A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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    CORTICAL REPRESENTATIONS OF INTELLIGIBLE AND UNINTELLIGIBLE SPEECH: EFFECTS OF AGING AND LINGUISTIC CONTENT
    (2023) Karunathilake , I.M Dushyanthi; Simon, Jonathan Z.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Speech communication requires real-time processing of rapidly varying acoustic sounds across various speech landmarks while recruiting complex cognitive processes to derive the intended meaning. Behavioral studies have highlighted that speech comprehension is altered by factors like aging, linguistic content, and intelligibility, yet the systematic neural mechanisms underlying these changes are not well understood. This thesis aims to explore how the neural bases are modulated by each of these factors using three different experiments, by comparing speech representation in the cortical responses, measured by Magnetoencephalography (MEG). We use neural encoding (Temporal Response Functions (TRFs)) and decoding (reconstruction accuracy) models which describe the mapping between stimulus features and the cortical responses, which are instrumental in understanding cortical temporal processing mechanisms in the brain.Firstly, we investigate age-related changes in timing and fidelity of the cortical representation of speech-in-noise. Understanding speech in a noisy environment becomes more challenging with age, even for healthy aging. Our findings demonstrate that some of the age-related difficulties in understanding speech in noise experienced by older adults are accompanied by age-related temporal processing differences in the auditory cortex. This is an important step towards incorporating neural measures to both diagnostic evaluation and treatments aimed at speech comprehension problems in older adults. Next, we investigate how the cortical representation of speech is influenced by the linguistic content by comparing neural responses to four types of continuous speech-like passages: non-speech, non-words, scrambled words, and narrative. We find neural evidence for emergent features of speech processing from acoustics to linguistic processes at the sentential level as incremental steps in the processing of speech input occur. We also show the gradual computation of hierarchical speech features over time, encompassing both bottom-up and top-down mechanisms. Top-down driven mechanisms at linguistic level demonstrates N400-like response, suggesting involvement of predictive coding mechanisms. Finally, we find potential neural markers of speech intelligibility using a priming paradigm, where intelligibility is varied while keeping the acoustic structure constant. Our findings suggest that segmentation of sounds into words emerges with better speech intelligibility and most strongly at ~400 ms in prefrontal cortex (PFC), in line with engagement of top-down mechanisms associated with priming. Taken together, this thesis furthers our understanding on neural mechanisms underlying speech comprehension and potential objective neural markers to evaluate the level of speech comprehension.
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    Effects of Aging on Cortical Representations of Continuous Speech
    (2022) Karunathilake, I.M Dushyanthi; Simon, Jonathan Z.
    Understanding speech in a noisy environment is crucial in day-to-day interactions, and yet becomes more challenging with age, even for healthy aging. Age-related changes in the neural mechanisms that enable speech-in-noise listening have been investigated previously; however, the extent to which age affects the timing and fidelity of encoding of target and interfering speech streams are not well understood. Using magnetoencephalography (MEG), we investigated how continuous speech is represented in auditory cortex in the presence of interfering speech, in younger and older adults. Cortical representations were obtained from neural responses that time-locked to the speech envelopes using speech envelope reconstruction and temporal response functions (TRFs). TRFs showed three prominent peaks corresponding to auditory cortical processing stages: early (~50 ms), middle (~100 ms) and late (~200 ms). Older adults showed exaggerated speech envelope representations compared to younger adults. Temporal analysis revealed both that the age-related exaggeration starts as early as ~50 ms, and that older adults needed a substantially longer integration time window to achieve their better reconstruction of the speech envelope. As expected, with increased speech masking, envelope reconstruction for the attended talker decreased and all three TRF peaks were delayed, with aging contributing additionally to the reduction. Interestingly, for older adults the late peak was delayed, suggesting that this late peak may receive contributions from multiple sources. Together these results suggest that there are several mechanisms at play compensating for age-related temporal processing deficits at several stages, but which are not able to fully reestablish unimpaired speech perception.
