Opposing serial effects of stimulus and choice in speech perception scale with context variability

Here, we disentangled stimulus and choice history revealing opposing effects on perception: Choice history attracts and stimulus history repels current speech perception. The repulsive effect of stimulus history was only revealed once choice history was accounted for. On top of that, stability in the speaker context strengthens the repulsive effects of stimulus history.

In this study, we investigated serial effects on the perception of auditory vowel stimuli across three experimental setups with different degrees of context variability. Aligning with recent findings in visual perception, our results confirm the existence of two distinct processes in serial dependence: a repulsive sensory effect coupled with an attractive decisional effect. Importantly, our study extends these observations to the auditory domain, demonstrating parallel serial effects in audition. Furthermore, we uncover context variability effects, revealing a linear pattern for the repulsive perceptual effect and a quadratic pattern for the attractive decisional effect. These findings support the presence of adaptive sensory mechanisms underlying the repulsive effects, while higher-level mechanisms appear to govern the attractive decisional effect. The study provides valuable insights into the interplay of attractive and repulsive serial effects in auditory perception and contributes to our understanding of the underlying mechanisms. 

Ufer, C., & Blank, H., (2024) Opposing serial effects of stimulus and choice in speech perception scale with context variability. iScience.

The pupil dilation response as an indicator of visual cue uncertainty and auditory outcome surprise

Expectations can be induced by cues that indicate the probability of following sensory events. The information provided by cues may differ and hence lead to different levels of uncertainty about which event will follow. In this experiment, we employed pupillometry to investigate whether the pupil dilation response to visual cues varies depending on the level of cue-associated uncertainty about a following auditory outcome.

Also, we tested whether the pupil dilation response reflects the amount of surprise about the subsequently presented auditory stimulus. In each trial, participants were presented with a visual cue (face image) which was followed by an auditory outcome (spoken vowel). After the face cue, participants had to indicate by keypress which of three auditory vowels they expected to hear next. We manipulated the cue-associated uncertainty by varying the probabilistic cue-outcome contingencies: One face was most likely followed by one specific vowel (low cue uncertainty), another face was equally likely followed by either of two vowels (intermediate cue uncertainty), and the third face was followed by all three vowels (high cue uncertainty). Our results suggest that pupil dilation in response to task-relevant cues depends on the associated uncertainty, but only for large differences in the cue-associated uncertainty. Additionally, in response to the auditory outcomes, the pupil dilation scaled negatively with the cue-dependent probabilities, likely signaling the amount of surprise. 

Becker, J., Viertler, M., Korn, C., & Blank, H., (2024) The pupil dilation response as an indicator of visual cue uncertainty and auditory outcome surprise. European Journal of Neuroscience, 1-16.

Pupil diameter as an indicator of sound pair familiarity after statistically structured auditory sequence

Inspired by recent findings in the visual domain, we investigated whether the stimulus-evoked pupil dilation reflects temporal statistical regularities in sequences of auditory stimuli. Our findings suggest that pupil diameter may serve as an indicator of sound pair familiarity but does not invariably respond to task-irrelevant transition probabilities of auditory sequences.

We conducted two preregistered pupillometry experiments (experiment 1, n = 30, 21 females; experiment 2, n = 31, 22 females). In both experiments, human participants listened to sequences of spoken vowels in two conditions. In the first condition, the stimuli were presented in a random order and, in the second condition, the same stimuli were presented in a sequence structured in pairs. The second experiment replicated the first experiment with a modified timing and number of stimuli presented and without participants being informed about any sequence structure. The sound-evoked pupil dilation during a subsequent familiarity task indicated that participants learned the auditory vowel pairs of the structured condition. However, pupil diameter during the structured sequence did not differ according to the statistical regularity of the pair structure. This contrasts with similar visual studies, emphasizing the susceptibility of pupil effects during statistically structured sequences to experimental design settings in the auditory domain. 

Becker, J., Korn, C.W. & Blank, H. (2024) Pupil diameter as an indicator of sound pair familiarity after statistically structured auditory sequence. Scientific Reports, 14, 8739.

Multivariate analysis to unravel predictive processes in speech

Speech perception is heavily influenced by our expectations about what will be said. In this review, we discuss the potential of multivariate analysis as a tool to understand the neural mechanisms underlying predictive processes in speech perception.

Speech perception depends on speaker priors

When different speakers articulate identical words, the physical properties of the produced sounds can vary substantially. Fundamental frequency (F0) and formant frequencies (FF), the two main parameters that discriminate between voices, also influence vowel perception. In this study, we investigated how we use speaker prior information when decoding the speech stream.
While, in everyday circumstances, we have no difficulties decoding a speaker’s intended word from the audio stream, listeners benefit from a general familiarity with target speakers when comprehending speech. We used a twofold approach to investigate the influence of speaker expectations on vowel perception. On the one hand, we showed how shifts in FF and F0 influenced vowel perception depending on the context, i.e., when the FF and F0 voice characteristics of the speaker could or could not be anticipated. On the other hand, we observed that different expected speaker FF and F0 characteristics influence the perception of identical, unshifted vowels in a contrastive manner. In conclusion, our findings support the view that knowledge about a speaker’s voice characteristics influences vowel perception.

Krumbiegel J, Ufer C, Blank H (2022) Influence of voice properties on vowel perception depends on speaker context. The Journal of the Acoustical Society of America 152:820–834.

