Vision - Physiology of retina and visual cortex. Flashcards
What are saccadic eye movements?
Rapid and jerky (3-5/s) as gaze moves from one object to another.
In saccadic eye movements is our visual perception turned off or on?
Visual perception turned off.
What is smooth pursuit eye movement?
Tracking moving objects.
What is the sensitive part of the retina called?
The fovea.
What are the 3 types of photoreceptors and how many of each are there?
Rods (1), cones (3), photoreceptive ganglion cells - the output cells of the retina (1).
What do rods and cones contain stacks of? and how are these formed?
Disc membranes, formed by invagination of plasma membrane.
Why is the optic disc is referred to as the blind spot?
Because it contains no receptors.
Which photoreceptors are involved in night and which are involved in day vision? Which are involved in central and which in peripheral vision?
Night and peripheral = rod. Day and central = cone.
Fovea centralis has a very high cone density.
What is the photopigment found in rods?
Rhodopsin.
What is scotopic vision?
Rods being very sensitive to low level light.
What do rods have none of?
No colour or vision acuity.
Cones have 3 opsin photopigments, why?
Because each is sensitive to a different wavelength of light. Red, green and blue. High density in foveal region.
When are cones responsive and rods maximised/saturated?
In high illumination levels.
Loss of cone photopigment results in what?
Colour blindness = X-chromosome mutation.
Transduction of light energy into chemical energy occurs where? and leads to what?
Photopigment molecular complex. Leads to subsequent activation of biochemical second messenger cascade which leads to hyperpolarisation in receptor membrane potential.
In light what are the biochemical occurrences?
There is a decrease in levels of cGMP, this closes Na+ ion channels and shuts off the ‘dark current’, which hyperpolarises photoreceptor cell and alters the transmitter release .
In the dark what biochemical changes take place?
cGMP levels increase and cGMP binds to the membrane Na+ channel protein, this allows dark current (carried by Na+) to flow into the cell (rod or cone).
What are the 4 anatomical targets of central projection of retinal ganglion cell axons? Describe the function of each.
Geniculo-cortical pathway (dorsal lateral geniculate (LGNd) and visual cortex) - visual recognition, cells respond to specialised features of a stimulus.
Superior colliculus - visual localisation, cells respond to existence of of a stimulus at a certain place, play a role in eye and head movements.
Suprachiasmatic nucleus (SCN) in hypothalamus - biological clock, responds to light intensity and and duration.
Pretectal region - visual proprioception, cells respond to movement of the visual field as a whole, sends info to the vestibular system and cerebellum, involved in compensatory postural reactions.
How many layers does the LGNd have?
6
What layers are involved in the ipsilateral retinal input and which involved in the contralateral retinal input?
Ipsi - 2,3,5.
Contra - 1,4,6.
What cells are found in layers one and two?
Magnocellular cells, 20% of population, code brightness and contrast.
What cells are found in layers 3-6?
Parvocellular cells. 80% of population, code for colour contrast.
Area 17 is known as what?
Brodmann’s area 17, primary visual cortex, V1. (V2-30 found in nearby areas 18 and 19).
What is mapping?
When responses recorded with micro-electrodes positioned in visual pathways and RF properties studied by projecting stimuli onto a screen or computing system. Eg: V1 neurne selective for moving edge orientated at 270 degrees.
Edge selective cells are orientated how in V1?
Neurons concerned with a specific orientation are organised into vertical ‘orientation’ columns.
Many V1 cells receive binocular input, what does this help?
Providing info about 3-D shape of objects, stereopsis, object distance.
Binocular-driven cells are organised how?
Into columns called ocular dominance columns, independant of orientation columns.
What is stereoblindness?
Inability to see in 3D using stereo vision, resulting in an inability to perceive stereoscopic depth by combining and comparing images from the two eyes.
What is retinal disparity?
Visual cortical binocular driven neurons computing the difference of image position in each eye.
V1 transmits info to 2 primary pathways, what are they?
Ventral stream. V1 - V2 - V4 - V8 - inferior temporal lobe. What pathway. Associated with object recognition and long-term memory.
Dorsal stream. V1 - V2 - V3 - middle temporal area (V5) - inferior parietal lobe. How pathway. Associated with motion perception and representation of object location. controls eyes and arms - visual information used to guide saccades/reaching.
Lesion in V5 leads to what?
Motion blindness, jittery movement.
Lesions in visual area V8 leads to what?
Achromatopsia - see in shades of grey only.
Parietal area - V7 processes what?
Spatial relationships of objects in visual space.
Lesion in V7 leads to what?
Neglect syndrome.
Area 21 - inferotemporal area is involved in what?
Facial recognition and interpretation.
