vision- theories of visual processing Flashcards
discuss classifications and types of receptive fields
Receptive fields can be unimodal e.g. excitatory only or bimodal some areas acting excitatory and others being inhibitory.
Receptive fields can be on or off center or ahve complex morphology e.g. color opponent cells
Area of receptive field inversely proportional to the acuity of that area.
E.g. fingers or small visual buckets near fovea
state the order of neurons in a pathway from photoreceptors to the thalmus
photoreceptors -> bipolar cells -> ganglion cells -> lateral geniculate nucleus (LGN)
explain how receptive fields can have temporal as well as spatial structure
Some continuous activation for a present stimulus
Some have transient response - detect change in stimulus
You can also have a non linear receptive field with no clear excitatory or inhibitory regions.
why are cat models not a perfect match for human vision
Cat and monky models are often used to elucidate human vision however neither is a perfect system e.g. cats have no colour vision.
explain the following terms
Simple cells
Complex cells
hypercomplex cells
Simple cells- Respond to flashed bars or edges
Complex cells- respond to a bar/shape at specific orientation in a certain direction
Hypercomplex cells- stimulus must now be a set length
within the 6 layered cortical structure where are simple and complex cells predominantly found
Layer 4 houses lots of simple cells while complex cells make up the surround.
What are visual receptive fields for?
- -receptive fields as templates ‘Fly detectors’ in frog’s retina (Barlow 1953) ‘Bug detectors’ in frog’s retina and optic tectum (Lettvin et al 1959) Hubel’s & Wiesel’s template matching model of visual cortex
- -receptive fields as ‘filters’ (Lettvin et al 1959) Lateral inhibition (Ratliff & Hartline) Mach Bands Spatial frequency analyzers (Campbell & Robson 1968) –FHS Part 1
- -receptive fields as “zero crossing” detectors (Marr & Hildreth 1980)
explain the theory of receptive fields as templates
Successive processing and combing creates increasingly complex interpretations from each neuron of a visual stimulus.
Receptive fields would converge creating larger receptive fields which does happen.
evaluate the template matching theory of receptive fields
- no one has ever been able to trace one of these suggested pathways.
–there is far more overlap, or convergence, between the LGN and striate cortex than implied by the model
–Feedback as well as feed forward not included in model.
–Timing- the latencies of responses of simple, complex and hyper-complex cells don’t conform to the sequence implied by the model
–Logical extension of theory is ‘grandmother cells’ which only respond to one specific stimulus clearly not correct.
Combinational explosion- responses are not as specific as they need to be but impossible to test all possible stimuli.
–too few neurons to encode all possible combinations of features (combinatorial explosion)
explain ensemble coding
best model is ‘ensemble’ encoding by means highly abstracted combinations of features
state the difference between additive and subtractive colour mixing
additive is with light subtractive is with pigments
explain the color matching experiment
Get participants to match light colour with pure wavelength light by mixing primary light
Primary light= cannot be matched by combination of two other primaries.
Inference humans are trichromatic
-> at the time thought that particles were omitted from the eye
explain the color matching experiment
Get participants to match light colour with pure wavelength light by mixing primary light
Primary light= cannot be matched by combination of two other primaries.
Inference humans are trichromatic
-> at the time thought that particles were omitted from the eye
peak wavelength absorbance of the blue cone
420nm
peak wavelength absorbance of green cone
534nm
peak wavelength absorbance of red cone
564nm
most common form of colour blindness is lack of red or green photo-pigments what are these disorders called and how might they arise
deuteranopia- missing green or protanopia- missing red resulting in difficulty distinguishing longer wavelengths of light; this occurs in ~2% of the population.
erroneous ‘ unequal homologous recombination:
Prevalence of this disorder is largely due to the high level of homology between the red and green photopigments (98% of aminoacids in each sequence are the same). The genes are also located in adjacent stretches of the X chromosome, this and the homology of the two photopigments gives a relatively high likelihood of errors arising during recombination.
what is a lack of blue photopigment called
Tritanopia or lack of blue cones comparatively is extremely rare, occurring in less than 0.01% of the population.
Explain the principle of univarience
Three photoreceptors needed as:
One photoreceptor would conflate a weak optimal signal with a strong sub optimal signal- metamulisum (gives identical response to visual system hence cant distinguish.
dicromatsalso have trouble distinguishing
Three cones is a trade off between acuity and metabolic cost.
More cones less likely to get metamers
why are we forveal dichromats
Dichromats not blue cones as prioritizes spatial acuity
Colour opponent channels:
and how can this be easily demonstrated?
red – green
blue – yellow
light - dark
some colours mix while others cancel each other out e.g. red yellow = orange, blue yellow =white
how is colour oponency achieved and how can it be modeled
This is achieved by LGN systems center surround frequency sampling
it can be modeled therefore on a 3D axis
what is colour consistency state some evidence for it
Where a perceived colour is the same irrespective of illumination
Lan’s ‘colour mondrian’ demonstration
how can colour consistency be explained
Local adaption colours are swapped to be consistent with static local precursors
Local corrections only happen in local part of the retina
helmhotz’s theory of unconscious inference; that our brain adapts our perception of the imagenbesed of what we know or predict.
