15. LIGHT FALLING UPON THE RETINA CAUSES
PHOTOCHEMICAL CHANGES
RHODOPSIN BLEACHING
RHODOPSIN REGENERTION
VISUAL CYCLE
GENERATION OF RECEPTOR POTENTIAL
PHOTOTRANSDUCTION
16. STIMULI FOR VISUAL SENSATION
INADEQUATE STIMULI
• PRODUCE GLOWING SENSATION CALLED PHOSPEHENES
• EX. MECHANICAL STIMULUS BY PRESSURE ON SCLERA – PRESSURE
PHOSPHENE,ELECTRIC CURRENTS- ELECTRICAL PHOSPHENES,ETC
ADEQUATE STIMULI
• FORMED BY VISIBLE PORTION OF ELECTROMAGNETIC RAADIATION (400nm-
750nm)
17. VISUAL PIGMENTS
• SUBSTANCES HAVING THE PROPERTY TO ABSORB LIGHT
• PEAK OF EACH PIGMENTS ABSORPTION CURVE IS CALLED ITS
ABSORPTION MAXIMUM
• MADE OF PROTEIN OPSIN AND RETINENE(RETINAL) , THE
ALDEHYDE OF VITAMIN A
RHODOPSIN (VISUAL
PURPLE)
CONE PIGMENTS
SCOTOPIC VISION TRICHROMATIC PIGMENTS
ABSORPTION SPECTRUM
493nm-505nm
PEAK ABSORPTON WAVELENGTH
BLUE-435NM (CYANOLABE)
GREEN-535NM (CHLOROLABE)
RED-580NM (ERYTHROLABE)
PRESENTS IN DISC OF OUTER ROD
SEGMENTS
BLUE SENSITIVE CONES- LEAST
PREVALENT
18. VISUAL CYCLE
EQILIBRIUM BETWEEN PHOTODECOMPOSITION AND REGENRATION OF VISUAL
PIGMENTS
RHODOPSIN
BLEACHING
RHODOPSIN
REGENERATION
PHOTODECOMPOSITION
19. METARHODOPSIN II
ACTIVATES PHOSPHODIESTERASES
CONVERSION OF c GMP TO GMP
REDUCTION IN c GMP IN PHOTORECEPTOR
PRODUCTION OF ELECTRICAL RESPONSE(RECEPTOR POTENTIAL)
BLEACHING OF THE RETINAL PIGMENTS OCCURS UNDER THE INFLUENCE OF
LIGHT WHEREAS REGENERATION OCCURS INDEPENDENT OF LIGHT
TOTAL ROD BLEACHING OCCURS BEFORE SIGNIFICANT BLEACHING CAN BE
OBSERVED IN CONES SETTING ASIDE SCOTOPIC ROD FUNCTION FROM THE
PHOTOPIC PORTION WHICH FUNCTIONS DURING BRIGHT LIGHT
20. PHOTOTRANSDUCTION
STANDING POTENTIAL
• INNER SEGMENT OF PHOTORECEPTOR CONTINOUSLY PUMPS Na+ FROM INSIDE TO
OUTSIDE REATING NEGTIVE POTENTIAL INSIDE CELL
• Na+ CHANNELS IN OUTER SEGMENT ARE KEPT OPEN BY c GMP CAUSING Na+ FROM ECF
TO FLOW INTO IT IN DARK
• OUTER SEGMENT IS HYPOPOLARISED COMPARED TO INNER SEGMENT THAT IS CURRENT
FROM INNER SEGMENT TO OUTER SEGMENT
• IN DARK THERE IS INFLUX OF Ca+ INTO THE CELL BY c GMP GATED CHANNELS
HYPERPOLARISING RECEPTOR POTENTIAL
• DECREASE IN c GMP IN PRESENCE OF LIGHT CLOSES SOME OF THE Na+ CHANNELS
CAUSING HYPERPOLARISING RECEPTOR POTENTIAL
• IN DARK ELECTRONEGATIVITY IN PHOTORECEPTORS IS 40mV AND AFTER EXCITATION
REACHES 70mV
• IT IS A GRADED POTENTIL
• CONE RESPONSES ARE PROPORTIONAL TO STIMULUS INTENSITY AT HIGH LEVEL OF
