E.2 - Perception of Stimuli

Biology Journal 3/17/2014

Suppose that you are driving on the freeway and notice that the car in front of you has stopped. You react by slamming on the breaks. But, this “reaction time” process has taken up a certain amount of time. What nervous system processes needed to happen? Describe it, including what your motor neurons, sensory neurons, and relay neurons did during that process.

Biology Journal 3/18/2014

The back of your eye is full of specialized neurons called rods and cones. What kind of neuron do you think these cells are? What do you think would be different about their dendrites?

E.2 Perception of Stimuli E.2.1

Outline the diversity of stimuli that can be detected by human sensory receptors, including:  Mechanoreceptors, chemoreceptors, thermoreceptors, photoreceptors Details of how each receptor functions are not required.

E.2.2

Label a diagram of the structure of the human eye. The diagram should include:  sclera, cornea, conjunctiva, eyelid, lens, choroid, aqueous humour, pupil,  iris, vitreous humour, retina, fovea, optic nerve, blind spot

E.2.3

Annotate a diagram of the retina to show the cell types and the direction in which light moves. Include names of rod and cone cells, bipolar neurons and ganglion cells.

E.2.4

Compare rod and cone cells. Include:  use in dim light versus bright light  one type sensitive to all visible wavelengths versus three types sensitive to red, blue and green light  passage of impulses from a group of rod cells to a single nerve fibre in the optic nerve versus passage from a single cone cell to a single nerve fibre

E.2.5

Explain the processing of visual stimuli, including edge enhancement and contralateral processing.  Edge enhancement occurs within the retina and can be demonstrated with the Hermann grid illusion.  Contralateral processing is due to the optic chiasma, where the right brain processes information from the left visual field and vice versa. This can be illustrated by the abnormal perceptions of patients with brain lesions.

E.2.6

Label a diagram of the ear. Include:  Pinna, eardrum, bones of the middle ear  oval window, round window, semicircular canals  auditory nerve, cochlea

E.2.7

Explain how sound is perceived by the ear, including the roles of the eardrum, bones of the middle ear, oval and round windows, and the hair cells of the cochlea.

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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