Physiological Psychology: Vision
Receptive Fields in Striate Cortex
V1 cells are more responsive to spatial frequencies than to bars of light
Coding of Visual Stimuli: Receptive Fields
The region in the visual field that when stimulated causes a particular sensory cell to respond
Sclera
The white of the eye
Retinal Disparity
These cells respond more strongly when input from each eye differs In normal viewing, the information coming from each eye is slightly different In both diagrams both eyes converge on a box but the angle of viewing is slight different for each eye This facilitates depth perception Many V1 cells respond to this
Cornea
Translucent protective front of the eye that allows light to get through
Opponent Processing Theory
In ganglion cells the 3 color code gets translated 4 different ganglion cells exist These receptive fields are also center-surrounded where pairs of colors are in opposition
Bonnet's Syndrome
No damage to V1 but they experience hallucinations even though they know they are blind
Dorsal Stream
(Where) Damage to these areas often affect visually guided movements and or detection of movement and images across space E.g. Balint's syndrome, neglect
Ventral Stream
(what) Damage to these areas will affect the ability to detect specific features (shapes, colors, etc.) E.g. Achromotopsia, Prosopagnosia Color perception o V4: color constancy Colors remain the same under all lighting conditions o V8: color perception in "real objects" - Responds only when colors are appropriate Globs- color sensitive hot spots in visual association cortex
Grandmother Cell
A cell that only responds to images of your grandmother And other cells only respond to other specific images etc.
Dorsal Stream: Movement and Perception
Area MT/ V5 • Selective for motion direction and velocity • Inputs from V1/V2 and superior Colliculus • Cannot see movement (its like digital world) Area MST • Radial movements: optic flow • As you move along in your environment things flow past you
Hypercomplex Cell in V1
As the length of the stimulus increases there is a decrease in response Cells are also sensitive to orientation, motion, and direction
Blindspot
Axons merge to reach rest of the brain and there are no photoreceptors (rods and cones)
Illusions
Blind spot illusion- Red X disappears and black bar continues Ebbinghaus illusion - Which yellow dot is bigger? Ponzo illusion- Depth cues
Optic Nerve
Bundle of neurons leaving the eye that goes into the brain
Spatial Frequencies
By changing the size of the image you can make them look the same Farther away you lose the high frequencies - Einstein and Monroe's photo
Ganglion Cells
Cells that take information from the eye to the brain First neurons that have action potentials in the eye
Early Depictions of the Eye
Cicero (107-43 BC)- Earliest version of "eyes are the windows to the soul" Vesalius- represents eye in I De Fabrica (1568) Da Vinci Descartes (1598-1650) - remarkably accurate in his theory
Lens
Composed of transparent, flexible tissue and is located directly behind the iris and the pupil It is the second part of your eye, after the cornea, that helps to focus light and images on your retina.
Anton's Syndrome
Confabulation- they insist they can see things but they are blind
Cilary Muscle
Controls the lens
Acuity vs. Sensitivity
Convergence of cones and rods on retinal ganglion cells o Low convergence in cone-fed pathways o High convergence in rod-fed pathways o One cell-one signals gives cones greater acuity vs. higher sensitivity for rods E.g. best way to see something in the dark is if you don't look directly at it
Bipolar Cells
Convey signal from photoreceptors to ganglion cells
Color Coding in the Striate Cortex
Cytochrome oxidase (CO) reveals patters of blobs (v1) and stripes (V2) Neurons within these stained areas are responsive to color
What We (as humans) Can See
For humans light is a narrow band of the spectrum of electromagnetic radiation Electromagnetic radiation with a wavelength of between 380 and 760 nm (a nanometer, nm, is one-billionth of a meter) is visible to humans Other animals can detect different ranges of electromagnetic radiation Insects and some birds can see lower (ultraviolet) wavelength
Striate to Extrastriate Cortex
From V1, information flows to V2 and beyond (Extrastriate cortex= visual association cortex) Flattening algorithms let us see these areas in 2D
Dorsal and Ventral Streams
From V1, there are 2 basic streams of information flow: dorsal and ventral
Hermann Grid Illusion Explained
Ganglion cells whose receptive fields fall into the intersections of the grid produce a smaller response than ganglion cells whose receptive fields fall on lines because intersection by ganglion cells are more inhibited by light in the surround Why the smudges disappear when you look directly at them? Receptive Fields of ganglion cells that receive input from the fovea are much smaller ABOVE IS WRONG If it were correct then we should still see the smudges with wavy lines but they aren't there! So a 150 year old illusion remains a mystery
Center Surround Concept
Happens at retinal level (not in the brain) Receptive fields consist of either ~on center off surround~ or ~on surround off center~
Attributes of Color
Hue- color Brightness Saturation- how pure the color is
Magnocellular Layers of Dorsal LGN
