Visual Cognition I basic processesWhat is perception good for?Slide 3Object Recognition System must be flexible to recognize variations in object shapesBasic processes in the Visual SystemHierarchical OrganizationSlide 7Primary and Secondary Visual Cortex (V1 and V2)Retinotopic maps in V1Slide 10Revealing retinotopic maps with fMRISlide 12Receptive FieldsA wiring diagram for building simple cells out of on-off cellsThere is some evidence for cells that recognize facesSlide 16Illusory ContoursMach Bands and Lateral InhibitionLateral InhibitionLateral Inhibition enhances edgesThrowing Away InformationFunctional SpecializationSensory Binding ProblemSlide 24Face RecognitionSlide 26Is Face Recognition Special?Neuropsychological evidenceSlide 29Brain Imaging EvidenceSlide 31Visual Expertise?ResultWhat about real-world experts?EvaluationGrouping PrinciplesPerceptual Grouping and Gestalt LawsCommon FateFigure–Ground SegregationFilling in the GapsSlide 41Visual Cognition Ibasic processesWhat is perception good for?•We often receive incomplete information through our senses. Information can be highly ambiguous•Perceptual system must resolve ambiguities by drawing inferences from a large set of perceptual cues and conceptual knowledge of the worldSlide from Rob GoldstonePerceptual system is always looking for meaningful patternsObject Recognition System must be flexible to recognize variations in object shapesBasic processes in the Visual SystemHierarchical OrganizationVisual InputLow Level VisionHigh Level VisionOriented bars/edges, Motion, Texture, DepthObject/Face/ScenerecognitionKnowledgePrimary and Secondary Visual Cortex (V1 and V2)•Retinotopic maps •Receptive fields:–On-off cells; Off-on cells–Simple cells•Lateral inhibitionRetinotopic maps in V1•Retinotopic mapping: locations on retina are mapped to cortex in orderly fashion. Note: more of visual cortex is dedicated to foveal visionResponse in monkey primary visual cortex (V1) measured by radio-active tracersStimulus pattern Tootell, R. B., M. S. Silverman, et al. Science (1982)StimulusCortical Mapping: Left HemisphereCortical Mapping: Right HemisphereRevealing retinotopic maps with fMRIFrom: Geoff Boynton, SALK instituteRevealing retinotopic maps with fMRIFrom: Geoff Boynton, SALK instituteReceptive Fields•The receptive field (RF) of a neuron is the area of retina cells that trigger activity of that neuronOn-off cells and off-on cells: Simple cells (bar detectors)A wiring diagram for building simple cells out of on-off cellsHierarchical organization of the brain: by aggregating responses over several on-off cells, the brain can detect more complicated features (e.g. bars and edges)There is some evidence for cells that recognize facesBruce, Desimone & Gross (1981)Hierarchical OrganizationIllusory Contours•One can see a square even though there is no square  illusory contour•Bar detectors in visual cortex are activated by partial contours. •Higher areas assemble lines into objects. The visual system fills in the lines in the corners+ + + + + +- - - - - -Mach Bands and Lateral InhibitionLateral Inhibition•Lateral inhibition sets up competition between neurons so that if one neuron becomes adept at responding to a pattern, it inhibits other neurons from doing so.DEMO APPLETS: http://www.psychology.mcmaster.ca/4i03/demos/lateral-demo.htmlhttp://serendip.brynmawr.edu/bb/latinhib_app.html ++++- -------++++- -------++++- -------++++- -------++++- -------Light:On-Off Cells with lateral inhibition:Response Edge detectionLateral Inhibition enhances edgesThrowing Away InformationThe Craik-Cornsweet-O’Brien illusion (a) A gray rectangle is shown with a special edge in the middle. The rectangle seems to be divided with a lighter region on the left and darker region on the right. If you look closely, you will see that actually the two regions are not uniform. There is a gradual transition in each side, producing a sharp change from light to dark in the middle. (b) This is the same figure as in (a), but with the middle region covered by a black rectangle. Now you can see that the gray regions are actually the same. Try putting your finger over the middle of (a) to reveal the illusion.Functional Specialization•Spatially different areas are functionally specialized for processing visual attributes such as shape, color, orientation, and direction of motion•Achromatopsia (damage to V4)•cortical color blindness all color vision is lost and the world appears in shades of gray. And in achromatopsia, unlike as in blindness caused by damage to the eyes or optic nerve, even memory of color is gone•Akinetopsia (damage to V5 or MT)•or motion blindness—the loss of the ability to see objects move. Those affected report that they perceive a collection of still images.Sensory Binding Problem•If spatially different areas are functionally specialized for processing visual attributes such as shape, color, orientation, and direction of motion….•then how does the brain then “bind” together the sensory attributes of an object to construct a unified perception of the object?  Binding ProblemBinding ProblemFace RecognitionPainting from ArchimbaldoFace processing involves configural processing – analyzing configurations of featuresWe detect faces when the configurations of features are presentIs Face Recognition Special?•Arguments have been made for both functional and neuroanatomical specialization for face processing (different from objects)•Sources of evidence:–Behavioral experiments–Brain injury–Brain imagingNeuropsychological evidence•Face specific deficits found in prosopagnosia•Subjects can identify most objects, but are impaired at recognizing faces•Can name faces from touch and verbal descriptions so prosopagnosia is not a naming deficit  visual pattern recognition problem•Is it specific to faces or any difficult recognition problem?Prosopagnosics can perform well on difficult object discrimination tasksBrain Imaging Evidence•In humans, face perception is uniquely associated with activity in the fusiform face area located in the fusiform gyrus in the inferior temporal lobe •This finding is more robust in the right vs. left hemisphere.FFA (face)V4 (color)FFA (face)Visual Expertise?•Gauthier and colleagues suggest that faces are special because we have become experts at within-category discriminations•Claims that becoming an expert at "Greeble" discrimination involves the