Experimenting on Mechanisms Review 1 A mechanism consists of parts entities and their operations activities organized to produce a phenomenon Organization plays a critical role in coordinating the component parts and their operations so as to perform an activity Organization also creates systems that have an identity and act as a unit in the world Often scientific inquiry spans multiple levels of organization The level at which a mechanism as a whole interacts with other entities The level of the component parts and their operations The level of the subcomponents and their operations Review 2 Causation at multiple levels The level at which the mechanism engages the world The level at which its parts perform their operations The level at which the parts of the parts perform their operations 1 Review 3 Small world phenomenon If most connections between nodes of a network are local but a few are long range one can have a system of separate mechanisms that are nonetheless highly coordinated Scale free networks If the mean number of connections follows a power law develop networks robust against random damage Discovering How Mechanisms Work What are organized in mechanisms are parts entities performing operations activities When it is possible the most effective way to understand parts and their operations is to experiment Experimentation involves intervention and manipulation of a possible cause to determine what effect it has and CONFOUNDS remain the worry The only differences are This is being done within the context of a organized system The goal is to understand how the parts contribute to the working of the mechanism First step Getting the Phenomenon Right Pick a card and think real hard about it Don t forget it Whoops I thought you were cheating and tried to shoot you But I got your card instead Check it out did I take out your card 2 Correctly characterizing perception One view Perception is like photography creating a picture for a homunculus to view This fits our phenomenal awareness but that may be a false lead As you just saw Remember change blindness Alternative Perception involves extraction of information in format useable by down line systems There may be no place where all the information comes together We may not in fact even perceive much of the visual scene Intervening to discover how a mechanism works The operations of the components of a mechanism result in causal interactions between components Just as with simple causal interactions between a independent and dependent variable Correlated activity suggests causal linkages But the best evidence for causation comes from manipulating and so controlling changes Manipulating the input to the mechanism and determining the effects Manipulating components of the mechanism and determining their effects The Mechanism Your task figure out how this mechanism produces the phenomenon 3 Figuring out how it works Three basic strategies for figuring out what the components of a mechanism do Recording from an individual component while the mechanism is operating and inferring from the conditions in which it is active what operation it might perform Intervention Detection q p u t v r s w Figuring out how it works Inhibition lesion Inhibition lesion or ablation studies q p Lesioning or ablating a r s t u component w v and inferring from the deficit in the behavior Detection of the whole what Excitation operation the component performed Excitation or stimulation studies stimulating a component and inferring from its affect on the whole system what operation it performed Start with Lesions Until the 1940s no way to record the activity in an area of the brain Crude stimulation with the electrodes of the 19th century activated very wide areas and so not sufficiently specific That left ablation or lesion as initially the tool of choice 4 Occipital Lobe Bartolomeo Panizza 1855 Proposed occipital lobe as locus of vision based on patients who experienced blindness after strokes experiments of nature Hermann Munk 1870s Unilateral removal of the occipital lobe in dogs resulted in partial blindness Bilateral removal resulted in total blindness Getting More Detailed Tatsuji Inouye a young Japanese ophthalmologist was assigned to assess visual loss in Japanese soldiers who had suffered brain injury so as to determine how large their pensions would be but he decided to make the job more interesting and map deficits onto the area damaged The Russians had developed a new high velocity rifle Mosin Nagant Model 91 that fired a 7 62 mm hard jacketed bullet This bullet pierced the skull without shattering leaving tidy entrance and exit wounds This made it possible to trace the trajectory and determine just what parts of the occipital lobe was damaged Retinotopic map of the visual field Inouye correlated the parts of the visual field in which his patients were blind with areas of brain damage and mapped the visual field onto the visual cortex Gordon Holmes 1918 constructed a similar map based on studies of soldiers injured during World War I 5 Much Later Recording Studies Confirm Retinotopy So Visual Cortex is Needed but What does it do Lesions can show that an part of the mechanism seems to be necessary for it to perform a specific phenomenon But are not able to show more specifically what the damage part does in normal situations A powerful complement is to record from the component while it is in operation to see what elicits its activity Development of electrodes wired to amplifiers and speakers Single cell recording 6 Single cell recording in Retina and LGN Stephen Kuffler applied the technique of single cell recording to the retina and lateral geniculate nucleus LGN and found cells that fired either to Light dots with dark backgrounds on center off surround Dark dots with light backgrounds off center on surround Turning to Cortex When a technique works once it makes sense to try it again David Hubel and Thorsten Wiesel tried to replicate Kuffler s achievements in occipital lobe And failed and failed and FAILED BUT one day while they were inserting a glass slide into their projecting ophthalmoscope it stuck creating a bar of light on the screen Hubel reports that over the audiomonitor the cell went off like a machine gun Bars of light edges not dots activate occipital cortex Hubel and Wiesel s Simple Cortical Cells Many of the cells Hubel and Wiesel tested in occipital lobe responded to bars of light If they were properly oriented on off on off