Chapter 8: Spatial and Temporal Properties of fMRI

 

 

 

Study Questions

 

 

 

 

 

 

 

1.   What are the major divisions of fMRI data, from subjects down to voxels?

-spatial and temporal information; functional and structural information; many subjects for more complex studies, less for simpler studies; single sessions or multiple sessions; less slices for studying specific regions or more for studying the whole brain; block designs or event-related designs; voxelwise analyses or region-of-interest analyses

 

 

 

 

 

 

 

 

 

2.   What are disdaqs, and why are they sometimes incorporated into imaging protocols?

-“discarded data acquisitions”

 

 

 

 

 

 

 

 

 

3.   What are the disadvantages of high spatial resolution in fMRI?

-reduced signal to noise ratio and increased acquisition time

 

 

 

 

 

 

 

 

 

4.   What are partial volume effects?

-the combination of signal contribution from two or more tissue types within one voxel

 

 

 

 

 

 

 

 

 

5.   What are large vessel effects, and why do they matter for fMRI?

-signal changes in veins that drain a functionally active region but are distant from the neuronal activity of interest

 

 

 

 

 

 

 

 

 

6.   What factors, aside from voxel size, influence spatial resolution in fMRI?

-spatial scale; smoothing; normalization; use of ROI analysis

 

 

 

 

 

 

 

 

 

7.   What happens to estimates of the hemodynamic response as repetition time (TR) is reduced from very long (i.e., 4 s) to very short (i.e., 500 ms)?

-they become more accurate

 

 

 

 

 

 

 

 

 

8.   Is there a preferred TR for fMRI? Does this depend on whether the design is event-related or blocked?

- there are basic preferred TR for fMRI studies (1-2s); event-related studies benefit from shorter TR but only up to a point; blocked studies benefit from longer TR

 

 

 

 

 

 

 

 

 

9.   What are the disadvantages of high temporal resolution in fMRI?

-limiting spatial coverage, reducing signal amplitude, reduction of number of trials obtained for each condition

 

 

 

 

 

 

 

 

 

10. What is interleaved stimulus presentation? (Note: not interleaved slice acquisition.)

 A method to increase temporal resolutions, where experimental stimuli are presented at different points within a TR on different trials. It increases the effective sampling rate of an experiment at the expense of fewer trials per condition.

 

 

 

 

 

 

 

 

 

11. Which is easier to study using fMRI, absolute event timing or relative event timing? Why?

 It is easier to study relative event timing, because of the understanding of the temporal relation between the different regions of the brain. Though they won't get an immediate HDR they can understand the relation between the responses of the activation of the other sections of the brain.

 

 

 

 

 

 

 

 

 

12. How have small event timing differences been measured using fMRI? What are some caveats for such studies?

 They measured with a gradient-echo ech-planar fMRI at high field (4 T) and high temporal resolution (TR of 100 ms), using hemifields. Hemifields were projecting to half a visual display and they were either shown at the same time or one before the other in order to see if the correspondence between stimulus introduction and brain activation was in correspondence between the two sides. From this the examined the different areas of the brain for their activation times. The caveat to this is that differences in vascular properties contribute to the timing of activity.

 

 

 

 

 

 

 

 

 

13. Name and define the two properties of a linear system.

Scaling: the principle that states that the magnitude of the system output must be proportional to the system input

Superposition: the principle of linear system that states that the total response to asset of inputs is equivalent to the summation of the independent responses to the inputs

 

 

 

 

 

 

 

 

14. How well is the fMRI response to a long-duration stimulus (e.g., 12 s) predicted by that to a short-duration stimulus (e.g., 3 s)?

 The 12 s stimuli indicated linear superposition, but when reducing the stimuli to 3s, the response was larger than expected. This is tought to be caused by the neuronal adaptation effects, which is when the neurons decrease in activity over the first few seconds of a stimulus.

 

 

 

 

 

 

 

 

 

15. Why did Dale and Buckner describe the fMRI response as roughly linear?

 When testing the hemodynamic responses evoked by increasing amounts of stimuli, they saw the the fMRI BOLD response adds across stimuli in a "roughly linear" manner in intervals similar to the intervals of experimental testing. So the principle of superposition held.

 

 

 

 

 

 

 

 

 

16. Approximately how long does the fMRI refractory period last?

 Approximately  6 s.

Refractory period: a time period following the presentation of a stimulus during which subsequent stimuli evoke a reduced response.

 

 

 

 

 

 

 

 

 

 

17. How do refractory effects change the amplitude and latency of hemodynamic response?

 The amplitude is reduced and the latency is delayed. For instance, at 1 s difference between the preceding stimulus, the amplitude was down 40% and the latency was 1 s late. For 2 s, the amplitude was down slightly more than 20% and around a second late as well.

 

 

 

 

 

 

 

 

 

18. How might one use the refractory effect to study neuronal adaptation? What information do these sorts of studies provide about brain function?

 By understanding adaption, if we alter aspects of the stimuli presented we can understand what aspects of the stimuli are being adapted to. For instance, if we are given a face stimuli and certain aspects of the face are altered (size, position in sight, etc...), and there is no real change in response, we understand what aspects are being adapted to. This likewise can show that  other aspects (such as lighting and view-points) which provide new neuronal stimulation, are not included in the process of adaptation in the area examined.