Perception of colour, form, depth and movement; organization of associative visual fields. Seminar Prof Maja Valić

Transcription

1 Perception of colour, form, depth and movement; organization of associative visual fields Seminar Prof Maja Valić

2 Perception of colour, form, depth and movement binding problem in the visual system: how information conveyed in parallel but separate pathways is brought together into a coherent perception? Motion, depth, form, and color-are coordinated into a single visual image The magnocellular (M) and parvocellular (P) pathways feed into two extrastriate cortical pathways: a dorsal pathway and a ventral pathway.

3 The Parvocellular and Magnocellular Pathways Feed Into Two Processing Pathways in Extrastriate Cortex P pathway continues in the ventral cortical pathway that extends to the inferior temporal cortex M pathway becomes the dorsal pathway that extends to the posterior parietal cortex. motion and depth, mediated in large part by the dorsal pathway to the posterior parietal cortex (M) Perception of contrast and contours, mediated largely by the ventral pathway extending to the inferior temporal cortex (P).

4 The pathways arising from M (magnocellular neurons) are associated with identifying the location of the visual image. (where?) the P (parvocellular pathway) is associated with form and color. (what?) Recognition of faces and other complex forms depends upon the inferior temporal cortex. (P)

5 Posterior parietal area Prestrial area Primary visual cortex (area striata) Inferotemporal area

6 Dorsal pathway M Primary visual cortex Ventral pathway P

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8 Role of visual cortex 1) V1: separation of visual information for color, form, movement and depth. 2) V2: sending visual information to the parietal or temporal pathway

9 3) Asocitive visual fields involved in color and form perception (P-system); associative visual fields involved in movement perception (Msystem) 4) Asociative visual field V4 mediates the information from P-pathway into the temporal region; field V5 mediates information from M-pathway into the parietal region.

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11 Color Vision Color is subjective experience tied to the spectral composition of the light Starts with the fotoreceptors in retina

12 The role of photoreceptors human retina consists of two types of photoreceptors: the rods and cones functional regions: an outer segment, an inner segment, and a synaptic terminal

13 The outer segment is located toward the outer surface of the retina and is involved in phototransduction contain light-absorbing photopigments The outer segments are constantly being renewed The inner segment contains the nucleus and most of the biosynthetic mechanisms. The synaptic terminal makes synaptic contact with the other cells.

14 Cones are responsible for daylight vision mediate color vision have a fast response, and their integration time is short they are concentrated in the fovea

15 Rods highly sensitive and can detect dim light specialized for night vision The loss of rods results in night blindness and loss of peripheral vision.

16 PHOTOTRANSDUCTION cyclic guanosine monophosphate (cgmp) gated Na+ (sodium) channels cgmp binds directly to the cytoplasmic side of the channel, which causes it to open, allowing an influx of Na+

17 During darkness, the presence of high levels of cgmp in photoreceptors results in opening of Na+ channels, and an inward current carried by Na+ flows into the outer segment of the photoreceptor. photoreceptors remain depolarized during darkness K+ (potassium) flows out across the inner segment of the receptor membrane through nongated K+ (leakage) channels.

18 DARK LIGHT Rodopsin is not activated cgmp keeps Na+ channels open Sodium (Na+) ions enter the cone and depolarization occurs Light affects rodopsin cgmp is degraded and Na+ channels are closed Na+ does not enter so cones are hyperpolarized Cones release glutamate Release of glutamate is decreased

19 Cones for Short lenght waves - blue Cones for Medium lenght waves green and yellow Cones for Long lenght waves - red

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22 P and M system perception of color and form

23 When a special stain (cytochrome oxidase) for identifying mitochondrial enzymes is applied to the visual cortex, it reveals two types of staining patterns. Electrophysiology experiments reveal different function in different staining patterns.

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25 blobs (V1) and narrov lines (V2) are part of P system for color and form wide lines (V2) are part of M system for perception of movement

26 PET scan immages and functional Magnetic resonance reveal functional distinction of visual cortex in human.

27 Color Blindness CAUSE: congenital, hereditary 8% men and about 0.05% women Coupled to the x-chromosom mothers give it to their sons HEALTHY cones are trichromate

28 3 types of dichromates 1) Protanopia: loss of the L cones (red) 2) Deuteranopia: loss of the M cones (green) 3) Tritanopia: loss of the S cones (blue)

29 Milde impairments 1) Protanomalia: dammage of the L cones (green) 2) Deuteranomalia: dammage of the M cones (green) 3) Tritanomalia: dammage of the S cones (green)

30 Monochromate color blindness Completely color blind

31 Ishihara Test for Color Blindness

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34 How do they see it? Normal Protanopia Deuteranope Tritanope

35 Normal Protanope Deutanope This is how numbers look to a dichromate (only two photopigments) on a color vision test.

36 Normal Protanomaly Deuteranomaly This is how numbers look to an anomalous trichromate (three photopigments, one pigment is just a little off) on a color vision test. The defect is not as severe compared to a dichromate. In fact, some of the test numbers can be seen by an anomalous trichromate.

37 Normal Protanope Deuteranope This is how objects look to a dichromate

38 Color Normal Dichromat

39 Color Normal Dichromat

40 Color Normal Dichromat

41 Perception of Form Distinction beetween the form and the background Gestald psychology

42 Perception of the edges Mach bands

43 Perception of the motion Peripheral retina can only see moving object Motion is analysed in the Dorsal Pathway to the Parietal Cortex Motion is represented in the Middle Temporal Cortex Visual system perception of the motion : a) based on the movement of the object b) based on the neural signals from the moving eyes and had

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45 Depth Vision Depends on monocular cues and binocular disparity

46 Monocular cues 1) Familiar size 2) Occlusion (one person is partialy hiding another) 3) Liner perspective 4) Size perspective 5) Distribution of shadows and illumination 6) Motion parallax

47 Information from the two eyes is first combined in the primary visual cortex (V1). Brain calculates the disparity between the images seen by the two eyes and then estimate the distance based on simple geometric relations.

48 Size perspective Occlusion (one person is partialy hiding another) Familiar size

49 Gradient of the texture

50 Form is constant

51 Effects of Lesions of the Occipital and Temporal Regions of the Cortex Primary Visual Cortex a total lesion of the visual cortex will produce a contralateral homonymous hemianopsia lesion restricted to the inferior bank of the calcarine sulcus will cause an upper quadrantanopia

52 Secondary Visual Areas (in Occipital Cortex) visual agnosia (failure to understand the meaning or use of an object) color agnosia (inability to recognize)

53 Inferotemporal Cortex prosopagnosia = the loss of the ability to recognize familiar faces Middle Temporal Cortex movement agnosia = the patient cannot distinguish between objects that are stationary and those that are moving.

54 Visual ilusions No scientific explanation

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80 Thank you!