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The sensory system
The visual system
Perceptual Organs I
Li Liu
Stimulation Transduction Processing Perception Recognition Action
Perceptual process
Energytransform
Signaloperation
Informationarrived andpercievedin the brain
Place an object at aRight category
Move
Smile
talk
Natural
Sensors
Important properties of receptors
• Transduction
• Receptive field
stimulus
Transduction
The stimulus energy must be transformed into usable neural electrical activity, in order for the neural system to detectthe presence of a stimulus.
The receptors work as transducers.
Transduction mechanisms – are basically the same at one level:
ReceptorNa+
K+
Electricity
Permeability of the cellMembrane is changed
Membrane potentialis formed
Receptive field (RF)
The area of a sensory surface that must be stimulatedin order to elicit a change in activity in a neuron.
A stimulus that falls within the RF, can activate the neuron; A stimulus that falls outside the RF, can not.
RF exists in many neurons at different levels in a sensorypathway, even those in the brain.
Different shapes of RF
Ganglion cell
LGN cellSimple cell
On-center RF
Function of Perceptive Fields
Image Analysis
The Visual System
What is light?
The structure and function of the eye
The visual pathway
What is light?
The start point of vision is light—the physical stimulus that activatesthe visual system
Light is electromagnetic radiation, which can be expressed as wavelength (nm). Neither very longnor very short wavelength are visible.
Visible light, that our humans can perceive, has wavelength ranging from 400 to 700 nm.
The structure of the eye
Cones and rods distribute differently on the retina
Rods
vs.
Cones
• night vision• very sensitive• one type only• no colour vision• 120 million • absent from fovea• Low acuity
day visionless sensitivethree typesallow color vision6 millionconcentrated in foveaHigh acuity
The Visual Pathway
LGNVisual cortex
Eye
LGN = Lateral Geniculate Nuclei
The visual processing on the retina (eye)
Visual pigment
Activated pigment
Thousands of chemical reactions
Trigger
Electrical signal in the receptor
Generate
•
high convergence
120:1•
low acuity
Rodbipolar cellRetinal Ganglion cell
Rod Pathway
Cone Pathway
•
low convergence
6:1•
high acuity
The receptive field of a ganglion cell
Two types of receptive field
The visual processing in the LGN (thalamus)
LGN1 2 3
45
6
Parvocells
MagnocellsM-layer
largeP-layer
small
Information aboutwhere an object is,e.g. motion.
Information aboutwhat an object is,e.g. color, texture, shape, depth
(Lateral Geniculate Nuclei)
35
Ganglion cellsP-type
The brainstem
2
M-type M-type Channel of LGN: 1, 2
P-type Channel of LGN: 3, 4, 5, 6
Left eye Right eye
Information of flow in the LGN
LGNRetina Visual cortex
Brain steam Visual cortex75%
25%
10 inputs 4 outputs
The visual processing on the visual cortex
Visual cortexLGN
Primary visual cortex
Visual cortex extracts the orientation, movement, directions.