ILLUMINATION WHEN ROD RESPONSES ARE MAXIMAL AND CANNOT CHANGE CAUSING
CONES TO PRODUCE GOOD RESPONSES TO CHANGE IN LIGHT INTENSITY
21. IN PRESENCE OF LIGHT
DECREASE Na+ CONCENTRATION CAUSING HYPERPOLARISAION OF CELLS
Ca+ CONCENTRATION DECREASES CAUSING DECREASE IN RELEASE OF
NEUROTRANSMITTER
DEPOLARISATION OF ON CENTRE BIPOLAR CELLS (RODS AND CONES ) AND
HYPERPOLARISATION OF OFF CENTRE CONE BIPOLAR CELLS
ACTIVATION OF RETINAL GANGLION CELLS
SIGNAL TO BRAIN
SOME BIPOLAR CELLS RECEIVE SIGNAL INDIRECTLY THROUGH HORIZONTAL CELLS
WHICH BEIG INHIBITORY CELLS INVERSES THE POLARITY OF RESPONSE
24. LATERAL INHIBITION
• LIGHT FALLING ON RETINA EXCITES ONLY CENTRAL MOST AREA WHERAS THE
AREAAROUND IS INHIBITED
• HORIZONTAL CELLS TRANSMIT SIGNALS HORIZONTALY INBOUTER
PLEXIFORM LYER FROM RODS AND CONES TO BIPOLAR CELLS
• RECIPROCAL RELATIONSHIP BETWEEN DEPOLARISING AND
HYPERPOLARISING BIPOLAR CELLS
TEMPORAL PROCESSING
• BY AMACRINE CELLS AT SYNAPSE BETWEEN BIPOLAR CELL WITH GANGLION
CELL
• ADJUST THE BIPOLAR CELL IN NEGATIVE FEEDBACK ARRANGEMENT
• AMACRINE CELLS PRODUCE DEPOLARISING POTENTIAL
25.
26. GANGLION CELLS
W GANLION CELLS X GANGLION CELLS
(SUSTAINED CELLS)
Y GANGLION CELS
(TRANSIENT CELLS)
40% 55% 5%
SMALL SIZE MEDIUM SIZE LARGE SIZE
DENDRITES
WIDESPREAD – BRAOD
FIELD
SMALL FIELD MAXIMUM BROAD
DENDRITIC FIELD
MAXIMUM
EXCITATION FROM
RODS TRANSMITTED
FROM BIPOLAR AND
AMACRINE CELLS-
ROD VISION IN DARK
AND DETECTING
DIRECTIONAL
MOVEMENT
RECEIVE INPUT FROM
AT LEAST ONE CONE
CELLS-COLOUR
VISION
REPOND TO RAPID
CHANGES IN VISAUL
IMAGES AND LIGHT
INTENSITY
28. RECEPTIVE FIELD- SINGLE OPTIC NERVE FIBRE CAN BE EXCITED ONLY BY
SPECIFIC STIMULUS FALLING ON AREA SPECIFIC AREA ON RETINA
LATERAL GENICULATE BODY
• RELAY STATION-GENICULHYPERCLCARINE TRACT
• SIGNALS FFROM TWO EYES ARE KEPT APART IN LGB
• GATE THE TRANSMISSION OF SIGALS BY GETTING SIGNALS(INHIBITORY)
FROM
CORTIFUGAL FIBRES FROM VISUAL CORTEX
RETICULAR FORMATION FROM MESENCEPHALON
MAGNOCELLULAR LAYER
• RECEIVE FROM Y GANGLION CELLS
• PROVIDES RAPIDLY CONDUCTING PATHWAY
• CARRY SIGNALS FOR DETECTION OF MOVEENT
• COLOUR BLIND
• NEGATIVE FOR POINT TO POINT TRANMSISSION
PARVOCELLULAR CELLS
• RECEIVE FROM X GANGLION CELLS
• TRANSMIT COLOUR VISION
• GOOD POINT TO POINT SPATIAL TRANSMISISON
• TEXTURE, SHAPE AND DEPTH VISION
29.
