Brain & Cognition 🧠Flashcards
the retina prre-processes the rod and cone signals
via bipolar cells to ganglion cells
ganglion cells
pass the preprocessed signals to the brain
long wavelength cone
responds well to red or yellow
medium wavelength cone
responds best to green less to yellow
short wavelength cone
responds best to blue
rhodopsin
translaties light into the closing of Na+ channels so that the membrane hyperpolarizes > neural signal that is sent to bipolar >? ganglion cell
retinal color blindness
absence of a particular cone type
fovea
cup shaped highest density photoreceptors, mainly cones; sharpest vision, color vision
age related macular degeneration
older age, smoking diet, genetic , loss of central vision, acqity loss, pigment epithelium (receptors) are lost due to accumulation of toxic products,
dry macular degeneration
damage to the fovea, yellow deposits (drum) accumulate in macula
why are rods and cones not switched with the photoreceptors placement?
the pigment epithelium prevents light scatter, so that sharper vision is possible (also provides nutrients )
RGC fibers lying on top causes the blind spot
place where all retinal ganglion cell fibers pass through the eye (optic disk)), and no receptors are present
glaucoma
increase of pressure inside of the eye, damage of nerve fibers of the RGC’s : optic nerve, loss of peripheral vision first (but may vary), treatment : eyedrops, surgery
the need for data compression
optic nerve only contains 1 mil nerve fibers and data compression gebeurt in het oog van 130 mil.
how is retinal information compressed?
the photoreceptor responds to light by hyperpolarization (closing of Na+ channels) to dark by depolarisation (opening of Na+ channels) : a graded potential signal
the one and off systems originate at the level of the bipolar cells
the receptors make sign conserving synapse with the off bipolar cells and sign inverting synapses with on bipolar cells that have an unique neurotransmitter receptor site (mGluR6)
horizontal cells
receive signals from widespread region of receptors, . they provide negative feedback on the receptors
the retinal network
from luminance (receptors) to contrast (bipolar and ganglion cells). from graded potentials (hyper/depolarization) in cones, bipolars, horizontals, to action potentials (in ganglion cells, because of long axons)
retinal ganglion cells
encode contrast, luminance is discarded
Hermann grid illusion
‘side effect’ of the data compression by the retina (and higher visual areas), comparable to artefacts caused bt JPEG compression
how the retina solves the data compression problem
- contrast coding (ON and OFF center surround Retinal Ganglion cells)
- rod signals pass through the same RGC’s as cone signals
- colours are coded as R/G, G/R or B/Y contrasts
- parallel pathways (M/P>P/M) for color //bw, detail/global
how do rods work? because they are mainly working in the dark
- rods mostly connect to rod bipolar cells
- they do not connect directly to RGC’s
- they connect to cone driven bipolar cells via the amacrine cells
- so bipolar and RGC’s have (overlapping) receptive fields from cone and rod iputs
- horizontal cells also give the rods a suppressive surround > Mexican hat RF profile
diagram of primary rod-driven signal pathways via four synapses
rods -> rod bipolar cell (RBC) -> all amacrine cell (AII-AC) -> OFF or ON (cone) bipolar cell (OFF-BC, ON-BC) -> OFF or ON ganglion cell (OFF-GC, ON-GC).
rod cell
sensitive enough to respond to a single photon of light and is about 100 times more sensitive to a single photon than cones.
- sensative to single wavelength (hence are useless for color vision)
- rod bipolar receive input from multiple rods, hence have larger receptive fields
- therefore, vision in the dark is less sharp (also because rods sit mainly peripheral)
dark adaptation
- pupil dilation
- cone> rod transition
- ‘bleaching’ (depletion) of pigment (opsin) in photoreceptors that become undone
- less receptor signal > less negative feedback from horizontal cells
retinis pigmentosa
genetic disorder (>50 genes involved)
- progressive degeneration of receptors: rods first, followed by cones
- pigments deposits at affected parts of retina, depigmentation at vulnerable sites
- night blindness* > loss of peripheral vision > tunnel vision > full blindness
two types of ganglion cells
midget(X, small receptive fields) and parasol cells (Y, larger receptive fields)
midget cells
small receptive field, single cone center and surround: color contrast selective, slow sustained responses, tuned to high spatial frequencies// parvocellular layers of the LGN
parasol cells
large receptive field, many cones input to center and surround; not color sensitive, fast transient response, tuned to low spatial frequencies// Magnocellular layers of the LGN
continuous spectrum, yet color opponent of retinal ganglion cells:
sampling of red/green, green/red, blue vs yellow (not yellow blue because yellow is made up off of two cones and that cannot be put in the center)
the concept of spatial frequency decomposition
every image/ contour can be decomposed into the spatial frequencies it contains
- sharp edges (square waves) contain both low and high spatial frequencies
- so do small spots of light (impulse function)
spatial frequency sensitivity depends on
- contrast & brightness
- receptive field size
center component of RGC is sensitive from
low to high SF’s, surround from low to intermediate SF’s, combined, this gives the characteristic SF running of RGC’s
