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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 Visualoftransduction The beginning vision a flash of light in to photoreceptors a photoreceptor(hyperpolarization) produces Photons-presenting produce electrical events an electrical response In darkness, there’s a continuous current in the outer segment caused by the circulation of sodium. In light, sodium circulation slows down and receptors hyperpolarize Disks in outer segments called 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 interactions in the retina help with several 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’s simple hierarchical model of 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 and the 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 I Schneider’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’ Visual processing streams III –Weiskrantz and blindsight 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. 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’s Two visual cortical streams Milner and Goodale’s “Posting” experiment