How do Northern Tai dialects differ from other Tai languages in their use of aspirated consonants?

In the study of Tai languages, Northern Tai dialects are distinguished from other groups by their phonological development regarding aspirated consonants.

Unlike other Tai language groups, Northern dialects have lost the distinction between aspirated and unaspirated voiceless stops. While aspiration—the puff of breath accompanying a consonant—is a feature maintained in other Tai dialects, it is absent in the Northern group. It is important to note, however, that this feature may be reintroduced in some Northern dialects through later linguistic borrowing or secondary developments.1