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