Superior Temporal Sulcus

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 STS has been shown to produce strong responses when subjects perceive stimuli in research areas that include theory of mind, biological motion, faces, voices, and language.

Language Processing

Spoken language processing

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.

Sign language processing

Research shows that the Broca's area of the brain is activated during sign language production and processing. Although Broca's area is found in the frontal lobe, it receives connection from the superior temporal gyrus, including the STS. Native signers are people who learned and have been using sign language, such as American Sign Language (ASL), from birth, and/or use it as their first language. They often learn sign language from their parents and continue its use throughout their lifetime. Sign language activates language regions of the brain, including the STS. There have been studies that show activation of the STS while deaf and hearing native signers perceive sign language, suggesting the STS is tied to the linguistic processing aspect of sign language. Studies also show that there is greater activation of the middle STS in both deaf and hearing signers who acquired ASL earlier than those who acquired it later.

Social processing

Studies reveal multiple social processing capabilities. Research has documented activation in the STS as a result of five specific social inputs, and thus the STS is assumed to be implicated in social perception. It showed increased activation related to: theory of mind (false belief stories versus false physical stories) voices versus environmental sounds, stories versus nonsense speech, moving faces versus moving objects, and biological motion. It is involved in the perception of where others are gazing (joint attention) and is important in determining where others' emotions are being directed.

Theory of mind

Neuroimaging studies examining the theory of mind, otherwise known as the ability to attribute mental states to others, have identified the posterior superior temporal sulcus of the right hemisphere as being involved in its processing. Activation of this region in the theory of mind has been found to be best predicted by independent ratings from other groups of participants, or more specifically, how much each item in the study made them consider the protagonist’s point of view.Dodell-Feder D, Koster-Hale J, Bedny M, Saxe R. fMRI item analysis in a theory of mind task. Neuroimage. 2011;55(2):705-712. doi:10.1016/j.neuroimage.2010.12.040. Reports noted in other studies suggest a number of inconsistencies with the localization of theory of mind processing, such as the middle and anterior portions of the superior temporal sulcus having increased activation in response to theory of mind tasks. Thus, further research is needed to expand upon the precise functional role of the superior temporal sulcus in the perception of theory of mind.

Face perception

A recent study identified a region of the posterior superior temporal sulcus that is preferentially activated in the interpretation of facial expressions.Direito B, Lima J, Simões M, et al. Targeting dynamic facial processing mechanisms in superior temporal sulcus using a novel fMRI neurofeedback target. Neuroscience. 2019;496:97-108. doi:10.1016/j.neuroscience.2019.02.024 Similarly, another study found that transcranial magnetic stimulation disrupted the neural response to faces, but not the neural response to bodies or objects.Pitcher D, Japee S, Rauth L, Ungerleider LG. The Superior Temporal Sulcus Is Causally Connected to the Amygdala: A Combined TBS-fMRI Study. J Neurosci. 2017;37(5):1156–1161. doi:10.1523/JNEUROSCI.0114-16.2016 The patterns of activations found in this study suggests that facial information is processed by projections in the right hemisphere from the posterior superior temporal sulcus, through the anterior superior temporal sulcus, and into the amygdala. Another study showed that the resting-state functional connectivity between the right posterior superior temporal sulcus, the right occipital face area, early visual cortex, and bilateral superior temporal sulcus was positively correlated with each subject’s ability to recognize facial expression.Wang X, Song Y, Zhen Z, Liu J. Functional integration of the posterior superior temporal sulcus correlates with facial expression recognition. Hum Brain Mapp. 2016;37(5):1930-1940. doi:10.1002/hbm.23145.

Face-voice audiovisual integration

Many studies have suggested that the posterior superior temporal sulcus is associated the crossmodal binding of auditory and visual stimuli. The activation of this posterior portion of the superior temporal sulcus was reported in the detection of audio-visual incongruences and in voice perception. The posterior superior temporal sulcus has also been shown to be preferentially activated by lip reading.Uno T, Kawai K, Sakai K, et al. Dissociated roles of the inferior frontal gyrus and superior temporal sulcus in audiovisual processing: top-down and bottom-up mismatch detection. PLoS One. 2015;10(3):e0122580. doi:10.1371/journal.pone.0122580 An area of the right posterior superior temporal sulcus was characterized by a recent study by a stronger response to audiovisual stimuli compared to that of auditory or visual stimuli alone.Watson R, Latinus M, Charest I, Crabbe F, Belin P. People-selectivity, audiovisual integration and heteromodality in the superior temporal sulcus. Cortex. 2014;50(100):125–136. doi:10.1016/j.cortex.2013.07.011 This study also identified this same region to preferentially activated in the processing of stimuli associated with people, such as faces and voices. Another fMRI study found that the neural representations of audiovisual integration, non-verbal emotional signals, voice sensitivity, and face sensitivity are all localized to separate regions of the superior temporal sulcus.Kreifelts B, Ethofer T, Shiozawa T, Grodd W, Wildgruber D. Cerebral representation of non-verbal emotional perception: fMRI reveals audiovisual integration area between voice- and face-sensitive regions in the superior temporal sulcus. Neuropsychologia. 2009;47(14):3059-3066. doi:10.1016/j.neuropsychologia.2009.07.001. Likewise, this study