Cortical sensory systems

Transcription

1 Cortical sensory systems Motorisch Somatosensorisch Sensorimotor Visuell Sensorimotor Visuell Visuell Auditorisch Olfaktorisch Auditorisch Olfaktorisch Auditorisch Mensch Katze Ratte

2 Primary Visual Cortex 1 c m Area 17 1 c m Area 17 Macaca Mensch

3 I II III A α β B C V VI IV Substance blanche

4 «Take Home Message» Primary Visual Cortex The primary visual cortex (V1) is in the occipital lobe, corresponding to Brodmann s area 17 As most other cortical areas, V1 is formed by six horizontal cortical layers (I on the surface to VI adjacent to the white matter)

5 V1 connectivity I II III IVA IVB IVC V VI α β I II III IVA IVB IVC V VI Weisse Substanz Andere kortikale Areale Pons und superior colliculus LGN

6 «Take Home Message» V1 connectivity Layer IV is the layer receiving ascending inputs from the LGN Layer V and VI are layers of origin of the corticofugal projections back to thalamus Layers II, III and V are layers sending (and receiving) cortico-cortical projections

7 V1 receptive fields Ecran Stimulation lumineuse Limites du champ visuel Enregistrement par microélectrode de l'activité des cellules du cortex strié

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9 «Take Home Message» V1 receptive fields Experimental procedure to establish the receptive field of a single retinal ganglion cell or of a single neuron of LGN or V1 (single neuronal recording in responses to small light spots whose position is changed in the visual field) Receptive fields of ganglion cells and LGN neurons are circular In contrast, receptive fields of V1 neurons are elongated and thus orientation selective and have more diverse shapes and are subdivided in a more complex manner into activating and inhibiting sub-areas.

10 Hubel & Wiesel receptive field mapping V1 Simple cell

11 «Take Home Message» Simple and Complex Cells The orientation of the borders of a visual stimulus (spot or bar of light) influences the strength of neuronal responses The response is also modified depending on which part of the receptive field is illuminated Selectivity for orientation: a single neuron in V1 respond preferentially to a certain orientation of a light bar. If the same stimulus is presented at a perpendicular orientation, the neuron is silent (or inhibited) The information derived from several «simple» neurons in V1 converge onto a complex cells, characterized by sophisticated response properties.

12 Retinotopy

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14 «Take Home Message» Retinotopy Retinotopy: relationship between location of a neuron in V1 and the position of its receptive field in the visual field Retinotopy also in LGN Cortical Magnification: more area of cortex is devoted to the foveal representation of the visual field than to the periphery

15 V1 functional organization Ocular dominance columns Pial surface Blobs Layers } To extrastriate cortical regions 4 5 To superior colliculus 6 To lateral geniculate nucleus White matter Orientation columns Lateral geniculate nucleus 6(C) 5(I) 4(C) 3(I) 2(I) 1(C)

16 «Take Home Message» V1 functional organization The primary visual cortex (V1) is functionally organized in columns (vertical modules) Neurons in the same column have common functional properties There are 4 main types of columns in V1: - retinotopic columns - orientation columns - ocular dominance columns - Blobs (columns with neurons sensitive to colors)

17 Visual Pathways

18 «Take Home Message» Visual Pathways From V1, the visual information is transferred to higher order areas (V2, V3, V4, V5, etc), responsible for the processing of various visual attributes The dorsal pathway («where» pathway) is involved principally in the processing of the spatial information. This information is relayed to the parietal lobe The ventral pathway («what» pathway) is involved principally in the processing of the identity information (object recognition, including color). This information is relayed to the temporal lobe Finally, both pathways will converge to some extent in the frontal lobe, for a unified percept that may be relevant for action

19 Gestalt laws

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21 Similarity Good continuation

22 «Take Home Message» Gestalt laws The Gestalt principles of organization relate figural properties to perceived patterns. The main Gestalt principles are: Proximity, similarity, closure and good continuation.