Visual

•    Sensation and the Visual system:  transforming light energy into neural activity

•    Anatomy of the Visual System
Structure of the Eye

•      Cornea

•      Iris

•      Pupil

•      Lens

•      Retina

•    Anatomy of the Visual System
Structure of the Eye

•      Blind spot – where optic nerve leaves eye, no photoreceptors present

•      Fovea – at center of retina, specialized for high acuity (what does this imply about receptive field size?); its receptive fields are at center of eye’s visual field.

•    Photoreceptors:  absorb light

•      Rods

–      More numerous

–      Sensitive to low levels of light (dim light)

•      Cones

–      Specialized for color and high visual acuity

–      Highly responsive to bright light

•    Distribution of rods and cones over retina

•    Photoreceptors

•      3 Types of Cone Pigments that absorb light over range of frequencies, maximal absorptions are:

•  419 nm (“blue” or short wavelength)

•  531 nm (“green” or middle wavelength)

•  559 nm  (“red” or long wavelength)

–     Approximately equal #s red and green cones, fewer blue

•       Color-blindness

•    Neurons of the Retina

•          Photoreceptors (rods and cones)

•          Bipolar cell

•          Ganglion cell à optic nerve

•     Phototransduction:  changing light energy into neural activity

•       Photoreceptors are active (depolarized) in the dark.  Light causes cation channels to close, “turning off” the cell (causing hyperpolarization)

•        How is this signal coded by retinal circuits?

•       Mapping Visual Cortex

•       Beyond V1

•    Injury to the “What” Pathway
(Ventral Stream)

Agnosia = not knowing

•      Visual-Form Agnosia

–     Inability to recognize objects or drawings of objects

–     Visual form agnosia

•      Color Agnosia (achromatopsia)

–     Inability to recognize colors

•      Face Agnosia (prosopagnosia)

–     Inability to recognize faces

•    Injury to the “How” Pathway
(Dorsal Stream)

•     Optic Ataxia

–     Deficit in the visual control of reaching and other movements

–     Damage to parietal cortex

–     Can recognize objects normally

–     Patient with optic ataxia