exam 2 : vision

Sensation: stimulus detection

Receptors and receptive fields • Transduction: transform environmental energy into electrical energy

Perception: stimulus interpretation

Depends on biology • Depends on personal history

Light

Hue: wavelength • Brightness: intensity • Saturation: relative purity

ront of Eye The Eye • Cornea refracts • Pupils regulates • Lens accommodates

Front of Eye Disorders The Eye • Astigmatism: multiple focal points onto retina • Presbyopia: difficult to accommodate • Cataracts: cloudy lens

Back of Eye

Retina contains photoreceptors + optic disk

Optic disk:

where optic nerve leaves ey

Back of Eye: Photoreceptors

RodsCones

Rods

Periphery ▪ Scotopic ▪ Dim light . high synaptic convergence

Cones

Fovea ▪ Photopic ▪ Color, Fine Detail. low synaptic convergence

Back of Eye Disorder

Myopia: nearsightedness ● Hyperopia: farsightedness

Retina: Cellular Layers

Three main layers: photoreceptive, bipolar, ganglions

Retina: Cellular Layers

Layers connected by interneurons: horizontal cells, amacrine cells

Information in Retina: Bipolar and Ganglion Cells

Receptive Fields • ON-center and OFF-center action

Relative firing rate encodes lighting conditions

Bipolar and Ganglion Cells

Receptive Field

Overlap and varying size allow for lateral inhibition • Enhances contrast

Photoreceptor activation leads to glutamate release

Photoreceptors fire graded potentials onto bipolar cells

Transduction: Photopigments

help transduce light energy • Molecules: opsin (protein) + retinal (absorbs light) • Rhodopsin

Transduction: cGMP

Cyclic guanosine monophosphate (cGMP) keeps cation channels open • Dark current: constant flow of cations entering photorec

In the light: neuron becomes hyperpolarized

Rhodopsin → breakdown of cGMP • Breakdown of cGMP → cGMP-gated ion channels close → cell hyperpolarize

Optic Nerve and Visual Fields

Information leaves retina and down optic nerve ● Optic nerve = cranial nerve II

Portion from each eye decussates:

: Optic chiasm

Retinal Pathways

Fibers from retina make up several pathways

Hypothalamus

: sleep/wake

Pretectum

pupil size

Superior colliculus:

eye movements and orienting

Lateral Geniculate Nucleus

(LGN) of thalamus

Six Layers of Neurons in LGN • Three sets of layers

ParvocellularMagnocellular layers:Koniocellular s

Magnocellular layers

light and dark; movemen

Parvocellular layers

red and green cones; fine detail

Koniocellular sublayers

blue cones

Primary Visual Cortex (V1; Striate Cortex)

First cortical region to combine visual information

Sends information to visual association cortex

ex (V1; Striate Cortex)

Retinotopic organization

ection of retina maps onto section of V1 • Neurons sensitive to orientation

Visual Association Cortex

Combines information from V1 • Helps perceive objects and entire visual scenes

Reconstruction: Dual Streams

Ventral Stream: “What Pathway” ▪ RecognitionForms ▪ Shapes and color (V4) ▪ Connects with limbic system

Dorsal Stream: “Where and How Pathway”

Motion and LocationObject direction and location, visually guided reach ▪ Motion

Color Perception: Trichromatic Coding

Color perception due to 3 types of receptors • S-cones • M-cones • L-cones

Trichromatic Coding

Protanopia: no L-cones • Deuteranopia: no M-cones • Tritanopia: no S-cones • Monochromatic vision: no cone

Retinal Ganglion CellsOpponent-Process Coding

Retinal ganglion cells use an opponent-color system • Neurons respond to opposing pairs of primary colors • Red-green, yellow-blue, and black-white cells

Specific Ventral Stream FunctionsRecognizing

Faces and Bodies • Fusiform face area (FFA): facial recognition

Specific Ventral Stream Functions

Extrastriate body area (EBA): bodies and body parts

Visual Agnosia

Cannot identify common items by sight • Prosopagnosia a common version

Specific Ventral Stream Functions 2

Parahippocampal place area (PPA): places • Lateral occipital complex: things

Spatial Location: Role of the Retina

Monocular vision: depth cues requiring only one eye • Sample cues: occlusion, relative size • Binocular vision: depth cues involving two eyes • Stereopsi

Spatial Location: V1 and Dorsal Stream

Most neurons in V1 are binocular and respond to retinal disparity

Intraparietal Sulcus (IPS):V1 and Dorsal Stream

visually-guided motor movements

Flat visionV1 and Dorsal Stream

lesion to portion near parietal and occipital lobes

: Dorsal Stream Association Cortex

Area V5 (or area MT, for medial temporal): movement • Akinetopsia

Movement: Dorsal Stream

Area MST (medial superior temporal) receives information about movement from V5