sensory system
gives rise to sensory perceptions. basic functions include transduction and coding.
trandsduction
transformation of physical energy into neuronal activity. Occurs in receptor neurons.
coding
Information about stimuli (e.g., intensity, duration) are represented (coded) in the pattern of activity (action potentials) of the neurons.
laws of specific sense energies
Johannes Muller (1826). labeled pathways.
sensory receptor
A specialized neuron that detects a particular category of physical energy. Some transduce and encode (somatosensory, olfaction); some only transduce (vision, audition, taste).
sensory transduction
The process by which sensory stimuli are transduced into slow, graded receptor potentials.
receptor potential
A slow, graded electrical potential produced by a receptor cell in response to a physical stimulus.
receptive field
The region of the sensory surface that modulates the activity of sensory neurons.
visual system
answers what it is (recognition) and where it is (location). transforms a 2D and upside-down retinal image into the 3D world we perceive.
Wavelength (frequency)
perception of color
Intensity (amplitude)
perception of brightness
electromagnetic spectrum
colors and wavelengths visible to humans. violet (shortest, 400 nm) to red (longest 700nm)
lens
focuses light on the retina
ciliary muscles
alter the shape of the lens to focus
accommodation
adjusting the lens to bring images into focus
optic disk
produces a blind spot. normally solved by the phenomenon called completion. the person does not perceive a blind spot because it is filled with the information around it.
choroid membrane
has blood vessels and pigments
the retina is “inside-out”
light passes through several cell layers before reaching its receptors
travel of light through the retina
light -> retinal ganglion cells -> bipolar cells -> receptor cells
lateral communication
horizontal and amacrine cells
duplexity theory of vision
cones and rod mediate different kinds of vision
cones (8 mill)
photopic vision; lower sensitivity, high-acuity in day light. color information in good lighting.
rods (120 mill)
scotopic vision; high-sensitivity, low-acuity vision in dim light. lacks detail and color information.
high converge in rod system
increasing sensitivity while decreasing acuity
only cones are found at the fovea
low converge and low sensitivity but high acuity
scanning the visual world
because only the central region of the retina provides high resolution, we see the world by moving our eyes.
saccades
quick eye movements
perception of color and detail decrease beyond
20 degrees from the fixation point, unless you move your eyes
spectral sensitivity curve
shows how sensitivity depends on wavelength
phototransduction
conversion of light to neural signals by visual receptors. relatively slow.
when light strikes the retina
it interacts with light-sensitive photopigments in the rods and cones
photopigments
are contained in discs located in the rod and cone outer segments
rhodopsin
photopigment in rods; responds to light rather than neurotransmitters. made of retinal (from Vitamin A) and opsin, a protein.
rhodopsin (in the dark or DARK CURRENT)
phototransduction; Na+ channels are maintained partially open by cGMP. this partial depolarization (-40 mV/-50 mV) releases glutamate out of the synaptic terminal.
rhodopsin (when light strikes)
phototransduction; split rhodopsin activates a G-protein. Na+ channels close (cGMP is degraded). Rods hyperpolarize (returns to resting potential of -70 mV), inhibiting glutamate release.
amplification during transduction
one rhodopsin activates hundreds of transducin (G-proteins) molecules per second. each transducin activates a single phosphodiesterase. each phosphodiesterase degrades over 100 cGMP molecules per second. overall, one photon degrades over 105 cGMP molecules per second.
the visual pathway in humans
optic nerve -> optic chiasm -> optic tract -> LGN -> optic radiations -> occipital cortex
lateral geniculate nuclei layers (6-1)
contra, ipsi, contra, ipsi, ipsi, contra
dorsal lateral geniculate nucleus (dLGN)
layers are monocular. maps are in register. each layer only receives input from one eye.
cortical layers
1, 2, 3, 4a, 4b, 4c, 5, 6
lateral geniculate nuclei
neurons project to cells in layer IV (monocular) in V1.
layers II-III
biocular cells appear
layer V
projects to SC in mesencephalon
layer VI
projects back to the LGN.
retinotopic map in human V1
systematic organization of RF position. representation of fovea is magnified.
receptive fields of visual neurons
The area of the visual field within which it is possible for a visual stimulus to influence the firing of a given neuron in the retina or central visual system.
on and off channels
- Rods and cones contact bipolar cells.
- Bipolar cells contact ganglion cells.
- Some bipolar cells (off-center) are excited by glutamate and are depolarized by darkness and hyperpolarized by light.
