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Chapter 4: Cortical Organization An Exploration of Spatial Organization • Electronic map on V1 – Retinotopic map is an electron map of the retina on the cortex – Cortical magnification – a small area of the fovea is represented by a large area on the visual cortex Figure 4-1 p78 Figure 4-2 p78 Brain Imaging Techniques • Positron emission tomography (PET) – Person is injected with a harmless radioactive tracer – Tracer moves through bloodstream – Monitoring the radioactivity measures blood flow – Changes in blood flow show changes in brain activity Brain Imaging Techniques - continued • PET - subtraction method – Brain activity is determined by: • Measuring activity in a control state • Measuring activity in a stimulation state • Subtracting the control activity from the stimulation activity Brain Imaging Techniques - continued • Functional magnetic resonance imaging (fMRI) – Hemoglobin carries oxygen and contains a ferrous molecule that is magnetic – Brain activity takes up oxygen, which makes the hemoglobin more magnetic – fMRI determines activity of areas of the brain by detecting changes in magnetic response of hemoglobin • Subtraction technique is used like in PET Figure 4-3 p79 Figure 4-4 p79 The Cortex is Organized in Columns • Cortical magnification factor – Fovea has more cortical space than expected Figure 4-5 p80 The Cortex is Organized in Columns • Visual cortex shows: – Location columns • Receptive fields at the same location on the retina are within a column – Orientation columns • Neurons within columns fire maximally to the same orientation of stimuli • Adjacent columns change preference in an orderly fashion • 1 millimeter across the cortex represents entire range of orientation Figure 4-6 p80 Figure 4-7 p81 Figure 4-8 p81 Figure 4-9 p81 The Cortex is Organized in Columns continued • Visual cortex shows – Ocular dominance columns • Neurons in the cortex respond preferentially to one eye. How Do Feature Detectors Respond to a Scene? • Tiling – columns working together to cover the entire visual field. Figure 4-10 p82 Figure 4-11 p82 Figure 4-12 p82 Streams for Information About What and Where • Lesioning or Ablation Experiments – First, an animal is trained to indicate perceptual capacities. – Second, a specific part of the brain is removed or destroyed. – Third, the animal is retrained to determine which perceptual abilities remain. – The results reveal which portions of the brain are responsible for specific behaviors. Streams for Information About What and Where - continued • Ungerleider and Mishkin experiment – Object discrimination problem • Monkey is shown an object • Then presented with two choice task • Reward given for detecting the target object – Landmark discrimination problem • Monkey is trained to pick the food well next to a cylinder Streams for Information About What and Where - continued • Ungerleider and Mishkin - Using ablation, part of the parietal lobe was removed from half the monkeys and part of the temporal lobe was removed from the other half. – Retesting the monkeys showed that: • Removal of temporal lobe tissue resulted in problems with the object discrimination task - Where pathway • Removal of parietal lobe tissue resulted in problems with the landmark discrimination task - What pathway Figure 4-13 p83 Figure 4-14 p84 Streams for Information About What and Where - continued • What pathway also called ventral pathway • Where pathway also called dorsal pathway • Both pathways: – originate in retina and continue through two types of ganglion cells in the LGN. – have some interconnections. – receive feedback from higher brain areas. Streams for Information About What and How • Where pathway may actually be “How” pathway or action pathway – Dorsal stream shows function for both location and for action. – Evidence from neuropsychology • Double dissociations: two functions involve different mechanisms and operate independently Table 4-1 p85 Streams for Information About What and How - continued • Behavior of patient D.F. – Damage to ventral pathway due to gas leak – Not able to match orientation of card with slot – But was able to match orientation if she was placing card in a slot – Other patients show opposite effects – Evidence shows double dissociation between ventral and dorsal pathways Figure 4-16 p85 Behavior of People Without Brain Damage • Ganel experiment was designed to demonstrate a separation of perception and action in non-brain-damage subjects. Figure 4-17 p86 Modularity: Structures for Faces, Places, and Bodies • Module - a brain structure that processes information about specific stimuli – Rolls measured the response neurons in the Inferotemporal (IT) cortex in monkeys • Responds best to faces with little response to non-face stimuli – Temporal lobe damage in humans results in prosopagnosia. Figure 4-18 p87 Figure 4-19 p87 Figure 4-20 p88 Areas for Faces, Places, and Bodies in the Human Brain • Evidence from humans using fMRI and the subtraction technique show: – Fusiform face area (FFA) responds best to faces. – Parahippocampal place area (PPA) responds best to spatial layout. – Extrastriate body area (EBA) responds best to pictures of full bodies and body parts. Figure 4-21 p88 Figure 4-22 p89 Where Vision Meets Memory • MTL structures are extremely important in memory – H.M. – hippocampus Figure 4-23 p90 Figure 4-24 p90 Experience and Neural Responding – Experience-dependent plasticity in humans • Brain imaging experiments show areas that respond best to letters and words. • fMRI experiments show that training results in areas of the FFA responding best to: –Greeble stimuli –Cars and birds for experts in these areas Figure 4-25 p91