W7L1: Higher Visual Cortices (II) - Visual Disorders

Describe the receptive fields of simple cells, in terms of:

• What they are made of

• Their shape

• What they are tuned for

• The relationship b/w ON/OFF zones

• What they detect

• Made of concentric receptive fields

• Receptive fields are elongated

• Tuned for orientation

• Separate ON/OFF zones

• Detect bars/edges of correct orientation & polarity

Describe the receptive fields of simple cells, in terms of:

• What they are made of

• Their shape

• What they are tuned for

• The relationship b/w ON/OFF zones

• Made of concentric simple cell receptive fields

• Receptive fields are elongated

• Tuned for orientation

• Overlapping ON/OFF zones

• Detect bars/edges of correct orientation & any polarity

What are the implications of the visual pathway being very large?

Means there are many things that can go wrong/many places for things to go wrong

Define & describe amblyopia

• Amblyopia: reduced vision in an eye in the absence of any ophthalmologically detectable retinal anomaly or any disorder of the afferent visual pathways that might cause defect

  • Absence of adequate symmetrical stimuli to the two eyes → binocular reflexes cannot be developed

  • Something goes wrong at an early age → two eyes stop working together

When are children usually most susceptible to amblyopia?

Before the age of 6, but is particularly pronounced within the first few years of development (e.g. 1-2 y/o)

Provide some examples of what can contribute to amblyopia, & types of intervention to resolve the issue

• Causes:

  • Amblyogenic anisometropia

  • Constant unilateral or bilateral esotropia or exotropia

  • Amblyogenic bilateral isometropia

  • Amylogenic unilateral or bilateral astigmatism

  • Image degradation

• I.e. anything that leads to the imbalance/mis-integration of the input to 2 eyes

• Intervention:

  • Caught early: put eye patch over good eye

  • Example: even having an eye swollen for the week as an infant can cause amblyopia

Describe the effects of monocular deprivation on ocular dominance columns in V1

• Suture one eye shut soon after birth → block form vision from that eye

• Non-deprived eye: wider ocular dominance columns

• Deprived eye: thinner ocular dominance columns

  • Decreased V1 input (less colour input)

Describe why the timing of monocular deprivastion is critical

• Very pronounced effects at age 2-4 weeks

• Effects less pronounced at older ages (e.g. 10 weeks -14 months)

• Older age + shorter deprivation period → slight effect but not as pronounced

What is the Hubel & Wiesel Ocular Dominance Scale?

• Quantifies the electrophysiological responses of V1 cells to stimuli applies to the L/R eye

  • X-axis showing the ocular dominance class

  • Y-axis showing the number of cells responding to the stimulus

    • V1 cells are generally very strongly biased towards one eye or the other

  • 1 = input only from contralateral eye

  • 4 = equal responses to both eyes

  • 7 = input only from ipsilateral eye

Describe the Hubel & Wiesel Ocular Dominance Scale following monocular deprivation

Vast majority of cells are driven exclusively by the non-deprived eye

Describe the changes that occur during the development of V1 in infants

• Initially:

  • V1 initially has no retinotopic map

  • Long, straggly branches → spread over very wide areas of the cortex

  • No segregation of signals from the L & R eyes; all cells sending random branches everywhere

• Development:

  • Active connection of cells

  • Spontaneous, coordinated waves of neural activity in infants → groups of cells become more active → signal propagates in the retina → groups of cells in one eye become active together → strengthen the circuit of that group of cells

  • Segregation of separate zones b/w the L & r eyes is forced through differential strengthening of gaps of cells in L & R eyes → help build retinotopic map