parvocellular pathway
(from midget cells) has sustained response sees collor low contrast agin higher spatial resolution slower
magnocellular pathway
(from parasol cells) transient réponse monochrome high contrast gain lower spatial resolution faster
Y-type (parasol) RGC axons have …. conduction velocities than X-type (midget)
faster
magnocellular fibers of LGN … than parvocellular fibers
faster
global precedence:navon task (hierarchical letter stimuli)
global targets are detected faster than local targets. also, congruent stimuli are faster than incongruent
seems to be a hemispheric asymmetry in the processing of global versus local information
global information is faster in the right hemisphere (stimuli in left visual field)
local information is faster in the left hemisphere (stimuli in right visual field)
this corresponds to the finding that patients with right hemisphere damage have trouble copying the global shape, while patients with left hemisphere damage have trouble copying the local shapes
what is the purpose of the brain interpreting the incoming wavelengths (color is not wavelength)
color constancy as a brain code for ‘tasty’ or ‘nasty’
V4
color constant emerges at the level of V4. V4 receptive field is large enough to integrate numerous color opponent signals and discount the illuminant
achromatopsia
lesions to V4/V8 results in cortical color blindness// lesion in V4 also had deficits in discriminating complex shapes. he could distinguish luminance, orientation and motion but not illusory contours and complex shapes
simplified Reinhardt detector model for direction selectivity
the cell receives input from two cells that have spatially separate receptive fields. one of the cells has a delayed input. only when the stimulus moves in the right direction, the cell receives simultaneous input from both cells and will fire
computational power of the neuron
adding up all inputs - in a weighted manner- to generate an action potential or not
the neuron is a coincidence detector
only when enough inputs combine at the same time, an action potential is generated
direction of motion selectivity
V1, V3, MT
the aperture problem
detecting motion through an aperture is ambiguous, many motion vectors can yield the same motion through the aperture. V1 cells suffer from the aperture problem
pattern cell
MT 50%, the cell is ‘seeing’ the pattern motion, no longer the movement of the individual compontnetns
component cell
V1, the cell is not seeing the pattern motion but the movement of the individual components// so basically it is seeing things that you are not seeing, you perceive it as going left but the component cell perceives the two individual directions that it is made out of
MST
neurons répond selectively to particular motion flow fields. often caused by self-motion
biological motion
recognising species, sex, mood etc from movement
superior temporal sulcus
selectively activated by biological compared to non-biological (scrambled) motion
akinetopsia
bilateral MT,MST,STS lesion. zihl’s motion blind patient: sees only stationary scenes, with objects flashing from one position to the next
motion (visual area
component V1, V3, MT
optic flow MST
biological STS
color visual area
wavelength V1., V2
colour constancy V4
dorsal pathway
magno dominated
ventral pathway
parvo dominated
hypothalamus
regulation of circadian rhythms
pretectum
reflexive control of pupil and lens
superior colliculus
converts a retinatopic map into a saccade map too that stimuli are foveated //orienting movements of head and eyes
layers of LGN
magnocellular, parvocellular, koniocellular
magnocellular
y-type (parasol) input
parvocellular
X-type (midget) input
layers 1,4,6 of LGN
(depends on which LGN) contralateral eye
layers 2,3,5 of LGN
ipsilateral eye
retino-cortical expansion
there is much more area in your visual cortex looking at the central part of your visual field than the surrounding peripheral visual field
V1
starting point of projection to the other visual areas and the rest of the brain
definition of visual area
each area contains a separate, retinotopic, map of the visual field, need not to be a complete map, sometimes only central visual field or peripheral or only lower or upper.
oculair dominance columns
strong separation in layer 4, weaker in layers 2/3 & 5/6
amblyopia
pathological dominance fo one eye , if this happens during the critical period in development (2-6 years) input from that eye occupies less space in V1, the OD column of that eye becomes smaller. after that critical period, this reduced representation remains, and amblyopia (lazy eye) remains
receptive field tuning
a neuron will only respond to a stimulus within its receptive field if that stimulus has certain characteristics.
feature selectivity
orientation and direction of motion tuning in V1
orthogonal organisational components of primary visual cortecx
orientation columns
ocular dominance columns
CO blobs
hypercolumn
basic processing unit in V1 : Cytochome oxidase activity, Ocular dominance column, orientation column
Gabor filter
sinusoid combined with Gaussian envelope
simple cells
Gabor filters tuned to orientation and spatial frequency, with on and off zones
quadrature pairs
of simple cells combine to form complex cells
colour constancy
we perceive the same color despite differences in illumination. the wavelength of the light from the banana is very different in the morning versus the evening, yet it is perceived as yellow the whole day