physiologyofvision3-20052207035073637.pdf

PHYSIOLOGY OF
VISION
By- Dr. Priyanka Raj

• Visual impulse in photoreceptors
• Processing and transmission of visual impulse in retina
• Processing and transmission of visual impulse in visual pathway
• Analysis of visual impulse in visual cortex
• Three part system hypothesis of visual perception

Visual impulse in
photoreceptors

PHOTOTRANSDUCTION
STANDIN
G
POTENTI
AL OR
DARK
CURREN
T
HYPER-
POLARISING
RECEPTOR
POTENTIAL

Cone versus rod receptor potential
• Cone receptor potentisl has sharp onset and offset
• Rod receptor potential has sharp onset and slow offset
• Rod responses are proportionate to stimulus intensity at illumination levels below
threshold for cones rods detect absolute illumination
• Cone responses are proportionate to stimulus intensity at high levels of illumination
when rod responses are maximal generate response to change in light intensity
above background

Processing and
transmission of
visual impulse in
retina

• Receptor potential from photoreceptors electronic conduction other cells of
retina
• NEUROTRANSMITTERS IN RETINA-
• Glutamine excitatory. Rods & cones
• GABA
• Glycine
• Dopamine amacrine cells
• Acetylcholine
• Indolamine

Physiological activities in retinal cells
• HORIZONTAL CELLS
1. Phenomenon of lateral inhibition-
Horizontal transmission of signals in outer plexiform layer (PR->bipolar cells)
Minute spot of light central most area excited area around (surround)
inhibited
Enhance visual contrast by lateral inhibition
2. Concept of receptive field-
The influence area of a sensory neuron
Receptive field of horizontal cells is very large as compared to the
photoreceptor cell

BIPOLAR CELLS
• 1st order neurons of visual pathway.
1. Some bipolar cells depolarize while
some hyperpolarize provide
opposing excitatory and inhibitory
signals
1. Depolarizing cells respond to
glutamate (excitatory)
2. Some are directly stimulated by
photoreceptors excitatory
others are indirectly inhibited by
horizontal cells
1. Receptive field of bipolar cells has got
a centre-surround antagonism

AMACRINE CELLS
• Receive information at the synapse of
bipolar cell axon with ganglion cell
dendrites temporal processing at the
other end of bipolar cells
• Electrically produce depolarizing
potentials & spikes act as generator
potentials in ganglion cells
• Help in temporal summation and initial
analysis of visual signals

GANGLION CELLS
• Electrical response of Bipolar cells modified by amacrine cells ganglion cells
action potential signals to brain
• On & off centre ganglion cells
 these produce propagated spikes
 On centre increase their discharge
 Off centre decrease their discharge
• Depending upon their function W, X, Y ganglion cells

W X Y
Small (diameter <10μ) Medium (dia= 10-15) Large (dia upto 35)
40% of all ganglion cells Most numerous (55%) Fewest (5%)
Dendrites spread widely in IPL Small fields (dendrites do not
spread)
Very broad dendritic field
Pick up signals from rods Pick signals from at least one
cone
Pick up signals from widespread
retinal area
Responsible for Rod vision in
dark
Detect directional movements
Responsible for colour vision Respond to rapid change in
visual image
Sustained cells Transient cells

Processing and transmission
of visual impulse in visual
pathway

OPTIC NERVE, CHAISMA & OPTIC TRACT
• Axons of RGC optic nevre
• Single optic nerve fibre can be
excited only by a specific stimulus
falling on a restricted area of
retina receptive field

LAERAL GENICULATE BODY
• 2 principal functions
1. RELAY STATION-
 Relay visual information from optic tract to visual
cortex (geniculocalcarine tract)
 The signals from two eyes are kept apart in LGB
2. TO “GATE” THE TRANSMISSION OF
SIGNALS-
 Control the passage of visual signals to visual cortex
 Receive gating (inhibitory) controls from-
1. Primary visual cortex corticofugal fibres
2. Reticular area of mesencephalon

