  Arabic is a Semitic language which is widely spoken with many dialects. Given
the success of pre-trained language models, many transformer models trained on
Arabic and its dialects have surfaced. While these models have been compared
with respect to downstream NLP tasks, no evaluation has been carried out to
directly compare the internal representations. We probe how linguistic
information is encoded in Arabic pretrained models, trained on different
varieties of Arabic language. We perform a layer and neuron analysis on the
models using three intrinsic tasks: two morphological tagging tasks based on
MSA (modern standard Arabic) and dialectal POS-tagging and a dialectal
identification task. Our analysis enlightens interesting findings such as: i)
word morphology is learned at the lower and middle layers ii) dialectal
identification necessitate more knowledge and hence preserved even in the final
layers, iii) despite a large overlap in their vocabulary, the MSA-based models
fail to capture the nuances of Arabic dialects, iv) we found that neurons in
embedding layers are polysemous in nature, while the neurons in middle layers
are exclusive to specific properties.
