An interesting interview on the practice of meditating on the Four Brahmaviharas or the four divine abodes (cattāri brahmavihārā in Pail, चत्वारि ब्रह्मविहाराः in Sanskrit.)
In meditation practice, as it is commonly taught in the West, the breath is the sole object of meditation, but in this interview Bhikkhu Anālayo points out the effects of making the brahmaviharas the focus.
These are: mettā (मैत्री) or loving-kindness, karuṇā (करुणा) or compassion, muditā (मुदिता) or empathetic joy , upekkhā (उपेक्षा) or equanimity
Of all of these, upekkhā (equanimity) is the closest to the core of Zen because it is a precondition of the other brahmaviharas in many ways. It is a balanace of mind that neither likes nor dislikes.