Retinotopic projection
• Ganglion cell axons project a detailed
spatial representation of retin on LGB
with precise point-to-point localization
• LGB 6 layers
 1,4,6 input from contralateral eye
 2,3,5 input from ipsilateral eye
• Each layer  point-to-point
representation of retina present
• Along a line perpendicular to layers
receptive fields of cells are identical

P cells
• project to the parvocellular layers of
the lateral geniculate nucleus.
• known as midget retinal ganglion cells
small sizes of their dendritic trees and cell
bodies.
• 80% of all retinal ganglion cells
• part of the parvocellular pathway.
• receive inputs from relatively few rods and
cones.
• have slow conduction velocity
• respond to changes in color but respond only
weakly to changes in contrast unless the
change is great
M cells
• project to the magnocellular layers of the lateral
geniculate nucleus.
• known as parasol retinal ganglion cells large
sizes of their dendritic trees and cell bodies.
• 10% of all retinal ganglion cells
• part of the magnocellular pathway.
• receive inputs from relatively many rods and
cones.
• have fast conduction velocity
• can respond to low-contrast stimuli, but are not
very sensitive to changes in color

Electrophysiological properties
1. Receptive fields of P & I cells of LGB are similar to
RGCs & optic nerve axons
2. All geniculate receptive fields process on-center/off-
center configuration
3. Fields as sustained (X) & transient (Y) is maintained
4. High degree of peripheral suppression in geniculate
receptive field. Larger “off” periphery cancels effects
of “on” centre. They are sensitive in responding to
spatial differences in retinal illumination.
5. Geniculate relay cells have binocular receptive fields.

OPTIC RADIATIONS
• Composed of axons of LG relay cells which project to visual cortex on
same side
• Central portion macular fibres
• Dorsal fibres upper retinal quadrants
• Ventral fibres lower retinal quadrants

Analysis of visual impulse in
visual cortex

1. Retinopic organization
• Striate Area 17 visuosensory retina
• Peristriate area 18 & 19 visuopsychic area
Modified nomenclature
• V1 area 17
• V2 most of 18
• V3 narrow strip over anterior part of area 18
• V4 within area 19
• V5 posterior end of the superior temporal gyrus
point-to-point representation
Ganglion cell axons LGBvisual cortex

Layers of Primary Visual Cortex
• Six distinct layers –
• Layers I, II and III – are thin and contain pyramidal cells
• Layer IV – thickest layer. Further subdivided into a, b, ca and cb.
• Layer V and VI – relatively thin.

Connections of Primary Visual Cortex
• Geniculate afferents
• the axons from the lateral geniculate nucleus terminate generally in layer IV.
• The rapidly conducted signals from the Y retinal ganglion cells terminate in
layer IV ca.
• Visual signals from X ganglion cells in the retina terminate in layers IVa and
IVc. This pathway transmits accurate point to point and color vision.
• Subcortical connections
• Reciprocal connections returning from striate to LGB arise from pyramidal cells
of layer VI.
• Axons from pulvinar to striate cortex terminate among dendrites of layers I and
V.

• Corticocortical connections
• Fibres to extrastriate visual regions arise from pyramidal cells of layers II and III
os the striate cortex.
• Fibres to contralateral striate cortex also arise in layer III.
• Reciprocal connections from these regions are made predominantly by fibres
that terminate in layer II and III of striate cortex.

PHYSIOLOGY OF VISUAL CORTEX
• Retinal ganglion cells & lateral geniculate neurons respond to both diffuse
retinal and spot stimulus
• Cortical neurons stimulus in form of straight line, bar or edge presented in a
proper spatial orientation orientation & configuration receptive field differ in
visual cortex
• Aspects of physiology:
1. Concept of receptive field of striate cortex
2. Columnar organization of striate cortex
3. Serial v parallel analysis of visual image
4. Role of extra-striate cortex in visual functions
5. Psychophysiological aspects of visual functions

Concept of receptive field of striate cortex
• Hubel & Wilson named cortical cells as 3 receptive field types
Cortical
cells
simple
hypercomplex
complex

