The visual system Chapter 10
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Transcript of The visual system Chapter 10
The visual system
Chapter 10
The physical stimulus
Light is a wave… …and a particle
Psychological dimensions of light
Hue
Saturation
Brightness
The eye
•Cornea – the main focusing element
•Lens – adjustable focusing
•Iris – adjust sensitivity and depth of focus
•Retina – photosensitivity and much, much more
Structure of the retina
Visual transductionPhotons produce electrical events in photoreceptors (hyperpolarization)
In darkness, there’s a continuous current in theouter segment caused bythe circulation of sodium.
In light, sodium circulationslows down and receptorshyperpolarize
Disks in outer segmentscalled lamellae contain a photopigment
Rhodopsin -- the magic photopigment
Through the wizardry of biochemistry, sodium channels close
Photoreceptors come in different flavours
Spectral absorption curves
Lateral interactionsin the retina help withseveral problems
1. Contour sharpening
2. Enhancing sensitivity
Mach bands
A slightly misleading illustration
We understand the neural basis of lateral inhibition because of work on the horseshoe crab that is not feasible in mammals
The duplex retina
The cost of the duplex retina
Central visual pathways
The lateral geniculate nucleus of the thalamus
Primate lateral geniculate nucleus
Centre-surround antagonism is the mammalian analogue of lateral inhibition.
Hubel and Wiesel’ssimple hierarchical modelof visual cortical processing
Simple cells
Complex cells
Columnar organization of VI
Ocular dominance
The hypercolumn
Optical imaging of ocular dominance columns
Optical imaging of orientation tuning
Correlation between optical imaging and electrophysiological results for orientation tuning
Margaret Wong-Riley andthe cytochrome oxidase story
•autoradiography and activity•cytochrome oxidase and activity•intrinsic variability in cyo
Cytochrome oxidase in monkey VI and VII-blobs and stripes of every stripe
V2 and cytochrome oxidase stripes
Multiple visual representations in cortex
Visual agnosias
• Motion blindness
• Prosopagnosia
• Cortical colour blindness
• Visual object agnosia
Visual processing streams ISchneider’s Experiment
Tectal undercut Cortical ablation
Visual processing streams II
• Gordon Holmes– single patient studies -- it
was obvious that people without conscious vision were not ‘blind’
In a preliminary test, Weiskrantz positioned a stick in D.B.s blind spot, either sideways or straight up and down. He asked D.B. what he saw. The patient said, "I see nothing."Weiskrantz persisted. "Am I holding the stick sideways, or vertically?"D.B.: "I don't know -- I don't see a stick."Weiskrantz: "Guess."D.B.: "Sideways."Weiskrantz: "Now which way am I holding it?"D.B. "I don't see a stick."Weiskrantz: "Guess."This continued for 20 trials in which D.B.'s performance was perfect.
Visual processing streams III –Weiskrantz and blindsight
Weiskrantz recounts:
"In the interview that followed, and which was recorded, D.B. expressed considerable surprise. 'Did you know how well you had done?', he was asked. 'No,' he replied, 'I didn't -- because I couldn't see anything; I couldn't see a darn thing.' 'Can you say how you guessed -- what it was that allowed you to say whether it was vertical or horizontal?' 'No, I could not because I did not see anything; I just don't know.' (p 24)."
Pohl’s Experiment
Ungerleider and Mishkin’sTwo visual cortical streams
Milner and Goodale’s “Posting” experiment