sensation and perception - sensory1.ppt © 2001 laura snodgrass, ph.d. sensory coding outline...

Sensation and Perception - sensory1.ppt © 2001 Laura Snodgrass, Ph.D.

Sensory Coding Outline

• Elements of sensory systems

• Basic Neuron– parts– all-or-none law– spontaneous activity

• excitation and inhibition


Outline - continued

• Three basic efficiency mechanisms– Separate subsystems for specific functions

• ascending pathways– Ignore steady state information

• lateral inhibition• receptive fields

– Precode for critical features• receptive field wiring


Outline - continued

• Sensory Coding– Quantity - how much?– Quality - what?


Elements of Sensory Systems

Peripheralend organ

receptor transduction Adequatestimulus

Primaryprojection

area

Secondaryprojection

area

eyeball rods andcones

chemical Electromagnetic energy

occipital temporal

ear hair cells mechanical vibration temporal andnose olfactory

epithelium? airborn

chemicalsfrontal parietal

tongue taste bud chemical chem insolution

frontal for

skin many varied temperature,pressure,disruption

parietal all


Primary Projection Areas


Neuron

• Dendrites- receive info

• Cell body - maintains cell

• Axon - transmits info to next cell

• Synapse - gap between axon of one neuron and dendrite of next


All-or-None Law

• Neurons cannot change the intensity at which they fire

• They change the rate - number of impulses per second - to indicate increasing stimulus intensity


Spontaneous Activity

In (a) a strong excitatory signal causes a high firing rate.

In (b) through (d) as the inhibition is increased, the firing rate decreases.

In (e) the inhibition has caused the cell to cease firing. The rate of firing before and after the signal is the spontaneous rate.

Note that in © when the excitation equals the inhibition, the cell fires at the spontaneous rate


Importance Of Spontaneous Activity

• Same cell can code for two different perceptual experiences– excitation signals one quality– inhibition signals a different quality

• For example:– excitation = blue, inhibition = yellow– excitation = left, inhibition = right


Three Basic Efficiency Mechanisms

• Need to be efficient in sending signals to the brain– too much sensory information to process– too many receptors

• e.g. There are about 132 million receptors in each eye but only 1 million axons going to the brain. Thus 131 million bits of information must be condensed or lost at the level of the eye


Three Basic Mechanisms

• Subsystems for specific functions

• Ignore steady state information

• Precode for critical features


Subsystems

• In vision:– geniculostriate - form and detail– tectopulvinar - motion and location

• can lose one and maintain function of the other

• In audition:– several - language, music, sounds

• In touch:– pain in one, touch in another


Ignoring Steady State Information

• Ring demonstration


Lateral Inhibition

• Horseshoe crab (Limulus) - ommatidia

• Lateral inhibition accomplishes two tasks:– ignore steady state information– enhance borders


Mach Bands


Hermann’s Grid


Receptive fields

• Work with lateral inhibition to ignore steady state information

• Are the basis for the precoding of critical features

• Receptive fields - each visual cells has a “receptive field” of numerous receptors


Receptive Field Mapping

• Single cell recordings

• Map out responses on large sheets of paper


Hartline - Optic Nerve Of Frog

• “on” cells– sustained firing when light is on

• “off” cells– brief burst (transient) when light goes off

• “on-off” cells– brief burst for on and off

sensation and perception - sensory1.ppt © 2001 laura snodgrass, ph.d. sensory coding outline...

Documents

laura snodgrass

inhibition slide

spontaneous rate slide

eye slide

right slide

quality inhibition

sensory information

rate of firing