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