Cosyne 2007 Workshops
February 26-27, 2007
The Canyons, Utah
Nachum Ulanovsky
Hippocampal cellular and network activity in freely-moving echolocating bats
The hippocampus is crucial for episodic and spatial memory. In freely-moving rodents, hippocampal pyramidal neurons exhibit spatially-selective firing when the animal passes through a neuron’s ‘place field’, and theta-band oscillation is continuously present during locomotion. Here we report on the first hippocampal recordings from echolocating bats, mammals phylogenetically distant from rodents. These recordings revealed place-cells very similar to rodents, as well as rodent-like high frequency ‘ripple’ oscillations. Theta oscillation, however, differed from rodents in two important ways: (i) theta occurred when bats explored the environment without locomoting, using distal sensing through echolocation; (ii) theta was not continuous but occurred in short intermittent bouts. The intermittence of theta suggests that models of hippocampal function relying explicitly on continuous theta may not apply to bats. These data support the hypothesis that theta in mammals is involved in sequence-learning, and hence theta power increases with sensory-input rates -– explaining why theta-power correlates with running-speed in rodents and with echolocation-call-rate in bats. Finally, we demonstrate rapid modulation of hippocampal spatial representation in the bat, with a ~1-second timescale: Place fields were most spatially-selective immediately after an echolocation call, when the returning echoes from the environment brought a wealth of new sensory information –- and then the place-fields broadened, or ‘diffused’, with the passage of time after the echolocation call, when echo information was no longer arriving. Such rapid dynamics of place fields may account for out-of-field ‘noise’ spikes observed when activity is averaged over many minutes, and it might also underlie the high variability of place-cell discharges during individual passes through a place field.
