  We employ a number of statistical measures to characterize neural discharge
activity in cat retinal ganglion cells (RGCs) and in their target lateral
geniculate nucleus (LGN) neurons under various stimulus conditions, and we
develop a new measure to examine correlations in fractal activity between
spike-train pairs. In the absence of stimulation (i.e., in the dark), RGC and
LGN discharges exhibit similar properties. The presentation of a constant,
uniform luminance to the eye reduces the fractal fluctuations in the RGC
maintained discharge but enhances them in the target LGN discharge, so that
neural activity in the pair no longer mirror each other. A drifting-grating
stimulus yields RGC and LGN driven spike trains similar in character to those
observed in the maintained discharge, with two notable distinctions: action
potentials are reorganized along the time axis so that they occur only during
certain phases of the stimulus waveform, and fractal activity is suppressed.
Under both uniform-luminance and drifting-grating stimulus conditions (but not
in the dark), the discharges of pairs of LGN cells are highly correlated over
long time scales; in contrast discharges of RGCs are nearly uncorrelated with
each other. This indicates that action-potential activity at the LGN is subject
to a common fractal modulation to which the RGCs are not subjected.
