Biology subjects

Deouell, L. Y.

Publications and source records attributed to Deouell, L. Y..

5 recordsLinked to original sources

bioRxiv2018

Human posterior parietal cortex responds to visual stimuli as early as peristriate occipital cortex

Much of what is known about the timing of visual processing in the brain is inferred from intracranial studies in monkeys, with human data limited to mainly non-invasive methods with lower spatial resolution. Here, we estimated visual onset latencies from electrocorticographic (ECoG) recordings in a patient who was implanted with 112 sub-dural electrodes, distributed across the posterior cortex of the right hemisphere, for pre-surgical evaluation of intractable epilepsy. Functional MRI prior to surgery was used to determine boundaries of visual areas. The patient was presented with images of objects from several categories. Event Related Potentials (ERPs) were calculated across all categories excluding targets, and statistically reliable onset latencies were determined using a bootstrapping procedure over the single trial baseline activity in individual electrodes. The distribution of onset latencies broadly reflected the known hierarchy of visual areas, with the earliest cortical responses in primary visual cortex, and higher areas showing later responses. A clear exception to this pattern was robust, statistically reliable and spatially localized, very early responses on the bank of the posterior intra-parietal sulcus (IPS). The response in the IPS started nearly simultaneously with responses detected in peristriate visual areas, around 60 milliseconds post-stimulus onset. Our results support the notion of early visual processing in the posterior parietal lobe, not respecting traditional hierarchies, and give direct evidence for the upper limit of onset times of visual responses across the human cortex.

neuroscience↗

bioRxiv2018

Direct evidence for prediction signals in frontal cortex independent of prediction error

Predictive coding (PC) has been suggested as one of the main mechanisms used by brains to interact with complex environments. PC theories posit top-down prediction signals, which are compared with actual outcomes, yielding in turn prediction-error signals, which are used, bottom-up, to modify the ensuing predictions. However, disentangling prediction from prediction-error signals has been challenging. Critically, while many studies found indirect evidence for predictive coding in the form of prediction-error signals, direct evidence for the prediction signal is mostly lacking. Here we provide clear evidence, obtained from intracranial cortical recordings in human surgical patients, that the human lateral prefrontal cortex generates prediction signals while anticipating an event. Patients listened to task-irrelevant sequences of repetitive tones including infrequent predictable or unpredictable pitch deviants. The amplitude of high frequency broadband (HFB) neural activity was decreased prior to the onset of expected relative to unexpected deviants in the frontal cortex only, and its amplitude was sensitive to the increasing likelihood of deviants following longer trains of standards in the unpredictable condition. Single trial HFB amplitudes predicted deviations and correlated with post-stimulus response to deviations. These results provide direct evidence for frontal cortex prediction signals independent of prediction-error signals.

neuroscience↗

bioRxiv2018

Sustained High- and Low-Frequency Neural Responses to Visual Stimuli in Scalp EEG

What are the neurophysiological correlates of sustained visual processing in the scalp EEG signal? In a previous study using intracranial recordings in humans, we found that presentation of visual stimuli for prolonged durations (up to 1.5 seconds) was associated with two kinds of sustained neural activity patterns: a high-frequency broadband (>30 Hz) response that tracked the duration of the stimulus with high precision in early visual cortex (EVC), and with lesser temporal precision in downstream category-selective areas; and a sustained low-frequency potential shift appearing in a small subset of EVC sites. Using a similar approach of presenting images for variable durations to identify sustained activity, we provide the first comprehensive characterization of the manifestation of sustained visual responses as recorded with EEG. In a series of four experiments, we found that both high- and low-frequency sustained responses can be detected on the scalp. The high frequency activity could be detected with high signal to noise ratio only in a subset of individual subjects, in whom it was unequivocal and highly localized. The low frequency sustained response was sensitive to the size and position of the stimulus in the visual field. Both response types showed strong lateralization for stimuli on the left vs. right visual field, suggesting a retinotopic visual cortical source. However, different scalp topographies and different modulation by stimulus properties suggest that the two types of sustained responses are likely driven by distinct sources, and reflect different aspects of sustained processing in the visual cortex.

neuroscience↗

bioRxiv2017

Increasing Suppression Of Saccade-Related Transients Along The Human Visual Hierarchy

A key hallmark of visual perceptual awareness is robustness to instabilities arising from unnoticeable eye and eyelid movements. In previous human intracranial (iEEG) work (Golan et al., 2016) we found that excitatory broadband high-frequency activity transients, driven by eye blinks, are suppressed in higher-level but not early visual cortex. Here, we utilized the broad anatomical coverage of iEEG recordings in 12 eye-tracked neurosurgical patients to test whether a similar stabilizing mechanism operates following small saccades. We compared saccades (1.3{degrees}-3.7{degrees}) initiated during inspection of large individual visual objects with similarly-sized external stimulus displacements. Early visual cortex sites responded with positive transients to both conditions. In contrast, in both dorsal and ventral higher-level sites the response to saccades (but not to external displacements) was suppressed. These findings indicate that early visual cortex is highly unstable compared to higher-level visual regions which apparently constitute the main target of stabilizing extra-retinal oculomotor influences.

neuroscience↗

bioRxiv2016

Persistent neural activity encoding real-time presence of visual stimuli decays along the ventral stream

Neural populations that encode sensory experience should be persistently active for as long as the experience persists. However, research into visual neural activity has focused almost exclusively on onset-driven responses that cannot account for sustained perception. We used intracranial recordings in humans to determine the degree to which the presence of a visual stimulus is persistently encoded by neural activity. The correspondence between stimulus duration and response duration was strongest in early visual cortex and gradually diminished along the visual hierarchy, such that is was weakest in inferior-temporal category-selective regions. A similar posterior-anterior gradient was found within inferior temporal face-selective regions, with posterior but not anterior sites showing persistent face-selective activity. The results suggest that regions that appear uniform in terms of their category selectivity are dissociated by how they temporally represent a stimulus in support of real-time experience, and delineate a large-scale organizing principle of the ventral visual stream.

neuroscience↗