Vision (Lecture 5) Flashcards
vision pathway
light enters through the pupil and reaches the retina, the cornea refracts light onto the fovea (center of the retina with highest acuity)
equivalent of light
waves of photons
Photons
-emitted with different energies resulting in different wavelength and different quantities resulting in different amplitudes of waves
Visible Light Spectrum
- we only see (380-760nm)
- we perceive different wavelengths as different colors which is a creative process of the brain
light enters they eye through….
the pupil
lens
attached to ciliary muscles, important for contraction and reflection of light to the back of the eye
cornea
-refracts the light onto the fovea
ciliary muscle
- contort lens to focus on fovea
- far objects —> relax ciliary muscles
- close objects —> contract cilary muscles –> fat lens
- this is ACCOMMODATION
fovea
- center of the retina with highest acuity (ability to see details)
- focuses objects in center of visual field
- CONES ONLY
- thinning of ganglion cell layer reduces distortion
emmetropia
PERFECT??
hyperopia
images appear behind the retina, farsightedness
hyperopia correction
-convex lens
myopia
images appear before the retina, nearsightedness
myopia correction
-convave lens
convergence
-eyes must turn slightly inward when objects are close
binocular disparity
difference between the images on the two retinas = 3D vision
RETINA
thin strip of tissue (contains the fovea) along the back of the eye where light is converted to neural signals that can be sent to the brain through the optic nerve (blind spot)
blind spot
areas of no photoreceptors in the retina
- located at the optic disk
- completion
optic disk
-where axons of retinal ganglion cells gather together and leave the eyeball (optic nerve)
ganglion cells
-primary visual neurons
optic nerve
projects to brain
horizontal, bipolar, and Amacrine cells
-process info from rods and cones
photoreceptors
-rods and cones
-detect light
-contain light sensing photopigments in discs within their outer segments
middle (cell body), and synaptic ending-makes contact with bipolar cells
lateral communication
the synaptic points
(amacrine cell between ganglion and bipolar)
(horizontal cell between photoreceptors and bipolar)
ganglion cell axons
-project to forebrain
rods
- scotopic (nighttime vision)
- high-sensitivity to light because more photopigment (larger outer segment) allowing for low-acuity vision in dim light, but lacks detail and color information
- only one type
- in bright daylight rods are saturated and the cell can not be further hyper polarized
cones
- photopic (daytime) vision
- high acuity color information in good lighting
- different opsin proteins that are differentially activated by specific wavelengths of light
rods ratio
-many:1 ratio with retinal ganglion cells
Why do rods and cones differ in sensitivity and acuity?
- differences in amount of light sensing photopigments
2. convergence
cones ratio
-1:1 ratio with retinal ganglion cells
less of a light signal (dim/fewer photons)=
- greater change of exciting a rod-fed ganglion cell = greater sensitivity
- ganglion cell carries less information about where photons were revieced = less acuity
visual transduction
conversion of light to neural signals by photoreceptors
in dark the cell is depolarized and releases….
glutamate
light causes cell to hyper polarize glutamate….
-rate of release is reduced
lack of light (pathway)
- second messenger (cGMP) na+ channels open
- cell is depolarized
- glutamate is being released
optic nerve
-formed by asons of ganglion cells
optic chiasm
- the part of the brain where the optic nerves partially cross (located at the bottom of the brain below the hypothalamus)
- where part of image crosses over to opposite side
optic tract
continuation of the optic nerve and runs from the optic chiasm to the lateral geniculate nucleus
visual fields
- travel to contralateral(opposite) cortex
- fields are not defined by the eyes; cortex recieves info from ipsilateral (same side) and contralateral (opposite side) eye
retina-geniculate-striate system
-90% of axons of retinal ganglion cells
information received at adjacent portions of the retina remains adjacent = retinotopic
lateral geniculate body/nucleus (thalamus/LGN)
- thalamic relay center nucleus that deals with visual information
- asons from retinal ganglion cells synapse on other neurons here
P(parvocelluar) layers
- small cell bodies
- color and fine details
- stationary objects
- input from cones
M(magnocellular) layers
- large cell bodies
- detect movement
- primary input from rods
do retinal cell project anywhere else?
-some axons from the optic nerve go out of main pathway and relay into the hypothalmus(important fro sleep/wake cycle) and superior colliculus (orientation reflex)
primary visual cortex (occipital lobe)
- V1
- brodmann’s Area 17
- Striate Cortex
- axons from LGN (M?P cells) terminate in layer 4 of V1
- info arriving in layer 4 remains segregated(i.e. is either from one eye or the other) until it travels to other cortical layers for further processing
- most neurons in V1 are either simple or complex
Hubel and Wiesel (1979)
-mapped receptive fields of retinal ganglion cell, LGN cells, and layer 4 V1 cells
receptive field
-area in visual field within which it is possible for a visual stimulus to influence that neurons firing
receptive field of foveal areas vs periphery
foveal areas are smaller than those in periphery (due to convergence)
neurons receptive fields
- circular in shape
- neurons are monocular
- many neurons at each level had receptive fields with excitatory and inhibitory area
center-surrounded receptive fields
-bipolar and retinal ganglion cells and neurons in V1 layer 4
on-center (off-surround)
- cells fire more when light is in center of receptive field
- firing it inhibited when light is shone in surrounding receptive field
off-center (on-surround)
- cells fire more when light is shone in surround
- firing is inhibited when light is shone in center of receptive field
lateral inhibition
- capacity of excited neuron to reduce the activity of its neighbors
- used in vision to sharpen signals to the brain, it increases the contrast and sharpness in visual response
simple visual cortex receptive fields
-rectangular
-on and off regions (like cells in layer IV)
-orientation and location sensitive
all are monocular
complex visual cortex receptive fields
-rectangular
-do not have static on and off regions
not location sensitive
-motion sensitive
-many are binocular
larger receptive fields
secondary visual cortex
-association cortex
dorsal steam
- “how” pathway
- processing objects spatial location
- specialized for motion
- “where” stimuli are for the “control of behavior”
ventral steam
- specialized for attributes
- “what” stimuli are for “conscious perception”
posterior parietal cortex (dorsal stream)
- cells selective for linear motion, radial motion, or circular motion
- brain lesion in this area–>
inferior temporal lobe (ventral steam)
- cells are orientation selective and color selective
- important for shape and color perception
- what would happen if you sustain a brain lesion in this area???
fusiform face area
- specialized face recognition circuit
- shows enhanced activation in fMRI when subjects shown faces as opposed to other shapes
prosopagnosia
- damage to fusion face areas
- damage to other object recognition areas but no FFA results in recognition of face and not other objects
retinotropic
mapping of visual information from retina to neurons within the visual system