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    Bilaterally Reduced Rolandic Beta Band Activity in Minor Stroke Patients - Dataset
    (2022) Kulasingham, Joshua; Brodbeck, Christian; Khan, Sheena; Simon, Jonathan; Marsh, Elisabeth
    Stroke patients with hemiparesis display decreased beta band (13–25Hz) rolandic activity, correlating to impaired motor function. However, clinically, patients without significant weakness, with small lesions far from sensorimotor cortex, exhibit bilateral decreased motor dexterity and slowed reaction times. We investigate whether these minor stroke patients also display abnormal beta band activity. Magnetoencephalographic (MEG) data were collected from nine minor stroke patients (NIHSS < 4) without significant hemiparesis, at ~1 and ~6 months postinfarct, and eight age-similar controls. Rolandic relative beta power during matching tasks and resting state, and Beta Event Related (De)Synchronization (ERD/ERS) during button press responses were analyzed. Regardless of lesion location, patients had significantly reduced relative beta power and ERS compared to controls. abnormalities persisted over visits, and were present in both ipsi- and contra-lesional hemispheres, consistent with bilateral impairments in motor dexterity and speed. Minor stroke patients without severe weakness display reduced rolandic beta band activity in both hemispheres, which may be linked to bilaterally impaired dexterity and processing speed, implicating global connectivity dysfunction affecting sensorimotor cortex independent of lesion location. Findings not only illustrate global network disruption after minor stroke, but suggest rolandic beta band activity may be a potential biomarker and treatment target, even for minor stroke patients with small lesions far from sensorimotor areas.
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    Cortical Processing of Arithmetic and Simple Sentences in an Auditory Attention Task - Dataset
    (2021) Kulasingham, Joshua P.; Joshi, Neha H.; Rezaeizadeh, Mohsen; Simon, Jonathan Z.
    Cortical processing of arithmetic and of language rely on both shared and task-specific neural mechanisms, which should also be dissociable from the particular sensory modality used to probe them. Here, spoken arithmetical and non-mathematical statements were employed to investigate neural processing of arithmetic, compared to general language processing, in an attention-modulated cocktail party paradigm. Magnetoencephalography (MEG) data were recorded from 22 human subjects listening to audio mixtures of spoken sentences and arithmetic equations while selectively attending to one of the two speech streams. Short sentences and simple equations were presented diotically at fixed and distinct word/symbol and sentence/equation rates. Critically, this allowed neural responses to acoustics, words, and symbols to be dissociated from responses to sentences and equations. Indeed, the simultaneous neural processing of the acoustics of words and symbols were observed in auditory cortex for both streams. Neural responses to sentences and equations, however, were predominantly to the attended stream, originating primarily from left temporal, and parietal areas, respectively. Additionally, these neural responses were correlated with behavioral performance in a deviant detection task. Source-localized Temporal Response Functions revealed distinct cortical dynamics of responses to sentences in left temporal areas and equations in bilateral temporal, parietal, and motor areas. Finally, the target of attention could be decoded from MEG responses, especially in left superior parietal areas. In short, the neural responses to arithmetic and language are especially well segregated during the cocktail party paradigm, and the correlation with behavior suggests that they may be linked to successful comprehension or calculation.
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    Post-Stroke Acute Dysexecutive Syndrome, a Disorder Resulting from Minor Stroke due to Disruption of Network Dynamics - Dataset
    (2020) Marsh, Elisabeth B.; Brodbeck, Christian; Llinas, Rafael H.; Mallick, Dania; Kulasingham, Joshua P.; Llinas, Rodolfo R.; Simon, Jonathan Z.
