Publications by authors named "Deborah F Levy"

4 Publications

  • Page 1 of 1

Categorical Encoding of Vowels in Primary Auditory Cortex.

Cereb Cortex 2020 03;30(2):618-627

Department of Hearing and Speech Sciences, Vanderbilt University Medical Center, Nashville, TN 37232, USA.

Speech perception involves mapping from a continuous and variable acoustic speech signal to discrete, linguistically meaningful units. However, it is unclear where in the auditory processing stream speech sound representations cease to be veridical (faithfully encoding precise acoustic properties) and become categorical (encoding sounds as linguistic categories). In this study, we used functional magnetic resonance imaging and multivariate pattern analysis to determine whether tonotopic primary auditory cortex (PAC), defined as tonotopic voxels falling within Heschl's gyrus, represents one class of speech sounds-vowels-veridically or categorically. For each of 15 participants, 4 individualized synthetic vowel stimuli were generated such that the vowels were equidistant in acoustic space, yet straddled a categorical boundary (with the first 2 vowels perceived as [i] and the last 2 perceived as [i]). Each participant's 4 vowels were then presented in a block design with an irrelevant but attention-demanding level change detection task. We found that in PAC bilaterally, neural discrimination between pairs of vowels that crossed the categorical boundary was more accurate than neural discrimination between equivalently spaced vowel pairs that fell within a category. These findings suggest that PAC does not represent vowel sounds veridically, but that encoding of vowels is shaped by linguistically relevant phonemic categories.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1093/cercor/bhz112DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7306170PMC
March 2020

Neural representation of vowel formants in tonotopic auditory cortex.

Neuroimage 2018 09 31;178:574-582. Epub 2018 May 31.

Department of Hearing and Speech Sciences, Vanderbilt University Medical Center, Nashville, TN, USA. Electronic address:

Speech sounds are encoded by distributed patterns of activity in bilateral superior temporal cortex. However, it is unclear whether speech sounds are topographically represented in cortex, or which acoustic or phonetic dimensions might be spatially mapped. Here, using functional MRI, we investigated the potential spatial representation of vowels, which are largely distinguished from one another by the frequencies of their first and second formants, i.e. peaks in their frequency spectra. This allowed us to generate clear hypotheses about the representation of specific vowels in tonotopic regions of auditory cortex. We scanned participants as they listened to multiple natural tokens of the vowels [ɑ] and [i], which we selected because their first and second formants overlap minimally. Formant-based regions of interest were defined for each vowel based on spectral analysis of the vowel stimuli and independently acquired tonotopic maps for each participant. We found that perception of [ɑ] and [i] yielded differential activation of tonotopic regions corresponding to formants of [ɑ] and [i], such that each vowel was associated with increased signal in tonotopic regions corresponding to its own formants. This pattern was observed in Heschl's gyrus and the superior temporal gyrus, in both hemispheres, and for both the first and second formants. Using linear discriminant analysis of mean signal change in formant-based regions of interest, the identity of untrained vowels was predicted with ∼73% accuracy. Our findings show that cortical encoding of vowels is scaffolded on tonotopy, a fundamental organizing principle of auditory cortex that is not language-specific.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.neuroimage.2018.05.072DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6231402PMC
September 2018

Closed-loop stimulation of temporal cortex rescues functional networks and improves memory.

Nat Commun 2018 02 6;9(1):365. Epub 2018 Feb 6.

Department of Psychology, University of Pennsylvania, 433 South University Avenue, Philadelphia, PA, 19104, USA.

Memory failures are frustrating and often the result of ineffective encoding. One approach to improving memory outcomes is through direct modulation of brain activity with electrical stimulation. Previous efforts, however, have reported inconsistent effects when using open-loop stimulation and often target the hippocampus and medial temporal lobes. Here we use a closed-loop system to monitor and decode neural activity from direct brain recordings in humans. We apply targeted stimulation to lateral temporal cortex and report that this stimulation rescues periods of poor memory encoding. This system also improves later recall, revealing that the lateral temporal cortex is a reliable target for memory enhancement. Taken together, our results suggest that such systems may provide a therapeutic approach for treating memory dysfunction.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41467-017-02753-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5802791PMC
February 2018

Direct Brain Stimulation Modulates Encoding States and Memory Performance in Humans.

Curr Biol 2017 May 20;27(9):1251-1258. Epub 2017 Apr 20.

Department of Psychology, University of Pennsylvania, Philadelphia, PA 19104, USA. Electronic address:

People often forget information because they fail to effectively encode it. Here, we test the hypothesis that targeted electrical stimulation can modulate neural encoding states and subsequent memory outcomes. Using recordings from neurosurgical epilepsy patients with intracranially implanted electrodes, we trained multivariate classifiers to discriminate spectral activity during learning that predicted remembering from forgetting, then decoded neural activity in later sessions in which we applied stimulation during learning. Stimulation increased encoding-state estimates and recall if delivered when the classifier indicated low encoding efficiency but had the reverse effect if stimulation was delivered when the classifier indicated high encoding efficiency. Higher encoding-state estimates from stimulation were associated with greater evidence of neural activity linked to contextual memory encoding. In identifying the conditions under which stimulation modulates memory, the data suggest strategies for therapeutically treating memory dysfunction.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.cub.2017.03.028DOI Listing
May 2017