5 ms·

The study was regarding density, not size.

nonbel 9y ago
Where are you seeing that? Also, fine density/volume/mass, does it really change the argument? There is some optimal size for this region when it comes to certain functions, probably relative to the size of other regions. That size is probably suboptimal when it comes to other functions and there are tradeoffs going on. The press release seems to be assuming that thinner is bad automatically.
- MikkoFinell 9y ago
  Yeah I do believe it makes a difference, because brain atrophy is linked to a range of cognitive impairments, for example strongly correlated with Alzheimer's disease. But you're right on the last point though, about assuming thinner is bad automatically. Heavy and chronic cannabis usage has been linked to an increase in density, but a reduction in total volume, which resulted in a slight overall decrease in IQ. The takeaway from all this being: Cognitive function is affected by a variety of parameters and we got more neuroscience to do still.
  - nonbel 9y ago
    >"Heavy and chronic cannabis usage has been linked to an increase in density, but a reduction in total volume, which resulted in a slight overall decrease in IQ." I wouldn't put much stock into these type of non-quantitative explanations wherein "this makes that go up which makes this go down", etc. Usually the researchers measure a bunch of different things, analyze the data in a bunch of different ways, and only publish whatever is "significant". This works at the single lab level and multi-lab level since the "non-significant" results are considered boring and don't get published. By "usually" I mean this is standard behavior. You can call it p-hacking, file drawer effect, and more recently "forking paths": http://andrewgelman.com/2017/12/29/forking-paths-plus-lack-theory-no-reason-believe/ http://andrewgelman.com/2017/12/29/forking-paths-plus-lack-t... >"Cognitive function is affected by a variety of parameters and we got more neuroscience to do still." Sure, that was the case before these studies were done too though.
- always_good 9y ago
  > The press release seems to be assuming that thinner is bad automatically. Well, it's not unreasonable. I don't know of any literature that says that a sedentary lifestyle is healthy for the brain, but a lot of it has found the opposite. I would like to avoid anything that reproduces the neurological effects of being sedentary.
  - nonbel 9y ago
    >"I don't know of any literature that says that a sedentary lifestyle is healthy for the brain" Here are two I found: >"More sedentary behavior was strongly predictive of more depressive symptomatology and, unexpectedly, of better cognitive performance." https://journals.humankinetics.com/doi/pdf/10.1123/japa.13.3.294 https://journals.humankinetics.com/doi/pdf/10.1123/japa.13.3... >"Self-reported sedentary behavior was related to better performance on one cognitive task (trails A; p < .05)." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4861254/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4861254/ Obviously these are "undesirable" results, so you have to look a little bit deeper to find them.
- jonhendry18 9y ago
  They're using "thinning" in the sense of "thinning hair". Generally lower density would imply fewer neurons and fewer connections. Since this is happening late in life, rather than during the pruning stage of development in youth, this would tend not to be a good thing.
  - nonbel 9y ago
    Why do you think this?
nonbel 9y ago
It looks like this was the study (emphasis added): Sedentary behavior associated with reduced medial temporal lobe thickness in middle-aged and older adults http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0195549 http://journals.plos.org/plosone/article?id=10.1371/journal....
- MikkoFinell 9y ago
  As far as I know, thickness is a synonym for density. Quote from the article: To calculate thickness, for each gray matter voxel we computed the distance to the closest non-gray matter voxel. In 2D-space, for each voxel, we took the maximum distance value of the corresponding 3D voxels across all layers and multiplied by two. Mean thickness in each subregion was calculated by averaging thickness of all 2D voxels within each region of interest.
  - nonbel 9y ago
    I think it is literal thickness since they report the results in units of millimeters. (eg figure 2)
    - MikkoFinell 9y ago
      The authors explained quite carefully what they mean by thickness, as per the quote I provided. They are measuring the average distance between voxels in MRI scans. Since thickness is a measure of average distance in this case, it makes sense to provide it in millimeters like in the figure you mentioned. If you read the article, the authors make distinction between volume, surface area, and thickness under the "Discussion" part [0]. [0] http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0195549#sec010 http://journals.plos.org/plosone/article?id=10.1371/journal....
      - nonbel 9y ago
        Their description sounds like literal thickness. They took the distance to the border (white matter) of all the points in a region. This is like the radius of a circle, and then multiplied by 2, to get a value like the diameter of a circle. It does sound like quite a convoluted process but I don't see where you are getting anything like "density" from that description. Also, reading the methods closer makes me think these are cherry picked results: >"Once segmentation is complete, the original images are interpolated by a factor of 7, resulting in a final voxel size of 0.39 × 0.39 × 0.43 mm. Next, up to 18 connected layers of gray matter are grown out from the boundary of white matter, using a region-expansion algorithm to cover all pixels defined as gray matter." Why 7? Why 18? These magic numbers shouldn't be there without some kind of sensitivity analysis. Finally, you can see the dimensions of the voxels is something determined by their methods, not a property of the brains.

