Psych210 quiz2&exam

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Vision

Chapter Six

• Environmental stimuli – Transduced into action potentials for

interpretation by the nervous system – Perceived by the process of sensation and

interpreted by the process of perception • Bottom-up processing vs. top-down

processing

From Sensation to Perception

Evolution of Sensory Systems Is Species- Specific

• Electromagnetic radiation – Moving waves of photons

• Visible light – Wavelength determines the colors perceived – Amplitude determines the brightness

perceived • The advantages of light as a stimulus

– Electromagnetic energy is abundant, travels quickly and in straight lines

The Visual Stimulus: Light

Dimensions of Electromagnetic Radiation

• The electromagnetic spectrum – Range of energy visible to humans falls

between 400 and 700 nanometers • Absorption, reflection, and refraction

– Absorption and reflection of visible light by objects determine the colors we see

– Air and water refract, or change the direction, of traveling waves of light

The Electromagnetic Spectrum

Light Interacts with the Environment

• Protecting the eye – Located in bony orbit of the skull – Cushioned by fat – Eyelids, blinking, tears

• The anatomy of the eye – Sclera, cornea, anterior chamber (aqueous

humor), pupil, lens, vitreous chamber (vitreous humor), retina

The Structures and Functions of the Visual System

The Structure of the Eye

The Color of the Iris

Demonstrating the Blind Spot

• Contains visual interneurons and photoreceptors

• Optic disk • Macula • Fovea (central vision vs. peripheral vision)

Features of the Retina

• Ganglion cell layer • Inner nuclear layer • Outer plexiform layer • Outer nuclear area • Inner plexiform layer

The Layered Organization of the Retina

Landmarks of the Retina

The Structure of the Retina

• Transduction of visual stimuli is performed by photoreceptors

• Rods: scotopic vision – High density in the peripheral retina – Sensitivity to dim light

• Cones: photopic vision – High density in the fovea – Best visual acuity

The Structures and Functions of the Visual System: Photoreceptors

Rods and Cones

Scotopic and Photopic Vision

• Depolarized photoreceptors at rest – Dark current – Sodium channels are open due to cGMP – Stimulates glutamate release

• Light stimulus splits rhodopsin – Photoreceptor becomes hyperpolarized – Enzymes break down cGMP – Release less glutamate

• With return to darkness, rhodopsin rejoins

Transduction in Photoreceptors (Rods)

Transduction in Rods

• Rods – Contains rhodopsin photopigment – Sensitive to dim light in the blue to green

range of the electromagnetic spectrum • Cones

– Requires more light than rods in order to respond

– Three different cones • Blue/short, green/middle, red/long wavelength

Differences between Rods and Cones

The Responses of Rods and Cones to Different Wavelengths

• Horizontal cells – Form connections with photoreceptors and

bipolar cells – Graded potentials

• Bipolar cells: receptive fields – Antagonistic center-surround organization – Lateral inhibition – Two types: diffuse and midget – Graded potentials

Processing by Retinal Interneurons

• Amacrine cells – Form connections between bipolar, ganglion,

and other amacrine cells • Ganglion cells

– Receive input from bipolar, amacrine cells – Receptive fields replicate the information

passed to them by the bipolar cells – Antagonistic center-surround organization – Three types of ganglion cells: M, P, and K

cells

Processing by Retinal Interneurons (cont’d.)

Retinal Bipolar Cells Have Receptive Fields

Receptive Fields of Bipolar and Ganglion Cells

The Three Types of Ganglion Cells

• Optic nerves – Ganglion cell axons exit each eye through the

optic disk, forming an optic nerve leaving each eye

– Optic nerves cross at the optic chiasm • The lateral geniculate nucleus (LGN)

– Receives 90% of optic tract axons – Six distinct stacked layers – Keeps input from each eye separate

Optic Nerve Connections

• The superior colliculus – Guides movements of the eyes and head

toward a newly detected object

Optic Nerve Connections (cont’d.)

The Pathways from Eye to Cortex

The Lateral Geniculate Nucleus (LGN)

• Cortical mapping – Correlate location of neural activity with the

position of an object in the visual field • Cortical receptive fields

– Simple and complex cortical cells – End-stopped cells

• Cortical columns and hypercolumns • Cytochrome oxidase blobs • Cortical modules

The Striate Cortex (Primary Visual Cortex; V1)

Simple Cortical Cells

Complex Cortical Cells

Cortical Modules

• At least a dozen other areas of human cerebral cortex participate in visual processing

• The dorsal stream (“where”) – Important role in processing motion – Areas MT and MST; akinetopsia

• The ventral stream (“what”) – Important for object recognition – Areas V4, IT, and the fusiform face area (FFA)

Visual Analysis Beyond the Striate Cortex: V2

The Dorsal “Where” Pathway and the Ventral “What” Pathway

• Hierarchies – Simple cells input to increasingly complex

cells – Bottom-up processing

• Spatial frequencies – Spatial frequency analysis of contrasts and

frequencies in the visual field • Gratings

– Contrast sensitivity function

Visual Perception

Bottom-Up Processing?

Problems for the Hierarchical Model of Vision

Spatial Frequencies

A Cat’s View of the World

• Monocular cues – Perspective – Texture and shading – Comparison of size of familiar objects

• Binocular cues – Retinal disparity – Disparity-selective cells

Visual Perception: Perception of Depth

3-D Movies Take Advantage of Retinal Disparity

• Trichromatic theory – Interpretation of the output of three types of

cones • Opponent process theory

– Opposition of four colors: green/red, blue/yellow

Visual Perception: Coding Color

Mixing Lights

Color Afterimages Illustrate Opponency

Opponent Process Theory

• Dichromacy: two cone photopigments – Red-green is sex-linked and most common – Absence of blue photopigment is rare

• Monochromacy: one/no cone photopigments

• Anomalous trichromacy – The cone photopigments have peak

responses at abnormal wavelengths

Visual Perception: Color Deficiency

Looking Through the Eyes of a Dichromat

• Color contrast – The same color appears different depending

upon the context of surrounding colors • Color constancy

– Colors look the same regardless of the type of light illuminating the object

Visual Perception: Color Contrast and Color Constancy

Color Contrast

• Contrast sensitivity functions of children – Infants see less fine detail at a distance – Preferences for high-contrast, colorful objects

• Age-related vision changes – Presbyopia – “old sight” – Slower adaption to changes in light – Yellowing of the lens; smaller pupils – Less selectivity in cortical responses to visual

stimuli

The Life-Span Development of the Visual System

The Development of Contrast Sensitivity

• Ambylopia: lazy eye • Cataracts: clouding of lens of the eye • Visual acuity problems

– Myopia, hyperopia, astigmatism • Blindness

– Scotomas, blindsight • Visual agnosias

– Difficulty recognizing what is seen

Disorders of the Visual System

Eyeball Shape Influences the Quality of Vision

Scotoma