Perception of StimuliStephen Taylor

http://www.youtube.com/watch/?v=MWfbr0tthgU

http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html

Processing Visual Stimuli

http://www.nature.com/nrn/journal/v6/n3/fig_tab/nrn1630_F4.html

The lens focuses light onto the retina at the back of the eye, where it stimulates photoreceptors

(rods, sensitive in low light with low acuity; and cones, sensitive to colour in high light, with high

acuity).

Photoreceptors synapse with bipolar neurons. These feed into ganglion cells, carrying the impulse to the

visual cortex through the optic nerve.

Some ganglia are sensitive to impulses from the edge of the receptive field, where others are sensitive to

impulses from the centre.

Edge enhancement (due to lateral inhibition of cells in the retina) results in greater

contrast around edges.

Stimulus from the left visual field of each eye is processed in the right side of the brain

and vice versa. This is due to contralateral processing via the optic chiasm

Uses the retina and the brain.

Thanks to John Burrell & David Mindorff

Rod Cells Cone Cells

Many rod cells feed into one ganglion: all their action potentials are combined into a single impulse at the synapse. This means

each ganglion has a large receptive field, but low acuity (low ability to detect differences).

Rod cells are activated in low light conditions, but ‘bleached’ in high light

intensities. They do not detect colour.

Rods are distributed throughout the retina.

Cone cells feed into their own ganglion.This gives a small receptive field for each ganglion, leading to high visual acuity – small differences are easily detected.

There are three types of cone cells, receptive to different wavelengths (red, green, blue). These are only active in sufficient light.

Cone cells are concentrated in the fovea.

images adapted from http://www.fujifilmusa.com/products/digital_cameras/exr/eyes/page_03.html

images adapted from http://www.fujifilmusa.com/products/digital_cameras/exr/eyes/page_03.html

Receptive Fields and Processing Visual StimuliMany rod cells feed into one retinal ganglion. This means that many impulse converge to form a single signal which is sent to the brain. There is no distinction between stimuli which hit different sections of the same receptive field.

Some ganglia are stimulated by impulses sent from rod cells from the edge of their receptive field and inhibited by signals from the middle.

Other ganglia are inhibited by impulses sent from rod cells from the edge of their receptive field and stimulated by signals from the middle.

This allows for greater perception of contrast.Edge enhancement also plays a key role.

Explaining Edge EnhancementAlthough each band is uniformly shaded, regions around the edges are enhanced in your vision.

appears darker

appearslighter

retina

Light hits the photoreceptors.More light, more stimulation.

In these diagrams, as the receptor cells get brighter, is

shows a stronger signal.

Stimulated photoreceptors pass the action potential to the bipolar neuron and ganglion.

uniform signal

Explaining Edge EnhancementAlthough each band is uniformly shaded, regions around the edges are enhanced in your vision.

appears darker

appearslighter

retina

Light hits the photoreceptors.More light, more stimulation.

Neighbouring cells will inhibitthe neurons of each other.

Greater stimulation of the receptor means greater

inhibition of the neighbours.

This is called lateral inhibition.

If all neighbouring cells receive the same stimulus (and

therefore inhibition), they will produce a uniform signal.

Stimulated photoreceptors pass the action potential to the bipolar neuron and ganglion.

uniform signal

Explaining Edge EnhancementAlthough each band is uniformly shaded, regions around the edges are enhanced in your vision.

uniform weak signal(dark colour perceived)

uniform strong signal(light colour perceived)

If an edge falls within a visual field, edge enhancement occurs.

Receptors receiving a stronger stimulus will inhibit their neighboursmore strongly, and vice-versa.

So a neuron that is more inhibited than its neighbours will result in a darker colour being perceived (on the dark side of the edge), and vice versa, giving an enhanced contrast on the border between light and dark images.

Explaining Edge Enhancement

A B C D

Why is B darker than A?A receives the same weak stimulus as its

neighbours and so is inhibited equally by them.B is next to C, which recieves a stronger stimulus

and therefore inhibits C more. As a result, B is overall more inhibited than A, so is darker.

Why is C brighter than D?D receives the same strong stimulus as its neighbours and so is inhibited equally by them. C is next to B, which recieves a weaker stimulus and therefore inhibits C less. As a result, C is overall less inhibited than D, so is brighter.

Receptor A receives the same light stimulus as B.

Receptor D receives the same light stimulus as C.

It’s more like a gradient… see if you can explain why by annotating the diagram.

http://serendip.brynmawr.edu/bb/latinhib_app.html

images adapted from http://www.fujifilmusa.com/products/digital_cameras/exr/eyes/page_03.html

http://www.sumanasinc.com/webcontent/animations/content/visualpathways.html

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