Psychology 210 Lecture 4 Kevin R Smith

Vision Sensory System Perceptual System The eye Exactly what we sense from our environment Perceptual System The brain How we put together what we sense into a visual picture

The brain constructs our environment

Multiple experiences of an item lead to different interpretations

The Visual Sensory System Light is made of waves Wavelength Different colors have different wavelengths Amplitude Different amplitudes lead to differences in brightness Visible spectrum 400nm to 750nm ROY G BIV Short wavelengths are near blue and violet Long wavelengths are near red and orange

Properties of Light and Waves The color of an object is determined by its abilities across two dimensions Absorption Reflection Colors that are reflected are the colors that we see A red sweater is red because it reflects wavelengths that we perceive as red Other wavelengths would be absorbed and NOT visible as a color for this sweater

Properties of Light and Waves Black is a color that absorbs all other colors ie. It is the absence of reflected color White is a color that reflects all other colors ie. It is the presence of all colors prisms White object

Properties of Light and Waves Refraction The change of direction of the waves Occurs in water Different substances refract light differently

The eye Sclera Outer covering that protects the eye and gives it shape Cornea Protective covering for the eye Begins to bend the light waves and focus them Aqueous humor Fluid filled area behind the cornea Provides nutrients to the cornea and lens

Parts of the eye Pupil Lens Vitreous humor Area in the center of the eye Controls the amount of light that enters Iris The muscle that controls the widening or narrowing of the pupil Lens Bend to focus light onto the retina Accommodation The process of the bending of the lens to properly focus the image on our retina Vitreous humor Gives the eye its shape Does not regenerate The vitreous humor your born with is what you still have Floaters Debris that gathers in the vitreous humor and casts shadows onto the retina

Retina Translates light waves into an electrical signal our brain can process Concave Object on retina is translated upside-down Photoreceptors Rods and cones Optic Disk/ Optic Nerve Area in the retina where nerves and blood vessels exit the eye Forms a blind spot Fovea Area in center of retina

Layers of the Retina Four main types of visual processing neurons Ganglion cell layer Amacrine and bipolar cell layer Horizontal cells Visual Sensory Neurons Photoreceptors Rods Cones

Rods and Cones Where a sensory signal (light waves) gets changed into electrical energy Process called transduction Rods Sensitive to black and white Most of them in periphery Cones Sensitive to color Most of them in fovea

Rods Contain rhodopsin 120 million in a human eye Responsible for night vision Very sensitive to light Very poor clarity

Cones 6 million in the human eye Responsible for vision in bright light Excellent clarity 3 different types with different pigments

Cones Three different types Blue, short Green, medium Red, long

Horizontal Cells Integrate information from the photoreceptors Transfer that information to bipolar cells

Bipolar cells Receive input from horizontal cells and photoreceptors Amacrine Cells Bipolar cells Receive input from horizontal cells and photoreceptors Transmit information to amacrine cells Contain receptive fields Antagonistic center-surround organization Horizontal cells photoreceptors

Amacrine Cells Respond to changes in the visual environment Connect bipolar cells, ganglion cells, and other amacrine cells

Ganglion cells Receive input from bipolar and amacrine cells Same center-surround receptive fields found in bipolar cells On-center bipolar fields connect to on-center ganglion cells Off-center bipolar fields connect to off-center ganglion cells

What do receptive fields do? On-center and off-center fields provide for greater acuity Large unchanged surfaces don’t activate the neurons as well as changing surfaces with lines, cracks, and ends

Types of Ganglion cells Three types P cells: Parvocellular M cells: Magnocellular Non P or M cells

Pathway to the Brain Optic nerve Group of ganglion axons exit the eye Superior Colliculus (part of the tectum) Thalamus- Lateral Geniculate Nucleus Visual cortex- V1, V2, V3, V5 (MT)

Contralateral organization Visual hemifields in space are contralaterally organized Not the contralaterally organized by the eyes

The Thalamus The lateral geniculate nucleus (LGN) 6 Layers: Keep much of the information seperated Parvocellular and Magnocellular information go to different layers Non P and M projections go between the other layers Information from each eye goes to different layers Displays retinotopy

What is retinotopy? A B C D E F G H I J K L

Primary Visual Cortex Located in the Calcarine Sulcus (Occipital Lobe) Also referred to as Striate cortex Brodmann Area 17 V1

Cortical organization of V1 Cells don’t like spots! They like lines/edges Retinotopically organized Simple cortical cells Respond to edges from a specific location Complex cortical cells Respond to edges from a larger area than simple cells Orientation selective Often movement direction selective

Other properties of V1 Cortical magnification (half of V1 is devoted to fovea) Cells often respond to input from either eye, but with a preferred eye (“ocular dominance”) Ocular dominance columns M & P pathways project to different layers Response preferences are organized into columns (ocular dominance, orientation, visual field location)

Organization in V1

Other Visual Areas: V2, V3… Also retinotopically organized V3 encodes colors

Visual Pathways 2 Main pathways Dorsal stream Ventral Stream Also called the “where” stream Processes where things are in space Ventral Stream Also called the “what” stream Processes what things are

The “Where” Stream Contains MT (area V5) Visual motion area Integrates with the “where stream of the auditory system in the parietal lobe to construct a full picture of the environment

The “What” Stream Contains the Fusiform Face Area (FFA) Responsible for recognition of faces Much debated

The FFA debate

The FFA debate Expert vs face area Scientists argue that the FFA is not sensitive to faces, but instead is sensitive to identification of things that we can expertly identify We’ve all seen so many faces that we are effectively “face experts” Bird watchers show increased activation when identifying various types of birds

Greebles Face-like Train people about the different types and check their activation The debate is still going on whether a face area exists Nancy Kanwisher Isabel Gautier

Problems of the visual system Nearsightedness Focuses image before retina Can see close, not far Farsightedness Focuses image beyond retina Can see far, not close Astigmatism Aspherical cornea

Color processing theories Young-Helmholtz theory Trichromacy Found in the retina Opponent process theory Red-green, blue-yellow, and black-white Input from one color inhibits the other color Found beyond the retina Evidence from afterimages