Senses 2: Visual pathways Flashcards
Visual processing
takes up a large proportion of the human brain
important part of the evolution of the human brain has been ways to improve how we use vision to guide our actions (and understand the world)
large proportion of neuroscience research has been into vision so we understand the neural processing quite well
understanding visual pathways is probably the best way to understand how our brain works
what is vision?
detecting and interpreting patterns of electromagnetic radiation
differences in intensity – can see in bright and low light conditions
differences in wavelength – can see colours
evolution of vertebrate eyes
eyes evolved through a gradual sequence of improvements for detecting directions and forming an image
advanced types of eyes have evolved several times in the animal kingdom
fossil records date back to the Cambrian explosion
faster movement and navigation in animals required better vision
regulation of phys processes during day and night
light levels detected through eye are sent to the SCN (suprachiasmatic nucleus)
keeps the circadian clock in the SCN accurately timed with natural daily light cycles
what is the pineal gland?
unpaired midline structure near epithalamus
evolutionary old part of the brain that is found in nearly all vertebrates (the third or parietal ‘eye’)
how is vision used to make sense of the world?
if your brain can makes sense of what it sees, it can initiate or guide (hopefully) appropriate actions
eye and other areas of the visual brain codes and analyse regularities and patterns in spatial and temporal differences in light intensity and wavelengths
projections of light onto the retina
rod and cone cells form an array in the retina
human eyes contain 95 million receptor cells
steps in image processing?
retina could be compared to 95 Megapixel camera but with a larger, curved sensor chip and with much more sophisticated processing circuits.
lens to focus image
aperture to control light entering (Iris)
pixels to register image (photoreceptors)
filtering media (glass body, macula, pigment)
filter to protect lens (cornea)
lens cover for when not in use (eye lid)
cleaning mechanism (tears)
processing algorithms (retinal interneurons
light refraction in the eye
light passes through cornea, aqueous humor, lens and vitreous humor on the way to the retina.
at each boundary it is refracted
shape of lens is adjustable
the nearer the object, the stronger the lens needed to form a focussed image
lens become stiffer from age 40+
accomodation
changing the strength of the lens to form a focussed image
why do humans and animals move their eyes
saccades (jumps) and fixations
2-3 saccades per second
direct fovea to collect information about the visual scene
move centre of attention to centre of visual field
fovea
the central portion of the retina, packed with the most photoreceptors and therefore the center of our gaze.
eye movements in everyday actions (Land et al., 1999)
three subjects (a-c) showed similar sequences and locations where they fixated on objects
predictive saccades in anticipation of the next movement
saccades moved to particular locations when eyes engaged in visual search, more precisely if the subject knew what to find where
eyes disengaged from fixating hand and/or object before an action was completed
controlling movement of eyes
the field of view
we can move eyes and heads separately. Many animals cannot move their eyes (insects, birds) and they have to move their head and/or body to be able to see.
stabilising gaze for better vision
movement can be described as combination of three directions of translation and three directions of rotation
movement of head renders vision blurry
diff types of eye movements
saccades
smooth pursuit moevemnts
optokynetic nystagmus
vestibulo-ocular movements
saccades
move the eye very quickly to a new position between periods of gaze stabilisation (fixations) in order to scan the scene across the entire field of view
smooth pursuit movementd
slower, keeps a moving stimulus on the fovea
optokynetic nystagmus
brings the eye back from a peripheral to a more central position after it has followed a large-scale moving stimulus (whilst head is still)
vestibulo-ocular movements
compensate for the movement of the head by moving the eye the same distance but in the opposite direction in order to maintain a constant field of view
brain circuit for saccadic eye movements
conscious control of eye movements comes from the cortical frontal eye fields (FEF)
automatic control of eye movements comes from the superior colliculus
both use input from vision, but also from auditory and somatosensory systems
coping with changing light levels
two types of visual receptors: rod and cone cells
both types of receptors detect light in basically the same way
(opsins, cell shapes, regulatory processes are slightly different)
why have 2 types visual receptors?
cones are specialised for vision during the day
(1-100 million times brighter in sunlight than moonlight)
rods are specialised for vision during the night
what do humans have?
duplex retina with rods and cones
dim-light vision (rods) doesn’t use central fovea
acuity is proportional to
eye movements position the fovea in those positions of the visual field where it is most important to collect high-acuity information
at night high acuity is sacrificed for sensitivity, and it is more advantageous to have no rods in the fovea
visual transduction
light causes graded hyperpolarisation of receptor membrane
conformational change in rhodopsin activates G-protein (transducin)
activated rhodopsin activates a (G-protein) messenger (transducin). Through a series of steps, it causes Na channels to close. The membrane therefore becomes more polarised. – hyperpolarised
signal amplification in G-protein cascade
sequence of steps means that the absorption of single photon can close up to 200 Na channels
but the amount of amplification can be regulated so that fewer channels open - useful in bright light
types photopigments in rods and cones
opsin – light-sensitive protein (G-protein coupled receptor molecule) in the membrane of photoreceptors
opsins are covalently bound to a chromophore
three functional classes of cones
one functional class of rods
first steps in processing in retina
receptor cells have graded (non-spiking_ responses
3 layers on analogue processing (graded responses)
retinal ganglion cells send APs to brain
visual pathways
geniculate-striate pathway:
extrageniculate pathway:
primary visual cortex of rhesus monkey
ca. 1200 mm2 (1/3 of a credit card)
15% of area of whole cortex
disproportionate large foveal projection area in V1
given the higher density of cones in V1 (higher acuity), more processing power is required for information originating from the fovea
visual functions in blind humans and primates
damage to V1 (primary visual cortex) causes cortical blindness, the loss of conscious vision. Patients are able to perform visually-guided behaviours, like grasping or pointing to the location of objects, or avoiding obstacles, correctly at a level above chance.
what does the pineal gland do?
produces melatonin during darkness
what does vision require?
forming and processing an image in the eye
rod cells
are specialised for high sensitivity to see in dim light
large cells containing large amounts of photopigment
G-protein cascade produces high amplification
cone cells
specialised for high acuity and high speed of response to see in bright light
smaller in size than rods and contain less photopigment per cell
not saturated at higher light levels (e.g. low amplification)
photoreceptor recovers rapidly from change
where is acuity of vision highest
in fovea and decreases towards the periphery of the retina
what is acuity proportional to?
the density of receptor cells
in mammals what is the chromophore?
retinal - absorption of light causes a conformational change in retinal molecule to the activated form (all-trans retinal
functional classes of cones
(S-, M- and L-cones):
Cone opsins differ in their wavelength-specific affinity to absorb light (S, M and L opsins)
Only one opsin type is expressed per cone.
functional classes of rods
All rods express the same type of opsin (RH1, or rhodopsin)
geniculate striate pathway
retina
LGN (lateral geniculate nucleus) of the thalamus
V1 (primary visual cortex)
areas of the higher visual cortex (90% of retinal projections)
extrageniculate pathway
retina
Superior colliculus (SC)
several projections to areas of the higher visual cortex and the pulvinar nucleus of the thalamus (eye movement control and visual attention) (10% of retinal projections)
the field of view
is defined by the position and orientation of the eye ball, of the head and of the body
