Vision 3

What are 3 features of the retinotopic map within V1?

• inverted

• horizontally flipped (right VF in LH)

• distorted (cortical magnification)

What is cortical magnification?

there are more cells representing the fovea than the visual periphery

Why does fovea take up only 1% of retina but 50% of V1?

• only cones which have higher visual acuity as not many to one so good detail at the fovea

• at fovea only photoreceptors minimises bluriness so again higher visual acuity

• all good for evolution

Who mapped out the retina, focusing on peaks and declines of photoreceptors distribution in retina?

Curcio et al (1990)

How many layers are in the striate cortex (V1)?

6

Where do most inputs from the thalamus go to?

layers IV and V

What happens in other layers?

• higher processing in layers 2/3

• outputs mostly from layers 2/3 corticortical

• ouputs also from layers 5/6 subcortical

Whats one attribute V1 neurons show?

orientation tuning

Who investigated orientation tuning and how?

Hubel and Wiesel (1959)

• microelectrode recordings in cat V1 while presenting bars of light against dark background

• cells in V1 layer IV show orientation selectivity respond most strongly to bars in a particular direction

• most be located in particular location within neurons spatial receptive field to elecit response (simple cells)

How is simple cell orientation tuning computed?

• combining four centrre/surround fields produces an orientated bar receptive field in V1 simple cells

What is the difference in complex cells orientation tuning?

• they will respond to orientation anywhere when presented within neurons spatial receptive field

• no inhibitory surround - don’t have to be presented in middle of receptive field

What else are complex cells attuned for?

motion direction selectivity

How is orientation tuning organised within V1? (take sulcus and drive electrode perpendicular through)

• cells throughout cortical layers prefer the same orientation

• clusters of cells show similar preferred orientations in a ‘pinwheel’ arrangement across the cortical surface

Do cells in V1 have occular dominance?

yes - most cells respond more robustly to inputs from one particular eye

How is eye preference arranged?

into occular dominance columns across the V1 surface and throughout the cortical layers

How do you investigate ocular dominance?

• inject retina with marker

• taken upoptic tract

• lands in V1

• can do in one eye or other

• track where in visual cortex you’re getting input from each eye

What’s another thing that cells in V1 are attuned to?

colour contrast

How can we see colour contrast in those cells?

• some colour sensitive cells show circular receptive fields with centre excited by one colour and inhibited by the other

• in surround the pattern is reversed

Where might we find colour double opponent cells?

• in blobs

  • cytochrome oxidase staining in V1 shows ‘blobs’ of staining in layers 2/3

  • blobs get heavy input directly from the colour sensitive k-cells in LGN

What is a V1 hypercolumn?

a local region of V1 that may contain all the possible tuning information associated with one region of the visual receptive field

What is the extrastriate cortex?

the part of the visual cortex located next to the striate cortex involved in processing specific features of visual information

(higher areas may inegrate sensory/motor etc)

What are V2 neurons more responsive to?

• illusory contours, binocular disparity, figure/ground responses, patterns

• more complex receptive fields than V1

What are V4 neurons responsive to?

colour, orientation, spatial frequency, figure/ground, shapes

What can damage to V4 lead to?

lack of volour vision - achromatopsia

What are responses in V4 strongly modualted by?

attention and show long-term plasticity - change firing through learning process

What is the FFA specialised for?

• responding to faces

• also perhaps to visual images for which individual is an ‘expert’

  • Gauthier et al (2000) — cars and birds

• damage leads to prospagnosia

What is V5/MT specialised for?

• visual motion perception

• most to a particular direction

• most not orientation or colour selective

Two ways to measure V5/MT response?

• microelectrode (extracellular) recordings

  • response of macaque MT neurons to their preferred direction is dependent on dot coherence

• microstimulation

  • can cause monkey to think the dots are moving in their preferred direction

What are the dual processing streams (Mishkin and Ungerleider, 1983)

ventral = what the object is

dorsal = where the object is

What is cortical blindness?

individuals with lesions to V1 report being blind in the corresponding visual space yet can grasp objects, avoid walls etc

Possible explanation for cortical blindness?

• loss of visual awareness but not complete loss of vision

  • V1 = place of conscious perception of inputs

• extrastriate cortex respond to images presented in cortically ‘blind’ field

• information is reaching extrastriate visual cortex perhaps via

  • opptic tract - superior colliculus - secondary thalamus - EC

What is associative agnosia?

difficulty recognising visual objects

What damage causes associative agnosia?

• brain lesions located bilaterally in occipito-temporal regions (ventral stream)

• can copy rawingss and match obkects though so visual perception preserved, simply cannot identify

What is apperceptive agnosia?

• failure of visual perception

• cannot identify, copy or match visual objects

• patient DF

  • lesion to ventral

  • changed dorsal to ‘how’ stream

What is akinetopsia?

• damage in human V5 and macaque MT - motion blindness

• dorsal stream

• Zeki (1991) - coffee pouring

What is the double dissociation?

• dorsal lesions

  • goof visual recognition

  • poor movement vision and visual guidance of motor output

• ventral lesions

  • opposite