Movement Dependence and Layer Specificity of Entorhinal Phase Precession in Two-Dimensional Environments

As a rat moves, grid cells in its entorhinal cortex (EC) discharge at multiple locations of the external world, and the firing fields of each grid cell span a hexagonal lattice. For movements on linear tracks, spikes tend to occur at successively earlier phases of the theta-band filtered local field potential during the traversal of a firing field - a phenomenon termed phase precession. The complex movement patterns observed in two-dimensional (2D) open-field environments may fundamentally alter phase precession. To study this question at the behaviorally relevant single-run level, we analyzed EC spike patterns as a function of the distance traveled by the rat along each trajectory. This analysis revealed that cells across all EC layers fire spikes that phase-precess;indeed, the rate and extent of phase precession were the same, only the correlation between spike phase and path length was weaker in EC layer III. Both slope and correlation of phase precession were surprisingly similar on linear tracks and in 2D open-field environments despite strong differences in the movement statistics, including running speed. While the phase-precession slope did not correlate with the average running speed, it did depend on specific properties of the animal's path. The longer a curving path through a grid-field in a 2D environment, the shallower was the rate of phase precession, while runs that grazed a grid field tangentially led to a steeper phase-precession slope than runs through the field center. Oscillatory interference models for grid cells do not reproduce the observed phenomena

A Chandra Observation of the Obscured Star-Forming Complex W40

The young stellar cluster illuminating the W40 H II region, one of the nearest massive star forming regions, has been observed with the ACIS detector on board the Chandra X-ray Observatory. Due to its high obscuration, this is a poorly-studied stellar cluster with only a handful of bright stars visible in the optical band, including three OB stars identified as primary excitation sources. We detect 225 X-ray sources, of which 85% are confidently identified as young stellar members of the region. Two potential distances of the cluster, 260 pc and 600 pc, are used in the paper. Supposing the X-ray luminosity function to be universal, it supports a 600 pc distance as a lower limit for W40 and a total population of at least 600 stars down to 0.1 Mo under the assumption of a coeval population with a uniform obscuration. In fact, there is strong spatial variation in Ks-band-excess disk fraction and non-uniform obscuration due to a dust lane that is identified in absorption in optical, infrared and X-ray. The dust lane is likely part of a ring of material which includes the molecular core within W40. In contrast to the likely ongoing star formation in the dust lane, the molecular core is inactive. The star cluster has a spherical morphology, an isothermal sphere density profile, and mass segregation down to 1.5 Mo. However, other cluster properties, including a \leq{1} Myr age estimate and ongoing star formation, indicate that the cluster is not dynamically relaxed. X-ray diffuse emission and a powerful flare from a young stellar object are also reported.Comment: Accepted for publication in The Astrophysical Journal. 60 pages, 16 figure

Principles of local computation in the entorhinal cortex

Lebewesen sind jeden Tag Sequenzen von Ereignissen ausgesetzt, die sie sich merken wollen. Es ist jedoch ein allgemeines Problem, dass sich die Zeitskalen des Verhaltens und der Induzierung von neuronalem Lernen um mehrere Größenordnungen unterscheiden. Eine mögliche Lösung könnte "Phasenpräzession" sein - das graduelle Verschieben von Aktionspotential-Phasen relativ zur Theta-Oszillation im lokalen Feldpotential. Phasenpräzession ermöglicht es, Verhaltens-Sequenzen zeitlich zu komprimieren, herunter bis auf die Zeitskala von synaptischer Plastizität. In dieser Arbeit untersuche ich das Phasenpräzessions-Phänomen im medialen entorhinalen Kortex der Ratte. Ich entdecke, dass entorhinale Gitterzellen auf der für das Verhalten relevanten Einzellaufebene Phasenpräzession zeigen und dass die Phasenpräzession in Einzelläufen stärker ist als in zusammengefassten Daten vieler Läufe. Die Analyse von Einzelläufen zeigt zudem, dass Phasenpräzession (i) in Zellen aus allen Schichten des entorhinalen Kortex existiert und (ii) von den komplexen Bewegungsmustern der Ratten in zweidimensionalen Umgebungen abhängt. Zum Abschluss zeige ich, dass Phasenpräzession zelltyp-spezifisch ist: Sternzellen in Schicht II des medialen entorhinalen Kortex weisen klare Phasenpräzession auf, wohingegen Pyramidenzellen in der selben Schicht dies nicht tun. Diese Ergebnisse haben weitreichende Implikationen sowohl für das Lokalisieren des Ursprungs als auch für die m"oglichen Mechanismen von Phasenpräzession.Every day, animals are exposed to sequences of events that are worth recalling. It is a common problem, however, that the time scale of behavior and the time scale for the induction of neuronal learning differ by multiple orders of magnitude. One possible solution could be a phenomenon called "phase precession" - the gradual shift of spike phases with respect to the theta oscillation in the local field potential. Phase precession allows for the temporal compression of behavioral sequences of events to the time scale of synaptic plasticity. In this thesis, I investigate the phase-precession phenomenon in the medial entorhinal cortex of the rat. I find that entorhinal grid cells show phase precession at the behaviorally relevant single-trial level and that phase precession is stronger in single trials than in pooled-trial data. Single-trial analysis further revealed that phase precession (i) exists in cells across all layers of medial entorhinal cortex and (ii) is altered by the complex movement patterns of rats in two-dimensional environments. Finally, I show that phase precession is cell-type specific: stellate cells in layer II of the medial entorhinal cortex exhibit clear phase precession whereas pyramidal cells in the same layer do not. These results have broad implications for pinpointing the origin and possible mechanisms of phase precession

Grid cells exhibit phase precession in two-dimensional environments.

