Problems in the inner ear — where head position is perceived — are the most common cause of nystagmus, usually accompanied by vertigo. (source)
According to Macmillan Dictionary (marked as countable) and Cambridge Dictionary (most definitions say [countable] or [C usually singular] and but [C/U] also appears. All examples are countable usages), "position" is a count noun. Why is there no definite article before "head position" in this sentence? It seems the context calls for specificity. I thought the sentence should read:
Problems in the inner ear — where the head position is perceived — are the most common cause of nystagmus, usually accompanied by vertigo.