09-30 Intro to Cog Sci Lecture 9

09-30 Intro to Cog Sci Lecture 9 - Introduction to...

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Introduction to Cognitive Science Lecture 9: 9/30/09 We’ve now talked about the nature of fMRI, how fMRI works, the experimental logic sometimes used in fMRI studies, and examples of fMRI experiments. This is not a graph of the activity in the regions of the brain, it’s a graph of the differential activity – these numbers are the results of a subtraction (it’s a comparison). Part of the point of this is to see a cool example of cognitive science (a collaboration between a neuroscientist and a philosopher) and also as a great example of subtraction logic (in considering a moral dilemma, a whole bunch of stuff is going on, but this study controls for this by having two different moral dilemmas with only one difference. This is the standard of fMRI and the logic of many studies in science.) fMRI Limitations This is an important antidote to how fMRI is treated in the popular press. The first problem is more something you have to watch for: remember that you’re looking at the tip of an iceberg. You are looking at the parts of the brain that are most effective and every bounded field will exhibit a maximum. Sometimes, due to random activity, there will be a most active part of the brain. The second problem is a key limitation – so much of the magic of cognition happens quickly but we cannot use fMRI to isolate anything that happens in less than 1 second, because it lacks temporal resolution. Thirdly , there are some conceptual problems. Let’s talk about an abstract point – remember that fMRI is dependent on blood flow. In many places, we can distinguish between activation and inhibition. It turns out, unsurprisingly, that both require energy and blood flow. So using a blood area shows a pretty similar thing for using and not using a brain. Mental imagery – do visual processing without visual stimulus (recall Lecture 1: windows in your house) – is this a completely independent process or does it involve using visual processes – the same ones you use for seeing – when there is no external input. The battle has raged on but if you look at which brain regions are active, you find that many aspects of many visual areas of the brain are active. This was used in a paper to argue but critics said according to their theory, you’re actually inhibiting visual system in order to prevent contamination. We already talked about trying to get rid of vision – everyone looked up at ceiling to get rid of visual stimulus. So this is a great example of fMRI telling us that either visual areas are being used or being inhibited! Some of these points are just things to keep in mind. EEG is a direct measure, while fMRI is indirect measure – looks at the blood supply. fMRI cannot measure overall activity – just % change (if you look at Y-axis, it’s always % change from base-
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09-30 Intro to Cog Sci Lecture 9 - Introduction to...

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