Oxidation of 1,4,5,8-tetrahydronaphthalene by meta-chloroperoxybenzoic acid
As the following figure indicates, the terminal oxygen in a peracid (the oxygen involved in the epoxidation) has a partial positive charge ; therefore this oxygen is electrophilic.  



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The central double bond in your compound is tetrasubsituted, the lateral bonds are only disubstituted with alkyl substituents (remember that alkyl groups are electron donating).  Consequently the central double bond is the most electron rich and will be the preferred site of attack for the electrophilic oxygen.