Vision “El ojo que ves no es ojo porque tu lo veas, es ojo porque te ve” Antonio Machado “The eye you see is not an eye due to you seeing it, It’s an eye because it sees you”
Vision: Outline Light Eye Visual Path Visual Cortex
Perceptual Dimensions of Light Wave amplitude Purity of the wave Wave frequency UV rays
Vision: Outline EYE Anatomy Retina Receptive Field Edge Detection Color vision
Eye Anatomy We study this in the lab
transparent mediumair (cornea, aqueous humor, pupil, lens, vitreous humor) lens iris diaphragm retina filma focal point Similarity btw eye & camera known since 1600’s Eye anatomy: Functions
Near-sightedness (Myopia ): image falls too short of retina (eyeball too long) newborns Far-sightedness: focal point of light falls beyond retina (Eyeball is too short) Lasik Changes the shape of the cornea (Laser-Assisted In Situ Keratomileusis) Eye Anatomy: Abnormalities
Near-Sightedness nearby things are on focus
Cataracts Reduced illumination, acuity, and color saturation Deposits in the lens Common in older adults
Eye Anatomy: Retina fovea: center of the retina, high concentration of cones optic disk (blindspot) & direct view of arteries (clinical importance) photorreceptors: cones (color vision) and rods Red eye in photos due to dilated pupils Retina of diabetic patient
Concentration of Cones & Rods in Retina Visual Acuity Eye Anatomy: Retina
One Cones --> one ganglion cellhigh acuity (fovea) Many Rods --> ganglion cells. High sensitivity (periphery) (e.g, night vision)
Lateral visual field Medial Retina The eye is a device 'designed' to see, but the ‘blindspot’ reveals it is not perfect Eye Anatomy: Optic disc (blindspot)
Receptive field (RF) is that portion of the visual field (outside world) in which the presentation of visual stimuli will produce an alteration in the firing rate of a particular neuron
Each strip is uniform, Strips look lighter on the left Edge detection
: Center-surround receptive fields & Edge Detection Neither excitatory nor inhibitory parts of RF are stimulated Both excitatory & inhibitory parts of RF are stimulated, canceling each other
: Center-surround receptive fields & Edge Detection Inhibitory part of RF is stimulated: reduced firing A A Part of Inhibitory area of RF is not being stimulated: above baseline firing B B
Edge detection
Hermann Grid Inhibition (-) Excitation (+) + -
Tri-chromatic theory –Blue, red, & green “color” receptors –But some colors don’t mix! Peak sensitivities of the three cones Opponent process theory Red vs. Green; Blue vs. Yellow - Negative afterimage - But there is no ‘yellow’ receptor! COLOR VISION (-) BLUE (-) GREEN (+) RED (+) YELLOW
Test for Deuteranopia: Name number: (‘5’ or ‘2’) If you see a 2: Red/Green Color blindness (male) Most people who are color blind can see colors No ‘green’ cones
Vision: Outline Light Eye Visual Path & its deficits Visual cortex
Visual Paths
LGN thalamic organization Magnocellular –M ganglion cells –large receptive fields –motion detection –locating stimulus in space –dorsal cortical stream –parietal lobe Parvocellular –P ganglion cells –small receptive fields –Object identity –Color recognition –ventral cortical stream –temporal lobe Concentric receptive fields (center surround)
VISUAL PATHWAY Retinal Field: representation of visual field in the retina (reversed: right/left, up/down) Visual Field: outside world you see
Right homonymous most common Can also get upper and lower deficits and scotoma Visual Path: Lesions & Deficits Quadrantanopia Homonymous Hemianopia Bitemporal
Scotoma: A small blindspot in the visual field caused by a small lesion, usually in the occipital lobe Hemianopia – objects are bisected with ½ obscured experiencing the obscured part as “blank” or “void”
Vision: Outline Light Eye Visual Path & its deficits Visual cortex V1: Orientation sensitive –Ventral Pathway –Dorsal Pathway
Visual Cortex V1: primary visual cortex
Primary visual cortex (V1) V1 cells respond to lines –of particular orientations –of particular widths.
Primary visual cortex (V1) Orientation selective cells are organized in a topographic map in V1
How does orientation selectivity in V1 emerges? LGN cells with concentric receptive field provide input to simple cells in V1
Vision: Outline Light Eye Visual Path & its deficits Visual cortex Orientation sensitive –Ventral Pathway Area MT (motion), Object Recognition, Area V4 (color) synesthesia –Dorsal Pathway Spatial Attention Hemispatial Neglect
Complex & with multiple connections Over-simplified version: dorsal & ventral paths Cortical Connections of Visual areas
Ventral & Dorsal Paths ¼ of the brain is involved in visual processing, more than for all other senses
Ventral & Dorsal Paths & how
Ventral & Dorsal Paths
Ventral Path: Object recognition Lesion of ventral pathway Agnosia fMRI: Object recognition
R.V. has bilateral parietal lobe damage ventral lesion (patient DF): - Agnosia - Normal grip dorsal lesion (patient RV): - Normal recognition - poor grip Independence of Dorsal and Ventral paths: Neurpsychological evidence
Cerebral Achromatopsia: bilateral damage to V4 Color is more important of ‘what’ than for ‘where’ Synesthesia Ventral Pathway (V4): Color perception
Ventral Path: Objects vs. Faces Are faces very difficult objects or special ones (i.e., specific process)
Neuroimaging of face, bird and car experts “Face Experts” Fusiform Gyrus Fusiform Gyrus Car Experts Bird Experts Fusiform Gyrus Gauthier et al., 2000 Cars-ObjectsBirds-Objects
Control Group Autism Group Hypoactivation of fusiform face area Schultz, et al Faces Fusiform Gyrus Fusiform Gyrus Children with autism as face “novices”
Area MT: motion perception
Different parts of the visual cortex are specialized in the processing of specific features For example, movement, color. Objects Faces Location Binding problem: If the brain processes features separately, how does it bind those features into a single conscious representation: Answer: Attention (next week)
Spare slides Spatial frequency
Neurons respond to specific spatial frequencies ‘Blurry’ As nearsighted
3-D vision Retinal disparity Stereograms