Evidence for cortical reorganization in Albinos?

Question 1

what is albinism?

Answer 1

congenital disorder
reduced pigment melanin
results in array of visual abnormalities

Question 2

Visual abnormalities?

Answer 2

abnormally crossing temporal retinal fibres (TRF) at chiasm

Fewer ganglion cells in fovea

Question 3

Line of Decussation

Answer 3

LOD is shifted 2-8deg into temporal retina

Question 4

Three patterns of cortical reorganisation?

Answer 4

Contiguous / Boston (Hubel and Weisel, 1971)
Interleaved Suppressed / Midwestern (Kaas and Guiller, 1973)
Interleaved / True Albino (Hoffman et al, 2003)

Question 5

Siamese cats (Kaas, 2005)

Answer 5

Have a high number of abnormally crossing TRFs

Therefore, can be used to study albinism.

Question 6

Creel (1971)

Answer 6

Monocular enucleation on tabbies and siamese cats
After ME: Siamese cats had a strong contralateral response and diminished ipsilateral response
Whereas, Tabby cats have equally lateralised VEPS.

Question 7

Hubel and Weisel (1971)

Answer 7

Boston/Contiguous pattern
Siamese Cats
Found V1 representation of the contra. VF (normal)

Plus, 20 degrees extra ips. VF due to the extra input from the TRFs of the contralateral eye.

Binocular neurons with nearly match RF not found.

Extra 20 degrees represented in area along the boundary of V1/V2.

Terminating in a reverse topographic pattern.
Contiguous map of both ips. and contra. VFs in V1.

This representation requires pre-cortical plasticity and altered geniculostriate connections.

Question 8

Kaas and Guillery (1973)

Answer 8

Interleaved Suppressed representation/ Midwestern

Siamese cats

Interleaved representation of LGN inputs of opposing VFs.

Abnormal ips. VF inputs failed to activate any V1 neurons.

Indicates suppression of abnormal input.

This suppression indicates a more conservative geniculostriate correction.

Normal visual input as long as both eyes open

One eye = 90 deg.

Question 9

Hoffman (2003)

Answer 9

Interleaved Pattern/ True albino
Human PS
Abnormally crossing TRFs form a mirror image map in the striate cortex that is superimposed on the normal retinotopic map of the nasal retina
Same pattern in extra-striate areas
Little sign of suppression
THEREFORE
One voxel in the PVC represents two locations in space, one in each visual field at the same eccentricity.

Question 10

Hoffman et al (2012)

Answer 10

Human achiasma
Same pattern of organisation found as seen in the interleaved pattern:
Overlay of retinotopic maps from opposing hemifields in VC
Each cortical location represents two VF locations
Two non-mutally exclusive reasons for this:
1. bilateral receptive fields
2. hemifield dominance columns
THEREFORE
conservative developmental mechanisms
And subtle intra-cortical plasticity (to prevent errors that would otherwise occur from binocular summation)

Question 11

Von dem Hagen (2005)

Answer 11

Found decreases in grey matter volume at occipital poles in albinos
Location of decreases in grey matter corresponds to the cortical representation of central visual field
Reduction likely to occur as a direct result of decreased ganglion cell numbers in central retina

Question 12

Guillery et al (1974)

Answer 12

MD of 3 midwestern kittens
All three kittens developed 135 degrees of vision in non-deprived eye
THEREFORE
the cortex monitors inputs coming from the LGN in terms of sequences coming from individual layers, allowing two parts of the VF to be represented

Question 13

Welton (2017)

Answer 13

Found significant functional interhemispheric hyperconnectivity of the primary visual areas in the albinism group (p=0.012).
Also found differences in whole-brain network metrics in albino PS

signals from the right visual field may cross back to the right visual processing areas (and vice-versa) at some point “downstream”; the most direct route being between the primary visual cortices via the splenium of the corpus callosum