Biology subjects

Ozker, M.

Publications and source records attributed to Ozker, M..

6 recordsLinked to original sources

bioRxiv2026

The scaffolding of individual variability in language processing by domain-general neural networks

Language processing is supported by distributed neural systems. Yet most research examines these systems at the population-average level, obscuring how individual cognitive differences shape language-related brain activity. In this study, we combined comprehensive cognitive assessments and task-based fMRI in a large sample of healthy adults (N = 205) to examine how variability in linguistic knowledge, working memory, processing speed, and non-verbal reasoning influenced neural responses in four language tasks: lexical decision, picture naming, sentence comprehension, and sentence production. All tasks engaged canonical left-lateralized language regions. However, individual differences in cognitive skills were not associated with modulations within commonly activated regions, but rather with modulations in domain-general systems outside traditional perisylvian language areas, mainly the default mode and dorsal attention networks. Notably, activations in these domain-general regions were predominantly negatively correlated with cognitive skills, indicating that individuals with lower cognitive skills draw on these broader neural resources more than higher-skilled individuals, possibly as a compensatory mechanism. These results reveal that while canonical language regions are consistently engaged during language tasks, the recruitment of domain-general systems acts as a variable resource modulated by individuals cognitive skills. Overall, our findings demonstrate that individual cognitive profiles determine how distributed brain systems are dynamically engaged to scaffold language processing.

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bioRxiv2025

Individual Differences in Speech Monitoring: Functional and Structural Correlates of Delayed Auditory Feedback

Sensory feedback is essential for the fine-tuning of motor actions, and speech production is no exception. It depends on continuous self-monitoring to ensure that produced sounds match intended targets. Delaying auditory feedback (DAF) disrupts this alignment and impairs fluency, providing a powerful tool to investigate sensorimotor control. We combined functional and diffusion-weighted MRI in 31 participants performing a word-production task under delayed (DAF) and immediate (no-DAF) auditory feedback. While all participants slowed their speech under DAF, the extent of this effect varied across individuals and was quantified using a susceptibility index (SI). At the group level, DAF elicited increased activation in a right-lateralized network encompassing the superior temporal gyrus, supramarginal gyrus, inferior frontal gyrus, supplementary motor area, and left cerebellum. Incorporating individual differences revealed that higher susceptibility was associated with greater activation in left-hemisphere speech motor homologues and larger volume of the right long arcuate fasciculus, a white-matter pathway connecting auditory and motor speech regions. This pattern suggests that vulnerability reflects increased recruitment of neural resources and a stronger reliance on auditory-motor coupling. In contrast, resilience was associated with greater engagement of the bilateral angular gyrus and higher fiber density in the right posterior arcuate fasciculus, which connects auditory and somatosensory speech regions. This finding indicates that resilience is supported by a posterior circuit that efficiently integrates multi-modal sensory feedback. Together, these findings link functional dynamics with underlying structural connectivity to reveal how a right-lateralized network supports speech control, while accounting for individual differences in susceptibility to fluency disruption. Significance StatementFluent speech depends on the brains ability to monitor self-produced sounds and sensations from articulatory organs to adjust motor commands in real time. To uncover the neural basis of this process, we combined a fluency-disrupting paradigm, delayed auditory feedback (DAF), with functional and structural neuroimaging. This multimodal approach revealed that while DAF processing relies on a right-lateralized network, more susceptible individuals show enhanced recruitment of left-hemisphere monitoring regions. We also found that stronger white-matter connections in the right posterior arcuate fasciculus predict greater resilience and fluency. These findings provide an anatomically grounded account of how auditory and somatosensory feedback interact to support speech production, offering new insight into why some individuals are more susceptible to fluency breakdowns and related disorders.

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bioRxiv2025

Dissociating stimulus encoding and task demands in ECoG responses from human visual cortex

Brain responses to sensory stimuli depend on the specific task performed by the observer. Previously, fMRI measurements in ventral temporal cortex (VTC) showed that BOLD responses to visual input scale with task demands. This scaling of BOLD responses is thought to be driven by the engagement of cognitive functions required for the task. To better understand how these cognitive task demands influence neural activity in VTC, we measured electrocorticography (ECoG) responses to images of varying contrast in two human participants during either a fixation or image categorization task. ECoG high frequency broadband activity (>70 Hz) showed that local neuronal responses increased with image contrast as expected for sensory encoding, and were scaled by task demands approximately 200 ms after stimulus onset. In contrast, ECoG low frequency activity in the alpha/beta range (8-28 Hz) was insensitive to image contrast and showed larger decreases with increasing task demands. These results indicate that high frequencies in VTC reflect both the encoding of sensory inputs and task demands, similar to prior BOLD response measurements, whereas low frequency oscillations represent task demands, not sensory inputs per se. In line with the interpretation that low frequency oscillations reflect pulsed inhibition, we speculate that a power decrease in low frequency oscillations amplifies neural activity in VTC in support of visual task demands.

