arXiv subjects

Jonathan Z. Simon

Publications and source records attributed to Jonathan Z. Simon.

7 recordsLinked to original sources

arXiv2006

Temporal Symmetry in Primary Auditory Cortex: Implications for Cortical Connectivity

Neurons in primary auditory cortex (AI) in the ferret (Mustela putorius) that are well described by their spectro-temporal response field (STRF), are found also to have a distinctive property that we call temporal symmetry. For temporally symmetric neurons, every temporal cross-section of the STRF (impulse response) is given by the same function of time, except for a scaling and a "Hilbert rotation". This property held in 85% of neurons (123 out of 145) recorded from awake animals, and in 96% of neurons (70 out of 73) recorded from anesthetized animals. This property of temporal symmetry is highly constraining for possible models of functional neural connectivity within and into AI. We find that the simplest models of functional thalamic input, from the ventral Medial Geniculate Body (MGB), into the entry layers of AI are ruled out because they are incompatible with the constraints of the observed temporal symmetry. This is also the case for the simplest models of functional intracortical connectivity. Plausible models that do generate temporal symmetry, both from thalamic and intracortical inputs, are presented. In particular, we propose that two specific characteristics of the thalamo-cortical interface may be responsible. The first is a temporal mismatch between the fast dynamics of the thalamus and the slow responses of the cortex. The second is that all thalamic inputs into a cortical module (or a cluster of cells) must be restricted to one point of entry (or one cell in the cluster). This latter property implies a lack of correlated horizontal interactions across cortical modules during the STRF measurements. The implications of these insights in the auditory system, and comparisons with similar properties in the visual system, are explored.

q-bio.NC↗

arXiv2006

Concurrent encoding of frequency and amplitude modulation in human auditory cortex: MEG evidence

A natural sound can be described by dynamic changes in envelope (amplitude) and carrier (frequency), corresponding to amplitude modulation (AM) and frequency modulation (FM) respectively. Although the neural responses to both AM and FM sounds are extensively studied in both animals and humans, it is uncertain how they are co-represented when changed simultaneously but independently, as is typical for ecologically natural signals. This study elucidates the neural coding of such sounds in human auditory cortex using magnetoencephalography (MEG). Using stimuli with both sinusoidal modulated envelope (f_{AM}, 37 Hz) and carrier frequency (f_{FM}, 0.3 - 8 Hz), it is demonstrated that AM and FM stimulus dynamics are co-represented in the neural code of human auditory cortex. The stimulus AM dynamics are represented neurally with AM encoding, by the auditory Steady State Response (aSSR) at f_{AM}. For sounds with slowly changing carrier frequency ((f_{FM} < 5 Hz), it is shown that the stimulus FM dynamics are tracked by the phase of the aSSR, demonstrating neural phase modulation (PM) encoding of the stimulus carrier frequency. For sounds with faster carrier frequency change ((f_{FM} >= 5 Hz), it is shown that modulation encoding of stimulus FM dynamics persists, but the neural encoding is no longer purely PM. This result is consistent with the recruitment of additional neural AM encoding over and above the original neural PM encoding, indicating that both the amplitude and phase of the aSSR at f_{AM} track the stimulus FM dynamics. A neural model is suggested to account for these observations.

q-bio.NC↗

arXiv2005

Stimulus-invariant processing and spectrotemporal reverse correlation in primary auditory cortex

The spectrotemporal receptive field (STRF) provides a versatile and integrated, spectral and temporal, functional characterization of single cells in primary auditory cortex (AI). In this paper, we explore the origin of, and relationship between, different ways of measuring and analyzing an STRF. We demonstrate that STRFs measured using a spectrotemporally diverse array of broadband stimuli -- such as dynamic ripples, spectrotemporally white noise, and temporally orthogonal ripple combinations (TORCs) -- are very similar, confirming earlier findings that the STRF is a robust linear descriptor of the cell. We also present a new deterministic analysis framework that employs the Fourier series to describe the spectrotemporal modulations contained in the stimuli and responses. Additional insights into the STRF measurements, including the nature and interpretation of measurement errors, is presented using the Fourier transform, coupled to singular-value decomposition (SVD), and variability analyses including bootstrap. The results promote the utility of the STRF as a core functional descriptor of neurons in AI.

q-bio.NC↗

arXiv2005

Fully Complex Magnetoencephalography

Complex numbers appear naturally in biology whenever a system can be analyzed in the frequency domain, such as physiological data from magnetoencephalography (MEG). For example, the MEG steady state response to a modulated auditory stimulus generates a complex magnetic field for each MEG channel, equal to the Fourier transform at the stimulus modulation frequency. The complex nature of these data sets, often not taken advantage of, is fully exploited here with new methods. Whole-head, complex magnetic data can be used to estimate complex neural current sources, and standard methods of source estimation naturally generalize for complex sources. We show that a general complex neural vector source is described by its location, magnitude, and direction, but also by a phase and by an additional perpendicular component. We give natural interpretations of all the parameters for the complex equivalent-current dipole by linking them to the underlying neurophysiology. We demonstrate complex magnetic fields, and their equivalent fully complex current sources, with both simulations and experimental data.

q-bio.NC↗

arXiv1998

Measuring the dynamics of neural responses in primary auditory cortex

We review recent developments in the measurement of the dynamics of the response properties of auditory cortical neurons to broadband sounds, which is closely related to the perception of timbre. The emphasis is on a method that characterizes the spectro-temporal properties of single neurons to dynamic, broadband sounds, akin to the drifting gratings used in vision. The method treats the spectral and temporal aspects of the response on an equal footing.

q-bio.NC↗

arXiv1996

Hamiltonian thermodynamics of a Lovelock black hole

We consider the Hamiltonian dynamics and thermodynamics of spherically symmetric spacetimes within a one-parameter family of five-dimensional Lovelock theories. We adopt boundary conditions that make every classical solution part of a black hole exterior, with the spacelike hypersurfaces extending from the horizon bifurcation three-sphere to a timelike boundary with fixed intrinsic metric. The constraints are simplified by a Kuchař-type canonical transformation, and the theory is reduced to its true dynamical degrees of freedom. After quantization, the trace of the analytically continued Lorentzian time evolution operator is interpreted as the partition function of a thermodynamical canonical ensemble. Whenever the partition function is dominated by a Euclidean black hole solution, the entropy is given by the Lovelock analogue of the Bekenstein-Hawking entropy; in particular, in the low temperature limit the system exhibits a dominant classical solution that has no counterpart in Einstein's theory. The asymptotically flat space limit of the partition function does not exist. The results indicate qualitative robustness of the thermodynamics of five-dimensional Einstein theory upon the addition of a nontrivial Lovelock term.

gr-qc↗

arXiv1992

Einstein Equation with Quantum Corrections Reduced to Second Order

We consider the Einstein equation with first order (semiclassical) quantum corrections. Although the quantum corrections contain up to fourth order derivatives of the metric, the solutions which are physically relevant satisfy a reduced equations which contain derivatives no higher than second order. We obtain the reduced equations for a range of stress-energy tensors. These reduced equations are suitable for numerical solution, are expected to contain fewer numerical instabilities than the original fourth order equations, and yield only physically relevant solutions. We give analytic and numerical solutions or reduced equations for particular examples, including Friedmann-Lema\^ıtre universes with cosmological constant, a spherical body of constant density, and more general conformally flat metrics.

gr-qc↗