Perception of StimuliStephen Taylor

Processing Visual Stimuli

httpwwwnaturecomnrnjournalv6n3fig_tabnrn1630_F4html

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 amp 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 lsquobleachedrsquo in high light intensities

They do not detect colour

Rods are distributed throughout the retina

Cone cells feed into their own ganglionThis gives a small receptive field for each ganglion leading to high visual acuity ndash 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 httpwwwfujifilmusacomproductsdigital_camerasexreyespage_03html

images adapted from httpwwwfujifilmusacomproductsdigital_camerasexreyespage_03html

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 contrastEdge enhancement also plays a key role

Explaining Edge Enhancement Although each band is uniformly shaded regions around the edges are enhanced in your vision

appears darker

appearslighter

retina

Light hits the photoreceptorsMore 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 Enhancement Although each band is uniformly shaded regions around the edges are enhanced in your vision

appears darker

appearslighter

retina

Light hits the photoreceptorsMore light more stimulation

Neighbouring cells will inhibit the 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 Enhancement Although 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)

stronger signal brighter

weaker signal darker

If an edge falls within a visual field edge enhancement occurs

Receptors receiving a stronger stimulus will inhibit their neighbours more 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 AA receives the same weak stimulus as its

neighbours and so is inhibited equally by themB 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 DD 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

Itrsquos more like a gradienthellip see if you can explain why by annotating the diagram

Explaining Edge Enhancement Although each band is uniformly shaded regions around the edges are enhanced in your vision

appears darker

appearslighter

retina

Light hits the photoreceptorsMore light more stimulation

Neighbouring cells will inhibit the 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 Enhancement Although 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)

stronger signal brighter

weaker signal darker

If an edge falls within a visual field edge enhancement occurs

Receptors receiving a stronger stimulus will inhibit their neighbours more 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 AA receives the same weak stimulus as its

neighbours and so is inhibited equally by themB 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 DD 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

Itrsquos more like a gradienthellip see if you can explain why by annotating the diagram

Explaining Edge Enhancement Although 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)

stronger signal brighter

weaker signal darker

If an edge falls within a visual field edge enhancement occurs

Receptors receiving a stronger stimulus will inhibit their neighbours more 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 AA receives the same weak stimulus as its

neighbours and so is inhibited equally by themB 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 DD 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

Itrsquos more like a gradienthellip see if you can explain why by annotating the diagram

Explaining Edge Enhancement

A B C D

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

neighbours and so is inhibited equally by themB 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 DD 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

Itrsquos more like a gradienthellip see if you can explain why by annotating the diagram

Itrsquos more like a gradienthellip see if you can explain why by annotating the diagram

images adapted from httpwwwfujifilmusacomproductsdigital_camerasexreyespage_03html

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