from photons to phenomena (week 3) Flashcards
what are rods and how many do we have?
rods are photoreceptors for vision in dim light, we have 125 million and they are mostly located in our peripheral/outer regions of the retina
what are cones and how many do we have?
cones are photoreceptors for colour and sharpness of vision, we have 6 million and they are mostly located in the fovea
what are the steps in the eye to cortex process?
reception (absorption of physical energy) > transduction (physical energy converted into an electro chemical pattern in the neurons) > coding (one-to-one correspondence between aspects of the physical stimulus and aspects of the resultant nervous system activity
which light are our eyes most sensitive to?
light in the green range
what is trichromatic theory?
confirmed Hermann Von Helmholtz’ theory - there are three types of cones that prefer different wavelengths: short (blue), medium (yellow green), and long (red)
what is the opponent-process theory? (Hering, 1878)
this theory works at the level of the neurons - before red, green, and blue signals are processed in your brain, they are reprocessed and redescribed in an opponent-process manner: a scale from red to green, a scale from blue to yellow, and a scale from light to dark
what is colour constancy?
the tendency to perceive something as the same colour, regardless of what colour light is illuminating it - what cones know are different from what a person percieves
there are two pathways after the retina, the parvocellular (P) pathway, and the magnocellular (M) pathway, what is the difference?
the parvocellular (P) pathway receives most input from cones (sensitive to colour and fine detail), whereas the magnocellular (M) pathway receives most input from rods (most sensitive to motion)
what are receptive fields (properties of visual neurons)?
the region of sensory space (i.e., retina) within which light will cause a neuron to fire
what is retinotopy (properties of visual neurons)?
things that happen close to one another, are processed by neurons that are close together
what is lateral inhibition (properties of visual neurons)?
neurons that are next to each other can inhibit each other; this is useful for enhancing contrast at edges of objects
what is the information’s direction of travel after the retina?
retina > optic nerve > optic chiasm > lateral geniculate nucleus (LGN) > cortical area V1
what is the lateral geniculate nucleus (LGN)?
part of the thalamus and a subcortical relay for most of the brains sensory input and motor output - the LGN maintains a retinotopic map
what is the primary visual cortex (V1)?
extracts basic information form the visual scene and sends this information for later stages of processing of shape and movement etc. - maintains retinotopy
what does damage to V1 cause?
leads to clinical diagnosis of cortical blindness - the patient can’t consciously report the object presented in this region of space - however, the patient can still make judgments about things in this “blind” area of visual field, suggesting their are other routes from the eye to brain
what is the functional specialisation theory (Zeki, 1992;1993)?
argues different parts of the visual cortex are specialised for different visual functions
what are V1 and V2 specialised for? (functional specialisation theory)
early stage of visual perception e.g., shapes
what are V3 and V3a specialised for? (functional specialisation theory)
responsive to form (especially moving objects)
what is V4 specialised for? (functional specialisation theory)
responsive to colour - Zeki et al. (1991) found V4 more active for colour than greyscale images
what is V5/MT specialised for? (functional specialisation theory)
responsive to visual motion - Zeki et al. (1991) found V5 more active for moving dots compared to static - brain damage to V5/MT leads to akinetopsia
what is cortical achromatopsia?
patients with this cannot see colour due to damage to V4, but also often due to V2 and V3
what is the binding problem?
sighted people don’t perceive the colour of something separately to its shape, but our brain processes this separately - where in the brain is something perceived?
what is the ‘where’ pathway?
the where pathway: the parietal (or dorsal) processing pathway - concerned with movement processing - “vision for action”
what is the ‘what’ pathway?
the what pathway: the temporal (or ventral) processing pathway - concerned with colour and form processing - “vision for perception”
what are the four steps in the model of object recognition?
- early visual processing: colour, motion, edges etc.
- perceptual segregation grouping of visual elements (gestalt principles, figure-ground segmentation)
- matching grouped visual description onto a representation of the object stored in the brain (called visual descriptions)
- attaching meaning to the object (based on prior semantic knowledge)
what are the gestalt laws of perceptual organisation?
the law of proximity, similarity, good continuation, and closure
what is figure ground segregation?
ambiguous drawings for example, face-goblet illusion - whichever you perceive to be in front of the other, it is assumed that more attention is paid to the figure than the ground
what are some problems with gestalt psychology?
- segmentation processes aren’t always bottom-up and following the laws of perceptual organisation - perceptual segregation processing is faster with prior experience with the objects in question
- most evidence descriptive not explanatory
- relies heavily on introspection and evidence from 2D drawings
- some segmentation clearly occurs via top-down prior knowledge
what is agnosia?
the impairment in object recognition
what are the two types of agnosia and what are their differences?
apperceptive agnosia and associative agnosia
what is apperceptive agnosia?
impairment in processes that construct perceptual representation from vision - seeing the parts but not the whole - this is associated with lateral occipital lobe damage
what is associative agnosia?
impairment in the process which maps a perceptual representation onto knowledge of the object’s function and association - seeing the whole but not its meaning - this is associated with occipito-temporal damage
are faces “special” in terms of recognition?
we don’t know, but probably
1. there is evidence faces are processed more holistically than other objects
2. there is evidence for face-specific brain regions, thought this could be a visual expertise region too
3. after brain injury, some patients appear to have impairment in face but not object recognition, suggesting that faces are special
