The superior temporal sulcus (STS) is the sulcus separating the superior temporal gyrus from the middle temporal gyrus in the temporal lobe of the brain. A sulcus (plural sulci) is a deep groove that curves into the largest part of the brain, the cerebrum, and a gyrus (plural gyri) is a ridge that curves outward of the cerebrum.
The STS is located under the lateral fissure, which is the fissure that separates the temporal lobe, parietal lobe, and frontal lobe. The STS has an asymmetric structure between the left and right hemisphere, with the STS being longer in the left hemisphere, but deeper in the right hemisphere. This asymmetrical structural organization between hemispheres has only been found to occur in the STS of the human brain.
The superior temporal sulcus also activates when hearing human voices. It is thought to be a source of sensory encoding linked to motor output through the superior parietal-temporal areas of the brain inferred from the time course of activation. The conclusion of pertinence to vocal processing can be drawn from data showing that the regions of the STS are more active when people are listening to vocal sounds rather than non-vocal environmentally based sounds and corresponding control sounds, which can be scrambled or modulated voices. These experimental results indicate the involvement of the STS in the areas of speech and language recognition.
The majority of studies find it is the middle to the posterior portion of the STS that is involved in phonological processing, with bilateral activation indicated though including a mild left hemisphere bias due to greater observed activation. However, the role of the anterior STS in the ventral pathway of speech comprehension and production has not been ruled out. Evidence for the involvement of the middle portion of the STS in phonological processing comes from repetition-suppression studies, which use fMRI to pinpoint areas of the brain responsible for specialized stimulus involvement by habituating the brain to the stimulus and recording differences in stimulation response. The resulting pattern showed expected results in the middle portion of the STS.
Studies using fMRI analysis to measure superior temporal sulcus activation have found that phonemes, words, sentences, and phonological cues all lead to increased activation throughout a posterior-anterior axis in the temporal lobe. This pattern of activation, which most frequently occurs in the left hemisphere, has been termed the ventral stream of speech perception. Many studies consistently indicate that the superior temporal sulcus activation is associated with the interpretation of phonological signals. Although present research suggest that the left hemisphere of the superior temporal sulcus and its associated left ventral stream plays a role in phonological processing, the right hemisphere of the superior temporal sulcus has been connected to the perception of voice and the prosody of speech.
According to the audiological pathway model supplied by Hickok and Poeppel, after the spectrotemporal analysis conducted by the auditory cortex, the STS is responsible for interpretation of vocal input through the phonological network. This implication is shown in the activation of the region in tasks of speech perception and processing, which necessarily involves access to and continuance of phonological information. By manipulating the interactions of phonological data, represented by the provision of words with high or low neighborhood density (words associated with many or few other words), the fluctuation of activity of the STS region can be seen. This changing activation links the STS with the phonological pathway.