02 Lower-level Vision

Question 1

Which parts of the eye refract the light and to what extent?

Answer 1

• cornea: fixed, 80% of entire refraction
• pupil: variable, 20% of entire refraction

Question 2

Which eye muscles are there and what are their functions?

Answer 2

• medial rectus: moves eye towards nose
• lateral rectus: moves eye away from nose
• superior rectus: raises eye
• inferior rectus: lowers eye
• superior & inferior oblique: rotate eye

Question 3

What are the parts of the eye and what are their functions?

Answer 3

• choroid:
• ciliary body:
• iris: muscles controlling pupil dilation and thus light intensity
• lens:
• pupil: variable refraction, hole in the eye
• cornea: fixed refraction, translucient layer in front of the eye/lens
• conjunctiva:
• zonules:
• sclera: protection of the eyeball
• retina: projection of inverted image
• macula:
• fovea: center of the macula, highest visual acuity, color vision
• optic nerve: axons of the ganglion cells

Question 4

retinal layers from outer to inner

Answer 4

• pigment epithelium
• photoreceptors
• horizontal cells
• bipolar cells
• amacrine cells
• ganglion cells
• nerve fiber layer

Question 5

horizontal & amacrine cells: contextual processing

Answer 5

• center-surround processing
• integrate information horizontally across different locations

Question 6

photoreceptors

Answer 6

• rods: periphery, very sensitive, good at very low light levels, bleaching at high light levels, low sensitivity to long wavelengths (red) -> no bleaching, vision in dark, black & white
• cones: fovea, short-, medium- and long-wavelength, higher light levels, no vision in dark
• graded potentials: fire rate depends on wavelength and light intensity, colour

Question 7

What is skotopic, mesopic and photopic vision?

Answer 7

• skotopic: only rods, without daylight (black and white)
• mesopic: rods and cones
• photopic: only cones, daylight, rods bleached out, colour vision

Question 8

Purkinje Shift

Answer 8

• at lower light levels and with increasing dark adaptation, peak luminance sensitivity shifts to the short wavelength (blue) end of the spectrum with low sensitivity for long wavelengths (red) due to wavelength-dependent sensitivity differences between rods and cones
• in photopic vision, red looks brighter than green because sensitive to a much wider wavelength spectrum
• in scotopic vision, red looks darker than green because relying primarily on rods
• at low luminance levels, long wavelengths look darker because vision is taken over by rods, which are more sensitive to short wavelengths

Question 9

axial chromatic aberration

Answer 9

• shorter-wavelength light is refracted more strongly than longer-wavelength light
• blue light actually converges in front of the fovea leading to a blurred image

Question 10

Why do we not perceive the blind spot?

Answer 10

• spot is filled in based on the statistical properties of the surrounding visual field
• filling-in presumably takes place at the level of V1

Question 11

What is the classical receptive field of a neuron?

Answer 11

• region of visual space where a stimulus can directly cause a change in the spike rate

Question 12

What is the non-classical receptive field of a neuron?

Answer 12

• region of visual space (typically surrounding a classical receptive field) where a stimulus cannot directly cause a change in spike rate, but it can modulate the effects of another stimulus

Question 13

What is the mexican hat model?

Answer 13

• a model for the activity in retinal center-surround ganglion cells