Evidence of cortical reorganisation following abnormal visual input?

Question 1

Hoffman (2003)

Answer 1

(2003)
Found that input from ipsilateral VF formed a mirror image map in v1 that is superimposed on the normal map of contralateral inputs from the nasal retina
- Little sign of suppression seen
- Therefore, one voxel in PVC represents two locations in opposing visual fields
- Suggests an absence of reorganisation

Question 2

Hoffman et al (2012)

Answer 2

found a similar cortical pattern in Achiasmatic PS (no decussation)

• again, suggests no reorganisation.
• However, subtle intra-cortical plasticity must occur to prevent errors that would arise from attempted binocular summation of a now non-matching input in these PS

Question 3

Baseler et al (2002)

Answer 3

PS = have only rods (lack cones, therefore colour blind and central field deficit (due to no rods in fovea)
Results
LPZ (occipital pole) activated by stimuli in more eccentric locations than those that drove responses in controls
THEREFORE
V1 has reorganised by remapping representation of eccentricity

Question 4

Ferreira et al (2017)

Answer 4

investigated cortical remapping in 13 PS with Retinitis Pigmentosa, a degenerative disorder leading to peripheral visual degeneration

• Found retinotopic eccentricity shift from central inputs to more peripheral locations in V1
• The shift was associated with extent of visual loss, but not with age of onset.
• Suggests that regardless of the age of onset, the visual system retains plasticity to remap even when RP symptoms started in adulthood.

Question 5

Evidence of cortical reorganisation following MD has been observed under certain conditions…

Answer 5

The first is when PS with MD participate in an active visual task as seen in Baker et al (2005). They reported activity in the LPZ of 2 PS when viewing images presented to their PRL.
When Masuda et al (2008) investigated this further they only observed activation in the LPZ during the active task and not the passive task in 4 PS with JMD. This suggests that the activity observed is not reorganisation but instead, feedback signals as observed in normal processing.
However, when Baker et al (2008) performed a follow up study using the same PS who did a passive and an active task they found activation suggestive of reorganisation in both task types.

Question 6

One explanation for discrepancy may be PS with complete vs partial foveal deficits. Baker et al (2008) compared these 2 populations with an active task presented to PRL…

Answer 6

Only PS with complete central deficit elicited cortical responses in LPZ
THEREFORE
Remapping of visual processing only occurs after complete foveal loss
HOWEVER…
Masuda et al (2010)
Tested PS with peripheral deficit (foveal vision only) in active and passive task
Results
Active task caused activity in LPZ (passive task did not)
THEREFORE
Therefore task-dependent stimulus-related activity in LPZ is not just due to absence of foveal sensitivity
LPZ activity due to foveal sensitivity being an island within visual field deficit
- so long as this island receives different input from its surroundings

Question 7

Pascual-Leone and Torres (1993)

Answer 7

recorded somatosensory evoked potentials from proficient blind braille readers and demonstrated that the cortical representation of the index finger was larger than that of sighted controls and blind non-Braille readers.
Furthermore, it was enlarged compared to the homologous finger of the opposite hand.
These findings suggest that braille reading, in the absence of sight, creates new motor demands on the reading finger resulting in cortical ‘over-representations’.

Question 8

Veraat et al

Answer 8

(1990) found visual cortex activity was higher in EB PS compared to blindfolded controls
- This lead to the assumption that the visual brain does not lay dormant and unused - perhaps other senses have utilised this cortical space for enhanced processing?

Question 9

Weeks et al, 2000

Answer 9

Using PET, Auditory localization tasks activate the visual cortex (V5) of the congenitally blind more than in controls

Question 10

Renier et al (2013)

Answer 10

using fMRI looked at occipital recruitment of higher level odor tasks in early blind PS
- Strong activation of occipital cortex found during discrimination and categorization

Question 11

Ptito et al, 2005

Answer 11

investigated neural correlates of tongue stimulation in 6 congenitally blind PS vs blindfolded controls using PET

• Both blind and controls learned (following 7 days training) the letter orientation task to the same degree of accuracy
• After training only, large occipital rCBF increases were seen in blind PS only
• The fact that they were congenitally blind, and no such activity was seen in controls make the results unlikely to be due to mental imagery

Question 12

Sadato et al (1996)

Answer 12

showed that PVC is activated in EB PS performing a braille reading task

• activity not observed in controls or during passive sweeps
• As well as tactile crossmodality, this occipital recruitment may have also been driven by lexical processing…

Question 13

Amedi et al (2003)

Answer 13

found V1 activation during verbal-memory task

- Magnitude of activation highly correlated with blind individuals ability, suggesting a functional role

Question 14

Support for functional role of occipital recruitment…

Answer 14

TMS to occipital cortex disrupts Braille reading only in blind PS (Cohen et al, 1997)