Parallel processing strategies of the primate visual system

Transcription

1 Parallel processing strategies of the primate visual system

2 Parallel pathways from the retina to the cortex Visual input is initially encoded in the retina as a 2D distribution of intensity. The retinal images are then transmitted to the visual cortex to compute the sensory cues and inferred attributes. Due to the bottleneck of the optic nerve, retinal output must be efficiently condensed by appropriate strategy. What strategy???

3 Parallel pathways from the retina to the cortex Each ganglion cell type has a distinct pattern of stratification in Inner plexiform layer (IPL) Each ganglion cell type has a unique structure, which determines the properties of each ganglion cell type. Strategy: 13 distinct ganglion types project in parallel to the LGN and on to V1!!!

4 Parallel pathways from the retina to the cortex, cont. Each ganglion cell type tiles the entire retina. In each point, multiple ganglion cell types signal complementary visual information in parallel to the visual cortex

5 Parallel pathways from the retina to the cortex, cont. Midget, parasol and bistratified ganglion cells constitute 90% all ganglion cells. Bistratified ganglion cells: Origin of the koniocellular pathway. convey a blue-on, yellow-off colouropponent signal to koniocellular layers of LGN and on to blobs of layer ⅔ Midget ganglion cells: Origin of the parvocellular pathway Convey a red-green colour-opponent signal to the parvocellular layers of the LGN and onto to layer 4C_beta, Parasol ganglion cells: Origin of the magnocellular pathway. Convey a broadband, achromatic signal to the magnocellular layers of the LGN and on to layer 4C_alpha.

6 V1 and parallel processing strategies V1 uses multiple strategies to transform the inputs into multiple outputs The parallel inputs maintain strict segregation and are directly passed on to the outputs The parallel inputs converge indiscriminately in V1 and lack any systematic relationship with the outputs. The parallel inputs converge in an organized and specific way so that the parallel outputs are systematically conveyed to the extrastriate cortex

7 V1 and parallel processing strategies, cont. Early model of V1 proposed that magnocellular(yellow) and parvocellular(red) pathway inputs remained strictly segregated Parvocellular pathway forms the basis for colour and form process in CO blob and interblob in layer 2/3 The magnocellular pathway forms the basis for motion- and depth-related processing in layer 4B Recent studies have shown that Parvocellular, Magnocellular and koniocellular pathways converge indiscriminately in V1 and lack any systematic relationship with the outputs.

8 [video of cortical neurons] edu/departments/neurosci/courses/bio330/h %26w.html

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10 Processing strategies in the extrastriate cortex Outputs from V1 and V2 begin to separate signals into the dorsal and ventral streams. However, there still is crosstalk between the two pathways. The two pathways process the same visual attributes, but for different behavioral goals. They do process some things the same: e.g. binocular disparity

11 Distribution of P and M cell influences on different areas of the visual cortex. IT, inferotemporal area; MST, medial superior temporal; MT, medial temporal; V1, striate cortex; V2, V4, higher-order visual areas. source:

12 Dorsal Stream (where?) extrastriate cortical areas in the downstream of MT, possibly two separate subcircuits. dorsal stream s goal is to mediate navigation and the visual control of skilled actions directed at objects Evidence: Lesions on the dorsal stream affect smooth eye movements speed discrimination complex motion perception accurate encoding of visual space

13 Ventral Stream (what?) extrastriate cortical areas in the temporal cortex downstream of V4 the ventral stream s goal is to transform visual inputs into representations Evidence: Lesions on the ventral pathway Orientation, complex shape discrimination perceptual invariance

14 Future Unanswered questions: How many parallel pathways between retina and v1, and what are their properties? what is the functional contribution of output pathways from the v1 to the dorsal and ventral streams in the extrastriate cortex?