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Cortical sensory systems Motorisch Somatosensorisch Visuell Sensorimotor Sensorimotor Visuell Visuell Olfaktorisch Auditorisch Auditorisch Olfaktorisch Mensch Katze Auditorisch Ratte Primary Visual Cortex Area 17 1 cm Macaca 1 cm Area 17 Mensch I II III A B IV α β C V VI S u bstance b lan ch e « 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) V1 connectivity I I II II III III IVA IV A IVB IV B IVC IV C V VI α β V VI Weisse Substanz Andere kortikale Areale Pons und LGN superior colliculus « 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 V1 receptive fields E cran S tim u lation lum ineu se L im ites d u ch am p visuel E nregistrem ent par m icroélectrod e de l'activité des cellu les du cortex strié « 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. Hubel & Wiesel receptive field mapping V1 Simple cell « 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. Retinotopy « 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 V1 functional organization Ocular dominance columns Blobs Pial surface Layers 1 2 3 extrastriate } To 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) « 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) Visual Pathways « 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 Gestalt laws Similarity Good continuation « 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.
