Vision and hearing Flashcards
Sensation
The detection of external or internal stimuli through the stimulation of specialized receptors.
Perception
what your brain makes of these stimuli
ex : your experience of colours etc..
Photons
Particles that create light and travel in waves
Electromagnetic energy
Light energy measured in wavelength
Electromagnetic spectrum
The range of wavelength of light
it ranges from gamma rays to extremely low frequency wavelength
Visible Spectrum
The narrow band of the electromagnetic spectrum ranging from 400 to 700 nm that can be perceived as colours ( by humans)
it has not been really decided on because it also depends on the light’s intensity and on the sensitivity of the observer.
Photoreceptors & Transducers
Colour perception results from the brain processing the input of specialized receptors called photoreceptors they are transducers which are devices the converts on form of energy to the next
HERE : light energy from he visible spectrum to action potentials
The pathway of the light to the human eye
1- Light enters the eye through the pupil .
2- Ciliary muscles control the zone fibres that pull on the lens allowing light to focus on the retina at the back of he eye.
3- the fovea is a little pit at the centre of the retina that picks up the light from the point in space being looked at ( a lot of photoreceptors
4- the optic nerve have a bundle of gangland cells that bring the visual information to the brain through the optic disk
Blind spot
light refracted directly to the optic disk cannot be processed since there are no photoreceptors. It is not perceived since the visual s system fills it in with info processed by receptors in the surrounding area
Refraction
the change in direction of the light rays as they travel from one medium to the next .
the cornea and then by the lens which result into a convergence of light rays into the middle of the retina
Pigment epithelium
The layer of cells that nourishes the photoreceptors
Accommodation
the ability of the eyes to keep objects in focus as they vary in distance
when the object moves towards the eyes , the ciliary muscle contract , the lens becomes more convex and increases is focusing power.
when the object moves away from the eyes , the ciliary ,muscles relaxes and the lens flattens decreasing its focusing power.
The pathway of light from the eye to the brain
The flow off information in the retina begins the activation of the photoreceptors ( rods and cones ) which activate the bipolar cells and the retinal ganglion cells . Their axon form the optic nerve that sends info to the brain
types of ganglia cells
midget cells : small size and are associated with colour vision .
Parasol cells : do not process info about colours but are sensitive to dark and light contrasts.
Rods
they are used for peripheral vision . there are no rods at the fovea ( centre of retina )
they do not mediate colour perception but are sensitive to low levels of light.
rods do not have acuity : the precision of vision
Found on the periphery of retina
Dark adaptation ( rods )
the eye’s ability to adjust to low levels of light .
Cones
they are concentrated at the fovea and are adapted for daytime vision and are not very effective in low light conditions ( less degree in dark adaptation )
they are responsible for colour vision and are highly acuity
Photopigments
Light absorbing molecules
produced by opsin
3 types of cones
S cones ( blue ) : short wavelength M- cones( green ) : medium wavelength L- cones ( red ) : long wavelength
Spectral sensitivity ( cone)
Each cone is differentially sensitive to a range of wavelength surrounding the one to which it best responds
Photopigments in rods
it is known as rhodopsin and the spectral sensitivity is medium length
Bleaching
when exposed to light photopigments undergo a certain amount of degeneration ( bright to light room )
it reduces the ability to transduce light .
when 20 min have passed the regeneration of rods is faster in rods
Photo transduction
the process by which light rays are converted into nerve impulse
Photo transduction in rods
No light
when rods are not stimulated by light, they are penetrated by a constant flow of Na 2+ ions through cGMP-gated Na + channels
( constant depolarization in the dark )
they release glutamate which inhibits bipolar neurons
Photo transduction in rods
With light
the photopigment rhodopsin is activated by light, the G-protein transducin releases guanosine triphosphate (GTP). GTP activates the effector enzyme phosphodiesterase. Phosphodiesterase causes the second messenger cGMP to be converted to guanosine monophosphate (GMP). This results in the closing of cGMP-dependent Na+ channels.
hyperpolorization sips the release of glutamate and revives the inhibitory influence on them.
Convergence
The ratio of connectivity between photoreceptors and retinal ganglion cells
Convergence in rods
Rods confer low acuity because the rods that converge to excite a single retinal ganglion cell come from slightly different places on the retina ( lack of precision)
Rods are more sensitive to low light conditions because the summated input from the rods to retinal ganglion cells amplifies the light signal
Convergence in Cones
a single cone receives info from a single ganglion cells so the location is quite precise .
there is less sensitivity to light since the summation does not occur.
