Mixed-effects modelling of response data (multiple stimuli&subjects) in Matlab

#1
Mixed-effects modelling of response data in Matlab

I have preference ratings (integers from 1-to-7) for k=80 stimuli, obtained from N=30 subjects. I want to use a mixed-effects model to test how well the following variables - as fixed effects - can predict the responses(ratings):
Predictor 1: Parncutt value, describing each stimulus
Predictor 2: GMSI score, describing each subject
Subject and stimulus number are to be considered random effects.

I used the Matlab command:
lme = fitlme(tbl,formula)
where the data is in the tbl attachment, and where:
formula = 'ratings_pls ~ Parncutt + GMSI + (1|subjectNr) + (1|stimulusNr)';

I'm not sure whether I should also have a term in the model for
A) random slopes: if allowing for differences between subjects and stimuli (random intercepts in the model), then why not also allow the slopes for both factors to be random?
B) interaction between any of the subject-wise factors (SubjectNr and GMDI) and any of the stimulus-wise factors (StimulusNr and Parncutt)

Many thanks for any help - very much appreciated!
 
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