Rhythms of the Brain: The "Neurophysics" of Space and Learning

What does space have to do with learning? And what does either of them have to do with brain rhythms? This is the subject of a recent experiment funded by the W. M. Keck Foundation, conducted by a team of researchers at UCLA led by Professor Mayank Mehta. Surprisingly, the team found the same hippocampal gamma oscillations that increase with running speed are also known to increase with learning, attention and awareness! Thus, these findings connect the internal world of attention with the external world of speed, which can be readily measured and manipulated. In fact, prior work by Mehta and colleagues has shown that brain rhythms are indispensible for learning.

Just like Einstein answered the question of the nature of physical space by postulating the general theory of relativity, the UCLA team is exploring the nature of neural representation of space and how it relates to learning and memory. They intend to solve this question at the interface of physics and neuroscience, in an emerging field of research called “Neurophysics”. A brain region called the hippocampus is known to be critical for spatial navigation. Interestingly, hippocampal damage also results in profound deficits in learning and memory of facts and events. Specific neurons in the hippocampus fire in a spatially selective fashion and are called “place cells.”

Figure 1. Hippocampal place cell: A mouse walked on a narrow, 1.5 meter long track. Each tiny grey dot shows the position of the mouse. Each tiny red dot shows the position of the mouse when a hippocampal neuron fired a spike. The red dots are concentrated in a small region of space to the right. Hence these neurons are called 'place cells'.

It was thought that the spatial selectivity of place cells is sufficient to mediate navigation. Along with graduate students Zhiping Chen and Evgeny Resnik, Mehta argued that in order to navigate, it is not only necessary to know where you are, but equally important to know how fast you are traveling. They further theorized that as position and speed are independent variables, they should be represented by independent mechanisms in the brain. The team measured the activity of neurons from the hippocampus using microwires, while mice navigated in space and they found that as running speed increased, the degree of rhythmic activity in the 25-100Hz range, called the gamma rhythm, increased as well. Since different frequencies are mathematically independent, the gamma rhythmic response to speed is mathematically independent of the average activity levels of neurons, thereby providing a representation for speed independent of representation of space. This research raises many new and exciting questions and brings us closer to understanding the “neurophysics” of space and memory.

Figure 2. The magnitude of gamma rhythm is shown as a function of the running speed (x-axis) and the phase of low frequency (~8Hz) theta rhythm. Warmer colors indicate greater amplitude. Not only does the magnitude of gamma rhythm increase with running speed, the phase of theta rhythm where the highest gamma amplitude occurs, shifts to lower values at higher running speeds (white dotted line). This shows that both the magnitude of gamma rhythm and the phase of theta rhythm where the gamma occurred contained information about the running speed.

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