Amblyopia: Crowding & Visual Function

Question 1

What is crowding?

Answer 1

Seeing objects amongst other objects harder than seeing them in isolation. In differentiating the target when surrounded be other visual stimuli
Stuart and Burian (1962)

The presence of adjacent contours influences our ability to discriminate / recognise objects both in normal and amblyopic eyes (Irvine, 1945)

Question 2

What are the features of amblyopic vision?

Answer 2

• Extensive crowding
• Reduced visual acuity,
• Contrast sensitivity, and position acuity
• Relative scotoma (often paramacular that responds less sensitively)
• Distorted vision
  Ciuffreda, Levi, & Selenow (1991); McKee, Levi, & Movshon (2003)

Question 3

What is Contour Interaction?

Answer 3

More similar the contours look, the more difficult they are to discriminate leading to a reduction in acuity as a result of an interference effect of flanking patterns.

Critical spacing is the smallest distance between flankers & target that allows correct identification (Jeon et al, 2010)

Question 4

What is visual masking?

Answer 4

Impaired ability to detect a stimulus when a ‘mask stimulus’ is presented briefly before, during or after the target, at the same flanking locations (Enns & Di Lollo, 2000)

Adding denser contours or, in visual masking, present something briefly and then cover with another target (differentiate by time differences). This is felt by amblyopes. Masking and crowding are related. Masking is more artificial so we don’t talk about this as much.

Question 5

What is visual masking related to?

Answer 5

Crowding (Petrov and McKee, 2006) as both refer to the effect of a “mask” pattern on the discriminability of a target stimuli (Pelli et al., 2004)

Question 6

Why are amblyopes so affected by visual masking/crowding?

Answer 6

Reduction in amblyopia thought be due to an interference effect caused by the flanking patterns

Question 7

How is VA related to crowding?

Answer 7

Stuart & Burian (1962) described a relationship between visual acuity & separation difficulty, so the lower the vision the greater the difficulty

Question 8

What are the neuronal theories as to why amblyopes experience crowding?

Answer 8

Miller (1954,1955) suggested that amblyopic eyes showed a loss of lateral inhibition (more lateral in the VF).

Flom et al (1963): Depth of the ability to discriminate is relative to the receptive fields that are simulated. In the periphery, vision is reduced due to larger receptive fields – “Scale Shift”. However, this is thought not to be the case due to the fact that crowding is independent of
letter/target size

Question 9

What is the lateral integration theory into the crowding effect in amblyopia?

Answer 9

Polat (1997) suggested that crowding may be caused by deficiencies in long-range spatial interactions (connections throughout the different cortical areas; may be beyond a V1 mechanism)

Second stage integration –pooling from the different visual cortical areas, V1 – V5 etc working together = long-range spatial interactions in visual cortex which may be affected in amblyopia. Lateral integration is mediated by these interactions to see clearly so in amblyopes we instead get lateral suppression and thus reduced VA.

Question 10

What is the fixation nystagmus theory into the crowding effect in amblyopia?

Answer 10

Burian (1969) attributed it to the fixation nystagmus in amblyopes being greater therefore the letter being fixated might be smudged by those adjacent to it (greater oscillation when looking at a visual target in terms of amplitude and thus may be less sensitive to visual change)

Fixation instability makes it more difficult to read and this further increases saccades when reading (Krista et al., 2017)

Question 11

What are some cortical theories of amblyopia?

Answer 11

Two-stage model

Detection – likely within the V1 cortex when integration occurs

• Threshold-elevation after-affect was reduced (after effect of the masking; finding the true stimuli harder to find after masking but if separating the eyes with 1 eye masked and 1 eye true or 1 eye crowded and 1 eye not then we still get masking/crowding effects; binocular interactions) (Blake 2006; Flom et al., 1963) = suggests cortical issue with the pooling of info between cortexes
• fMRI effects are evident in V2 (Arman, Chung and Tjan 2006)
• Crowding occurs similarly in chromatic and achromatic stimuli (tripathy and Cavanagh 2002)
• Cross-crowding between first and second order targets and flankers
  (Chung Li and Levi 2007)

Question 12

More than VA is affected in amblyopia. What do we mean?

Answer 12

1) Hyperacuity (dense stimuli)
2) Contrast Sensitivity
3) Orientation
4) Motion
5) Positional Acuity
6) Saccadic Latency

Question 13

How is hyperacuity affected in amblyopia?

Answer 13

Birch & Swanson (2000)

There is evidence that measurement of this shows abnormally low thresholds in strabismic amblyopia as compared to anisometropic.

May also suggest different aetiologies

Question 14

How is contrast sensitivity affected in amblyopia?

Answer 14

Pang et al. (2021)
Mild amblyopes = higher spatial freq affected
More severe amblyopia = all spatial frequencies more likely to be affected

Question 15

How is orientation affected in amblyopes?

Answer 15

Simmers et al (2005) showed a global orientation deficit in human amblyopia

Barret et al (2003) claimed that horizontal orientations least affected

Question 16

How is motion affected in amblyopia?

Answer 16

Simmers et al (2005) also found a global motion deficit, especially with second order contrast stimuli in human amblyopia

Orientation and motion deficits occur independently of each other

Question 17

How is position acuity affected in amblyopia?

Answer 17

Studies have also shown a positional inaccuracy amongst amblyopic subjects. However, it has been shown to improve with training (Li and Levi, 2004). Has been found to improve with training; improve behavioural receptive field. Scale shift improvement.

Behavioral receptive fields of the amblyopic brain change with perceptual learning.

Question 18

How is saccadic latency affected in amblyopia?

Answer 18

McKee et al. (2016)
Bigger saccadic latency in strabs and strab-aniso’s and intermittent strabismics
Even former strabismics they have improved CS but they don’t have improved saccadic latency

Different classifications of amblyopia are felt differently in the eyes!

Increased latency in strabismic and combined amblyopes compared to anisometropic amblyopes (saccadic latency only present with the presence of strabismus)
▶ Although high correlation with interocular difference of acuity, this is not the case for anisometropics so must be something else (only in strabismics!)
▶ Suggests frequent microsaccades (with the fixation nystagmus/visual smudge theory) and accompanying attentional shifts may be the cause.

Question 19

What is perceptual learning?

Answer 19

Hussain et al. (2012)
Getting rid of the flanking stimulus from the target stimulus – training supports plasticity in amblyopia even in adults. Learn visually to improve attentional perception i.e. what’s true vision and what’s attentional demand?

Comparing unflanked vs flanked VA. Found flanked acuity is lower than our unflanked VA but our data is closer together at the periphery than at the fovea = amblyopia is felt more at the central (which fits in with the X cells feeling amblyopia more than the Y cells)

Question 20

What about games like Dig Rush?

Answer 20

Four Large RCTs in paediatric amblyopes have recently reviewed the effect of both games

1) Not found any significant difference in visual acuity improvement or stereopsis between games and control groups

2) Compliance issues with both games in the paediatric cohort