<p>(A) Trajectory (white line) of a rat over 10 minutes in a 1 m² square enclosure together with the firing pattern (black dots) and color-coded firing-rate map of a single grid cell. Data from Sargolini et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100638#pone.0100638-Sargolini1" target="_blank">[26]</a>. Note that many different paths traverse each grid field. (B) Spike phase relative to the local field potential for all passages through three example grid fields. Runs with varying directions and originating from various points in the two-dimensional environment are pooled. The position along each path within the firing field is normalized by the path's total length. (C) Three examples of single runs with different phase-precession slopes <i>m</i> and circular-linear correlation values <i>r</i>. Circular-linear regression lines are indicated. (D) Running direction has no consistent influence on the phase-precession slope. Histogram of p-values of the correlation between entry direction of the animal into a firing field and single-run phase-precession slope. The analysis is restricted to straight runs. Red dashed line indicates significance level p = 0.05. (E) Comparison of single-run phase precession and phase precession assessed by pooling all runs through a particular grid field. Each dot represents a single run; the left panel shows the place-phase correlation, the right panel depicts the slope of phase versus location. A negative slope implies phase precession; note the large variability across different runs. Red crosses denote the average correlation and the average slope. The diagonal line marks the identity. (F) Single-run phase precession in one and two-dimensional environments. (<i>left</i>) Distribution of circular-linear correlation values for runs on a linear track (dashed lines) and in the square arena (full lines). (<i>right</i>) Distribution of phase-precession slopes for the same two conditions. Despite the large speed and movement differences between the linear track and the open field, the phase-precession statistics are similar.</p

Cell Type-Specific Differences in Spike Timing and Spike Shape in the Rat Parasubiculum and Superficial Medial Entorhinal Cortex

The medial entorhinal cortex (MEC) and the adjacent parasubiculum are known for their elaborate spatial discharges (grid cells, border cells, etc.) and the precessing of spikes relative to the local field potential. We know little, however, about how spatio-temporal firing patterns map onto cell types. We find that cell type is a major determinant of spatio-temporal discharge properties. Parasubicular neurons and MEC layer 2 (L2) pyramids have shorter spikes, discharge spikes in bursts, and are theta-modulated (rhythmic, locking, skipping), but spikes phase-precess only weakly. MEC L2 stellates and layer 3 (L3) neurons have longer spikes, do not discharge in bursts, and are weakly theta-modulated (non-rhythmic, weakly locking, rarely skipping), but spikes steeply phase-precess. The similarities between MEC L3 neurons and MEC L2 stellates on one hand and parasubicular neurons and MEC L2 pyramids on the other hand suggest two distinct streams of temporal coding in the parahippocampal cortex

Phase precession in different cortical layers.

<p>Phase-precession slope generally does not depend on the cell's cortical layer (A and C). However, phase precession is decreased in layer III, as measured by the correlation (B and D). The single-run correlation of phase precession is lowest in layer III, and in layers II, V and VI the place-phase correlation is similar. Single-run effects are reproduced when the analysis is restricted to significantly correlated runs (cross-hatched bars). All bars show mean values, error bars depict one s.e.m. and asterisks indicate statistical significance (p<0.05). (E) Spikes show a preferred theta phase. The theta-phase preference is mild, and the weakest phase locking is encountered in layer III. (F) The first spike in a grid-field traversal generally occurs late in the theta cycle for layer II and VI, while it occurs rather early in layers III and V. In (E) and (F), the spike count histogram is normalized so that the sum of all ten bins equals 1. Colors label the cortical layer. Black arrows indicate the vector strength of the spike-phase theta modulation. All spikes were included in the analysis; no prior selection was made. The analysis is based on a total of 95 cells: 20 cells for layer II, 36 from layer III, 10 from layer V and 29 from layer VI.</p

Salient features of the animal's path through a grid field affect phase precession.

<p>(A) The shorter the path is, the steeper the phase precession becomes. (B) The path length and phase precession correlate on a grid field by grid field basis, not just on average across grid fields. (C) First-half slopes are steeper than second-half slopes. The histogram in the inset shows the distance of data points from the diagonal, which is skewed towards smaller slopes in the second half of runs. (D) The phase range increases with path length and saturates at about 210°. (E) More meandering runs (increasing tortuosity) exhibit a less pronounced phase precession. As tortuosity correlates with the path length in a firing field, this finding is consistent with (A). (F) The animal's speed affects the phase-precession slope only weakly, and this effect primarily reflects a correlation between speed and tortuosity. For straight runs through the field, a statistically significant effect of speed on phase precession was not found. (G) Tangential paths lead to steeper phase precession than paths through the center of the field. The eccentricity measures the shortest distance between the path and the center of the firing field. For straight runs, the effect is not statistically significant. (H) Summary of the observed phenomena, with asterisks indicating statistical significance (p<0.05). For all investigated measures, restricting the analysis to straight runs weakens the effects. Error bars indicate one s.e.m. and are slightly offset for clarity in (A), (F), and (G).</p