Is this visual system plastic in childhood only? Flashcards
Intro
Based on work by Hubel and Weisel (1963), it was assumed that visual system (VS) plasticity was confined to a ‘critical period’ early in life.
Short term visual deprivation after that time causes no permanent damage
Too late to offset the effects of early visual deprivation after about 6 years of age when acuity is adultlike (Birnbaum et al, 1977)
However, surprising evidence for adult plasticity can be observed in the following scenarios: adult recovery from early blindness; adult recovery from amblyopia; evidence suggesting remapping in the adult brain following Macular Degeneration; evidence suggesting cross-modal plasticity following the loss of vision.
Fine (?) - Huber et al (?)
Gregory and Wallace (?)
Fine (2003) Patient MM
Lost vision at 3.5 years old, restored at 46 years old in remaining eye
Normal perception of colour and motion (supported by fMRI)
Severe deficits in complex form, object and face processing (supported by fMRI)
After 10 years of visual experience…. No improvements (Huber et al, 2015)
THEREFORE
Period of sensitivity for these preserved abilities may end prior to 3.5 years
High-level visual abilities still sensitive to deprivation at 3.5 years
No improvement = limited plasticity in adulthood
Similar results have been reported in other cases of recovery from early blindness (e.g. Patient SB) which suggest that the visual system has limited plasticity in adulthood (Gregory and Wallace, 1963).
Ostrovosky (?)
Ostrovosky (2006) - Patient SRD
Bilateral cataract from birth, restored at 12, tested when 30
High performance on form and face perception = suggests plasticity
However, long period in between restoration and testing. Therefore, SRD may have become proficient at utilising contextual cues to compensate for deficient vision
Moreover, recovery happened in adolescence, so any plasticity driven recovery suggests plasticity in the adolescent brain rather than the adult brain.
Amblyopia is assumed to be untreatable in adults, However… To et al (?)
Cases of adult amblyopia are also a useful tool for examining adult visual system plasticity.
Amblyopia is reduced acuity in an otherwise normal eye as a result of abnormal visual experience during childhood.
It was previously assumed that amblyopia if left untreated during childhood, caused irreversible deficits (Repka, 2003).
However, recent evidence suggests that the visual system retains enough plasticity to allow for improvements in acuity and depth perception in adult amblyopes following perceptual learning.
For example, To et al (2011) developed a training paradigm based on the popular game ‘Tetris’ using a lenticular array surface meaning that the two eyes saw different views.
They calibrated a balance point of contrast in which the amblyopic eye was contributing to the percept as well as the good eye and this balance point was adjusted daily with the end goal of having equal contrast between the two eyes.
At the end of the study, contrast between the two eyes was sig. Reduced for all PS and 6/9 PS could tolerate the same contrast to both eyes
Sig. improvement in acuity and depth perception
Suggests that the VS retains plasticity in adulthood
So, behavioural plasticity seems achievable for some recovering blind people (SRD) and man amblyopic PS. What about plasticity as measured by cortical reorganisation?
-Schumacher et al (?)
Age related macular degeneration results in atrophy of the macular, resulting in loss of central foveal vision. Due to this loss, patients develop a prefered retinal locus in the periphery, which acts as a compensatory ‘pseudo fovea’.
Schumacher et al (2008) stimulated PRL nd non-PRL regions in MD suffers
RESULTS
Stimuli presented to PRL elicited activity at occipital pole (normally representing foveal vision)
5/6 patients suffered from AMD suggesting that this cortical remapping can occur later in life after disease onset
Evidence for adult visual system plasticity
Ferreira et al (?)
Ferreira et al (2017) 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.
Burton et al (?)
Cohen et al (?)
There have been numeous reports of crossmodal plasticity following the onset of blindness.
For example, Burton et al (2002) reported that the visual cortex is activated during tactile stimulation (Braille reading)in blind patients
While some evidence suggests that this plasticity only occurs if they have early-onset blindness (Cohen et al, 1999), there is some evidence to suggest that the cortex retains plasticity in adulthood
Pascual-Leone and Hamilton (?)
Pascual-Leone and Hamilton (2001) showed that a five day period of complete blindfolding was enough to induce functionally relevant occipital recruitment in response to tactile processing in healthy adult subjects
It was defined as functionally relevant as it was correlated with improvement in braille reading proficiency across the five day period and the blindfolded PS showed an advantage compared to controls
This supports the notion that the VS retains plasticity well into adulthood as even a short period of visual deprivation led to dramatic changes in cortical functioning.
Clearly the speed at which these changes occurred suggests that this plasticity is mediated by an unmasking of latent connections rather than the establishment of new connections
Nonetheless, this is evidence of plasticity at the cellular level in that the weighting of synapses is changed in response to changed input, even in the adult brain.
Conclusion
Recovery from early blindness studies suggest that if vision is restored late in adulthood, there will be limited recovery of higher order functions as seen in patient MM.
However, if vision is restored in adolescence, there is more visual system plasticity and greater improvements can be expected.
Recent cases of adult amblyopia treatment suggest that the visual system retains plasticity in that improvements in acuity and binocular function are observed following behavioural training which aims to alleviate suppression of the bad eye which outlast the treatment itself.
The extent to which the visual cortex ‘remaps’ itself following retinal lesions in adulthood remains unclear.
- Some studies (Ferreira et al (2017)) report large scale visual remapping and conclude that this is evidence of adult plasticity.
- However, caution must be applied in interpreting the significance of the observed activation as it may simply reflect that changes were caused by the absence of visual stimulation alone and are thereby not evidence of cortical remapping or plasticity (Baseler et al, 2011)
Cross-modal plasticity reports are mixed with some suggesting that it only occurs in early-blind PS.
However, blindfolding study suggests that even in adulthood, the visual system and show plasticity on the cellular level in light of visual deprivation.
In conclusion, it seems that behavioural and cortical plasticity in adulthood is possible although limited. The adult visual system is certainly far more plastic than was concluded previously by authors such as Hubel and Weisel 91963) and has the capacity to recover at least somewhat under the right conditions.
However, whether this is indicative of lower order neural adaptations or higher order cortical remapping remains unclear.
Vereecken et al
1984
Plasticity in adults with amblyopia is also evident in the report of amblyopic patients whose visual acuity spontaneously improved in the wake of visual loss due to macular degeneration or other forms of vision loss in the fellow (non-amblyopic) eye.
