Lecture Notes on Vision and Pattern Recognition

outline - Agnosia- insight into pattern/object recognition - Theories of pattern recognition - Gestalt principles - Template-matching and feature analysis - Recognition-by-component theory - Neural mechanisms of pattern recognition - Contextual effects agnosia - Inability to...

outline - Agnosia- insight into pattern/object recognition - Theories of pattern recognition - Gestalt principles - Template-matching and feature analysis - Recognition-by-component theory - Neural mechanisms of pattern recognition - Contextual effects agnosia - Inability to recognise objects - In agnosia, processes such as colour, shape, and motion perception are intact - Recognising a whole object is more than just recognising its parts Two cortical pathways for vision figure\_06\_03.jpg Two cortical pathways for vision - Ventral pathway - Occipital temporal - Processes information about object appearance and identity - Important for object perception - Dorsal pathway - Occipital parietal - Processes spatial information about objects - Important for guiding action - Patient DF - Severe agnosia - Her visual acuity and nonvisual object recognition are intact - Optic ataxia - Intact object recognition - Inability to use visual information to guide action - Associated with lesions in the dorsal pathway (typically in the parietal cortex) Face perception - Face perception requires holistic possessing - Results from study  Gestalt principles fig\_2.13.jpg 00056FCDPM4\_061 BBE9038F: - Similarity - Closure - Good continuation - Proximity  Template-matching and feature analysis - We tend to recognise different stimuli as the same object irrespective of superficial variation - Gestalt principles don't really help explain way - Another theory of patten recognition, template matching, doesn't explain this either  Feature analysis - A visual pattern is perceived as a combination of elemental features - Selfridges pandemonium model  - What would happen when this pattern is presented on the retina without eye movements? - Whole feature tend to disappear simultaneously Recognition-by-competent theory - Similar mechanisms have been proposed for 3-D object recognition - An object is first segmented into a set of basic sub-objects (called *geons*), and then recognised as a pattern composed of geons  Neural mechanism of pattern recognition - Neurons in higher-order visual areas respond to increasingly complex pattern   Neural mechanism of pattern recognition - If we keep going down the visual information processing pathway, can we eventually find a cell that responds to a very, very, very specific stimulus? - "Grandmother cell" hypothesis - Weaknesses of the grandmother cell hypothesis - The final percept of an object is coded by a single neuron - However, each neuron's firing is not so reliable - If that neuron is lost, our perception of the corresponding object would be lost - Perception of novel objects cannot be explained well - Flexibility of object recognition cannot be explained well - Object cognition result from the firing of an ensemble of cells (ensemble coding) - However... - More recent evidence suggests that grandmother-cell-like neurons might exist (Quiroga et al., 2005) - But not necessarily in the visual object recognition system per se Top-down and bottom up Word- superiority effect - Superior recognition of letters in a word context than alone - Indicates top-down influence on pattern recognition Phoneme-restoration effect - "The state governors met with their respective legi...latures convening in the capital city." - Only 1 in 20 participants reported hearing this tone - And this one participant was unable to locate the tone correctly