SIMPLE CELLS
• Found mainly in layer IV of the primary visual cortex (area 17)
• Form the 1st replay station within the visual cortex
• Respond to bars of light, lines or edges in a particular orientation only
• The orientation of a stimulus most effective in evoking a response is called
“receptive field axis orientation”
• Receptive fields  arranged in parallel bands of “on-areas” & “off-areas”
• FUNCTION:
1. Role in detection of lines and borders in different areas of retina
2. Detect orientation of each line/border horizontal/vertical/inclined

COMPLEX CELLS
• Found in cortical layers above and below layer IV of areas 17, 18, 19
• Require preferred orientation of linear stimulus but are less dependent upon the
location of a stimulus in the visual field
• Respond maximally when stimulus is moved laterally without change in
orientation
• On and off areas cannot be mapped in their receptive fields
• Receive input from both eyes called binocular
• 4 types of receptive field a/c preferred stimulus
1. Activated by a slit-nonuniform field
2. Activated by a slit-uniform field
3. Activated by an edge
4. Activated by a dark bar

• FUNCTION-
1. Detection of lines, bars and
edges specially when they are
moving
2. Perception of features,
orientation and movement of
objects
3. Simple + complex cells =
feature detectors

HYPERCOMPLEX CELLS
• Found in cortical layers II & III of areas 17, 18, 19
• All properties of complex cells + require the line stimulus to be of specific length
• Hubel & wiesel = 6 types (4 lower + 2 higher) hypercomplex cells
• Dreher = class I & class II

Columnar organization of striate cortex
• ORIENTATION COLUMNS
• “vertical grouping of cells with identical
orientation specificity”
• Unit of organization in the cortex
• Several million vertical columns in
visual cortex
• On moving column-to-column
sequential changes in orientation
preference of 5-10 degress

• Depth perception 2 separate column systems
 Constant depth column contains binocular units with exactly same retinal
disparity for properly oriented stimuli
 Constant direction columns points perpendicular to the center of
contralateral eye
 Together localize points in a 3D space

• OCULAR DOMINANCE COLUMNS
• Independent system of columns which exist in
visual cortex with respect to binocular input to
cortical cells
• Simple cells uniocular input;
complex+hypercomplex cells binocular
input
• Neurons with receptive fields dominatd by
one eye are grouped alternately into left and
right eye columns
• A group of binocular complex and
hypercomplex cells in layers II, III, V & VI that
receive a stronger input from one of the two
eyes, along with their cells in layer IV
receiving uniocular input from the same eye
are known as ocular dominance column

• THE COLOUR BLOBS
• Primary areas for deciphering colours
• Interspersed among the primary visual columns
• Receive lateral signals from adjacent visual
column and respond specifically to colour signals
SERIAL Vs PARALLEL ANALYSIS
• Hierarchical model for cell interconnection
• Columnar organization of cortex
• Simple (monocular) complex (binocular) 
hypercomplex

EXTRASTRIATE CORTEX
• Neurons of straite cortex (area 17 or VI) 
extrastraite cortex [area 18 (V2), area 19
(V2), V3 V4 MT] strait cortex
• Pontifical cells receive information from the
feature detectors (simple & complex cells)
• Specialized extrastriate areas
1. Colour processing area V4 (rhesus
monkeytrial)
2. Movement processing area MT. cells
show strong preference for stimuli
moving in a particular direction
3. Stereoscopic depth perception area V2
& V3

PSYCHOPHYSIOLOGICAL ASPECT OF VISUAL
FUNCTIONS
• Vision is related to verbal language and reading
• Visual cortex connects with tactile sensory motor
auditoy, olfactory and speech areas
• Angular gyrus (area 40)of parietal lobe acts as visual
memory centre for words by forming associations
between visual and auditory centres
• Corpus callosum connets the two hemispheres and help
perceieve the several qualities simultaneously and
synthesize a unified picture
• Brain’s response to stimuli is in the form of an over all
picture

“THREE-PART-SYSTEM”
HYPOTHESIS OF VISUAL
PERCEPTION

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