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 transduction Photons produce electrical events in photoreceptors (hyperpolarization)

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 undercutCortical 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’s Two visual cortical streams

Milner and Goodale’s “Posting” experiment