visual processing in the brain Flashcards
what does the brain want to know from visual information
- conscious experience of our environment involves knowing; what things are,
- the retinogeniculate pathway is largely responsible for conscious visual experience.
visual input to the PVC
- majority of ganglion cell axons go back to the PVC
contralateral arrangement
signals from one side of the visual field are processed by the opposite side of the brain.
retinotopic organization of the primary visual cortex
- extracellular animal studies
retinotopic organization - cells close together get input from same region of the retina
- where an object is important for vision
- cortical magnification: more cortical cells devoted to the fovea than periphery
functional organization of PVC
- visual cortex: 6 layers based on visible anatomical differences
- input to layer 2/3 and 4 from the LGN
cortical column modules - process information in small region of space
- but their tile the entire visual field
blobs- - color
interblobs - orientation, motion, depth
specialized visual areas that revive input vial the PVC
- modular organization: each area has neurons with receptive fields that tile the visual field
modules have slightly different function, focusing on a particular aspect of the visual scene
hierarchal feed forward model
- neurons responses become more specific
- although lots of feedback and interconnection
the two-streams hypothesis of visual processing in the brain
- Dorsal and ventral stream:
- the ventral stream computed a detailed map of the world from visual input
- the dorsal ‘action’ stream transforms incoming visual information for action.
- also referred to as the ‘where’ and ‘what’ pathway
- the independence of the two streams had been overemphasized.
colour processing pathway
- wavelength sensitivity in the retina. speech sensitivity of L, M and S cones.
- congenital color blindness
- circulate center-surround cone opponent receptive fields in the retinal ganglion cells.
- functional segregation evident in different layers of the LGN
R-G opponent ganglion cell
- excited by red light
- inhibited by green light
- (also have G-r)
Y-B opponent ganglion cell
- excited by yellow light
- inhibited by blue light
areas responsive to color
- area V4 is highly responsive to color, subsequent work has shown that V4 is also responsibe to changes in shape and curvature
- damage to corresponding region in human cortex causes color blindness.
- achromatopisa
motion processing pathway
- retinal ganglion cells and LGN neurons respond to moving stimuli but
- stimulus could be moving in any direction
- stimulus could even just be turning off and on
- although Mangnocelluar neurons are particularly responsive to motion.
SO cells in V1 are orientation selective: respond to moving bars or edges with specific orientation
MT neurons influence the perception of movement
- the medial temporal-temporal (MT) area in the monkey has neurons with large receptive fields that are direction selective
- damage to the MT in the monkey reduces the ability to judge movement
- stimulation of individual neurons in MT biases perceived direction of motion
- motion selective neurons show adaptation.
MST integrates local motion
- the media-superior-temporal (MSt) area in the monkey has neurons that are
- sensitive to optic flow (movement of the world cause by self motion)
- responsive to complex movement
- associated with the perception of biological motion.
motion blindness (akinetopsia)
- caused by bilateral damage to MT (rare due to small size and location of MT)
- case of LM
- sees the world in snap shops
- unable to judge speed and therefore predict future position of moving objects
- BUT could see biological motion (so MT is not involved in perceiving structure from motion)
types of colour blindness
Deuteranopia= green cones are absent or less functional
Protonopia= red cones are absent, means its difficult to distinguish between shades
retinotopic arrangement