Lateral inhibition
how receptors in the surround of on-centre receptive fields have inhibitory connections with receptors located in the centre of the same receptor fields
white light
what is perceived when all wavelength of the visual spectrum are mixed.
Trichromatic theory
.the theory that the brain produces our perception of colours by reading out the combined input of three types of cone each responding best to a specific wavelength.
blue > short rejected and all others absorbed
red > long reflected and all others absorbed
white > all reflected
black > all absorbed
Across neuron response
the idea that the precise information about stimuli is encoded not by a single neuron but by the response pattern of population of neurons
why ? each type of cone responds to a range of other wavelength with less intensity than he does attributed to them
Protanopia
A type of red-green color deficiency due to the absence of L-cones.
Deuteranopia
A type of red-green color deficiency due to the absence of M-cones.
Tritanomaly
A type of blue-yellow color deficiency due to a limited number of functioning S-cones
Monochromacy
Color blindness due to the absence or failure of two of the three types of cones.
Rod monochromacy
Color blindness due to the absence of cones.
Color afterimage
An image of an object that persists after having stared at it for several seconds.
Nasal retina
portion of retina closest to the nose
The axons from the nasal retina of each eye cross over to the other side of the brain (contralateral) at the optic chiasm to form connections with the LGN ( lateral geniculate nucleus)
Beyond the optic chiasm, the optic nerves are renamed optic tract,
temporal retina
the portion of the retina that is nearest to each of the temples
he axons from the temporal retinas remain on the same side (ipsilateral)
Beyond the optic chiasm, the optic nerves are renamed optic tract,
Direction of the vision
The optic nerve from the nasal retina in the left eye and the temporal retina of the right eye convey information from the left visual field.
the optic nerve from the nasal retina in the right eye and the temporal retina of the left eye convey information from the right visual field.
Information from the center of the visual field is conveyed by projections from the fovea of both eyes to form the foveal image.
Parvocellular ( P-cells)
the dorsal layers of the LGN. These consist of layers 3, 4, 5, and 6 and are known as the parvocellular layers. P-cells receive color information from the axons of midget ganglion cells of the retina.
process color > midget cells
Magnocellular ( M- cells)
the ventral layers of the LGN. These consist of layers 1 and 2 and are known as the magnocellular layers. They convey information about motion and depth. Contrary to P-cells, M-cells are sensitive to contrast. They receive their information about contrast from the parasol ganglion cells
no color > parasol cells
Koniocellular (K-cells)
They are located in six thin layers sandwiched in between each of the dorsal and ventral layers. Cells of the LGN that receive information from short-wavelength cones in the retina as well as from bistratified retinal ganglion cells, which convey information about spatial resolution.
Color opponent cells
firing of cells located in the LGN depends on the wavelengths of light that illuminate the center or surround the receptive fields. The frequencies to which these receptive fields respond correspond to the pairs of opposing hues : blue /yellow, red/green ,
each of the pairs was found to activate and inhibit retinal ganglion cells in the pattern of opposition.
ex : Some of the P-cells in the LGN were found to be activated when the center of the receptive field of midget retinal ganglion cells in the retina was illuminated with yellow light. The same P-cells were inhibited by illumination of retinal midget ganglion cells’ surround with blue light
Orientation selectivity
A property associated with simple cells in the primary visual cortex, which best respond when bars of a particular orientation are projected onto the retina.
it also depends on the combined input of sets of LGN neurons known as concentric cells
Directional selectivity
A property associated with complex cells in the primary visual cortex, which best respond when bars of light moving in a particular direction are projected onto the retina.
the directional selectivity of these cells was found to depend on the combined input of several simple cells with the same orientation selectivity.
P-pathway
The visual information pathway from the retina to the visual cortex that connects midget retinal ganglion cells to P-cells of the LGN.
In V1, the P-pathway processes information about form and color. However, in extrastriate areas such as V2, the P-pathway processes information about depth and form but not color. In V4, the P-pathway processes information about form and color.
M-pathway
The visual information pathway from the retina to the visual cortex that connects parasol retinal ganglion cells to P-cells of the LGN.
one branch of it ends up in V2, which processes motion, depth, and form. The M-pathway then projects to area MT, where information about depth and motion is processed.
the what are where
damage to the visual pathway that leads to the temporal lobe were impaired in the identification of objects, ( what ) whereas those that had damage to the pathway leading to the parietal lobes were impaired in assessing the positions of objects in space ( where)
