Biomedical Engineering Reference
In-Depth Information
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FIgURE 2.10:
Examples of neuron tuning. Left plot is the tuning plot with neuronal tuning at 315°
and depth of 1. The right plot is for a neuron with tuning at 225° and a depth of 0.93.
To assess the degree of tuning, a metric called the “tuning depth” is often used. This quantity
is defined as the difference between the maximum and minimum values in the cellular tuning. For
an impartial comparison of the cellular tuning depth, the tuning curves are usually normalized by
the standard deviation of the firing rate. This measurement relates how “ peaky ” the tuning curve is
for each cell and is an indicator of how well modulated the cell's firing rate is to the kinematic pa-
rameter of interest. The normalized tuning depth for 185 neurons computed from three kinematic
vectors, hand positions, hand velocities, and hand accelerations, is presented in Figure
2.11
. The
cortices where the microelectrodes were implanted are also shown in the figure. From this analysis,
one can clearly see that most tuned neurons are in the primary motor cortex for all of the kinematic
vectors used to calculate the tuning depth.
In one-dimensional kinematic spaces, the vector space is restricted to vector magnitude. Typ-
ical histograms are ramp functions that saturate because neurons tend to increase their activity to
the maximum firing rate, so the range is still finite [
69
]. Despite these differences, the tuning depth
of each cell can still be used as a measure of information content provided by all neurons.
14
Because
computing the tuning depth from finite data is noisy, the curves are also usually smoothened by
14
We have observed that the tuning of a cell can vary as a function of the delay between the generation of the neural
activity and the physical movement of the hand. Because we are interested in tuning depth, we are looking for cells
that have the smallest variance in their tuning function. After computing tuning variance across all cells for delays up
to 1 sec, we found that the sharpest tuning occurs at the 0th delay or the instantaneous time alignment.
