3 Cortical Structure And Function Notes

Organisation of visual cortex

1. Retinotopic maps

2. Magnification factor

3. Orientation columns

4. Ocular dominance columns

5. Hypercolumns

Striate cortex = V1 = visual area retinotopic organisation within striate cortex

Extrastriate cortex = V2-V5

V1 in occipital lobes

V2 in dorsal and ventral lobes = prestriate cortex – primary region visual assoc

V3 in dorsal and ventral lobes – located in Broadmann’s area 19

V4 in ventral lobe

V5 in temporal lobe

Retinotopic maps

= measuring the spatial rep of the retinal image in the cortex helps to understand how the visual world is represented

Describes where diff locations on the retina are represented

Tootell et al., 1982: V1 is retinotopically organised

• Injected a monkey with deoxiglucose that is taken up by active neurons

• Showed the monkey a flickering grating looked at the cortical tissue

• Saw a grid-like pattern of activity in the cortical tissue after the monkey had looked at a grating for a prolonged period

visual stimuli are laid out in an orderly and organised way in the visual cortex = retinotopic organisation

Spatial RFs:

When measuring the RF locations of V1 neurons that are in the same column perpendicular to the cortical surface = they are in approx. the same location

A move of 1mm = separate RF

When moving along the cortical surface = RF locations will change in an orderly fashion, corresponding to the locations in the visual image

Hypercolumns = contains neurons sensitive to all orientations and to both eyes

Each hypercolumn represents all features in one location of visual space

Cortical magnification

There are more neurons that have RFs in the fovea than there are neurons with RFs in the periphery

ie there is more cortical tissue devoted to analysing foveal input

= allows us to resolve fine details in the fovea better acuity in fovea than periphery

Magnification factor = the factor by which the foveal representation is magnified

highest in the fovea and decreases monotonically toward the periphery

Why don’t we have such a high acuity rep of the whole visual space?

Lack of resources

We do not have enough brain to have a high-res rep of both fovea and periphery

evolution has favoured the foveal rep

Orientation columns

= the columns that contain neurons with the same orientation preference

Most neurons in V1 have an orientation preference either vertical or horizontal bars

Neurons are well organised according to their preferences in the cortical tissue

Neurons in the same vertical column have the same:

• RF location

• Orientation preference

similar orientation selectivity in a radius of 30-100micrometer

Moving along the cortical surface orientation preferences change gradually covering the whole 180 degrees in 0.5mm of tissue

Each column contains simple, complex, and end-stopped cells

Visualising columns preferring vertical orientations:

Revealed using deoxyglucose staining

Distance between columns = 0.5mm

Wong-Riley, 1979:

Used a cytochrome oxidase stain

CO blobs are revealed with a particular staining method

Blobs = sections of the visual cortex where groups of...

Buy the full version of these notes or essay plans and more in our Perception (2nd year) Notes.