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help > RE: seed-to-voxel coordinate for F-contrast
Jan 10, 2017 05:01 AM | Alfonso Nieto-Castanon - Boston University
RE: seed-to-voxel coordinate for F-contrast
Hi Teddy,
Those differences are likely due to having slightly different clusters in the one-sided vs. two-sided results. You should find that, even for those clusters that still show the same coordinates, their cluster sizes will vary depending on whether you use one-sided vs. two-sided analyses (assuming here that you are using height thresholds based on p-values, e.g. a uncorrected p<.001 height threshold). The coordinates are simply reporting the peak voxel within a cluster, so even if a cluster is slightly larger it may often still have the same peak coordinates. The reason for the different cluster sizes is that the same p-value threshold results in two different T- value thresholds, depending on whether you are using one-sided or two-sided statistics, and these will naturally result in slightly different suprathreshold clusters. To be precise, if you are using a p<.001 height threshold in your two-sided analyses, and you then want to look at one-sided results and make sure that you are looking at exactly the same clusters, then you should use p<.0005 height thresholds in each of your two one-sided analyses.
Hope this helps
Alfonso
Originally posted by Theodore Turesky:
Those differences are likely due to having slightly different clusters in the one-sided vs. two-sided results. You should find that, even for those clusters that still show the same coordinates, their cluster sizes will vary depending on whether you use one-sided vs. two-sided analyses (assuming here that you are using height thresholds based on p-values, e.g. a uncorrected p<.001 height threshold). The coordinates are simply reporting the peak voxel within a cluster, so even if a cluster is slightly larger it may often still have the same peak coordinates. The reason for the different cluster sizes is that the same p-value threshold results in two different T- value thresholds, depending on whether you are using one-sided or two-sided statistics, and these will naturally result in slightly different suprathreshold clusters. To be precise, if you are using a p<.001 height threshold in your two-sided analyses, and you then want to look at one-sided results and make sure that you are looking at exactly the same clusters, then you should use p<.0005 height thresholds in each of your two one-sided analyses.
Hope this helps
Alfonso
Originally posted by Theodore Turesky:
Hi Alfonso et al.,
Before describing my question, I want to thank you and the other CONN folks for providing this wonderful tool and for maintaining this forum. It's a remarkable feat!
Earlier today, I noticed a discrepancy in CONN's seed-to-voxel results explorer and I was hoping to receive some clarification.
I'm noticing that when I start with the effect of group (contrast vector: [1]) in the main GUI and then, in the results explorer, go from the one-sided positive (t) contrast to the two-sided (f) contrast, that the coordinates for the positive clusters stay the same. In contrast, if I compare the coordinates from the one-sided negative (t) contrast to the two-sided (f) contrast, that the coordinates for the negative clusters change.
If I type the F-contrast matrix into the main GUI (e.g., [1; -1] rather than [1]) or use the spm display button to define a new F-contrast, the coordinates are the same as both positive and negative one-sided contrasts. I'm wondering whether the positive one-sided coordinates should be updating when going from one- to two-sided or the negative one-sided coordinates are not updating accurately in the results explorer or I'm misinterpreting in some way. Any guidance you can provide would be greatly appreciated and definitely don't hesitate to have me clarify anything that is unclear.
Many thanks!
Teddy
Before describing my question, I want to thank you and the other CONN folks for providing this wonderful tool and for maintaining this forum. It's a remarkable feat!
Earlier today, I noticed a discrepancy in CONN's seed-to-voxel results explorer and I was hoping to receive some clarification.
I'm noticing that when I start with the effect of group (contrast vector: [1]) in the main GUI and then, in the results explorer, go from the one-sided positive (t) contrast to the two-sided (f) contrast, that the coordinates for the positive clusters stay the same. In contrast, if I compare the coordinates from the one-sided negative (t) contrast to the two-sided (f) contrast, that the coordinates for the negative clusters change.
If I type the F-contrast matrix into the main GUI (e.g., [1; -1] rather than [1]) or use the spm display button to define a new F-contrast, the coordinates are the same as both positive and negative one-sided contrasts. I'm wondering whether the positive one-sided coordinates should be updating when going from one- to two-sided or the negative one-sided coordinates are not updating accurately in the results explorer or I'm misinterpreting in some way. Any guidance you can provide would be greatly appreciated and definitely don't hesitate to have me clarify anything that is unclear.
Many thanks!
Teddy
Threaded View
| Title | Author | Date |
|---|---|---|
| Theodore Turesky | Jan 7, 2017 | |
| Alfonso Nieto-Castanon | Jan 10, 2017 | |
| Theodore Turesky | Jan 10, 2017 | |
| Alfonso Nieto-Castanon | Jan 11, 2017 | |
| Theodore Turesky | Jan 11, 2017 | |