THE VISUAL WORLD! Visual (Electromagnetic)

Perceived color of is determined by ___3 characteristics (properties of electromagnetic energy): 1. ____: the spectrum (wavelength) of light (color) 2. ______: height of wavelength (intensity) 3. ______: “purity” of wavelength

ANATOMY OF THE EYE AND A. Eye Ligament______

Cornea______

______Blood vessels Pupil______(opening ______of iris) ______Sclera (white of eye) Cross-section of the eye

______

Front of Back of Light eyeball eyeball

To blind spot and optic nerve Cross-section of the retina

1 Special organization of retina at _____

Normal vision: focal length perfect

Nearsightedness (myopia): eyeball

Farsightedness (hyperopia): eyeball

Astigmatism: uneven cornea/

2 PHOTORECEPTORS and TRANSDUCTION ______Cone

Connecting______cilium ______Bipolar cells ______Rod

______Connecting ______Nucleus ______cilium ______Mitochondria Back of retina How light energy is transformed transformed (transduced) into neural impulses: Rhodopsin______Rod____ molecules______Inactive______In the dark: Lamella______phosphodiesterase______1. High levels of ______; 2. Keep cation channels open.

______Cyclic GMP (cGMP) holds ion ______(cation) channel open

Transducin (G protein) ______Cations (Na+, Ca++) ______the ______and keep membrane depolarized

When light enters eye: 1 1. A photon strikes a photoreceptor (rhodopsin): 2. Rhodopsin molecule splits, 2 retinal binds with and activates transducin 3. Transducin activates phosphodiesterase 3 4. Phosphodiesterase destroys cGMP, closes ion channel 5. Cations (Na+, Ca++) no longer 4 5 enter, membrane ______.

Transmission of information in Retina Back of eye (retina)

Photoreceptor 1. NO cell

Light (photon) Information Bipolar cell 1. NO action potential

Ganglion cell 1. Action potential -senditid axon via optic Optic nerve nerve to brain to brain Front of eye (cornea) - photoreceptors (cones and rods) only produce ______(similar to postsynaptic potentials); - bipolar cells ____ produce postsynaptic potentials; - ganglion cells are the _____ cells to produce action potentials.

3 CENTRAL vs. PERIPHERAL vision

1. Receptive field in center of retina at fovea_____: ______- mostly populated by ______(color vision – 3 subtypes) - ______very little convergence of bipolar cells onto ganglion cells (not ______very sensitive).

2. Receptive field in “periphery” (outside of fovea): ______- very sensitive to light, mostly populated by ____ (B&W vision) - ______high level of convergence from several rods (via bipolar cells) ______onto ganglion cells - provides higher “sensitivity”.

Receptive field in center of retina (fovea)

Photoreceptors Bipolar cells

GlillGanglion cells

Receptive field in periphery of retina

VISUAL PATHWAY Region of overlap of two visual fields Right

______Optic chiasm

Information from left half of visual N T field T N Left visual field

Optic______nerve Optic______tract ______Lateral geniculate nucleus Information from right half of visual field Primary______visual cortex (Occipital______lobe) •Axons from ganglion cells located in inner half of retina (______) cross through the optic chiasm to the other side of the brain; •Axons from ganglion cells located in outer half of retina (______) remain on the same side of the brain; •Besides primary retino-geniculo-cortical pathway (geniculostriate), axons from retina also contact hypothalamus (suprachiasmatic nucleus) to synchronize 24-hr rhythm, and , which projects to pulvinar (tectopulvinar pathway) to control muscles involved in head and eye movements, iris and lens size.

4 DETECTING SHAPES: RECEPTIVE FIELDS ______Area of visual field within which it is possible for a visual stimulus to influence the firing of that neuron. - receptive fields in retino-geniculo-cortical pathway are circular. - ganglion cells normally have a ______level of activity, which can ______. - ganglion cells, lateral geniculate cells and cells in lower layer IVc of the have ______

The retinal ganglion cells all respond to light with circular receptive fields that are ______

5 EDGE DETECTION AND

A B C D E F

- although there are no differences in brightness within each separate bands, you perceive, from left to right, lighter to darker shades in each band.

A B C D E F

Mach Bands: ______- firing rate of ganglion cells is proportional to light______intensity - ganglion cells ______neighboringinhibit ganglion cells

- ex. above: B and C cells should fire at same rate; because from D is stronger than B, C fires less than B - D and E cells should fire at same rate; but lateral inhibition from C is less than E, so D fires more than E

6 RECEPTIVE FIELDS Simple cortical cells: Cells in primary visual cortex () that are not part of lower layer IV (IVc) of cortex.

- have ______“on” and “off” receptive fields - have ______rather than circular receptive fields - often respond best to ______- respond only to one eye (monocular)

RECEPTIVE FIELDS (CONTINUED) Complex cortical cells: Similar to simple cortical cells with the following exceptions: 1. Complex cells have ______2. Complex cells respond better to ______moving lines ______or rectangles across their receptive fields; 3. Complex cells are the first cells of the to show binocular______responses (both eyes) 4. Do not show ______

Hypercomplex cortical cells are similar to ______but do show ______complex cortical cells antagonistic receptive field responses.

DEPTH = RtiRetina ldil disparity it detected by complex cortical cells which respond most strongly to slightly different retinal images from the two eyes - depth also perceived with monocular______: depth cues 1. ______Overlap 2. Relative______brightness 3. ______Linear perspective 4. ______Relative texture

7 COLOR vision 1. Trichromatic theory: - Thomas Young (1802) and von Helmholtz (1852) - __3 types of cones (due to different photopigments______): Red (long wavelength: ______)approx. 560 nm Green (medium wavelength: ______)approx. 530 nm Blue (short wavelength: ______)approx. 420 nm - Color vision is due to ______the relative activity of these 3 kinds of photoreceptor cells N.B. This is true for:______photoreceptors at the level of the retina

Color blindness: associated with X chromosome - because men have only one X chromosome, results in “color blindness” with higher incidence in men than women - defects makes red and green confusing

2. Opponent process theory: - Ewald Hering (1874) - Certain colors appear to be “linked together” - 3 types of bipolar and ganglion cells: a. ______b. ______c. ______d. Color is due to the relative activity of these 3 kinds of opponents NBN.B. This is true for: ______the res t o f the v isua l sys tem_ Example: RED (+) GREEN ((--)) centercenter--surroundsurround OPPONENTS

GREEN LIGHT Green cone Red cone RED LIGHT -Inhibits ganglion -Excites ganglion cell when in center cell when in center field; field; - Excites ganglion - Inhibits ganglion cell when in cell when in surround field. surround field.

Bipolar cell

Horizontal cell

Ganglion cell

Red excites in center field Green excites in surround field

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