Non-hexagonal neural dynamics in vowel space

Zeynep Kaya; Mohammadreza Soltanipour; Alessandro Treves; Zeynep Kaya; Mohammadreza Soltanipour; Alessandro Treves

doi:10.3934/Neuroscience.2020015

AIMS Neuroscience

2020, Volume 7, Issue 3: 275-298. doi: 10.3934/Neuroscience.2020015

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Non-hexagonal neural dynamics in vowel space

1.
SISSA–Cognitive Neuroscience, via Bonomea 265, 34136 Trieste, Italy
2.
NTNU–Centre for Neural Computation, Trondheim, Norway

Received: 29 April 2020 Accepted: 27 July 2020 Published: 04 August 2020

Are the grid cells discovered in rodents relevant to human cognition? Following up on two seminal studies by others, we aimed to check whether an approximate 6-fold, grid-like symmetry shows up in the cortical activity of humans who “navigate” between vowels, given that vowel space can be approximated with a continuous trapezoidal 2D manifold, spanned by the first and second formant frequencies. We created 30 vowel trajectories in the assumedly flat central portion of the trapezoid. Each of these trajectories had a duration of 240 milliseconds, with a steady start and end point on the perimeter of a “wheel”. We hypothesized that if the neural representation of this “box” is similar to that of rodent grid units, there should be an at least partial hexagonal (6-fold) symmetry in the EEG response of participants who navigate it. We have not found any dominant n-fold symmetry, however, but instead, using PCAs, we find indications that the vowel representation may reflect phonetic features, as positioned on the vowel manifold. The suggestion, therefore, is that vowels are encoded in relation to their salient sensory-perceptual variables, and are not assigned to arbitrary grid-like abstract maps. Finally, we explored the relationship between the first PCA eigenvector and putative vowel attractors for native Italian speakers, who served as the subjects in our study.
- grid cells,
- vowel space,
- formants,
- hexagonal symmetry,
- EEG,
- diphthongs
Citation: Zeynep Kaya, Mohammadreza Soltanipour, Alessandro Treves. Non-hexagonal neural dynamics in vowel space[J]. AIMS Neuroscience, 2020, 7(3): 275-298. doi: 10.3934/Neuroscience.2020015

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Abstract

Are the grid cells discovered in rodents relevant to human cognition? Following up on two seminal studies by others, we aimed to check whether an approximate 6-fold, grid-like symmetry shows up in the cortical activity of humans who “navigate” between vowels, given that vowel space can be approximated with a continuous trapezoidal 2D manifold, spanned by the first and second formant frequencies. We created 30 vowel trajectories in the assumedly flat central portion of the trapezoid. Each of these trajectories had a duration of 240 milliseconds, with a steady start and end point on the perimeter of a “wheel”. We hypothesized that if the neural representation of this “box” is similar to that of rodent grid units, there should be an at least partial hexagonal (6-fold) symmetry in the EEG response of participants who navigate it. We have not found any dominant n-fold symmetry, however, but instead, using PCAs, we find indications that the vowel representation may reflect phonetic features, as positioned on the vowel manifold. The suggestion, therefore, is that vowels are encoded in relation to their salient sensory-perceptual variables, and are not assigned to arbitrary grid-like abstract maps. Finally, we explored the relationship between the first PCA eigenvector and putative vowel attractors for native Italian speakers, who served as the subjects in our study.

Acknowledgments

Funded by Human Frontier Science Program RGP0057/2016. Discussions and assistance in particular with EEG methods by Yamil Vidal Dos Santos, Yair Lakretz, Massimiliano Trippa and others in the Human Frontier collaboration are gratefully acknowledged. Research in partial fulfillment of the requirements for the PhD degree of ZK. AT acknowledges the gracious hospitality of the Kavli Institute for Theoretical Physics in Santa Barbara, where this work was partially written up.

Conflict of interest

The authors declare no conflict of interest.

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