[pymvpa] Interpreting representation similarity results

[Vadim Axel]

I think we understood something different by "trivial" term :)

I observed correlated neural activity for two tasks in some region. For me
one trivial explanation would be for example, that all the brain increases
its activity during the task (e.g., general arousal or brain vasculature).
To rule out this, I show that correlation is only in a specific region.
Another trivial explanation, is that anatomical connectivity of my specific
region is that it is always activated similarly. To rule out this one, I
show that there is a task which does not result in correlated activity in
this region.

The trivial you refer to are the not-well-controlled experimental
manipulation. So, for example, if I hypothetically get correlated activity
in the FFA for white faces and white clocks, I cannot say that cognitive
processing of  faces and clocks is similar, because it can be processing of
white color which is similar. However, the latter one is still "cognitive
processing", and it was similar between tasks. It was just not the
processing I was interested. Clearly, the control conditions here need to
be very specific.

Makes sense? Do you have more trivial explanations of first type?



On Fri, Aug 21, 2015 at 11:59 AM, Nick Oosterhof <
n.n.oosterhof at googlemail.com> wrote:

>
> > On 20 Aug 2015, at 14:25, Vadim Axel <axel.vadim at gmail.com> wrote:
> >
> > Suppose, I can similarity between two tasks  a) only in specific region,
> but not other regions and b) do not get similarity in this region when I
> use some control task. Do you see a trivial, non-cognitive explanation to
> this?
>
> With respect to the control task:
>
> - how specific you can be about inferences of the main task versus control
> depends on how good the control is. *Anything* that is different between
> the main task and the control task could potentially explain such effects.
> This may include differences in low-level and high-level features for the
> stimuli, memory and attention demands, task difficulty, predictability of
> conditions, etc. As you did not specify what types of tasks you used, I
> cannot be more specific about potential trivial explanations.
>
> - showing that task A gives a significant effect but task C (control) does
> not, is rather weak and uninteresting. This can be a case of p=0.049 versus
> p=0.051 (with alpha=0.05). More informative is whether task A shows a
> stronger effect (similarity, in your case) than task C, for example through
> a paired t-test.
>
> - interpreting BOLD signals in terms of cognitive mechanisms is not
> straightforward. It may be possible that in a region, certain neural
> processing is not detectable in the BOLD signal, even when single unit
> recordings show that such processing does take place there. Finding BOLD
> pattern differences between two tasks clearly suggests differences in
> processing at the neural level, but the step to cognitive mechanisms is
> more difficult.
>
> >
> > Thanks for refs. So, you show that vision and action have similar neural
> representation.
>
> It goes a step further. The first reference shows that for two different
> actions A and B, the neural pattern of A when performed (executed) is more
> similar to neural pattern when A is observed than to the neural pattern
> when B is observed. In other words, it shows cross-modal (across vision and
> execution), action-specific patterns. The second refs shows a similar
> effect for imagery versus execution and observation.
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