30. OPTIC RADIATIONS
• COMPOSED OF AXONS OF LGB WHICH PROJECT INTO THE VISUAL CORTEX
• ARRANGEMENT OF FIBRES IN OPTIC RADIATIONN
31. ANALYSIS OF VISUA IMPULSE IN VISUAL CORTEX
PRIMARY VISUAL CORTEX(BROADMANN AREA 17)- STRIATE CORTEX
SECONDARY VISUAL CORTEX(BROADMANN AREA 18,19)- VISUAL
ASSOCIATION AREAS ANTERIOR,SUPERIOR AND INFERIOR TO PRIMARY
VISUAL CORTEX
33. RECEPTIVE FIELD OF VISUAL CORTEX
CORTICAL CELL AS THREE RECEPTIVE FIELD TYPES
• SIMPLE
• COMPLEX
• HYPERCOMPLEX
SIMPLE CELLS
• IN LAYER 4 OF PRIMARY VISUAL CORTEX
• FORM THE FIRST RELAY STATION IN VISUAL CORTEX
• RESPOND TO BAR OF LIGHT , LINES OR EDGES HAVING PARTICULAR
ORIENTATION
• RECEPTIVE FILED AXIS ORIENTATION- ORIENTATION OF STIMULUS MOST
EFFECTIVE IN EVOKING RESPONSE
• RECEPTIVE FIELD ARE ARRANGED IN PARALLEL BANDS OF ON AREAAND OFF
AREAS
• RECEPTIVE FIELDS HAVE CENTRAL BAND THAT IS EITHER AN ON REGION OR
OFF REGION WITH PARALLEL FLANKING REGION ON TWO SIDES THAT ARE
OPPOSITE
• ROLE IN DETECTION OF LINES,BORDERS,ORIENATION OF LINE AND BORDER
THAT IS VERTICAL, HORIZONTAL,INCLINATION,ETC
34. COMPLEX CELLS
• IN CORTICAL LAYERS BELOW AND ABOVE LAYER 4 OF AREAS 17,18,19
• REQUIRE PREFERRED ORIENTATION OF THE STIMULUS BUT LESS DEPEMDENT OF
LOCATION OF STIMULUS IN VISUAL FIELD
• RESPOND MAXIMALLY WHEN LINEAR STIMULUS IS MOVED LATERALLY WITHOUT
CHANGE IN ORIENTATION
• COMPLEX CELLS RECEIVE INPUTS FROM BOTH EYES AND ARE BINOCULAR
• SIMPLE AND COMPLEX CELLS TOGETHER ARE KNOWNAS FEATURE DETECTORS
HYPERCOMPLEX CELLS
• IN CORTICAL LAYERS 2,3 OF CORTICAL AREA 17,18,19
• RETAIN PROPERTIES OF COMPLEX CELLS WITH FEATURE OF LINE STIMULUS
REQUIRED TO BE OF SPECIFIC LENGTH
• ROLE OF DETECTING LINES OF SPECIFIC LENGTH,ANGLES,SHAP
35. COLUMNAR ORGANISATION OF STRIATE CORTEX
• VERTICAL GROUPING OF CELLS WITH IDENTICAL ORIENTATION SPECIFICITY
• VISUAL CORTEX IS ORGANISED STRUCTURALLY IN MANY VERTICAL COLUMNS
OF NEURONL CELLS
• SEQUENTIAL CHANGES IN ORIENTATION PREFERENCES OF 10 DEGRESS FROM
COLUMN TO COLUMN ACROSS THECORTEX
• FOR EACH GANGLION CELL RECEPTIVE FIELD IN THE VISUAL CORTEX,THERE IS
COLLECTION OF COLUMN IN A SMALL AREA OF VISUAL CORTEX REPRESENTING
THE POSSBLE PREFERRED ORIENTATION AT SMALL INTERVALS
36.
37. COLOUR BLOBS
• IN PRIMARY VISUAL CORTEX
• RESPOND SPECIFICALLY TO COLOUR SIGNALS
ROLE OF EXTRASTRIATE CORTEX IN VISUA FUNCTIONS
• INFORMATION FROM STRIATE CORTEX NEUROS (AREA 17 OR V1) GOES
TO NEURONS IN ARE 18(V2),19(V2),V3,V4.MT
• FIBRES FROM EXTRASTRIATE AREA GO BACK TO STRIATE CORTEX,THEY
ARE CONNECTED TO EACH OTHER AND ALSO RECEIVE VISUAL INPUT
FROM PILVINAR
• THESE CELLS RECEIVING INFORMTON FROM FEATURE DETECTORS ARE
CALLED PONTIFOCAL CELLS
• COLOUR PROCESSING AREAS- V4
• MOVEMENT PROCESSING AREA –MT (MIDDLE PORTION TEMPORAL
LOBE)
• STEREOSCOPIC DEPTH PERCEPTION AREA-V2,V3
38. THREE PART SYSTEM HYPOTHESIS OF VISUAL PERCEPTION
• FIRST SYSTEM- CONCERNED WITH PERCEPTION OF MOVEMENT,LOCATION AND
SPATIAL ORGANISATION
• SECOND SYSTEM-PERCEPTION OF COLOUR
• THIRD- PERCEPTION OF SHAPE
• INFORMTION FROM SYSTEMS IS INTEGRATED INTOASINGLE VISUAL
PERCEPTION
