The 'trough effect': an ultrasound study

Bilabial stops often show a lowering of the tongue in symmetrical VCV sequences. The causes of this phenomenon, sometimes called the 'trough effect', are unknown. However, it could have important implications for the study of time-varying aspects of speech events. Ultrasound is a non-invasive technique that has allowed us to image the shape of the tongue in real time and measure the actual tongue displacement that occurs in the C of a VCV sequence. Five repetitions of symmetrical V(1)CV(2) sequences with the bilabial stops /b, p/ were obtained from 10 British English speakers. Results showed not only differences in the direction and degree of the tongue displacement but also differences in the tongue contour configuration between subjects. This study demonstrates the effectiveness of ultrasound as a technique in phonetic research, making possible the analysis of tongue surface movement for large amounts of data from multiple subjects.