PSYC 304 Vision

light path thru eye

• light enters thru cornea

• passes thru pupil, controlled by iris

• focused by lens (via ciliary muscles)

• light hits retina, meeting the photoreceptors

pathway of light from retina to V1

• retina → optic nerve → optic chiasm (visual field crosses)

• → LGN in thalamus

• → optic radiation → V1

retinotopic map

adjacent points on retina have adjacent neurons in LGN and V1

• this is the physical layout of how the retina is preserved

• spatial layout of visual field is preserved into adjacent neruons on the brain

fovea representation on retinotopic map

overrepresented because it is very detailed, requireing more cortical space for processing power

visuotopic map

how locations in visual space (what we see) are mapped in the brain

• because the retina receives a flipped image:

  • left visual field is mapped to right brain and vice versa

• this map represents one’s view of the world, not the retina’s layout

hierarchy of visual processing

• layered from simple to complex features

• each stage builds on info processed at early stages, creating a whole percept

stages of visual processing

1. RGCs respond to light in circular fields

2. V1 perceives objects and events (building blocks)

3. V2 combines building blocks of V1

4. V4 creates a strong response to concentric and radial stimuli, sometimes colour

5. V5/medial temporal for motion perception

6. inferior temporal for complex shapes, sometimes colour/textures

RGC receptive fiedls

• on-center or off-center organization

• this helps detect light/dark spots, contrast

V1 simple cells

respond to bars and edges of specific orientation

• only fire when stimulus is exactly within receptive field

V1 complex cells

respond to specific width and orientation

• but, bars can be anywhere within a larger area of visual field

IT neurons

• large receptive fields (half/most of visual field)

• shape and object selective

• detect whole objects, not just lines or contrast

  • experience-dependent

cortex is organized into…

columns of 1mm wide vertical units

neurons of a column

respond to the same visual feature, eg: orientation

neocortex

• organized into columnar structure of 6 layers

• 1 = outermost, 6 = deepest

layer 4

receives all sensory input from thalamus, which distributes vertically wtihin the cortex

layer 2-3

cortico-cortical communication

• receive input from layer 4 and send/receive signals from different cortical areas

layer 5

sends info to subcortical areas (below cortex, eg: superior colliculus, brainstem, etc)

hypercolumn

full sets of columns for a small part of the visual field

• eg: full range of orientation (cells for every angle)

• each column alternates between left and right dominance

• contains blobs

• each hyper column covers 1 part of visual field, about 1 degree

blobs

cells in hypercolumns that respond to colour

ventral stream pathway

V1 → V2 → V4 → inferior temporal cortex

ventral stream function

• object identity: shape, colour, sound info, etc.

• important for semantic knowledge

posterior ventral stream features

perceptual properties (what it looks like)

• shape, colour, texture, etc.

anterior ventral stream features

associative, memory properties (what it means)

• recognizing face, linking objects to memory, identifying familiar things

damage to ventral stream results in…

visual agnosia

dorsal stream pathway

V1 → V2 → V5 (medial temporal) → parietal lobe

dorsal stream function

motion, spatial awareness, action, location of objects

damage to dorsal stream (also posterior parietal) results in…

optic ataxia

optic ataxia

know what the object is, but cannot interact with it accurately

• eg: cannot guide hand to grab an object in space

optic ataxia is caused by…

dorsal stream damage, posterior parietal

visual agnosia is caused by…

ventral stream damage - inferior temporal

visual agnosia

can interact with an object, but cannot identify or name what it is

subtypes of visual agnosia

• apperceptive agnosia

• associative agnosia

apperceptive agnosia is caused by…

damage to occipitotemporal area

• more posterior (lower-level processing)

apperceptive agnosia

• cannot form full percept

• difficulty recognizing shapes, names of objects

associative agnosia is caused by…

damage to fusiform gyrus

• more complex process (more anterior) to associate the identity of an object with its meaning

associative agnosia

cannot link percept to its meaning

• person can perceive an object, know what to do with it, but cannot name/identify it

• eg: finishing a puzzle, but not knowing what the picture is of