Biology subjects

Issa, N. P.

Publications and source records attributed to Issa, N. P..

5 recordsLinked to original sources

bioRxiv2026

Real-time detection of N2 and N3 sleep from scalp and intracranial EEG

Sleep rhythms such as slow oscillations and spindles arise predominantly during N2 and N3 sleep (N2+N3). Closed-loop stimulation targeting these rhythms is a promising strategy to enhance memory. Such stimulation requires online sleep stage classification. Conventional staging labels completed 30-s epochs using past and future data and cannot support online staging. We developed a causal tracker that, every 2 s, estimates the current stage from the preceding 10 s of scalp or intracranial EEG. We also evaluated a simulated rule that permits stimulation after three consecutive estimates indicating N2+N3 sleep. In six adults with drug-resistant epilepsy with simultaneous scalp EEG and stereo-EEG, real-time N2+N3 discrimination remained close to offline scoring of the same recordings (scalp AUC 0.953 versus 0.958). Thalamic channels alone reached AUC 0.934 and exceeded mesial temporal channels in every patient, and scalp plus thalamus performed best (0.967). The scalp tracker, retrained in 28 healthy adults, reached 0.976. When patients transitioned into N2+N3 sleep with stimulation initially disabled, the rule permitted stimulation within 5 min in 63-73% of transitions, with a median delay of 42-64 s among those detected. After leaving N2+N3, stimulation was disabled after a median of 19-22 s. The rule also switched on incorrectly 1.4-1.8 times per hour outside N2+N3 and remained active during 28-31% of REM in patients, compared with 2% in healthy adults. These findings show that both scalp and thalamic EEG can support real-time N2+N3 detection and inform when stimulation is permitted. Translationally, implanted thalamic leads, increasingly used for neuromodulation in epilepsy, could support sleep-stage-dependent stimulation without scalp EEG, while scalp EEG might be used for noninvasive neuromodulation. Closed-loop systems should be evaluated not only by sleep-stage classification accuracy but also by how reliably and promptly they enable and disable stimulation.

neuroscience↗

bioRxiv2026

Thalamic stereo-EEG detects sleep stages N2 and N3 with discrimination approaching scalp EEG

Sleep and epilepsy interact strongly during sleep stages N2 and N3, when synchronization of brain rhythms is increased. Sleep staging relies on scalp EEG, which is often absent or incomplete during stereo-EEG monitoring and unavailable from implanted devices, while thalamic leads are increasingly implanted for epilepsy neuromodulation. Given the role of the thalamus in spindle generation and slow oscillation dynamics, we hypothesized that thalamic channels would identify N2 and N3 (N2+N3) better than other intracranial sites. We analyzed 477 h of stereo-EEG with simultaneous scalp EEG from six adults, including 46 thalamic bipolar channels on 11 electrodes. Two classifiers were evaluated on identical channels: a random forest with hidden Markov decoding (RF + HMM) trained leave-one-participant-out, and the pretrained SleepSEEG algorithm developed without thalamic channels. The primary endpoint was discrimination of N2+N3 from other stages as classified from scalp EEG by a sleep expert. Among regions sampled in all participants, thalamic channels performed best (area under the curve [AUC] 0.94 with RF + HMM and 0.93 with SleepSEEG across whole recordings; 0.88 and 0.86 at night). With RF + HMM, thalamic channels outperformed mesial temporal and temporal neocortical channels in all six participants across whole recordings. The advantage persisted against channel-count-matched subsets and a white-matter control with both classifiers. Individual thalamic electrodes reached AUCs of 0.79-0.99 with RF + HMM (10 of 11 at least 0.89). These offline findings support thalamic N2+N3 detection and motivate real-time evaluation for sleep-targeted neuromodulation.

