[pymvpa] Interpreting representation similarity results

[Nick Oosterhof]

> On 20 Aug 2015, at 10:59, Vadim Axel <axel.vadim at gmail.com> wrote:
> 
> Very simple question: to what extent representation similarity can be interpreted as similarity of cognitive processing? 
> 
> Consider a toy sample, where I have two experiments. In Exp.1 there is task A and baseline1. In Exp.2 there there is task B  and baseline2. For each experiment, I generate t-contrasts:  A > baseline1 and  B > baseline2. To check for similarity between tasks A and B, I can run conjunction analysis (spatial overlap). For stronger evidence, I can for each experiment, extract t-values for some predefined ROIs. Then, I run Pearson correlation across voxels within a ROI. Using across subjects statistics I can show that in some ROIs the correlation between experiments is above 0. Can this result be interpreted, as having similarity of cognitive processing during two tasks?

It would indicate that *something* is similar (at a pattern level) between the two tasks. You may possibly interpret this as cognitive processing, but cognitive processing is a rather broad concept. Pattern similarity can arise through a variety of different mechanisms, including trivial ones.

> Also, does someone know about papers that examined similarity between experiments using a contrast (and not Haxby_2001_like_style of patterns of single faces vs cats). In my case, Exps 1 and 2 have very different designs, so A and B cannot be compared directly. In general, good references for citing are highly appreciated.

This may be considered as shameless self-promotion, but I have done some work on executing versus observing different manual actions [1], and imagery and execution/observation of such actions [2]. 

[1] Oosterhof, N. N., Wiggett, A. J., Diedrichsen, J., tipper, S. P. & Downing, P. E. Surface-based information mapping reveals crossmodal vision-action representations in human parietal and occipitotemporal cortex. J. Neurophysiol. 104, 1077–1089 (2010).
[2] Oosterhof, N. N., Tipper, S. P. & Downing, P. E. Visuo-motor imagery of specific manual actions: A multi-variate pattern analysis fMRI study. Neuroimage 63, 262–271 (2012).