THE VISUAL WORLD! Visual (Electromagnetic) Stimulus Perceived color of light 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 RETINA 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/lens 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 cell ____________________________________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 action potential 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 visual field ________________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 superior colliculus, 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 visual cortex have ______________________ The retinal ganglion cells all respond to light with circular receptive fields that are ___________________________ 5 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. 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 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 6 RECEPTIVE FIELDS Simple cortical cells: Cells in primary visual cortex (occipital lobe) 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 visual system 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 PERCEPTION = RtiRetina ldil dispar itity 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 8.