Inner 2 layers o M-pathway o Rod input o Most responsive to movement
Ventral Stream: Forming Perceptions
Lateral occipital complex- Responds to a wide variety of shapes and objects Fusiform face area • Recognition of faces • Prosopagnosia- Inability to recognize faces due to damage to FFA Autism: FFA not activated when viewing faces William's syndrome: larger FFA Extrastriate body area- perception of human body and body parts other than faces
Cones
Less sensitive to light Color vision Excellent acuity Found in the center of the retina
Photoreceptors
Light- responsive cells that transduce photic energy into electrical potentials Rods and Cones
Vision without Photoreceptors
Loss of cones- age related muscular degeneration Loss of rods- Retinitis Pigmentosa - loss of basically all peripheral vision
Kuffler (1950's)
Many ganglion cells have receptive fields with a center surrounded organization: excitatory and inhibitory regions on the retina separated by a circular boundary
Rods
More sensitive to light No color Poor acuity Found in periphery of retina
Ocular Dominance Column
Most V1 neurons are binocular (respond to input from both eyes) , but respond best to one eye or the other
Iris
Muscle that expands and contracts to let light in- colored part
Retina
Neural tissue and photo receptive cells located on inner surface of posterior portion of the eye Composed in an "inside-out" arrangement of photoreceptors, bipolar cells and ganglion cells While light enters from the front to the back, the information flow is back to front
Trichromatic Theory
Originally proposed by Young in 1802 There are 3 types of cones: blue, red, and green Each has a different photopigment
Parvocellular Layers of Dorsal LGN
Outer 4 layers o P-pathway o Cone input o Color
Photopigments
Outer segment has stacks of membranes (lamellae) where photopigments are found Photopigments composed of a molecule of retinal plus opsin o Rods: Rhodopsin o Cones: photopsin
Parts of the Eye
Pupil Iris Lens Cornea Cilary muscle Sclera Retina Optic nerve Fovea Blind spot
Lateral Geniculate Nucleus (Dorsal)
Receives input from retina and projects to primary visual cortex 6 layers- Magnocellular and Parvocellular Sublayer below each layer is Koniocellular (blue cones)
Center Surround Organization
Receptive field of cells in the retina have a center-surround organization Cells in the retina that respond to colors receive input from cones These receptive fields are also center-surrounded where pairs of colors are in opposition (opponent processing) Red Blue and Green cells- characterized by differences in pigments
Simple Cell in V1
Receptive fields are rectangular with "on" and "off" regions and respond to location
Complex Cell in V1
Rectangular, with larger receptive fields, and respond best to a particular stimulus anywhere in their receptive fields
Pupil
Regulates amount of light coming through
Optic Disk
Represents the beginning of the optic nerve and is the point where the axons of retinal ganglion cells come together Responsible for blind spot
Fovea
Site at the very back of the eye Region of the retina that mediates the most acute vision of birds and higher mammals Color-sensitive cones constitute only type of photoreceptors found here Thinning of the ganglion cell layer reduces distortion due to cells between the pupil and retina (light does not have to penetrate through that many cells to get to the photoreceptors)
Hierarchical Streams of Processing
So far: information flows from retina → LGN→ V1 Receptive fields of cells in each area become more and more selective in what they will respond to From V1 to "higher level" visual processing regions, cells tend to respond to specific stimuli Is there a cell at the end of the stream that is selective for one thing only, the so called "grandmother cell"?
Retinotopy
Specific area of V1 process information from specific area of the visual field More space is dedicated to processing images from the fovea
Visual Cortex
Striate cortex= V1 6 layered Retinoptopic (special relationships are preserved)
Perceptual Constancy
Tendency to perceive objects as having a constant shape, size, and brightness under varying conditions
Distribution of Photoreceptors
The distribution of cones and rods over the human retina is shown as the number of cones and rods per square millimeter as a function of distance from the center of the fovea
Modular Organization of the Striate Cortex
V1 is composed of ~ 2500 'modules' that receive input from each eye Layer specific input from the LGN CO blobs: cells respond to color and low special frequencies Cells in the vertical column respond to stimuli of the same orientation Ice cube model o At 1 mm2 region of cortex that can process all features of one small region of visual space
Striate Cortex
V1 is primary visual cortex V2 is 1st part of primary association area In striate cortex neurons with circular receptive fields are rare Most neurons in V1 are either simple, complex, or hypercomplex
From the Retina to the Brain
Visual information crosses at the optic chiasm Axons from the outer half of the retinas stay on the same side Axon from the inner half cross over Target: LGN
Optic Chiasm
Where information from the left eye crosses to the right hemisphere and where the right eye crosses to the left hemisphere
