Specfiying fixed and random factors using Linear MIXED model in SPSS

Hello all,

I'm a bit stuck at specifying my model using MIXED in SPSS (to account for the non-independence of data/MLM structure). My goal is to figure out the levels at which the different data sit so that I can run the most appropriate model.

Here's a description of the setup/data: Participants' neural activity (main outcome variable) was recorded after being randomly assigned to one of two conditions. Each participant i) performed the task and observed others perform (perform vs. observe), and ii) interacted separately with ingroup and outgroup members (ingroup vs. outgroup).

I'm not sure which levels to specify the data, and consequently, which factors are fixed or random.

Any sort of nudge would be much appreciated, and I can always provide more details information if it's necessary.