Extrastriate Cortex Criteria For a Visual Area

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1 Extrastriate Cortex Criteria For a Visual Area
• Cyto-, myeloarchitecture • Connectivity New Way to Gain a Clear View of the Brain New York Times October 10, 2011 • Retinotopy -complete or partial map of visual space -represent a point in space only once? -smoothly varying? -orthogonal axes? • Specialized Function • Topography

2 Doctrine of the Receptive Field
Τα R.Fs μεγαλώνουν καθώς προχωρούμε σε ανώτερες περιοχές Retinotopy V2 Okay, so even if our stated focus is the human visual system, the fact is that we approach this topic with many expectations based on animal studies. The beginning of modern physiological thinking is Hubel and Weisel. On what basis was the terms V1, V2 and V3 used? Look at size differences! V3 V1 Multiple “areas” V1, V2, V3

3 Functional Division of Labor?
(motion) (color)

4 V1 blob cells: Some V1 cells resemble kLGN neurons. They are
monocular (i.e., respond to stimulation of one eye only).  color sensitive.  characterized by small, concentric receptive fields. (άρα όχι επιλεκτικά στον προσανατολισμό). found in clusters (.e., blob cells).  a special target of the kLGN axon terminals.  Stimulated by spots of light The P-stream information processed by the V1 blob cells is used in color perception, color discrimination and the learning and memory of the color of objects. The blob cells are the "color" processing cells of V1.

5 V1 interblob cells: Most V1 interblob cells are
binocular (i.e., respond to stimulation of either eye) but exhibit ocular dominance (i.e., respond best to stimulation of a preferred eye). not color sensitive.  characterized by elongated (rectangular-shaped) receptive fields that may or may not have a center-surround type organization. (σύνθετα & απλά κύτταρα της V1σημασία ο προσανατολισμό και μόνο για τα απλά η θέση του S στο R.F του κυττάρου) found around the clusters of color-sensitive V1 blob cells.  exhibit orientation specificity (i.e., respond best when the stimulus is oriented in a particular plane). Location specific V1 interblob cells: One subset of V1 interblob cells responds best when the stimulus is in a specific location of the receptive field (i.e., they also exhibit location specificity). The P-stream information processed by the V1 interblob cells that exhibit orientation and location specificity but are not motion sensitive is used in object perception, discrimination, learning and memory or in spatial orientation. These interblob cells are the "shape/form" processing cells and the "location" processing cells of V1. Movement sensitive V1 interblob cells: A second subset of interblob cells respond best to moving stimuli (i.e., exhibit movement sensitivity, Figure 15.8) without a preference for the direction of movement. Direction specific V1 interblob cells: A third subset displays a preference for movement in a particular direction (i.e., some also exhibit directional sensitivity, Figure 15.9).The M-stream of information processed by the motion sensitive V1 interblob cells is used to detect object movement and direction/velocity of movement and to guide eye movements. These motion-sensitive interblob cells are the "motion detecting” cells of V1.

6 “What” versus “Where” Pathways
The information from the “color”, “shape/form”, "location" and “motion” detecting V1, neurons are sent to different areas of the extrastriate cortex  Cross talk remains, and feedback is prevalent

7 Ventral Pathway – Two parallel channels for form and color
Επεξεργάζεται πληροφορίες για χρώμα & μορφή από το P-channel Ventral Pathway – Two parallel channels for form and color Parvocellular – interblob system (form): (V1) L4 (4Cb) → (V1) L2/3 interblob → (V2) pale interstripe → V4 → IT(κάτω κροταφικός) Parvocellular – blob system (color)x: (V1) L4 (4Cb) → (V1) L2/3 blob → (V2) thin stripe → V4 → IT

8 V4 --- Inputs mainly from foveal regions of V1(blobs) and V2 (thin stripes)color sensitive ΌΜΩΣ αντίθετα με V1, V2 δεν αποκρίνεται στη σύνθεση μήκους κύματος της φωτεινής πηγής αλλά σε συγκεκριμένο χρώμα (χρωματική σταθερότητα: πχ αντίληψη κόκκινο άσχετα με το αν ο φωτισμός κόκκινος, μπλε κλπ). ---Also some coding for orientation (may be colour specific) Achromatopsia damage to V4 causes an inability to perceive colour patients “see the world in black and white” also an inability to imagine or remember colour

9 • Inferotemporal Cortex
–---  Cells respond to a single complex stimulus such as an apple –---  Lesions here leads to inability to identify an object (visual agnosia), picking it up is no problem Superior Temporal Cortex –--- Lesions here lead to inability to recognize faces (prospagnosia) Recognition of objects dependent on their form but independent of scale (distance), orientation, illumination

10 . Dorsal Pathway - motion and depth Processing(στις MT , MST)
MofLGNV1(απλά&σύνθετα κυττ επιλεκτικά στην κατεύθυνση κίνησης)ΜΤ, MSTposterior parietal cortex(PPC) MT: 1. Direction-selective cells for moving bar or moving dots 2. Columnar organization of direction selectivity(αποκρίνονται σε 1 κατεύθυνση κίνησης αλλά όχι στην αντίθετη κατεύθυνση). 3.Specialized for detection of speed and overall motion of entire objects. 4. Some MT cells (20%) are “pattern direction-selective” 5. Lesion of MT cells impairs motion perception--- perception is frozen (Cerebral akinetopsia) PPC: attention to an object intense response of PPC cells Saccade to the object for better examination intense response of SC, V1 cells. .

11 The Organization of the Visual Cortex
Dorsal stream V5 Posterior parietal Cx Superior colliculus Ventral stream V3 V3A STS Eye Dorsal LGN V1 V1 V2 V4 TEO TE Striate Cortex Extrastriate Cortex Inferior Temporal Cortex STS Superior temporal sulcus TEO Inferior temporal cortex TE Inferior temporal cortex Evidence of a hierarchical organization of function within the dorsal and ventral streams

12 Illusory contours can trip firing of V2 cells, while only real contours fire V1 cells.

13 --- Receives input from thick stripe and interstripe areas of V2
--- No thin stripe (Blob) input, generally color insensitive --- Edges of a particular orientation (λόγω interstripe-input από Pcells?) --- Some motion perception(λόγω thick stripe-από M cells?) --- Depth perception (λόγω thick stripe-από M cells?)  First stage in the building of object form Code for component aspects of object recognition e.g. edges, orientation, spatial frequency (visual angle)

14 Extrastriate Visual Areas
Μέσω thin- στέλνει col σε V4 • V2 μέσω nter- ΄στέλνει ori -σεV4 Μέσω thick – στέλνει ori, dis – σε MT • V3d and V3A also magno-like • MT - MST magno input, V1 (4B), thick, dir, dis, motion, depth • LIP - 7a Optic flow input, large RF, multimodal, project to frontal • V4 Central field input V1, V2, col ori, form primitives • IT input V2, V4, object features, face cells, project to multimodal Very large RF, object invariance, color constancy, training effects