neuroscience↗

bioRxiv2026

Visuospatial coding by theta oscillations in human hippocampus

The hippocampus has been proposed to support visual processing and perception, challenging longstanding accounts that emphasize navigation or declarative memory. A key prediction of visual-processing accounts is that the hippocampus should exhibit similar visuospatial coding properties to those of higher-order visual neocortical areas, such as sensitivity to the size of visual stimuli and contralateral visual field biases. We tested for these properties using intracranial EEG to measure hippocampal neural activity during a retinotopic mapping task. The hippocampus exhibited characteristic slow ([~]2 Hz) and fast ([~]8 Hz) theta oscillations throughout the task. Fast theta was responsive to the presence but not the amount of visual stimulation. In contrast, slow theta did not generally respond to stimulus presence but scaled with the size of the visual stimulus, consistent with larger receptive fields. Slow theta also showed a contralateral bias, an effect that was specific to the right hippocampus. None of these effects were attributable to microsaccades or performance of the concurrent vigilance task. These findings provide electrophysiological evidence for visual field coding by human hippocampus, supporting accounts of hippocampal function that emphasize its role atop the visual hierarchy. Visual processing of this kind may combine with self-motion, memory, and other signals to support the broader spatial and mnemonic functions with which hippocampal theta oscillations have long been associated.

neuroscience↗

bioRxiv2024

Visual speech enhances auditory onset timing and envelope tracking through distinct mechanisms

Seeing a speakers face facilitates speech recognition in challenging listening environments. Prior work has shown that visual speech contains timing information to aid auditory speech processing, yet how these signals are integrated within the auditory system during audiovisual speech perception remains poorly understood. Observation of preparatory mouth movements may initiate phase reset of intrinsic oscillations, potentially sensitizing the auditory system for receptive speech processing, while observation of mouth movements post speech onset may facilitate entrainment to the speech envelope. Yet, little work has been done to test whether visual speech enhances encoding of auditory speech onset, speech envelope tracking, or both, and through independent or overlapping mechanisms. To investigate this, we examined the ways in which visual speech timing information alters theta band power and phase using human intracranial electroencephalography (iEEG) recordings in a large group of patients with epilepsy (n = 21). Prior to speech onset, preparatory mouth movements elicited theta phase reset (increased inter-trial phase coherence; ITPC) throughout the superior temporal gyrus (STG), which is thought to enhance speech onset encoding. Following speech onset, visual speech modulated theta ITPC only at anterior STG electrodes while theta power was modulated at posterior STG electrodes. Pre- and post-speech onset were spatially and temporally dissociated, consistent with the hypothesis that audiovisual speech onset encoding and envelope tracking mechanisms are partially distinct. Crucially, congruent and incongruent visual speech, designed here to have identical visual timing information about speech onset time, but different visual mouth evolution, produced only a small difference in the phase of theta band oscillations in the anterior STG, highlighting a more restricted role of visual speech in ongoing auditory entrainment. These results support the hypothesis that visual speech improves the precision of auditory speech encoding through two separate mechanisms, with auditory speech onset encoded throughout the entire STG and ongoing speech envelope tracking within anterior STG.

neuroscience↗

bioRxiv2020

Visual speech differentially modulates beta, theta, and high gamma bands in auditory cortex

Speech perception is a central component of social communication. While principally an auditory process, accurate speech perception in everyday settings is supported by meaningful information extracted from visual cues (e.g., speech content, timing, and speaker identity). Previous research has shown that visual speech modulates activity in cortical areas subserving auditory speech perception, including the superior temporal gyrus (STG), potentially through feedback connections from the multisensory posterior superior temporal sulcus (pSTS). However, it is unknown whether visual modulation of auditory processing in the STG is a unitary phenomenon or, rather, consists of multiple temporally, spatially, or functionally distinct processes. To explore these questions, we examined neural responses to audiovisual speech measured from intracranially implanted electrodes within the temporal cortex of 21 patients undergoing clinical monitoring for epilepsy. We found that visual speech modulates auditory processes in the STG in multiple ways, eliciting temporally and spatially distinct patterns of activity that differ across theta, beta, and high-gamma frequency bands. Before speech onset, visual information increased high-gamma power in the posterior STG and suppressed beta power in mid-STG regions, suggesting crossmodal prediction of speech signals in these areas. After sound onset, visual speech decreased theta power in the middle and posterior STG, potentially reflecting a decrease in sustained feedforward auditory activity. These results are consistent with models that posit multiple distinct mechanisms supporting audiovisual speech perception and provide a crucial map for subsequent studies to identify the types of visual features that are encoded by these separate mechanisms.

neuroscience↗