Vision 1

How we guide our visual behaviour can be an indicator of how we make use of visual information.

This is usually in relation to how we guide our attention.

A good way of looking at perception in vision is to look at visual search behaviour by tracking eye movements.

Land & McLeod (2000):

• For a fast bowler, it takes 600 ms for the ball to reach the batsman
• It takes an expert batter about 200 ms to adjust the shot
• So, the shot must be selected at least during the first 400 ms of flight
• When a batsman plays a shot, the batsman must judge the vertical position of the ball to within +/- 3 cm

The visual field is the view seen by the two eyes without movement of the head.

The left visual field projects on the right half of each retina.

The right visual field projects on the left half of each retina.

Located at the back of the eye.

Contains the photoreceptors.

Photoreceptors are neurons that detect light. If sufficient light strikes them, an action potential occurs.

Small depression in the retina.

Detects light from the centre of the visual field.

Visual field is the area of space that can be seen by the eye.

Usually smaller than the area blocked by a person's thumb at arm's length.

We are incredibly sensitive to low levels of light but still can operate in bright sunlight.

Rods:

• Detect dim light and are specialised for night vision
• Not present in fovea
• Achromatic (colour blind)
• Have low acuity

Cones:

• Are specialised for day vision
• Are concentrated in fovea
• Mediate colour vision
• Provide greater temporal and spatial resolution than rods

Light activates visual pigments in the outer segments of rods and cones, which causes the receptor to fire.

Ganglion cells feedback into the thalamus and then the visual cortex.

The more photoreceptors there are, the more visual information the ganglion cell can feedback to the visual cortex about.

Small cells.

Small receptive field.

Slow conduction rate.

Colour sensitive.

Concentrated in fovea.

Important for:

• Detailed form analysis
• Spatial analysis
• Colour vision

Associated with fovea/central vision.

Large cells.

Large receptive field.

Fast conduction rate.

Colour blind.

Concentrated in periphery.

Important for:

• Motion detection
• Temporal analysis
• Depth perception 

Associated with peripheral vision.

• Light bounces off an object and hits photoreceptors in the eye which are specialised neurons
• Photoreceptor fires, sending signal to the next layers of neurons in the eye, bipolar neurons and ganglion neurons
• Axons of ganglion neurons exit the eye via the optic nerve and go to the thalamus area of the brain
• Neurons in the thalamus send the signal to the primary visual cortex of the brain, where the initial processing of the visual information takes place