or the light-sensitive surface in the camera
-Too little light entering the eye produces a problem of visual illuminance, in which objects are too dim and it is hard to see any image at all
-The reason objects appear blurry in low illuminance is likely that we are mostly using rods, which provide a less sharp image
-Is typically a complication of aging eyes; it cannot be cured by corrective lenses
-As we age, the eye’s lens and cornea allow less light through, so less strikes the retina
-Estimated that between ages 20 and 40, people’s ability to see in dim light drops by 50%-and over each additional 20 years, by a further 50%
~Seeing in dim light becomes increasingly difficult, especially at night
-The only way to compensate for visual illuminance is to increase light
-Statistics show a marked drop in the number of people who drive at night in each successive decade after age 40
The Three Cone Types
-Are distributed more or less randomly across the retina, making our ability to perceive different colors fairly constant across the visual field
-The number of red and green cones are approximately equal, but blue cones are fewer
~As a result, we are not as sensitive to wavelengths in the blue part of the visible spectrum as we are, to red and green wavelengths
The Differences in these two red coned
-Appear minuscule, but it does make a functional difference in some human females’ color perception
-The gene for the red cone is carried on the X chromosome
~Males have only one X chromosome, so they have only one of these genes and only one type of red cone
-The situation is more complicated for females, who possess two X chromosomes; although most women have only one type of red cone, those who have both are more sensitive than the rest of us to color differences at the red end of the spectrum
-Could say that women who have both red cone types have a slightly rosier view of the world: their color receptors construct a world with a richer range of red experience, but they also have to contend with seemingly peculiar color coordination by other
Photoreceptors
-Are connected by two layers of retinal neurons
-The first layer contains three cell types
~Bipolar
~Horizonal
~Amacrine
-Horizontal cells link photoreceptors to bipolar cells
-Amacrine cells link bipolar cells with cells in the second neural layer
Retinal Ganglion Cell (RGC)
-One of the groups of retinal neurons with axons that give rise to the option nerve
-Axons collect in a bundle at the optic disc and leave the eye to form the optic nerve
-Are especially sensitive to increased intraocular pressure, which can lead to blindness
-A tiny subset about 1% contain melanopsin, a light-sensitive protein, and thus form a third type of photoreceptor in the eye
~These photoreceptors function to synchronize circadian rhythms, regulate pupil size, and regulate melatonin release
-Form the optic nerve, the road into the brain
~Road forks off to several places; destinations of these branches give us clues to what the brain is doing with visual input and how the brain constructs our visual world
Retinal Ganglion Cells Two Major Categories
- Magnocellular cells (M cells)
- Parvocellular cells (P cells)
M cells
- Large visual system neuron sensitive to moving stimuli
- Receive their input primarily from rods and so are sensitive to light but not color
- Found throughout the retina, including the periphery, where we are sensitive to movement but not to the color of fine detail
P cells
- Small visual system neuron sensitive to differences in form and color
- Receive their input primarily from cones and so are sensitive to color
- Found largely in the region of the fovea, where we are sensitive to color and fine details
Muller Cells
-That span from the retina’s inner membrane at the front to the photoreceptors at the back of the retina and act as optical fibers, channeling light to the buried photoreceptors
Glaucoma
-An eye disease that destroys the optic nerve is the most common cause of irreversible blindness and a prime target for research to restore vision
-The optic nerve begins with the axons of the retinal ganglion cells (RGCs), and if they are dead or dysfunctional, vision is impossible
-Clinical goal is to repair or replace RGCs after injury
-Challenge is that RGC axons do not spontaneously regenerate after injury, and the RGC die, they are not replaced
-One strategy for restoring vision is to stimulate RGCs to regenerate axons
~Huberman used both genetic and visual stimulation to enhance neural activity in the RGC of mice with severed RGC axons
~mTOR Signaling Pathways
-The second strategy for vision restoration is to replace RGCs by transplanting health RGCs from recently deceased donors
~Studies in rats have shown that transplanted RGCs thrive, respond to light signals, and extend axons into the brain to reach usual targets
*Although this is not ready to head to the clinic, it appears to offer a clinically viable strategy for curing blindness related to RGC death
-Not all blindness originating in the eye is related directly to lost RGC
~If photoreceptors are dysfunctioning or dying, as happens in retinitis pigmentosa (RP), blindness will occur
*To implant prosthetic devices into the eye to convert light to electrical signals and then pass to into RGCs
*To introduce light-senstive ion-gated channels to repair the receptors
