[ST:NB] W04 - Visual Fields

from retina to cortex

• thalamus is the translator between retina and cortex

• the visual cortex is at the back end of the brain

• the left visual field is processed on the right side of the cortex
  • and vice-versa
• each eye can be thought to have two visual fields
  • the right visual field
  • the left visual field
• the setup of the visual field is homonymous

left visual field pathway

• right retina’s right half receives the left visual field
• goes to the right half of the thalamus

• then goes to the right cortex

• right half of the left retina goes to the right half of the thalamus
• the cross happens across the optical chiasm

lesions in pathway

• between eye and thalamus (the optic nerve)
  • the eye in question goes blind
  • i.e. lesion between right eye and right thalamus makes the right eye blind
• between thalamus and cortex
  • the field related to the cortex goes blind
  • i.e if lesion is on the right side, the left half of both eyes go blind and vice verse
• pituitary gland sits right below the optic chiasm
  • a tumor of the pituitary gland can push up on the optic chiasm
  • one the right side, the right visual field is lost
  • one the left side, the left visual field is lost

alignment of eyes

• poor vision results from misaligned eyes
  • results in blurry vision
• stereoscopy of the eye pair gets distorted
  • individual eye vision remains clear

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edges

• the pathway is not the entirety of enabling vision
  • it is built to tell the differences between dark and light

center-surround organization

• the eyeball and the thalamus together are sensitive to
  • light in the center and surround
• they are inhibited by the darkness in the surround and center
  • they make a distinction between the center and the surround
• in the cortex, the center and surround gets an actual edge
  • the shape is elliptical
• this helps is detecting edges and maintaining vision

perfectly still in time

• if the observing eye and the scene was perfectly still in time, the vision would face
• eyes continuously make extremely small twitches to hold vision
  • this forces detecting edges using the center-surround system
  • cells only detect edges, nothing else

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visual processing for perception

• information passes in the following order
  • cornea
  • retina
  • thalamus
  • primary visual cortex
• primary visual cortex is also called the striate cortex because it has a stripe
  • it is along the medial surface of the brain
  • it is termed V1
• any visual processing done outside the striate cortex
  • is called extra-striate cortex
• riddoch syndrome is when the primary visual cortex is not functional at all
  • movement perception without any ability to see form (shape or color)
• perception happens in the extra-striate cortex

extra-striate cortex

• two steams come out of the V1 towards the front of the brain
  • a ventral stream
    • the lower pathway going towards the sides (temporal lobes) of the brain
    • “what?” perception processing pathway
  • a dorsal stream
    • the upper pathway going toward the front top of the brain
    • “where?” perception processing pathway

ventral stream

• lines, edges, colors etc are processed to identify what the vision holds

• visual agnosia:
  • all the components are perceived and processed
  • but the object the component belongs to cannot be identified
• prosopagnosia:
  • faces are not recognized
  • they may not even know that they are looking at faces

dorsal stream

• movement and location is perceived
  • not just sensed
• processes how to move to get to some place
• information that feeds into the motor cortex
• big part played in learning how to use tools

interpretation

• perceiving the visual world takes attention

• hemi-spacial neglect
  • lesions in the right hemisphere
  • entire left side of the world does not exist
  • their visual system is fine but they cannot respond to the things that happen in the left half of their visual world
• the whole process involves taking in light through the eyes, converting it into neural energy in the retina, processing it in the primary visual cortex and paying attention of the perception that occurs in the extra-striate cortex

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learning to see

• the process of processing visual information has to be learnt
  • how to perceive
  • how to interpret
  • figure out what it represents
• a person that loses their corneas at 3 years of age
  • and regains corneas through corneal transplant at 30 years of age
  • will be unable to use the information seen by the eye
• learning to see has to be learned during the “sensitive period”
  • this is the age before puberty
  • where learning happens easily and automatically
• a baby doesn’t have to intentionally learn a language
  • it just picks it up
• learning to see is picked up at the same time
  • happens automatically
• congenital cataracts cause loss of vision in babies
  • but removing the cataract later will not result in a fully developed sense of vision
• albino people have trouble with acuity
  • because of lack of acuity training
  • too much light floods their eye’s camera box
  • this hampers their ability to distinguish details