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bioRxiv2024

Susceptibility to auditory feedback manipulations and individual variability

Monitoring auditory feedback from hearing ones own voice is important for fluent speech production as it enables detection and correction of speech errors. The influence of auditory feedback is best illustrated by manipulating it during speech production. A common temporal manipulation technique, delaying auditory feedback (DAF), leads to disruptions in speech fluency, while a common spectral manipulation technique, perturbing the pitch of auditory feedback (PAF), results in vocal alterations. Previous research involving clinical populations has revealed diverse susceptibility profiles to auditory feedback manipulations, yet the extent of such diversity within the neurotypical population remains unclear. Furthermore, different types of manipulations elicit distinct speech errors (i.e. fluency/coordination versus acoustic errors), which may be processed by distinct error correction mechanisms. It is yet to be understood whether individuals affected by one manipulation are similarly impacted by the other. Lastly, based on evidence from clinical studies, which demonstrated that visual feedback can improve impaired speech production, it is an open question whether visual feedback can alleviate the disruptive effects of altered auditory feedback. We recorded voice samples from 40 neurotypical participants during both a DAF and a PAF task. DAF significantly prolonged articulation duration and increased voice pitch and intensity. In some trials, participants received immediate visual feedback, however visual feedback did not alleviate but rather strengthened the disruptive effects of DAF. During the PAF task, participants adjusted their voice pitch in the opposite direction of the perturbation in majority of the trials to compensate for the perturbation. We assessed susceptibility of the participants to the effects of DAF and PAF by examining articulation duration and compensatory vocal response magnitude, respectively. Susceptibility varied widely among participants for both manipulations, but individuals susceptible to one manipulation did not consistently exhibit susceptibility to the other, indicating distinct processing mechanisms for these different types of auditory feedback manipulations.

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bioRxiv2023

Speech-induced suppression and vocal feedback sensitivity in human cortex

Across the animal kingdom, neural responses in the auditory cortex are suppressed during vocalization, and humans are no exception. A common hypothesis is that suppression increases sensitivity to auditory feedback, enabling the detection of vocalization errors. This hypothesis has been previously confirmed in non-human primates, however a direct link between auditory suppression and sensitivity in human speech monitoring remains elusive. To address this issue, we obtained intracranial electroencephalography (iEEG) recordings from 35 neurosurgical participants during speech production. We first characterized the detailed topography of auditory suppression, which varied across superior temporal gyrus (STG). Next, we performed a delayed auditory feedback (DAF) task to determine whether the suppressed sites were also sensitive to auditory feedback alterations. Indeed, overlapping sites showed enhanced responses to feedback, indicating sensitivity. Importantly, there was a strong correlation between the degree of auditory suppression and feedback sensitivity, suggesting suppression might be a key mechanism that underlies speech monitoring. Further, we found that when participants produced speech with simultaneous auditory feedback, posterior STG was selectively activated if participants were engaged in a DAF paradigm, suggesting that increased attentional load can modulate auditory feedback sensitivity.

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bioRxiv2018

Converging Evidence from Electrocorticography and BOLD fMRI for a Sharp Functional Boundary in Superior Temporal Gyrus Related to Multisensory Speech Processing

Although humans can understand speech using the auditory modality alone, in noisy environments visual speech information from the talkers mouth can rescue otherwise unintelligible auditory speech. To investigate the neural substrates of multisensory speech perception, we recorded neural activity from the human superior temporal gyrus using two very different techniques: either directly, using surface electrodes implanted in five participants with epilepsy (electrocorticography, ECOG), or indirectly, using blood oxygen level dependent functional magnetic resonance imaging (BOLD fMRI) in six healthy control fMRI participants. Both ECOG and fMRI participants viewed the same clear and noisy audiovisual speech stimuli and performed the same speech recognition task. Both techniques demonstrated a sharp functional boundary in the STG, which corresponded to an anatomical boundary defined by the posterior edge of Heschls gyrus. On the anterior side of the boundary, cortex responded more strongly to clear audiovisual speech than to noisy audiovisual speech, suggesting that anterior STG is primarily involved in processing unisensory auditory speech. On the posterior side of the boundary, cortex preferred noisy audiovisual speech or showed no preference and showed robust responses to auditory-only and visual-only speech, suggesting that posterior STG is specialized for processing multisensory audiovisual speech. For both ECOG and fMRI, the transition between the functionally distinct regions happened within 10 mm of anterior-to-posterior distance along the STG. We relate this boundary to the multisensory neural code underlying speech perception and propose that it represents an important functional division within the human speech perception network.

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