Lecture11_vision2 - 1,238,432 x 9,423 = Which face was in...

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1,238,432 x 9 ,423 = ?
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Which face was in the porthole? images from cvdazzle.com
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HARD PROBLEM SIMPLER PROBLEMS Vision is a hard problem, and the brain solves it by dividing it up into many simpler problems, which in turn get divided into even simpler problems, and so on Divide & Conquer EVEN SIMPLER PROBLEMS subsystems visual system circuits
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Pioneering Visual Neurophysiologists David Hubel Torsten Wiesel 1981 Nobel Prize for Physiology & Medicine
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“Simple” Cells in V1 The preferred stimulus for a simple cell is a line, bar, or edge with the following properties: Centered in the receptive field (A is good, B is not) Oriented at the preferred angle (A is good, C is not) Preferred Antipreferred Nonpreferred No stimulus Postinhibitory Rebound Spikes A B C D The directional selectivity of a simple cell can be quantified by plotting its orientation tuning curve
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Building a simple cell from center-surround cells Center-surround receptive fields of LGN cells Inhibitory Interneuron in V1 Orientation tuned receptive of V1 simple cell Excitatory projections Excitatory Projection Neuron in V1 A V1 simple cell receives excitatory input from a row of center-surround LGN neurons; the simple cell will then be excited by a bar that “covers” the row of LGN receptive fields. The simple cell also receives inhibitory input from a V1 interneuron, which is excited by center-surround LGN neurons with receptive fields that surround the bar.
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Fast spiking No spiking Baseline spiking Non-preferred or no stimulus Preferred Stimulus Antipreferred Stimulus When no stimulus or a non-preferred stimulus is present, the simple cell receives either no input or equal excitation and inhibition, so it fires at a low baseline rate. The preferred stimulus activates the simple cell by stimulating only its excitatory but not its inhibitory inputs. The antipreferred stimulus turns off the simple cell by stimulating only its inhibitory but not its excitatory inputs. Responses to stimuli
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“Complex” Cells The preferred stimulus for a complex cell is a line, bar, or edge with the following properties: Oriented at the preferred angle, but anywhere in the receptive field is OK (both A and B are good; this is a very simple example of invariant recognition ) Sometimes a specific direction of motion is preferred, and the opposite direction is antipreferred (‘C’ excites, but ‘D’ inhibits) Preferred Preferred Preferred Antipreferred A B C D Diagram at left shows responses of a complex cell to different stimuli. The cell is excited by a stationary vertical bar anywhere in the gray receptive field (A and B), or by a bar moving leftward across the receptive field (C), but not rightward (D). This cell would never respond to any non-vertical bar, or to any stimulus that lies entirely outside of the gray shaded receptive field area.
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