sensory transduction◦ - conversion of physical energy from the

environment into changes in electrical potential

sensory coding-◦Making sense of that input

vision - light waves -

taste – chemicals in fluid – hearing – sound waves- touch- pressure, temperature changes,

smell- chemical in air

vision -

taste - - hearing - -

touch - smell -

Receptors show adaptation◦most sensitive to changes rather than constant

stimulation

◦why is this important?

General pathway for most sensory information:

◦sensory neurons – sensory nerves spinal tracts –

thalamus – primary cortex – higher association cortex

Certain sensory neurons have a spontaneous firing rate.

For these cells any change in their firing rate will convey important info (i.e. color vision)

Different rhythms of firing also can convey different information

* most highly developed sense in humans

optic nerve for one eye - 1,000,000 axons auditory nerve contains about 30,000

axons

adaptability and plasticity of visual system - make sense out of nonsense

iris ◦ largely a muscle that expands and contracts pupil

in response to light◦phenotypically unique –

iris scan sclera◦ tough opaque tissue

pupil◦often used to determine neurological function

light waves along the visual spectrum

1. inverted image on retina

2. region important for transduction is at very back of the eye

retina - structure of eye important for transduction

- retina contains neurons, glial cells and two types of photoreceptors

responsible for transduction

numerous differences between rods and cones

rods shaped like a rod

cones shaped like a cone

a low ratio of synaptic connections between neurons ensures higher definition and sharpness compared to a higher ratio

less sharp focused visual input

rods shaped like a rod insensitive to color work well under low

illumination 20,000,000/eye location: found around the

periphery of the retina requires extended time

until optimal function

cones shaped like a cone sensitive to color work best in bright light

5,000,000/eye location – found around

the fovea of the retina responsible for sharp

images and vision works optimally very

quickly

there are at least two levels of communication within the neural cells of the eye◦ rods and cones – bipolar cells – ganglion cells

(axons make up the optic nerve) to CNS

there are at least two levels of communication within the neural cells of the eye◦ rods and cones – bipolar cells – ganglion cells

(axons make up the optic nerve) to CNS◦across a single layer (rods and cones

communicate with each other; bipolar cells communicate with each other; etc)

optic nerve (ganglion cell axons) – make a blind spot on each eye!

8 inches

component (trichromatic ) or Young-Helmholz◦occurs at level of cones

3 different cones more sensitive to different wavelengths (ie colors)

trichromatic or Young-Helmholz◦occurs at level of cones

explains major type of color blindness◦deficits in certain types of cones can explain

major type of color blindness

At level of cones- GREAT!

◦ there are different cones that produce greater changes in electrical potentials depending on the color (wave)

◦ abnormalities in cones can explain red/green color blindness

Very rare to see complete color blindness - only usually seen with brain injury

~ 7% of US males (10,000,000) compared to 0.4% women - red/green

X-linked phenomenon

X Y

X XX XY

Xb XXb XbY

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What happens in hereditary color

deficiency?

Red or green cone peak sensitivity is shifted.

Red or green cones absent.

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B RG

437 nm 564 nm533 nm

37

B RG

437 nm 564 nm

(green shifted toward red)

5% of Males

At level of cones- GREAT!

negative afterimage –◦phenomenon that occurs as a result of

overactivity or inhibition of neurons (due to color stimulation)

opponent process theory◦occurs at level of bipolar cells and higher

black/white, red/green; yellow/blue; one color excites bipolar cell; other color inhibits it

says nothing about complexity as information reaches occipital lobe –

prestriate – primary occipital cortex; multiple layers of higher association cortex

Copyright © 2006 by Allyn and Bacon