*Based on independent parallel work by two different research groups on mouse models of RP, which experiments used CRISPR to reprogram genes expressed in rods, leading to an increase in conelike cells, with the restoration of visual function
mTOR Signaling Pathways
-While at the same time repeatedly exposing the eye to high-contrast black-and-white images; this procedure stimulated the RGC to re-establish many of the lost connections to their correct target and allow a partial restoration of vision
Optic Chiasm
- Junction of the optic nerves, one from each eye, at which the axons from the nasal halves of the retinas cross the brain’s opposite sides
- About half the fibers in each eye cross in such a way that the left half of each optic nerve goes to the left side of the brain, and the right half goes to the brain’s right side
Nasal Retina
-Middle path crosses the opposite side
Temporal Retina
-Lateral path travels straight back on the same side
Vision Field
- Light falls on the right half of each retina actually comes from the left side of the visual field
- Information from the left visual field goes to the brain’ right hemisphere
- Information from the right visual field goes to the left hemisphere
- Half of each retina’s visual field is represented on each side of the brain
Geniculostriate System
-Projections from the retina to the lateral geniculate nucleus to the visual cortex
-All of the ganglion and some of the M ganglion axons form a pathway
~This pathway goes from the retina to the lateral geniculate nucleus (LGN) of the thalamus and then to layer IV of the primary visual cortex in the occipital lobe
Striate Cortex
-Primary visual cortex (V1) in the occipital lobe; shows stripes (striations) on staining
-The primary visual cortex shows a broad strip across it in layer IV
-The geniculostriate system, therefore, bridges the thalamus (geniculate) and the striate cortex
~The striate cortex, the axon pathway divides
*One route goes to vision-related regions of the parietal lobe
*The other route goes to vision-related regions of the temporal lobe
Tectopulvinar System
-Projections from the retina to the superior colliculus to the pulvinar (thalamus) to the parietal and temporal visual areas
-Second pathway leading from the eyes is formed by the axons of the remaining M ganglion cells
~These cells send their axons to the midbrain’s superior colliculus, which send connections to the pulvinar region of the thalamus
-It runs from the eye through the midbrain tectum to the pulvinar
~The pulvinar sends connections to the parietal and temporal lobes, bypassing the occipital visual area
Retinohypothalamic Tract
- Neural route formed by axons of photosensitive retinal ganglion cells (pRGCs) from the retina to the suprachiasmatic nucleus; allows light to entrain the SCN’ rhythmic activity
- Between 1 and 3% of RGCs are unique in that they are photosensitive: they act as photoreceptors
- pRGCs, contain the pigment melanopsin, absorb blue light at a wavelength different from the wavelengths of rods or cones
- Axons of pRGCs form a small third visual pathway
- Retinohypthalamic tract synapses in the tiny suprachiasmatic nucleus (SCN) in the hypothalamus, next to the optic chiasm
- Photosensitive RGCs participate both in regulating circadian rhythms and in the pupillary reflex that expands and contracts the pupil in response to the amount of light falling on the retina
Two Distinct Visual Pathways that Originate in the Striate Cortex
- Ventral Stream
- Dorsal Stream
Ventral Stream
- Visual processing pathway from V1 to the temporal lobe for object identification and perceiving related movements
- Pathway to the temporal lobe
Dorsal Stream
- Visual processing pathway from V1 to the parietal lobe; guides movements relative to objects
- Pathway to the parietal lobe
Geniculostriate Pathway
-RGC fibers from the two eyes distribute their connections to the two lateral geniculate nuclei (left and right) of the thalamus
-The fibers from the left half of each retina go to the left LGN; those from the right half of each retina go to the right LGN
~Fibers from each eye do not go to exactly the same LGN location
-Each LGN has six layers, and the projections from the two eyes go to different layers
~Layers 2,3, and 5 receive fibers from the ipsilateral eye (eye on the same side)
~Layers 1,4, and 6 receive fibers from the contralateral eye (eye on the opposite side)
*This arrangement provides for combining the information from the Pand M ganglion cells
-Axons from the P cells are responsive to color and fine details, LGN layers 3 through 6 must be processing information about color and form
-M cells mostly process information about movement, so layers 1 and 2 must deal with movement
Visual Cortex
-Where the LGN cells from the thalamus send their connection
-Layer IV is the main afferent (incoming) layer of the cortex
~Layer Iv has several sublayers, two of which are known as IVSa and IVCb
*LGN layers 1 and 2 go to IVCa
*LGN layers 3 through 6 go to IVCb
-A distinction between P and M functions thus continues in the striate cortex
Cortical Columns
- Anatomic organization that represents a functional unit six cortical layers deen and approximately 0.5 mm square, perpendicular to the cortical surface
