Physiology of Vision

Question 1

Describe the functional anatomy of the eye

Answer 1

• Cornea and Lens: produce a focused image on the retina
  
  • focus varies as the shape and power of the lens is adjusted
• Iris acts a diaphragm: varies its diameter by 4x and thus the retinal intensity by 16x
• retinal pigment epithelium: behind the retina layer absorbs unwanted light

Question 2

Give examples of Refractive errors in the ye

Answer 2

• Hypermetropia (long-sightedness): eyeball too short or lens system too weak.
• Myopia (short-sightedness): eyeball too long or lens system too strong.
• corrected with glasses or contact lenses
  
  • there is a reciprocal of focal length in metres a 2D lens has a focal length of 0.5m

Question 3

Explain the structure of the retina

Answer 3

• in vertebrates the retina evolved backwards: ganglion cells and blood vessels are in the light path to the photoreceptors (not in fovea) -
• There are processing layers
  
  • 3 direct layers
    
    • Receptors: Rods and Cones
    • Bipolar cells
    • Ganglion cells
  • 2 transverse layers: involved in signal processing including lateral inhibition
    
    • Horizontal cells
    • Amacrine cells

Question 4

Explain the function/ action of Rhodopsin

Answer 4

• photosensitive pigment in the rods.
• when hit by a photon the retinal in the rhodopsin molecule flips from 11-cis to all-trans
• eventually results in closure of cGMP-gated nonselective cation channels that are open in the dark,
• leads to the hyperpolarization of the photoreceptor and reduction in the release of the neurotransmitter glutamate.

Question 5

Explain the ganglion cell response to light

Answer 5

• they respond very weakly to changes in overall light intensity
• they respond to local contrast: light on dark background and vice-versa
• On-Centre ganglion: the area of focus is lighter than the background/ surroundings
  
  • depolarise in response to light increments
  • hyperpolarize in response to light decrements (opposite of photoreceptors)
• Off-Centre ganglion: the area of focus is darker than the background/ surroundings
  
  • depolarise in response to light decrements
  • hyperpolarize in response to light increments (same as photoreceptors)

Question 6

What types of Colourblindness are there

Answer 6

Inherited colour anomalies are mostly seen in red and green pigments as these are X -linked and not as easily matched whereas as blue is on chromosome 7 and is matched

• Protonopia: no expression in the red (long) pigment
• Deuteranopia: no expression in the green (medium) pigment
• Anomalous trichromats: they need all three light sources (short, medium, long) to make all possible colour matches. the intensity of the wavelengths needed may need to be higher for each pigment
• Central Achromtopsia: caused by damage to the cortical colour processing area - V4

Question 7

Give an overview of the Central visual pathway

Answer 7

• Optic nerve from each retina divides into L & R halves.
• In the optic chiasm L halves from both eyes combine as do the R halves
• Optic tracts relay in the lateral geniculate nuclei of the thalamus
• Part of each optic tract goes to the superior colliculus in the mid-brain
• The output of each lateral geniculate goes almost exclusively to the striate cortex in the occipital lobe (V1).
• Here the image of one half of each combined visual field is represented in one half of V1
• The representation of the foveal region is hugely exaggerated
• Thereafter the cortical input passes on to areas that process depth, motion, colour etc.

Question 8

Explain the organization of the primary visual cortex (V1/ BA 17)

Answer 8

• has three overlapping patterns
• Ocular dominance columns driven by the left or right eye
• Orientational columns which are smaller: orientation of optimal stimulus varies systematically across the surface
• Colour ‘blobs’: Colour information is kept separate from orientation and passed on to other regions such as V4

Question 9

Give examples of partial loss of vision

Answer 9

• Left eye blindness: lesion across the left optic nerve
• Homonymous hemianopia: lesion across the optic tract
• Bitemporal hemianopia: lesion across the Optic chiasm
• Scotoma: caused by retinal damage, lesions in the visual cortex, or by pressure from tumours restricting the optic nerve, chiasm, optic tract or optic radiation.

Question 10

What are the roles of the dorsal and ventral streams in the visual cortex?

Answer 10

• The dorsal stream, from occipital to parietal cortex, is concerned with location, motion and action.
• The ventral stream, from occipital to temporal cortex, is concerned with object (and face) identity, and with conscious perception.

Question 11

What is visual agnosia?

Answer 11

• the inability to perceptually recognize or interpret visual information
  
  • the opposite is optic ataxia
  • caused by a disorder in the parietal lobes
• Prosopagnosia: the inability to recognise familiar faces
  
  • associated with the disorder in the fusiform gyrus on the underside of the temporal lobe

Question 12

What visual reflexes are there?

Answer 12

• Vestibulo-ocular reflex (VOR): stabilizes gaze by countering movement of the head
• Optokinetic reflex (OKR): stabilizes the image of a moving object on the retina
• Pupillary reflex: the contraction of one eye when one is illuminated

Question 13

How can the Pupillary reflex be used to identify disorder?

Answer 13

• both pretectal nuclei and the Edinger Westphal nuclei receive signals from both eyes
• damage to one optic nerve will prevent light in that eye from closing the pupil (direct response) but light in the other eye will still do so (consequential response)
• damage to one oculomotor nerve will prevent pupil contraction in that eye, but stimulation of either eye will cause contraction in the pupil in the second eye