perceiving our world

retinogeniculate pathway

transmits info from retina to primary visual cortex via LGN = largely responsible for conscious visual experience

primary visual cortex

striate cortex/V1 in occipital lobe

damage to V1

often due to stroke in posterior cerebral artery = cuts off oxygen and glucose to V1 = irreversible damage = loss of visual processing in affected cortical area

hemianopia

loss of vision in 1 half of visual field

• damage to right V1 = loss of left visual field

• compensate by making head and eye movements toward blind side

Retinotopic map

spatial organisation of visual input in V1 that preserves spatial layout

calcarine sulcus

deep longitudinal groove located in the occipital lobe that contains the primary visual cortex (V1) - visual mapping is contralateral and split vertically

• upper bank of calcarine - lower visual field

• lower bak = upper field

cortical magnification

unequal amount of V1 devoted to different parts of the visual field

• more cells for processing info from fovea than peripheral = importance of fovea for understanding details

visual cortex organisation

6 layers organised in 2500 cortical column modules - each module processes info from small portion of visual field. modules contain cytochrome oxidase blobs and interblobs

visual cortex layers

each layer receives different inputs from LGN

• magnocellular - layer 4Cα

• parvocellular - layer 4Cβ

• koniocellular - layers 2 and 3

output to visual areas from layer 3

cytochrome oxidase blob

mainly 1 eye input and for colour processing

interblob regions

processes orientation, motion and depth/binocular disparity and not sensitive to colour

simple cells

neurons in primary visual cortex and receives input from LGN neurons with adjacent receptive field = respond to presence of bars, lines, edges and orientation of visual stimuli.

• has ON and OFF cells

• if light passes thru ON and OFF = no change in firing

Hubel and Weisel

found neurons are orientation-selective after used electrodes to record cat’s visual cortex

• these neurons respond most strongly to bears or edges at specific angles

• different from retinal and LGN that respond to light and constrast

• 1st evidence that V1 begins analysis of form and shape in perception

major specialised visual areas

• V1 - initial processing of visual features

• V4 - colour

• Middle temporal area (MT) - motion perception

• inferior temporal cortex (IT) - complex objects and facial recognition

two-stream hypothesis

After V1 info processed along 2 major pathways

• ventral (what) - objects and visual recognition

• dorsal (where/how) - location and action

they crosstalk

ventral stream

• V3 + VP - further analysis from V2

• V3A - info from contralateral visual field

• V4 - analysis of form and colour constancy

• V8 - lateral occipital complex - colour perception

• LO - object recog

• fusiform face area - facial and expert object recog

• parahippocampal place area - place/scene recog

• extrastriate body area - percept of body parts and shape

Dorsel stream

• V7 - attention and controls eye movements

• Medial temporal/medial superior temporal - perception of motion and bio motion/optic flow in spec subregions

• lateral intraparietal area - attention and controls saccades

• ventral intraparietal area - attention to spec locations and controls eye pointing

• anterior intraparietal area - hand movements

• middle intraparietal - reaching

cone opponency

• red-green (R-G) - parvocellular - excited by red light and inhibit by green (als G-R)

• yellow-blue (Y-B)- koniocellular - excited by yellow and off by blue (also B-Y)

achromatopsia

after damage to V4 loss of colour in contralateral field (1/2 in colour and ½ black and white)

Why are retinal, LGN, and V1 neurons insufficient for accurate motion perception?

• retinal and LGN - ambigious response to moving stimuli = could be moving in any direction

• V1 cells are orientation selective and respond to moving bars but not good enough for full perception

medial temporal-temporal area

V5 - has neurons that respond to movement and can adapt when exposed to a continously moving stimuli = motion aftereffect

motion aftereffect

prolonged exposure to a moving stimulus = stationary object appears to moving in opposite direction

medial superior temporal area (MST)

neurons sensitive to optic flow (movement of world due to self motion) and perception of biological motion

akinetopsia

bilateral damage to MT = unable to perceive motion

• LM case = saw world in snapshots and can’t judge speed = can’t predict future position of objects. could still see biological motion