2 Overview of QuestionsHow can brain damage affect a person’s perception?Are there separate brain areas that determine our perception of different qualities?
3 Figure 4.1 (a) Side view of the visual system, showing the three major sites: the eye, the lateral geniculate nucleus, and the visual cortex. (b) Visual system showing how some of the nerve fibers from the retina cross over to the opposite side of the brain at the optic chiasm.
4 Pathway from Retina to Cortex Signals from the retina travel through the optic nerve to theLateral geniculate nucleus (LGN)Primary visual receiving area in the occipital lobe (the striate cortex)And then through two pathways to the temporal lobe and the parietal lobeThe instructor should also note the location of the superior colliculus which controls eye movements and receives about 10% of the fibers from the optic nerve.
8 How do we study visual cortex? Hubel and WieselSingle cell recording in visual cortex.Implant one cell in visual cortex.Shine light on retina.See is we can get that cell to respond.What does the single cell like to “see”.
9 Figure 2.17 Recording electrical signals from a fiber in the optic nerve of an anesthetized cat. Each point on the screen corresponds to a point on the cat’s retina.
10 Receptive fields are determined by monitoring single cell responses. Area of receptors that affects firing rate of a given neuron in the circuitReceptive fields are determined by monitoring single cell responses.Research example for visionStimulus is presented to retina and response of cell is measured by an electrode.Important to emphasize that the receptive field is on the retina. Students tend to forget this as you work your way through the explanation of more specifically tuned neurons further into the system.
13 The Map on the Striate Cortex Cortex shows retinotopic map.Electrodes that recorded activation from a cat’s visual cortex show:Receptive fields on the retina that overlap also overlap in the cortex.It might be helpful here to explain how the Hubel and Wiesel experiment works. Stimuli are shown on specific points on the retina to find the stimulus and the receptive field that stimulates a neuron in the cortex that is being monitored by an electrode. This is also a good place to discuss ethics in the use of the animals for research, if the instructor desires to do so.
14 Neurons in Striate Cortex Simple cortical cellsSide-by-side receptive fieldsRespond to spots of lightRespond best to bar of light oriented along the length of the receptive fieldOrientation tuning curvesShows response of simple cortical cell for orientations of stimuli
15 Figure 4.6 (a) The receptive field of a simple cortical cell. (b) This cell responds best to a vertical bar of light that covers the excitatory area of the receptive field. The response decreases as the bar is tilted so that it also covers the inhibitory area.
16 Orientation tuning curve of a simple cortical cell for a neuron that responds best to a vertical bar (orientation = 0). (From Hubel & Wiesel, 1959.)
17 Neurons in Striate Cortex - continued Complex cellsLike simple cellsRespond to bars of light of a particular orientationUnlike simple cellsRespond to movement of bars of light in specific direction
18 Figure 4.8 (a) Response of a complex cell recorded from the visual cortex of a cat. The stimulus bar is moved back and forth across the receptive field. The cell fires best when the bar is positioned with a specific orientation and is moved in a specific direction
19 Response of an end-stopped cell recorded from the visual cortex of the cat. The stimulus is indicated by the light area on the left. This cell responds best to a medium-sized corner that is moving up (*).
20 Neurons in Striate Cortex – Edge detector End-stopped cellsRespond to:Moving lines of specific lengthMoving corners or anglesNo response to:Stimuli that are too long
21 Feature DetectorsNeurons that fire to specific features of a stimulusPathway away from retina shows neurons that fire to more complex stimuliCells that are feature detectors:Simple cortical cellComplex cortical cellEnd-stopped cortical cell
23 Vision Visualized With FMRI Fovea Periphery Figure 4.17 (a) Red and blue areas show the extent of stimuli that were presented while a person was in an fMRI scanner. (b) Red and blue indicates areas of the brain activated by the stimulation in (a). (From Dougherty et al., 2003.)
24 Brain Imaging Techniques - fMRI Functional magnetic resonance imaging (fMRI)Hemoglobin carries oxygen and contains a ferrous molecule that is magneticBrain activity takes up oxygen, which makes the hemoglobin more magneticfMRI determines activity of areas of the brain by detecting changes in magnetic response of hemoglobinSubtraction technique is used like in PET
25 Figure 4.14 The magnification factor in the visual system: The small area of the fovea is represented by a large area on the visual cortex.
26 Maps and Columns in the Striate Cortex Cortical magnification factorFovea has more cortical space than expectedFovea accounts for .01% of retinaSignals from fovea account for 8% to 10% of the visual cortexThis provides extra processing for high-acuity tasks.
27 Figure 4.24 How a tree creates an image on the retina and a pattern of activation on the cortex.
28 Other Cortical Areas Vision begins to processed by V1-V5 Then goes to other lobes of the brain for further processing.What we have seen. Object identification.Where we have it. Locating object in world.
29 Figure 4.27 The monkey cortex, showing the what and the where pathways. The where pathway is also called the how pathway. (From Mishkin, Ungerleider, & Macko, 1983.)
30 What and Where (How) Pathways Where pathway may actually be “How” pathwayDorsal stream shows function for both location and for action.Evidence from neuropsychologySingle dissociations: two functions involve different mechanismsDouble dissociations: two functions involve different mechanisms and operate independentlyNote that neuropsychology involves the study of the behavioral effects of brain damage.
32 What and How Pathways - Further Evidence Rod and frame illusionObservers perform two tasks: matching and graspingMatching task involves ventral (what) pathwayGrasping task involves dorsal (how) pathwayResults show that the frame orientation affects the matching task but not the grasping task.
33 Figure 4. 30 (a) Rod and frame illusion Figure 4.30 (a) Rod and frame illusion. Both small lines are oriented vertically. (b) Matching task and results. (c) Grasping task and results.
34 Modularity: Structures for Faces, Places, and Bodies Module - a brain structure that processes information about specific stimuliInferotemporal (IT) cortex in monkeysResponds best to faces with little response to non-face stimuliTemporal lobe damage in humans results in prosopagnosia.
35 Figure (a) Monkey brain showing location of the inferotemporal (IT) cortex. (b) Human brain showing location of the fusiform face area (FFA), which is located under the temporal lobe.
36 Figure 4.33 Size of response of a neuron in the monkey’s IT cortex that responds to face stimuli but not to nonface stimuli. (Based on data from Rolls & Tovee, 1995.)
38 Evolution and Plasticity: Neural Specialization Evolution is partially responsible for shaping sensory responses:Newborn monkeys respond to direction of movement and depth of objectsBabies prefer looking at pictures of assembled parts of facesThus “hardwiring” of neurons plays a part in sensory systems