Imaging brain activity using voltage sensitive dyes has revealed propagating waves of neuronal activity in the brain. It’s been hypothesized that propagating waves of activity may contribute to cerebral cortical signal processing by determining when and where the cortex is depolarized in relation to a sensory or motor event.
Spiral waves are a particular kind of propagating wave that rotate around a center point. Spiral waves have been observed in turtle visual cortex and in brain slices but it wasn’t known if they may be found in the intact mammalian brain. Today’s paper “Spiral Wave Dynamics in Neocortex” (published December 9, 2010 in Neuron) reports on an investigation into the existence of spiral waves in intact rat brains.
The research team placed rats into two states and then looked for spiral waves emerging from the cerebral cortex. One state was induced by applying carbachol and bicuculline to the brain. Local excitatory connections in the cerebral cortex were greatly enhanced in this condition and oscillations at around 10 Hertz were induced. The other state was achieved with low levels of pentobarbital anesthesia. In this state, the cortex alternated between theta (around 6 Hz) and delta (1-4 Hz) rhythms that resembled the rapid eye movement (REM) state in rodent natural sleep.
Spiral waves were observed under both conditions. Furthermore, the emergence of spiral waves appeared to have a large impact on the oscillation frequency, spatial coherence, and amplitude of cortical activity.
The paper examined human touch perception. The authors looked at what happens in the sensory cerebral cortex devoted to the skin when a human detects being touched. The team recorded magnetic fields produced by electrical currents in the brain (magnetoencephalography or MEG) and carried out computational neural modeling.
A brief touch to a subject’s fingertip evoked a consistent MEG signal in their primary somatosensory cortex. The MEG signal predicted that the subject would detect being touched beginning about 70 milliseconds after the stimulus was initiated.
A simulation of the evoked response was created that reproduced all the major peaks recorded from the human subjects. The partial model of primary somatosensory cortex included 10 pyramidal cells and 3 inhibitory interneurons in layers 2 and 3 and another 10 pyramidal cells and 3 inhibitory interneurons in layer 5. Please see the paper for details about connectivity. The model provided a reasonable interpretation of the electrophysiological origin of the evoked primary somatosensory cortex response and the response characteristics correlated with human perception.
Note: Don’t forget to compile the files in the project folder. On the Macintosh computer you drag the project folder to NEURON’s mknrndll program icon.
NEURON should be displaying about eleven windows. Go to the window titled “RunControl” and click on the “Init & Run” button. You should see the model’s electrical activity being traced out as it’s generated in about five of those windows. For instance, one window should display the membrane potentials of a layer 2/3 pyramidal cell and a layer 5 pyramidal cell like in Figure 1 above. Another window should display a magnetoencephalography trace like in Figure 2 above.
Note: ModelDB is the computation neuroscience model repository within the larger SenseLab online data repository.
Figure 1. Example views of the “Pyramidal Neuron Deep, Superficial; Aspiny, Stellate (Mainen and Sejnowski 1996)” model downloaded from ModelDB and running in NEURON. (Left) A view of a reconstructed layer 5 pyramidal neuron. (Right) One second (1000 milliseconds) trace of the membrane potential voltage of a layer 3 pyramidal cell (blue) and a layer 5 pyramidal cell (red). The layer 5 pyramidal cell trace is from the neuron displayed left.
Neuron response properties are shaped by a combination of cell membrane conduction properties as defined by the types and distribution of ion channels and cell morphology as defined by the geometry of a neuron. The 1996 paper under review highlighted the importance of morphology by showing that neurons sharing common types and distributions of ion channels and differing only in the shapes of their dendrites could display many different firing patterns.
Note: Those with more than a passing interested in using NEURON may find the book or e-book by its creators useful “The NEURON Book.”
Note: NEURON runs under most computing environments. Details on setup and trouble shooting vary by platform but are well documented at the NEURON website.
Note: Don’t forget to compile the files in the project folder. On the Macintosh computer you drag the project folder to NEURON’s mknrndll program icon.
Load the model’s demofig1.hoc file.
The NEURON application should be running and displaying some windows. A window titled “Figure 1” should contain four buttons displaying the cerebral cortical layer and name of four different types of neuron. The letters a, b, c, and d correspond to the letters the paper’s Figure 1.
Click on the “a. L3 Aspiny” button. A new window showing the same reconstructed cell as in the paper’s Figure 1a should open.
Now go to the window titled “RunControl” and click on the “Init & Run” button. You should see the Aspiny neuron’s electrical activity actively being traced in a Graph window. When it’s finished, the 1 second (1000 millisecond) trace should be identical to the one in the paper’s Figure 1a.
This is how computational neuroscience papers should be published! Reconstruct the paper’s figures and play with the simulations while reading the paper itself.
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