    Stroke patients with small CNS infarcts often demonstrate an acute dysexecutive syndrome characterized by difficulty with attention, concentration, and processing speed, independent of lesion size or location. We use magnetoencephalography (MEG) to show that disruption of network dynamics may be responsible. Nine patients with recent minor stroke and 8 age-similar controls underwent cognitive screening using the Montreal Cognitive Assessment (MoCA) and MEG to evaluate differences in cerebral activation patterns. During MEG, subjects participated in a visual picture-word matching task. Task complexity was increased as testing progressed. Cluster based permutation tests determined differences in activation patterns within the visual cortex, fusiform gyrus, and lateral temporal lobe. At visit 1, MoCA scores were significantly lower for patients than controls (median (IQR)=26.0 (4) versus 29.5 (3), p=0.005), and patient reaction times were increased. The amplitude of activation was significantly lower after infarct and demonstrated a pattern of temporal dispersion independent of stroke location. Differences were prominent in the fusiform gyrus and lateral temporal lobe. The pattern suggests that distributed network dysfunction may be responsible. Additionally, controls were able to modulate their cerebral activity based on task difficulty. In contrast, stroke patients exhibited the same low-amplitude response to all stimuli. Group differences remained, to a lesser degree, six months later; while MoCA scores and reaction times improved for patients. This study suggests that function is a globally distributed property beyond area-specific functionality, and illustrates the need for longer-term follow-up studies to determine whether abnormal activation patterns ultimately resolve or another mechanism underlies continued recovery.
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    The Neural Dynamics of Amplitude Modulation Processing in the Human Auditory System
    (2010) Li, Kai Sum; Simon, Jonathan Z; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The neural, auditory amplitude modulation transfer function (MTF) is estimated from 3 - 50 Hz using magnetoencephalography (MEG). All acoustic stimuli are amplitude modulated (AM). Two different dynamical stimulus types are used: exponential sweeps with the AM rate changing from 2 up to 60 Hz, and 89 down to 3 Hz. Several carriers are also employed, including 3 pure-tone carriers (250 Hz, 707 Hz and 2 kHz) and 3 bandlimited pink-noise carriers (1/3, 2 and 5 octaves centered at 707 Hz). Neural response magnitudes, phases, group delays and impulse responses are all estimated. Our results show that the shape of modulation transfer function is flat but with a slightly low pass shape below 10 Hz. The phase of the response is approximately linear in many frequencies. The group delay is around 50 ms at 40 Hz for increasing-frequency sweeps and closer to 100 ms for decreasing-frequency sweeps.
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    Hearing VS. Listening: Attention Changes the Neural Representations of Auditory Percepts
    (2008-05-01) xiang, juanjuan; Simon, Jonathan Z.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Making sense of acoustic environments is a challenging task. At any moment, the signals from distinct auditory sources arrive in the ear simultaneously, forming an acoustic mixture. The brain must represent distinct auditory objects in this complex scene and prioritize processing of relevant stimuli while maintaining the capability to react quickly to unexpected events. The present studies explore neural representations of temporal modulations and the effects of attention on these representations. Temporal modulation plays a significant role in speech perception and auditory scene analysis. To uncover how temporal modulations are processed and represented is potentially of great importance for our general understanding of the auditory system. Neural representations of compound modulations were investigated by magnetoencephalography (MEG). Interaction components are generated by near rather than distant modulation rhythms, suggesting band-limited modulation filter banks operating in the central stage of the auditory system. Furthermore, the slowest detectable neural oscillation in the auditory cortex corresponds to the perceived oscillation of the auditory percept. Interactions between stimulus-evoked and goal-related neural responses were investigated in simultaneous behavioral-neurophysiological studies, in which we manipulate subjects' attention to different components of an auditory scene. Our experimental results reveal that attention to the target correlates with a sustained increase in the neural target representation, beyond well-known transient effects. The enhancement of power and phase coherence presumably reflects increased local and global synchronizations in the brain. Furthermore, the target's perceptual detectability improves over time (several seconds), correlating strongly with the target representation's neural buildup. The change in cortical representations can be reversed in a short time-scale (several minutes) by various behavioral goals. These aforementioned results demonstrate that the neural representation of the percept is encoded using the feature-driven mechanisms of sensory cortex, but shaped in a sustained manner via attention-driven projections from higher-level areas. This adaptive neural representations occur on multiple time scales (seconds vs. minutes) and multiple spatial scales (local vs. global synchronization). Such multiple resolutions of adaptation may underlie general mechanisms of scene organization in any sensory modality and may contribute to our highly adaptive behaviors.