Lecture 4- The Visual World

Receptive Fields

Receptive fields- sample sensory information and help locate that information in complex world

• Each sensory receptor (organ/cell) has a receptive field- an area that responds to a specific stimuli and selectively activates cells

• In the visual system, receptive fields are volumes in visual space (retina)

• Ensures accurate identification of information and has information properties

Structure of the Eye

Cornea- transparent structure at the front of the eye that helps to focus incoming light

Iris- has an adjustable circular opening (pupil) which can expand or contract to control the amount of light entering the eye

Crystalline lens- behind pupil, a colourless, transparent structure, refracts light to be focused on the retina by changing shape, changes the focal distance of the eye allowing for a sharp image

Ciliary muscles surround the lens- hold the lens in place

• When ciliary muscles relax, they pull on and flatten the lens, allowing the eye to see objects that are far away

• To see closer objects clearly, the ciliary muscle must contract in order to thicken the lens

Vitreous humour- interior chamber of the eyeball that is filled with a jelly-like tissue

• Vitreous humor maintains spherical shape of eyeball

• After passing through the lens, light must travel through this hum or before striking the sensitive layer of retinal cells

Retina- Light-sensitive tissue

• Photoreceptors turn the light into electrical signals

• Electrical signals travel from retina through the Optic Nerve

Digital camera in your head

• Light is focused by the cornea- like a camera lens

• The iris controls the amount of light reaching the back of the eye by automatically adjusting the size of the pupil

• Crystalline lens further focuses light through a process called accommodation- like an autofocus camera lens

• Reaches the retina (light-sensitive inner lining of the back of the eye)- acts like an electronic image sensor of digital camera, converts optical images into signals

• Optic nerve then transmits these signals to the visual cortex- the part of the brain that encodes vision

Light and the Visible Spectrum

• Visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye

• Electromagnetic radiation in this range of wavelengths is called visible light

• A typical human eye will respond to wavelengths from about 390 to 700 nanometres

Rods and Cones

Rods:

• Light sensitive

• Optimal in dim light

• More of them

Cones:

• Colour vision

• Optimal in high light

• Less of them

Functional Differences

Rods are extremely sensitive, and can be triggered by a single photon

• At very low light levels, vision is based on the rod signal

• This is why colours cannot be seen at low light levels- only one type of photoreceptor cells is active

Cones require significantly brighter light to signal

• Humans have 3 different types of cone cell- each responding to different wavelengths of light