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 s
pike-train pairs. In the absence of stimulation (i.e., in the dark), RGC an
d 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 activities in the pair cease to be mirror images of each other. A d
rifting-grating stimulus yields RGC and LGN driven spike trains similar in
character to those observed in the maintained discharge, with two notable d
istinctions: 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 drift! ng-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 o
f 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. (C) 2001 Academic Press.