Prediction errors during speech perception

Perception inevitably depends on combining sensory input with prior expectations, particularly for the identification of degraded input. However, the underlying neural mechanism by which expectations influence sensory processing is unclear. Predictive Coding suggests that the brain passes forward the unexpected part of the sensory input while expected properties are suppressed (Prediction Error). However, evidence to rule out the opposite and perhaps more intuitive mechanism, in which the expected part of the sensory input is enhanced (Sharpening), has been lacking.
We investigated the neural mechanisms by which sensory clarity and prior knowledge influence the perception of degraded speech. A univariate measure of brain activity obtained from functional magnetic resonance imaging (fMRI) was in line with both neural mechanisms (Prediction Error and Sharpening). However, combining multivariate fMRI measures with computational simulations allowed us to determine the underlying mechanism. Our key finding was an interaction between sensory input and prior expectations: For unexpected speech, increasing speech clarity increased the amount of information represented in sensory brain areas. In contrast, for speech that matched prior expectations, increasing speech clarity reduced the amount of this information. Our observations were uniquely simulated by a model of speech perception that included Prediction Errors.

Blank, H. & Davis, M. (2016). Prediction errors but not sharpened signals simulate multivoxel fMRI patterns during speech perception, PLOSBiology, 14(11) 

Slips of the Ear: When Knowledge Deceives Perception

The ability to draw on experience is important to keep up with a conversation, especially in noisy environments where speech sounds are hard to hear. However, these prior expectations can sometimes mislead listeners; convincing them that they heard something that a speaker did not actually say.
To investigate the neural underpinnings of speech misperception, we presented participants with pairs of written and degraded spoken words that were either identical, clearly different or similar-sounding. Reading and hearing similar-sounding words (like kick followed by pick), led to frequent misperception.

Using fMRI, we found that misperception was associated with reduced activity in the left superior temporal sulcus a brain region critical for processing speech sounds. Furthermore, when the perception of speech was more successful, this brain region represented the difference between prior expectations and heard speech (like the initial k/p in kick-pick).

Blank, H., Spangenberg, M., & Davis, M. (2018). Neural Prediction Errors Distinguish Perception and Misperception of Speech. The Journal of Neuroscience,  38 (27) 6076-6089.

Prediction error processing and sharpening of expected information across the face-processing hierarchy

The perception and neural processing of sensory information are strongly influenced by prior expectations. The integration of prior and sensory information can manifest through distinct underlying mechanisms: focusing on unexpected input, denoted as prediction error (PE) processing, or amplifying anticipated information via sharpened representation. In this study, we employed computational modeling using deep neural networks combined with representational similarity analyses of fMRI data to investigate these two processes during face perception.
Participants were cued to see face images, some generated by morphing two faces, leading to ambiguity in face identity. We show that expected faces were identified faster and perception of ambiguous faces was shifted towards priors. Multivariate analyses uncovered evidence for PE processing across and beyond the face-processing hierarchy from the occipital face area (OFA), via the fusiform face area, to the anterior temporal lobe, and suggest sharpened representations in the OFA. Our findings support the proposition that the brain represents faces grounded in prior expectations.

Garlichs, A., Blank, H. (2024). Prediction error processing and sharpening of expected information across the face-processing hierarchy. Nature Communications, 15, 3407.

Representations of face priors

In everyday life, we face our environment with several prior expectations about what we are going to encounter, for example, whom we are going to see most likely at a certain location. These expectations have to be weighted according to their probability, e.g., a student regularly entering our office will be expected with a higher probability than a shy colleague we rarely meet. In this study, we show that the human brain weights face priors according to their certainty in high-level face-sensitive regions.
We used functional resonance imaging (fMRI) in combination with multivariate methods to test whether the strength of face expectations can be detected alongside expected face identity in face-sensitive regions (i.e., OFA, FFA, and aTL) of the human brain. Participants used scene cues to predict face identities with different probabilities. We found evidence that representations of expected face identities were weighted according to their probability in the high-level face-sensitive aTL.

Blank, H., Alink, A., & Büchel, C. (2023). Multivariate functional neuroimaging analyses reveal that strength-dependent face expectations are represented in higher-level face-identity areas. Communications Biology, 6, 135.

Lipreading: How we “hear” with our eyes

In a noisy environment, it is often very helpful to see the mouth of the person you are speaking to. When our brain can combine information from different sensory sources, for example during lip-reading, speech comprehension is improved.

We investigated this phenomenon in more detail to uncover how visual and auditory brain areas work together during lip-reading. In the experiment, brain activity was measured using functional magnetic resonance imaging while participants heard short sentences. The participants then watched a short silent video of a person speaking. Using a button press, they indicated whether the sentence they had heard matched the mouth movements in the video. If the sentence did not match the video, a part of the brain network that combines visual and auditory information showed greater activity and there were increased connections between the auditory speech region and the STS. How strong the activation was depended on the lip-reading skill of participants: The stronger the activation, the more correct were responses. This effect seemed to be specific to the content of speech – it did not occur when the subjects had to decide if the identity of the voice and face matched.

Blank, H. & von Kriegstein, K. (2013). Mechanisms of enhancing visual-speech recognition by prior auditory information. Neuroimage, 65, 109-118"