Sulcalization
Sulcalization, in phonetics, is the pronunciation of a sound with a deep, longitudinal concavity down the back of the tongue, roughly opposite of the uvula. This is accomplished by raising the sides of the dorsum, and leaving a hollow along the mid-line.
This articulation has typically been associated with rhotics such as a 'bunched' or 'molar' and r-colored vowels, as well as 'dark' or 'throaty' quality sounds, either more velar-like or more pharyngeal-like.
No spoken language is known to make a phonemic distinction between sulcalized and ordinary vowels; though it has been reported that for some speakers of Received Pronunciation, the vowel, which is normally described as rounded, is pronounced with spread lips, and is instead given its characteristic quality through a "hollowing or sulcalization of the tongue-body." One scholar has also suggested that the vowel in the RP pronunciation of words like bird, typically transcribed, is actually a sulcal schwa, retaining the sulcality of the original rhotic consonant. Accordingly, the realization of the -element of the centring diphthongs,, in words such as near, pure and scare, is interpreted as the product of a loss of sulcality.
Grooved fricative
Some linguists have referred to grooved fricatives, a similar but distinct articulatory concept, as sulcalized, though this should not be confused with the more common definition described in the section above. Contrasting with slit fricatives, which are pronounced with the tongue flat, grooved fricatives also involve forming a groove down the center of the tongue.Unlike the more common definition of sulcalization, grooved fricatives involve the sides of the tongue focusing the airstream on the teeth, producing a more intense sound, typically associated with sibilants. J. C. Catford observed that the degree of tongue grooving differs between places of articulation as well as between languages; however, no language is known to contrast sibilants based purely on the presence or absence of tongue grooving.
Ultrasound imaging has shown to exhibit tongue grooving in English, despite being typically regarded as slit.