[pymvpa] Interpreting representation similarity results

[Nick Oosterhof]

> 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.