- The input from the ipsilaterally and contralateral connected parts of the LGN go to adjacent strips of the occipital cortex; these trips, which are about 0.5mm across
Primary Visual Cortex (V1)
- Striate cortex in the occipital love that receives input from the lateral geniculate nucleus
- Is its striations-its distinctly visible layers
- Wong-Riley stained region VI for the enzyme cytochrome oxidase, which has a role in cell metabolism, they found an unexpected heterogeneity
Extrastriate (Secondary visual) Cortex (V2-V5)
- Visual cortical areas in the occipital lobe outside the striate cortex
- With each region processing specific features f visual information
- Each occipital region has a unique cytoarchitecture (cellular structure) and unique inputs and outputs, we can infer that each must be doing something different from the others
Blob
- Region in V1 that contains color-sensitive neurons, ae revealed by standing for cytochrome oxidase
- The darkened region in the V1 layer
- Neurons take part in color perception
Interblobs
- The less-dark region in the V1 layer
- Neurons participate in the perception of form and motion
V1 input arriving from P cells and M Cell Pathway of the Grniculostriate System is Segregated into Three Separate Types of Information
-Color
-Form
-Motion
~All three types of information moves from V1 to the adjoining region V2
-The color, form, and motion input remain segregated, again seen through the pattern of cytochrome oxidase staining
~The staining pattern in region V2 differed from that in region V1
*Region V2 has a pattern of thick and thin stripes intermixed with pale zones
**The think strips receive input from the movement-sensitive neurons in region V1; the thin stripes receive input from V1’s color-sensitive neurons, and the pale zones receive input from V1’s form-sensitive neurons
The Visual Pathway
- Proceed from region V2 to the other occipital region and then to the parietal and temporal lobes, forming the dorsal and ventral stream
- Many parietal and temporal regions take part, the major ones are region G in the parietal lobe (thus called PG) and region E in the temporal lobe (thus called TE)
The simple records of color, form, and motion from the occipital regions
- Assembled in the dorsal and ventral streams to produce a rich, unified visual world of a complex object (such as faces and paintings) and complex skills (such as bike riding and ball catching)
- Think of the complex representations of the dorsal and ventral streams as consisting of how functions and what functions
Two Regions on the ventral surface of the temporal lobes
- One is specialized for recognizing face (fusiform face area (FFA))
- The other for analyzing landmarks such a building or trees (parahippocampal place area (PPA))
Three regions in the parietal lobe
-Eye movement ~Lateral intraparietal area (LIP) -Visual control ~Anterior intraparietal area (AIP) -Visual guided reaching movements ~Parietal reach region (PRR) -Damage to these regions can produce surprising specific deficits
Facial Agnosia (Face blindness)
-The inability to recognize faces
~Also called prosopagnosia
-Damage to the FFA
-A condition in which an individual cannot recognize faces
Visual Field
-A region of the visual world seen by the eyes
Ganglion Cell’s Receptors Fields
- By shining a light on the receptors the cells respond to stimulation on just a small circular patch of the retina
- The retina region on which it is possible to influence that cell’s firing
- Represents the outer world as seen by a single cell
- Each RGC only sees a small bit of the world, much as you would if you looked through a narrow cardboard tube
- Is composed of thousands of such receptive fields
How Receptive Fields Enable the Visual system to Interpret an Object’s Location
-Whan a tiny light shines on different parts of the retina, different ganglion cells respond
~Example
*When light shines on the top-left corner of the flattened retina, a particular RGC responds because that light is in its receptive field
*When light shines on the top-right corner, a different RGC responds
-Light comes above hits the bottom of the retina after passing through the eye’s lens, and the light from below hits the top of the retina
~Information at the top of the visual field stimulates ganglion cells on the bottom of the retina; information at the bottom of the field stimulates ganglion cells on the top of the retina
Connection from the Ganglion cells to the Lateral Geniculate Nucleus
-The LGN is not a thin sheet; it is shaped more like a sausage
~Each slice represents a layer of cells
-RGC that responds to light in the top-left region of the retina connects to the left side of the first slice
-RGC that responds to light in the bottom-right region of the retina connects to the right side of the last slice
Each LGN cell has a receptive field
- The region of the retina that influences its activity
- Two adjacent retinal ganglion cells synapse on a single LGN cell, the receptive field of that LGNcell will be the bum of two ganglion cell’s receptive fields
- Results from the receptive fields of LGN cells are bigger than those of RGCs
LGN projection to the striate cortex
- Region V1 also maintains spatial information
- Each cell representing a particular place, projects to region V1, a spatially organized neural representation- a topographic map-is produced in the cortex
