How important is early visual experience in shaping later vision? Flashcards
Stryker
(1978)- raised kitten with one eye viewing horizontal and one eye viewing vertical
Three key findings:
1. Majority of cells not selective for orientation
2. Cells with preferred orientation near the exposed orientations would respond only to the eye exposed to that orientation
3. Orderly arrangement of cells according to orientation
What do Stryker’s finding support?
Selectional Hypothesis
Cortical neurones that receive appropriate stimulation during early visual experience maintain their innate orientation preferences, while the remaining cells lose visual responsiveness or selectively
Supports a strict Hebbian view
Held and Hein
(1963)- Kitten Carousel
Only the kitten with active participation had depth perception
Supports Hebbian view - cat that didn’t actively use vision, lost some aspects of it.
Baker et al (2007)
Characterised by impaired vision in one eye without any obvious defect. Results in imbalance of inputs between the eyes.
If good eye is patched before the age of 6, visual function in the amblyopic eye has been found to improve (Birnbaum et al, 1977). However, improvement is limited if done after age of 6, suggesting that there is critical window of plasticity.
Consequently, EVE coupled with intensive unilateral input to the amblyopic eye seems paramount to gaining normal vision later in life for amblyopia sufferers.
However, Baker et al (2007) showed that EVE may not be so critical in improving amblyopic vision.
Found that when stimulus contrast to the amblyopic eye was increased, adult PS achieved normal BSRs. This suggests that rather than a complete loss of input from the amblyopic eye, the signal may just be attenuated due to an imbalence in monocular signals before the point of summation.
Therefore, if the critical period of improvement during EVE is missed, it may not result in a pruning and loss of binocular neurones, instead, the input from the good eye may just be dominating the input. For amblyopic PS, EVE may not be as crucial as once stressed.
LeGrand
(2001)- bilateral cataract from birth to 2-6m/o
No patterned visual input during this time
After 9 years of recovery, deficits in configural processing (spacing of features)
Other aspects of face processing normal (could distinguish faces varying only in shape of features)
Therefore, EVE is necessary for normal development of the neural architecture that will later specialise for the configural processing of faces, (a considerably higher-level function) but not all aspects of face processing.
Fine; Hubel
(2003;2015) - Patient MM - lost vision at 3.5y/o, restored at 46y/o
Normal perception of 2D form, colour and motion
-> concurrent fMRI
Deficits in complex form, face and object process
-> Lack fMRI catagory specific responses in ventral visual stream
- After 10 years, no improvements (Huber et al, 2015)
Therefore lower-level processing well developed before 3.5 and/or can develop without EVE, therefore less deficits to damage upon the loss of vision.
Higher order processes develop after 3.5 years and/or cannot develop without input, therefore HLV is based on visual architecture that is still sensitive to deprivation at 3.5 y/o. The fact that no improvement was seen, suggests limited plasticity in adulthood, stressings the importance of EVE for HLV.
Ostrovosky - in contrast to case MM
(2006) Patient SRD - bilateral cataract at 1.5 m/o, corrective surgery at 12 y/o
6 months post surgery, could identify family members
1 year later, could recognise household objects
20 years later, high performance in face and object processing
Some issues - e.g. RT on shape matching tasks longer and relied on head orientation to determine gaze direction
Therefore, visual cortex may retain plasticity up to 12 years
The previous research has been concerned with monocular deficits, but what about binocular deficits?
Children born with dense bilateral cataracts and then treated during the first year of life…
Children born with dense bilateral cataracts and then treated during the first year of life are normal at discriminating between large shapes (inc. facial features) and are normal at detecting direction of eye gaze. (Maurer et al, 1989; Geldart et al 1999)
- These skills exceed those present in birth in the visually normal child, therefore the neural circuits responsible for these skills have continued to develop in the absence of EVE and/or can recover completely after the deprivation.
- This suggests normal EVE is not necessary for the development of these particular faculties.
However, these children treated for bilateral cataracts have…
- Reduced acuity and impairments in higher visual functions persisting later into life (e.g. deficits integrating local elements into a global form) (Lewis et al, 2002)
- Suggests normal EVE is necessary for the development of higher-level visual functions.
- This can be seen in the following study… (Le Grand et al, 2001)
Evidence suggests that some aspects of vision rely heavily on normal EVE while other function seem to develop normally even in the absence EVE. For example…
(Global Motion)
Global motion (sensitivity to the overall direction of stimulus motion) can develop normally even when deprivation begins as early as 4 months of age.
- It is not the case however that at 4 months, global motion is fully developed: in fact, 4-month old infants are 7 times less sensitive than adults at detecting global motion. (Wattam-Bell, 1994)
- Therefore, there is a period of vast improvement in the absence of visual input, suggesting that for global motion, EVE is not a crucial prerequisite for normal motion detection later in life
- This is seen in case MM…
Conclusion
Early studies based upon cats’ visual cortex demonstrate classic Hebbian learning: those neurons tuned for orientations who did not receive appropriate input during EVE were pruned.
- Held & Hein also demonstrated if active participation in the environment of a cat is not achieved, the mechanisms governing depth perception are not formed. Becuase the “passive” cat was not able to “use” its depth perception to aid independent exploration, the neurons responsible were pruned.
However, evidence from Baker et al (2004?) contradicts this strict Hebbian view: It seems that under certain condition, binocular summation could occur. This suggests that the neurons bringing input from the amblyopic eye and those responsible for binocularity had not been pruned, instead, just attenuated.
Evidence from those with cataracts from an early age suggest that some visual functions are more reliant on normal EVE experience than others.
- Notably, it seems higher-level visual functions need a longer period of normal EVE, whereas lower level form and motion perception may crystallize earlier and also be able to develop without visual input.
In conclusion, EVE seems to guide Hebbian learning and determine which inputs should dominate. Whether the pruned input are permanently lost or just attenuated, is yet to be decided.
- Furthermore, EVE seems to be more critical for higher-level visual processes, than lower-level ones.
