Neural responses to a natural stimulus ensemble.

<p>At left is a schematic of the experimental setup (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000025#s4" target="_blank">Methods</a> for details). A fly was immobilized with wax, its body in a plastic tube, with its head protruding. Through a small hole in the back of the head an electrode was inserted to record extracellular potentials from H1, a wide field neuron sensitive to horizontal motion. This signal was amplified, fed through a slip ring system to a second stage amplifier and filter, and recorded by a data acquisition card. In synchrony with its master timer clock, the DAQ card generated a 500 Hz frame clock signal. Every 2 ms, through a bidirectional parallel port, this clock triggered a successive read of a divisor value from a file stored in the stimulus laptop computer. The Intel 8254 Counter/Timer chip used this divisor value to divide down the pulse frequency of a free running 8 MHz clock. In this way, in each successive 2 ms interval, and in strict synchrony with the data taking clock, a defined and evenly spaced burst of pulses was produced. These pulses drove the stepper motor, generating the angular velocity signal. A brief segment of this motion stimulus is shown in the top right panel, below which we plot a raster of action potentials from H1 in response to 100 repetitions of this stimulus. At bottom we expand the scale to illustrate (at left) that individual spikes following a transition from negative to positive velocity jitter from trial to trial by ∼1 ms: The standard deviations of spike times shown here are 0.72 ms for the first spike (•), 0.81 ms for the second spike (°), and 1.22 ms for the third spike (×). When we align the first spikes in this window, we see (at right) that the jitter of interspike intervals is even smaller, 0.21 ms for the first interval and 0.69 ms for the second interval. Our challenge is to quantify the information content of such precise responses.