VISUAL ANGLE

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VISUAL ANGLE 3.1

retina size is measured in degrees of visual angle

it incorporates size of object and its distance from the observer

as the object gets closer, the visual angle becomes larger

HOW TO CALCULATE VISUAL ANGLE

visual angle = 2 x (size/2x distance)

size = tangent (visual angle/2) x (2 x distance)

distance = (size/2)/(tangent (visual/2)

VISUAL ANGLE ILLUSION

stare at a black dot, look away then a shadow of the dot appears, when you hold a piece of paper in front of you the dot appears smaller

why? when you hold the piece of paper up your brain calculates that the distance of the after image has changed

when you view the object all your brain knows is the visual angle the object occupies on the retina

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FOVEAL ANGLE 3.2

the fovea is located directly behind the pupil and lens

provides the highest degree of visual acuity

the farther the image moves from the fovea into the periphery, the less clear it will be

WHY IS FOVEAL ACTIVITY BETTER THAN PERIPHERAL?

fovea packed with cones that have exclusive access to a single retinal ganglion cell

a large chunk of the striate cortex is dedicated to processing info from the fovea

the fovea is cortically magnified relative to other areas of the retina

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3.3 GABOR PATCHES

gabor patches are sine waves grating through a Guassian window

they match the receptive field properties of neurons in primary visual cortex

the light intensity alternates between its brightest and darkest values according to sine function

SPATIAL FREQUENCY

measured in cycles per degree

the number of times the sine wave repeats within one degree of visual angle

CONTRAST

refers to the difference in intensity between the lightest and darkest portions of the patch

high contrast: light regions are white, dark are black

low contrast: light regions are light grey, dark are dark grey

PHASE

the relative position or shift of the sine wave from left to right

as it cycles through the phases, it appears to move laterally

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3.4 STRIATE RECEPTIVE FIELDS

a receptive field is an area of the retina that the cell respond to along with a particular pattern of light that must be present in that area for the neurone to respond

the further away from the visual system, the receptive fields get larger

receptive fields not 'all or nothing' - e.g. a cell may respond best to a vertically orientated bar of light, if the bar is slightly tilted horizontally, the cell will still respond but not as well

striate cortex is the first area of the cerebral cortex to receive info from the eyes

neurones in the striate complex respond best to bars of light rather than spots of light that maximally stimulate retinal ganglion cells

Huber and Wiesel moved and rotated bars and discovered characteristics about receptive fields

RECEPTIVE FIELD CHARACTERISTICS

BAR WIDTH: some neurones respond best to narrow or wide bars of light, bar the wrong way the firing rate cut in half

BAR ORIENTATION: respond best to vertical, left/right oblique or horizontal bars, off by 45degrees, decreases firing rate by 80%

CELL TYPE: simple cells respond maximally when bar of light is in centre of the cells receptive field, complex cells respond the same anywhere

END STOPPING: end stopped cells respond best when the bar of light terminates within its receptive field, end stopped neurones however respond best when the bar extends through the entire receptive field

EDGE-AND BAR-DETECTORS: some respond best when bars surrounded by darkness, others when there is light on one side and darkness on the other

RECEPTIVE FIELD SIZE: extent of the area of which the visual field to which each cell responds

MOTION: some respond best when bars are in motion, others stable

COLOUR: some respond equally to any colour, others respond best to red etc.

OCCULAR DOMINANCE: respond best to left eye light, others right eye light

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3.5 HYPER COLUMNS

neurones in the striate cortex are organised into columns and hypercolumns

each column contains neurons with similar properties and each hyper column processes one small section of the visual world

COLUMNS

scientists pushed a recording electrode into the cortex, neurones had similar but not identical response properties

HYPERCOLUMNS

groups of columns function as a unit

each hypercolumns job is to perceive one small portion of the visual world

in each column half the cells respond strongly to left light, others right light

in these, some columns respond best to bars of light

some columns contain blobs involved in colour processing