Transcript Lecture 8
Computational Vision CSCI 363, Fall 2012 Lecture 8 Striate Cortex 1 Response of Retinal Ganglion Cells Retinal Ganglion cells and bipolar cells have receptive fields that exhibit a center-surround structure. Question: What is the center-surround structure good for? 2 From Retina to V1 3 Types of Cells in V1 Hubel and Wiesel identified 3 types of cells in V1: 1. Simple Cells 2. Complex Cells 3. End-stopped Cells 4 Simple Cells Simple cells have discrete regions that have on or off responses to a bar or an edge. 5 Simple Cell Responses Simple cells are selective for orientation and position. 6 Orientation response 7 A possible circuit for simple cells Hubel and Wiesel suggested this circuit to construct a simple cell from a group of LGN cells. 8 Complex Cells •Complex cell responses are not divided into discrete on and off regions. •Complex cells are orientation selective. •Many complex cells respond best to moving stimuli. 9 •Some complex cells are direction selective. Complex Cell Circuit Hubel and Wiesel proposed this circuit to construct a complex cell from simple cells. 10 End-stopped Cells Complex cells show increased response with the length of a line up to some maximum length. The response then levels off. End-stopped cells show inhibition when a line stimulus is too long. 11 Direction Selectivity Direction selective cells respond to motion in one direction, but not to motion in the opposite direction. The color of the bar (dark or light) does not matter for the direction selectivity. 12 Direction selective circuit Barlow and Levick proposed this circuit for direction selectivity. Neurons are connected by delayed inhibition along the null (nonresponding) direction of motion. 13 Ocular Dominance Columns Input from the LGN to layer 4C is segregated by eye. Inputs alternate between the left and right eyes. 14 Ocular Dominance columns on the cortical surface Labeling the cortex with a radioactive tracer from one eye reveals that the ocular dominance columns form stripes on the cortical surface. 15 Ocular Preference 1 => responds only to the contralateral eye. 7 => responds only to the ipsilateral eye. 4 => responds equally to both eyes. In V1, many cells respond to both eyes, but most prefer one eye over 16 the other. Orientation Columns Preferred orientation varies smoothly across the cortex. Cells in columns perpendicular to the surface have the same orientation preference. 17 Blobs and interblobs When the cortex is stained with cytochrome oxidase, patches on the surface stain more darkly than the rest of the cortex. The patches are called "blobs". Cells within the blobs have non-oriented receptive fields. They respond well to particular wavelengths of light (color). Blobs overlap the orientation and ocular dominance columns. 18 A Hypercolumn 19 Cortical Magnification About 25% of striate cortex processes the central 2.50 of the visual field. 20