Where are you seeing that? Also, fine density/volume/mass, does it really change the argument? There is some optimal size for this region when it comes to certain functions, probably relative to the size of other regions. That size is probably suboptimal when it comes to other functions and there are tradeoffs going on. The press release seems to be assuming that thinner is bad automatically.

MikkoFinell 9y ago
Yeah I do believe it makes a difference, because brain atrophy is linked to a range of cognitive impairments, for example strongly correlated with Alzheimer's disease. But you're right on the last point though, about assuming thinner is bad automatically. Heavy and chronic cannabis usage has been linked to an increase in density, but a reduction in total volume, which resulted in a slight overall decrease in IQ. The takeaway from all this being: Cognitive function is affected by a variety of parameters and we got more neuroscience to do still.
- nonbel 9y ago
  >"Heavy and chronic cannabis usage has been linked to an increase in density, but a reduction in total volume, which resulted in a slight overall decrease in IQ." I wouldn't put much stock into these type of non-quantitative explanations wherein "this makes that go up which makes this go down", etc. Usually the researchers measure a bunch of different things, analyze the data in a bunch of different ways, and only publish whatever is "significant". This works at the single lab level and multi-lab level since the "non-significant" results are considered boring and don't get published. By "usually" I mean this is standard behavior. You can call it p-hacking, file drawer effect, and more recently "forking paths": http://andrewgelman.com/2017/12/29/forking-paths-plus-lack-theory-no-reason-believe/ http://andrewgelman.com/2017/12/29/forking-paths-plus-lack-t... >"Cognitive function is affected by a variety of parameters and we got more neuroscience to do still." Sure, that was the case before these studies were done too though.
always_good 9y ago
> The press release seems to be assuming that thinner is bad automatically. Well, it's not unreasonable. I don't know of any literature that says that a sedentary lifestyle is healthy for the brain, but a lot of it has found the opposite. I would like to avoid anything that reproduces the neurological effects of being sedentary.
- nonbel 9y ago
  >"I don't know of any literature that says that a sedentary lifestyle is healthy for the brain" Here are two I found: >"More sedentary behavior was strongly predictive of more depressive symptomatology and, unexpectedly, of better cognitive performance." https://journals.humankinetics.com/doi/pdf/10.1123/japa.13.3.294 https://journals.humankinetics.com/doi/pdf/10.1123/japa.13.3... >"Self-reported sedentary behavior was related to better performance on one cognitive task (trails A; p < .05)." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4861254/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4861254/ Obviously these are "undesirable" results, so you have to look a little bit deeper to find them.
jonhendry18 9y ago
They're using "thinning" in the sense of "thinning hair". Generally lower density would imply fewer neurons and fewer connections. Since this is happening late in life, rather than during the pruning stage of development in youth, this would tend not to be a good thing.
- nonbel 9y ago
  Why do you think this?

nonbel 9y ago

It looks like this was the study (emphasis added): Sedentary behavior associated with reduced medial temporal lobe thickness in middle-aged and older adults http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0195549 http://journals.plos.org/plosone/article?id=10.1371/journal....

MikkoFinell 9y ago
As far as I know, thickness is a synonym for density. Quote from the article: To calculate thickness, for each gray matter voxel we computed the distance to the closest non-gray matter voxel. In 2D-space, for each voxel, we took the maximum distance value of the corresponding 3D voxels across all layers and multiplied by two. Mean thickness in each subregion was calculated by averaging thickness of all 2D voxels within each region of interest.
- nonbel 9y ago
  I think it is literal thickness since they report the results in units of millimeters. (eg figure 2)
  - MikkoFinell 9y ago
    The authors explained quite carefully what they mean by thickness, as per the quote I provided. They are measuring the average distance between voxels in MRI scans. Since thickness is a measure of average distance in this case, it makes sense to provide it in millimeters like in the figure you mentioned. If you read the article, the authors make distinction between volume, surface area, and thickness under the "Discussion" part [0]. [0] http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0195549#sec010 http://journals.plos.org/plosone/article?id=10.1371/journal....
    - nonbel 9y ago
      Their description sounds like literal thickness. They took the distance to the border (white matter) of all the points in a region. This is like the radius of a circle, and then multiplied by 2, to get a value like the diameter of a circle. It does sound like quite a convoluted process but I don't see where you are getting anything like "density" from that description. Also, reading the methods closer makes me think these are cherry picked results: >"Once segmentation is complete, the original images are interpolated by a factor of 7, resulting in a final voxel size of 0.39 × 0.39 × 0.43 mm. Next, up to 18 connected layers of gray matter are grown out from the boundary of white matter, using a region-expansion algorithm to cover all pixels defined as gray matter." Why 7? Why 18? These magic numbers shouldn't be there without some kind of sensitivity analysis. Finally, you can see the dimensions of the voxels is something determined by their methods, not a property of the brains.