Psych 253 Lecture 11 14 Wede Face Recognition Spring Faces An illustrative special case Face recognition seems to be special and different from object recognition Evidence that the process of face recognition differs from the process of object recognition 1 Potentially different brain regions for face and object recognition Agnosia A failure to recognize objects despite being able to see them Prosopagnosia An inability to recognize faces Double dissociation When one perceptual function can be damaged without affecting the other 2 Faces are difficult to recognize in photographic negatives but objects are not 3 Faces are more difficult to recognize down than are most objects Thatcher effect phenomenon in which local changes in a face are more difficult to detect when the face is upside down Object Recognition Objects in the brain Extrastriate cortex The region of cortex bordering the primary visual cortex and containing multiple areas involved in visual processing After extrastriate cortex processing of object information is split into a what pathway and a where pathway Where pathway dorsal pathway Concerned with the locations and shapes of objects but not their names or functions What pathway ventral pathway Concerned with the names and functions of objects regardless of where they are Two Processing Streams Ungerleider Mishkin 1982 Object Recognition Spatial Localization What and Where Pathways Object Recognition 1 Strong evidence that what and Where pathways support separate functions The interaction between the two is subject of lots of current research Psych 253 Lecture 11 14 Wede Spring Part of the cerebral cortex in the lower portion of the temporal lobe When IT cortex is lesioned it leads to agnosia Evidence in humans Severe apperceptive agnosia Difficulty in judging basic aspects of form or shape Damage throughout ventral stream Face perception Mooney images Images that when upright provide a clear percept of a face Inverting the image changes the percept dramatically What happens in the brain fMRI Stronger responses to faces Fusiform Face Area FFA Region in the fusiform gyrus temporal lobe So faces are special Damage to FFA leads to problems discriminating faces FFA activated when processing face stimuli These results do not necessarily mean the FFA is responsible for detecting faces Participants see a sequence of objects cars faces birds other judge similarity across trials Participants include car and bird experts everyone is a face expert Not the whole story Gauthier et al 2000 Similar results for Greebles Experts at Greeble discrimination show activation in FFA Lots of current research examining whether faces are special Are faces special No clear answer FFA might be activated anytime we are required to make fine discriminations Jennifer Aniston Cell Grandmother cells Could a single neuron be responsible for recognizing your grandmother Or any object 2 Psych 253 Lecture 11 14 Wede Short answer Likely not Spring Lecture 12 Intro to Color Basic Principles of Color Perception Color is not a physical property but a psychophysical property Most of the light we see is reflected Typical light sources Sun light bulb fire We see only part of the electromagnetic spectrum between 400 and 700 nm Objects appear colored because of the wavelengths reflected by those objects Perceived Colors and Corresponding Wavelengths 400nm purple 480nm blue 535nm green 580nm yellow 600nm orange 650nm red Problem of univariance An infinite set of different wavelength intensity combinations can elicit exactly the same response from a single type of photoreceptor Trichromacy Rods are sensitive to scotopic light levels All rods contain the same photopigment molecule Rhodopsin Therefore rods suffer from the problem of univariance Trichromatic Theory History of color vision Thomas Young 1773 1829 and Hermann von Helmholtz 1821 1894 independently discovered the trichromatic nature of color perception This is why trichromatic theory is sometimes called the Young Helmholtz theory James Maxwell 1831 1879 developed a color matching technique that is still being used today 3 Psych 253 Lecture 11 14 Wede Maxwell s color matching experiment Spring Young Helmholtz Trichromatic Theory 1802 Proposed that there are three different types of color receptors each responding maximally to a different wavelength Cone photoreceptors Three varieties S cones Short wavelengths blue cones M cones Middle wavelengths green cones L cones Long wavelengths red cones Respond to other wavelengths as well Metamers Different mixtures of wavelengths that look identical More generally any pair of stimuli that are perceived as identical in spite of physical differences Color space A three dimensional space that describes all colors HSB color space Defined by hue saturation and brightness RGB color space Defined by the outputs of long medium and short wavelength lights Hue The chromatic color aspect of light Saturation The chromatic strength of a hue Brightness The distance from black in color space Problem Hering s Opponent Process Theory 1878 Hering proposed three different types of receptors Red green cells Blue yellow cells Blac white cells Each cell is excited by one member of the pair and inhibited by the other Empirical facts which led to the Opponent Process OP Theory Some color combinations are impossible Adapting to one color produces afterimages of another color Opponent Processes Color opponent cell A neuron whose output is based on a difference between sets of cones 4 Psych 253 Lecture 11 14 Wede Spring In LGN there are color opponent cells with center surround organization Opponent color theory based on an opponency between two colors Some LGN cells are excited by L cone onset in center inhibited by M cone onsets in their surround and vice versa Red versus green Other cells are excited by S cone onset in center inhibited by L M cone onsets in their surround and vice versa Blue versus yellow Opponent process circuit Afterimages A visual image seen after a stimulus has been removed Negative afterimage An afterimage whose polarity is the opposite of the original stimulus Lecture 13 Color Perception Des Everyone See Colors the Same Way Yes General agreement on colors some variations because lens yellows as you age Some variation due to age lens turns yellow No About 8 of male population 0 5 of female population has some form of color vision deficiency Color blindness recessive gene Several types of color blind people