- Some bipolar cells (on-center) are inhibited by glutamate and
are depolarized by light
and hyperpolarized by
darkness.
lateral connections
allow cells in one part of the
retina to influence the activity of cells in another part
cortical simple cells
have elongated RFs with excitatory and inhibitory subregions. simple cell RFs would be formed by the convergence of input from circular, center surround RFs.
binocular disparity (retinal disparity)
for objects at different distances, each eye sees a slightly different view. grants us info about depth relationships between objects.
depth detectors
neurons in the visual cortex that are able to detect retinal disparity.
steropsis
depth perception through binocular vision
columnar organization of V1
- Cells with simple receptive fields send information on to cells with more complex receptive fields
- Functional vertical columns exist such that all cells in a column have the same receptive field and ocular dominance
- Ocular dominance columns – as you move horizontally, the dominance of the columns changes
- Retinotopic organization is maintained
hypercolumn
portion of cortex that has the machinery to analyze all properties of stimuli (orientation, motion, etc), for each point in the visual field, and for each eye.
component (trichromatic) theory
Proposed by Thomas Young (1802); refined by Hermann von Helmholtz (1852). Three types of receptors, each with a different spectral sensitivity. Color perceived depends on the ratio of activity in these three receptor types.
Protanopia (first color defect)
An inherited form of defective color vision in which red and green hues are confused; “red” cones are filled with “green” cone opsin. They see the world in shades of yellow and blue; both red and green look yellowish to them. Visual acuity is normal. affects men more.
Deuteranopia (second color defect)
An inherited form of defective color vision in which red and green hues are confused; “green” cones are filled with “red” cone opsin. Visual acuity is normal. affects men more.
Tritanopia (third color defect)
An inherited form of defective color vision in which hues with short wavelengths are confused; “blue” cones are either lacking or faulty. See the world in greens and reds. Blue looks green and yellow looks pink. Visual acuity is normal. affects men and women equally.
hierarchical organization
From receptors to striate (V1) and extrastriate (V2, etc.) cortex
functional segregation
cortical areas are functionally specific.
parallel processing
what and where are processed in different pathways that originate from the retina (magno, parvo)
Ganglion cells form two main retinal output channels
parvo cells and magno cells
parvo cells
mostly in the fovea and receiving
mainly cone inputs form the parvocellular pathway
magno cells
mostly in the peripheral retina
and receiving mainly rod inputs from the magnocellular pathway..
magnocellular pathway
Large ganglion cells in retina project to large neurons in the ventral (magnocellular) division of the dLGN. Information is passed from V1 to parietal cortex (dorsal stream). responds to motion and spatial info.
lesion of parietal cortex bilaterally (magnocellular pathway lesion)
inability to perceive movement (case LM)
balient syndrome (magnocellular pathway lesion)
Localization deficit due to lesion in dorsal stream
- Ataxia: deficit in visually guided movements
- Ocular apraxia: deficit in the ocular scanning of images
- Simultagnosia: perception of only one object at a time
parvocellular pathway
Small ganglion cells in retina project to small neurons in the dorsal (parvocellular) division of the dLGN. Information is passed from V1 to temporal cortex (ventral stream). Responds to shape, details (acuity), color.
lesions in temporal cortex (ventral stream) [parvocellular pathway lesion]
produce visual agnosias, which in the inability to recognize visual objects.
prosopagnosia (parvocellular pathway lesion)
inability to recognize faces. Often associated with damage to the ventral stream. Recognition deficits may not limited to faces. Prosopagnosics have different skin conductance responses to familiar faces compared to unfamiliar faces, even though they reported not recognizing any of the faces
when presented with face stimuli
activity change in the right temporal lobe of normal subjects. lesions in this area produce prosopagnosia.
Functional Areas of Extrastriate Visual Cortex
Anatomically distinct architecture and borders. Retinotopically organized. Neurons in each area respond to different visual cue: color, movement, shape, etc. Lesions of each area results in specific deficits
MT
motion area
motion is also used for the perception of form
breaking camouflage and biological motion
Intensity of a light stimulus is coded through
through increased frequency of action potential i.e. cells fires more often for intense light
After the chiasm, the optic nerve changes name to
optic tract
Optic nerve carries only
ipsilateral information (nasal and temporal)
Optic Tract carries
ipsilateral (temporal) and contralateral information (nasal)
pre-optic chiasm lesion
ipsilateral vision loss
ost-optic chiasm lesion
contralateral vision loss
Convergence of on/off bipolar cells-> leads to
on/off ganglion cells-> LGN on/off cells-> V1 on/off simple cells->V2 complex cells