Perception of Stimuli
Stephen Taylor
Processing Visual Stimuli
Uses the retina and the brain.
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
http://www.nature.com/nrn/journal/v6/n3/fig_tab/nrn1630_F4.html
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).
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.
Rod cells are activated in low light conditions,
but ‘bleached’ in high light intensities.
They do not detect colour.
There are three types of cone cells, receptive
to different wavelengths (red, green, blue).
These are only active in sufficient light.
Rods are distributed throughout the retina.
Cone cells are concentrated in the fovea.
images adapted from http://www.fujifilmusa.com/products/digital_cameras/exr/eyes/page_03.html
Receptive Fields and Processing Visual Stimuli
Many 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.
images adapted from http://www.fujifilmusa.com/products/digital_cameras/exr/eyes/page_03.html
Explaining Edge Enhancement
appears
darker
appears
lighter
Although each band is uniformly
shaded, regions around the edges
are enhanced in your vision.
Light hits the photoreceptors.
More light, more stimulation.
In these diagrams, as the
receptor cells get brighter, is
shows a stronger signal.
uniform signal
retina
Stimulated photoreceptors pass
the action potential to the
bipolar neuron and ganglion.
Explaining Edge Enhancement
appears
darker
appears
lighter
Although each band is uniformly
shaded, regions around the edges
are enhanced in your vision.
Light hits the photoreceptors.
More light, more stimulation.
Neighbouring cells will inhibit
the neurons of each other.
Greater stimulation of the
receptor means greater
inhibition of the neighbours.
uniform signal
retina
Stimulated photoreceptors pass
the action potential to the
bipolar neuron and ganglion.
This is called lateral inhibition.
If all neighbouring cells receive
the same stimulus (and
therefore inhibition), they will
produce a uniform signal.
Explaining Edge Enhancement
Although each band is uniformly
shaded, regions around the edges
are enhanced in your vision.
If an edge falls within a
visual field, edge
enhancement occurs.
Receptors receiving a
stronger stimulus will
inhibit their neighbours
more strongly, and viceversa.
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.
uniform weak signal
(dark colour perceived)
uniform strong signal
(light colour perceived)
Explaining Edge Enhancement
Receptor A receives the
same light stimulus as B.
A
B
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.
C
D
Receptor D receives the
same light stimulus as C.
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.
It’s more like a gradient… see if you can explain why by annotating the diagram.
images adapted from http://www.fujifilmusa.com/products/digital_cameras/exr/eyes/page_03.html
Wheels turning illusion from
http://www.newopticalillusions.com/moving-optical-illusions/two-wheels-new-optical-illusion/
@IBiologyStephen
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