Visual Senses Flashcards
what does vision require detection of
various light wavelengths
what is the role of lacrimal gland
- secretes saline tear
- fluid cleans and lubricates the front of the eye during a blink
- prevents cornea from drying out and frost injury during cold
- contains lysozomes and immunoglobulin A to protect against infections
what is the difference between optical and visual axis
- optical axis gives most optically clear image (crosses cornea, lens dead centre)
- visual axis to fovea gives best colour vision
what is the refraction of an image
light bends when passing from across the cornea, lens and vitreous humor
light information through a convergent lens are refracted and focused at a focal point
what is distance from the focal point behind the lens dependent on
the distance to the image
how do horses have full accomodation
they don’t have full accommodation ability
shape of retina means it can see near and far by moving head/eyes
what are subcortical reflexes
do not involve visual cortex
pupillary light reflex
dazzle reflex
what is the pupillary light reflex
pupillary reflexes indicate functional state of afferent and efferent that control the pupil
ex. if light stimulus directed to left eye elicits a consensual constriction in the right eye but not direct one in left eye –> afferent limb of the reflex (optic nerve is intact; but efferent limb to the left eye is damaged)
damaged oculomotor nerve
what is the dazzle reflex
blink or head movement
subcortical mediated brainstem response
palpebral fissure closes in response to sudden intense ilumination of eye –> ipsilateral response is greater than contralateral response
absence of a response indicates blindness
what are the cells in the retina
at back of eye there is either dark pigment or tapetum
what are the steps in intracellular phototransduction of rhodopsin
- photon hits rhodopsin (R)
- isomerization of retinal
- activation of alpha subunit of transducin
- activation of phosphodiesterase (PDE)
- decreases cGMP levels and closes ion channels (inhibitory)
–> hyperpolarizes the receptor cell
what are the distribution of photoreceptors in the retina
what are rods more sensitive to
light (irradiance)
one rod responds to one photon
there are multipile rods per bipolar cell
rods work in the scotopic region
rods are poor for spatial detail
what are the functions of cones
provide more detailed information –> including colour
but they need more light (photopic region)
ratio of cones to ganglion cells is 1:1
what are the function of ganglion cells
integrate multiple photoreceptor signals
what is the function of melanopsin
signals light control for daily rhythms
ganglion cells show intrinsic photoresponsiveness
melanopsin is a photoreceptors that response to irradiance information
mutations in melanopsin have detrimental effects on daily rhythms
what is the neural circuit from retina
from retina to lateral geniculate nucleus located in the thalamus
what are the layers of the LGN
6 layers of retinal map
consists of parvocellular (1-4) and magnocellular (5,6) cells
parvocellular cells are small, integrate signals from cones and necessary for colour and form
magnocellular cells are large, integrate signals from rods, involved in movement, depth and irradiance
what is the primary visual cortex
integrate multiple LGN cells
simple, complex and hyper-complex cells with different receptive fields
what is the visual control of motor movement
dorsal stream flows from V1 to V5/V3A
guides movements such as the hand postures for grasping a mug or pen
damage to ventral stream prevents identification of objects; damage to dorsal stream results in optic ataxia
reaching movements directed toward individaul body (tactile or proprioceptive feedback) remains intact
what are the 4 pathways of central visual and non-visual pathways
eye –> optic tract
- lateral geniculate nucleus –> visual cortex
- rostral colliculi –> dazzle reflex
- pretectum, pons –> pupillary light reflex
- suprachiasmatic nucleus –> hypothalamic, pineal gland
what are the additional visual pathways for proprioception and posture
visual proprioceptive system
linked to the vestibular apparatus
provides information by which head movement is sense
what is the trichromatic theory
colour vision that is based on the coding of the 3 basic colours (red, green and blue)
what is the opponent process theory
colour vision that emphasizes the importance of the opposition of pairs of colours: red versus green and blue versus yellow
where does colour vision start
start in ganglion cells
multiple colour opsins information is integrated by gangiolonic cells; then transmit info to LGN parvocellular cells
what is colour vision represented by in the cortex
by blobs
complex neurons in the visual temporal cortex (V4) respond to objects in the visual field that have colour that is dependent on orientation texture and shape
