Neuroscientist, Postdoctoral researcher
contact: ellapodvalny at gmail
The framework outlined above instructs my research aims and experiment design. I choose humans as my model organism because encoding of isolated objects in the human brain is reasonably understood, humans can engage in complex tasks without training (e.g., flexible goal-directed foraging/visual search), recordings of large-scale network activity with non-invasive methods, such as M/EEG, fMRI, are feasible, whereas questions concerning local brain activity can be answered with invasive, intracranial recordings (ECoG) in patients. Tracking the eye pupil size and position and objects in dynamic scenes allows tracking of arousal state and sensory input in naturalistic settings (Fig. 1).
Ongoing brain activity in object-related decision-making
How ongoing brain activity shapes decisions about forthcoming object stimuli? Using noninvasive brain recordings (MEG), we uncovered two ongoing (prestimulus) brain processes which shape object-related decisions in a dissociable manner: Content-Specific (CS) ongoing brain process predicts detection of objects from category A, but not B, while Non-Content-Specific (NCS) processes predicts detection of all object categories equally. Using additional images of phase-scrambled objects that preserve low-level category specific features, we found a double dissociation in these processes’ effect on behavior: the CS process correlates with perceptual sensitivity only (d’) and the NCS process correlates with decision bias only (c). These results imply that prior knowledge of object categories constrains ongoing brain activity and shapes perceptual decisions in conjunction with non-specific processes, such as arousal. We will further investigate the anatomical underpinning of these two processes using fMRI.
Podvalny, E., Flounders, M.W., King, L.E., Holroyd, T., He B.J. (2019) A dual role of prestimulus spontaneous neural activity in object recognition. Nature Communications 10, 1-13. DOI
Ongoing arousal and its effect on perceptual decision making
Arousal levels perpetually rise and fall to control behavior. How markers of arousal — pupil size and frequency content of brain activity — relate to each other and to subsequent perceptual decisions is not fully understood. We simultaneously monitored magnetoencephalography and pupil in healthy volunteers in rest and a perceptual decision-making task. Pupil size correlated with power of brain activity in all frequency bands and in all large-scale cortical networks. Prestimulus pupil size correlates with subsequent shifts in both detection criterion and sensitivity, which was fully explained by spectral power variation. When dissociated from pupil-linked state, Delta power of Dorsal Attention and Default networks predicts shifts in criterion only and Alpha power of Visual network predicts shifts in sensitivity only. Spontaneous constriction and dilation correlate with large-scale brain activity but not behavior. Our results illuminate the relation between central and peripheral markers of arousal and their respective roles in behavior in humans.
Podvalny, E., King, L.E., He B.J. (In preparation) Spectral signature and behavioral consequence of spontaneous fluctuations in pupil-linked arousal in human.
Object processing in real-world environment
Responses to objects in high-order visual cortex are more “immune” to changes in low-level object features, such as contrast or position. We hypothesized that in free viewing, such “immunity” may contribute to stability of our perceptual world despite the incessant shifts in the optical inputs induced by oculomotor movements as we engage in natural behavior. We used a novel experimental apparatus that enabled us to record intracranial neural activity, precise eye position and the visual scene simultaneously in human patients, while they engaged in their routine free behavior. Using machine learning tools, we were able to track people faces recorded in the video and determine object-related eye position. Our results show that neural response immunity to oculomotor factors (fixation duration and retinal image contrast) evolves gradually along the ventral stream.
Podvalny, E., Yeagle, E., Mégevand, P., Sarid, N., Harel, M., Chechik, G., Mehta, A.D. and Malach, R. (2017). Invariant temporal dynamics underlie perceptual stability in human visual cortex. Current Biology 27, 155-165. DOI: http://dx.doi.org/10.1016/j.cub.2016.11.024
Transformation of ongoing arrhythmic brain activity
In this project I studied how prestimulus brain activity, modeled according to its power spectrum, transforms into stimulus-triggered activity. Ongoing cortical activity exhibits a 1/f^"χ" power spectrum, whereas f is frequency and "χ" is a scaling exponent (on a double-logarithmic scale that would be a simple straight line with x as a slope). We discovered that when stimulus arrives, a transformation of this arrhythmic activity - specifically, a significant reduction in the exponent "χ" – underlies the major contribution to spectral response. We coined the term “spectral rotation” to describe a phenomenon of transformation from baseline to active cortical state. An increase in narrow-band high-gamma power correlated with the rotation amplitude. The rotation transformation was not specific to object stimuli and may be triggered by a more general phenomenon, such as phasic arousal. The transformation law we uncovered can be explained by a shift of temporal scale of pairwise correlations between neurons - that is, neurons synchronize on faster timescales on stimulus arrival.
Podvalny, E., Noy, N., Harel, M., Bickel, S., Chechik, G., Schroeder, C.E., Mehta, A.D., Tsodyks, M. and Malach, R. (2015). A unifying principle underlying the extracellular field potential spectral responses in the human cortex. J Neurophysiol 114, 505-519. DOI: http://dx.doi.org/10.1152/jn.00943.2014
Peer-reviewed research articles:
Podvalny, E., Flounders, M. W., King, L. E., Holroyd, T. & He, B. J. A dual role of prestimulus spontaneous neural activity in visual object recognition.
Nature Communications 10, 3910 (2019). DOI: https://doi.org/10.1038/s41467-019-11877-4
Podvalny, E., Yeagle, E., Mégevand, P., Sarid, N., Harel, M., Chechik, G., Mehta, A.D. and Malach, R. (2017). Invariant temporal dynamics underlie perceptual stability in human visual cortex.
Current Biology 27, 155-165. DOI: http://dx.doi.org/10.1016/j.cub.2016.11.024
Podvalny, E., Noy, N., Harel, M., Bickel, S., Chechik, G., Schroeder, C.E., Mehta, A.D., Tsodyks, M. and Malach, R. (2015). A unifying principle underlying the extracellular field potential spectral responses in the human cortex.
J Neurophysiol 114, 505-519. DOI: http://dx.doi.org/10.1152/jn.00943.2014
Submissions, preprints, articles in preparation, ongoing projects:
Podvalny, E., King, L.E., He B.J. (in review). Spectral signature and behavioral consequence of spontaneous fluctuations in pupil-linked arousal in human.
Levinson*, M., Podvalny*, E., Baete, S., He B.J. (in review). Distributed object category representations correlate with recognition of liminal objects.
Podvalny, E., Levinson, M., Baete, S., He B.J. (data analysis in progress). How activity in resting state networks shapes the response to object-related decisions?
* equal contribution.
Commitment to Diversity and Inclusion
Today’s academic environment unfortunately presents immense roadblocks for people from disadvantaged socioeconomic backgrounds and certain minority groups. Those who face discrimination, poverty, disability, are often set to fail in academic settings. I believe in active dismantling of mechanisms promoting marginalization of people from underprivileged backgrounds and am committed to improving diversity in academic environments.