Vishuddha, the especially pure, is described in the Sat-Cakra-Nirupana as a lotus of sixteen petals at the throat, bearing the sixteen Sanskrit vowels. Within it is a white circle, the region of akasha, ether or space, with the seed syllable HAM seated on a white elephant; the presiding deity is Sadashiva, in the half-male half-female form, with the shakti Shakini. The text says that one who meditates here becomes a great sage, eloquent and wise, and knows the four Vedas. The assignment of the vowels to this centre and of the consonants to the others reflects a much broader Indian idea, that the Sanskrit alphabet is itself a structure of the cosmos, a doctrine the atlas records separately under the matrika letters.
The throat centre carries a further, independent association in Tibetan Vajrayana practice: within the Six Yogas of Naropa, the throat is generally the centre used in dream yoga, the practice of maintaining awareness within the dream state, on the reasoning that the throat governs speech and the subtle currents associated with the dream body in that tradition's own physiology. As with the other Vajrayana correspondences noted elsewhere in this family of articles, this is an independent Buddhist tantric development rather than a variant reading of Purnananda's Hindu text, and the two should not be treated as describing the same underlying system.
Sources The Serpent PowerSir John Woodroffe (Arthur Avalon) with The Origins of Yoga and Tantra: Indic Religions to the Thirteenth CenturyGeoffrey Samuel