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fMRI Repetition (aka: “adaptation”, “priming”)

fMRI Repetition (aka: “adaptation”, “priming”). A method for probing lower levels of analysis with fMRI If assumptions are correct, can potentially overcome spatial resolution limits of fMRI

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fMRI Repetition (aka: “adaptation”, “priming”)

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  1. fMRI Repetition(aka: “adaptation”, “priming”) • A method for probing lower levels of analysis with fMRI • If assumptions are correct, can potentially overcome spatial resolution limits of fMRI • All started with an observation: the fMRI response to a repeated stimulus is smaller than an unrepeated stimulus

  2. Anatomical Functional Cortical Lobes Vision, Audition, Language “What vs. Where/Action” Recognizing words Pathways/Networks Cortical Areas Motion, Orientation, Color, Shape, Location, Movement Directions of motion, orientation Cortical Column Computations Lateral inhibition Neural Circuit Neuron Code

  3. fMRI adaptation: an example • We’ve identified an area that responds to visual motion. • We want to know if there are populations of neurons tuned to different directions of motion • What would happen if we did a subtraction comparison of leftward vs. rightward motion? • We reason: these populations of motion-tuned neurons (if they exist) are too small to be directly imaged using fMRI • Try an adaptation procedure…

  4. Our specific hypothesis • Neurons with different preferences for different motion directions

  5. fMRI adaptation assumptions • Presenting a stimulus adapts or habituates the neurons that respond to that stimulus • Firing rate gradually diminishes over time • Presenting a second, identical stimulus will tap into this adapted population  reduced response • Presenting a second, different stimulus will tap into an unadapted population  no reduced response

  6. fMRI Adaptation ISI Population Activity Time ISI Population Activity Time

  7. fMRI Adaptation Population Activity Time Population Activity Time

  8. fMRI Signal fMRI Adaptation • The amount of adaptation is proportional to the similarity in neural populations representing the two stimuli. “release from adaptation”

  9. Variety of Designs • Differences in timing • Short vs. Long initial stimulus • Short vs. Long interval between stimuli • “adaptation”, “priming”, “repetition effects” • Blocked (continuous repetition) vs. event-related • Though “repetition effects” are observed with all of these designs, not safe to assume they’re measuring the same thing.

  10. Two main theories • Viewer-centered model • populations of neurons “tuned” to different viewpoints (orientations) • Object-centered model • populations of neurons that are “invariant” to viewpoint (orientation)

  11. Is a particular area ‘invariant’ for a particular stimulus dimension? Does the object area “care about” the orientation of an object? OR fMRI response fMRI response identical identical rotated rotated

  12. Does the face area “care about” the identity of a face? OR fMRI response fMRI response identical identical different different

  13. Two main theories • Viewer-centered model • populations of neurons “tuned” to different viewpoints (orientations) • Object-centered model • populations of neurons that are “invariant” to viewpoint (orientation)

  14. Kalanit Grill-Spector, et al. Differential processing of objects under various viewing conditions in the human lateral occipital complex. Neuron, 24(1):187-20,1999. • Thomas W. James, et al. (2002) Differential Effects of Viewpoint on Object-Driven Activation in Dorsal and Ventral Streams. Neuron, Vol. 35, 793–801

  15. Two main theories • Viewer-centered model • populations of neurons “tuned” to different viewpoints (orientations) • Object-centered model • populations of neurons that are “invariant” to viewpoint (orientation) • Where to look and how to test? fMRI Adaptation

  16. Kalanit Grill-Spector, et al. Differential processing of objects under various viewing conditions in the human lateral occipital complex. Neuron, 24(1):187-20,1999. • Thomas W. James, et al. (2002) Differential Effects of Viewpoint on Object-Driven Activation in Dorsal and Ventral Streams. Neuron, Vol. 35, 793–801

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