THE VISUAL WORLD! Visual (Electromagnetic) Stimulus

Transcription

1 THE VISUAL WORLD! Visual (Electromagnetic) Stimulus Perceived color of light is determined by 3 characteristics (properties of electromagnetic energy): 1. Hue: the spectrum (wavelength) of light (color) 2. Brightness: height of wavelength (intensity) 3. Saturation: purity of wavelength

2 ANATOMY OF THE EYE AND RETINA A. Eye Ligament Cornea Iris Fovea Optic nerve Pupil (opening of iris) Cross-section of the eye Blood vessels Blind spot Retina Sclera (white of eye) Retinal ganglion cells Amacrine Horizontal Cells Bipolar cells Cones Rods cells Front of eyeball Light Back of eyeball To blind spot and optic nerve Cross-section of the retina

3 Special organization of retina at fovea

4 Normal vision: focal length perfect Nearsightedness (myopia): eyeball too long Farsightedness (hyperopia): eyeball too short Astigmatism: uneven cornea/lens

5 PHOTORECEPTORS and TRANSDUCTION Lamellae Cone Connecting cilium Lamellae Rod Bipolar cells Back of retina Connecting cilium Nucleus Mitochondria How light energy is transformed (transduced) into neural impulses: Rod Lamella Rhodopsin molecules Inactive phosphodiesterase In the dark: Cyclic GMP (cgmp) holds ion (cation) channel open 1. High levels of cgmp; 2. Keep cation channels open. Transducin (G protein) Cations (Na +, Ca ++ ) enter the cell and keep membrane depolarized When light enters eye: 1. A photon strikes a photoreceptor (rhodopsin): 2. Rhodopsin molecule splits, retinal binds with and activates transducin 3. Transducin activates phosphodiesterase 4. Phosphodiesterase destroys cyclic GMP, closes ion channel 5. Cations (Na +, Ca ++ ) no longer enter, membrane hyperpolarize

6 Transmission of information in Retina Back of eye (retina) Photoreceptor cell 1. NO action potential Light (photon) Bipolar cell Information 1. NO action potential Ganglion cell Optic nerve to brain 1. Action potential - send axon via optic nerve to brain Front of eye (cornea) - photoreceptors (cones and rods) only produce receptor potentials (similar to postsynaptic potentials); - bipolar cells only produce postsynaptic potentials; - ganglion cells are the first cells to produce action potentials.

7 CENTRAL vs. PERIPHERAL vision 1. Receptive field in center of retina at : fovea provides very clear, precise color vision. - mostly populated by cones (color vision 3 subtypes) - very little convergence of bipolar cells onto ganglion cells (not very sensitive). 2. Receptive field in periphery (outside of fovea): provides fuzzy, imprecise vision. - very sensitive to light, mostly populated by rods (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 Ganglion cells Receptive field in periphery of retina

8 Region of overlap of two visual fields VISUAL PATHWAY Right visual field Optic chiasm Left visual field T N N T Information from left half of 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 (Nasal retina) cross through the optic chiasm to the other side of the brain; Axons from ganglion cells located in outer half of retina (Temporal 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 superior colliculus, which projects to pulvinar (tectopulvinar pathway) to control muscles involved in head and eye movements, iris and lens size. Chap. 8-7

9 DETECTING SHAPES: RECEPTIVE FIELDS Receptive field: 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. - ganlion cells normally have a baseline level of activity, which can increase or decrease. - ganglion cells, lateral geniculate cells and cells in lower layer IVc of the visual cortex have circular receptive fields. Chap. 8-8

10 The retinal ganglion cells all respond to light with circular receptive fields that are integrated by the visual cortex. Chap. 8-9

11 EDGE DETECTION AND CONTRAST 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. Chap. 8-10

12 A B C D E F Mach Bands: Lateral inhibition - firing rate of ganglion cells is proportional to light intensity - ganglion cells neighboring inhibit ganglion cells - ex. above: B and C cells should fire at same rate; because lateral inhibition 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 Chap. 8-10

13 RECEPTIVE FIELDS Simple cortical cells: Cells in primary visual cortex (occipital lobe) that are not part of lower layer IV (IVc) of cortex. - have antagonistic on and off receptive fields - have lines or rectangular rather than circular receptive fields - often respond best to specific orientation - respond only to one eye (monocular) Chap. 8-11

14 RECEPTIVE FIELDS (CONTINUED) Complex cortical cells: Similar to simple cortical cells with the following exceptions: 1. Complex cells have larger receptive fields. 2. Complex cells respond better to moving lines or rectangles across their receptive fields 3. Complex cells are the first cells of the visual system to show binocular responses (both eyes) 4. Do not show antagonistic responses Hypercomplex cortical cells are similar to complex cortical cells but do show antagonistic receptive field responses. DEPTH PERCEPTION = Retinal disparity 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 Chap. 8-12

15 COLOR vision 1. Trichromatic theory: - Thomas Young (1802) and von Helmholtz (1852) - 3 types of cones (due to different ): photopigment 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 Chap. 8-13

16 2. Opponent process theory: - Ewald Hering (1874) - Certain colors appear to be linked together - 3 types of bipolar and ganglion cells: a. Red-Green opponents b. Blue-Yellow opponents c. Black-White opponents d. Color is due to the relative activity of these 3 kinds of opponents N.B. This is true for: the rest of the visual system Example: RED (+) GREEN (-) OPPONENTS GREEN LIGHT -Inhibits ganglion cell when in center field; - Excites ganglion cell when in surround field. Green cone Red cone RED LIGHT -Excites ganglion cell when in center field; - Inhibits ganglion cell when in surround field. Bipolar cell Horizontal cell Ganglion cell Red excites in center field Green excites in surround field Chap. 8-14