The exact timing of cortical afferent activity is instrumental for the correct coding
and retrieval of internal and external
stimuli. Thalamocortical inputs represent the most significant subcortical pathway
to the cortex, but the precise timing and
temporal variability of thalamocortical activity is not known. To examine this question,
we studied the phase of thalamic
action potentials relative to cortical oscillations and established correlations among
phase, the nuclear location of
the thalamocortical neurons, and the frequency of cortical activity. The phase of
thalamic action potentials depended on
the exact frequency of the slow cortical oscillation both on long (minutes) and short
(single wave) time scales. Faster waves
were accompanied by phase advancement in both cases. Thalamocortical neurons located
in different nuclei fired at
significantly different phases of the slow waves but were active at a similar phase
of spindle oscillations. Different thalamic
nuclei displayed distinct burst patterns. Bursts with a higher number of action potentials
displayed progressive phase
advancement in a nucleus-specific manner. Thalamic neurons located along nuclear borders
were characterized by mixed burst
and phase properties. Our data demonstrate that the temporal relationship between
cortical and thalamic activity is not fixed
but displays dynamic changes during oscillatory activity. The timing depends on the
precise location and exact activity of
thalamocortical cells and the ongoing cortical network pattern. This variability of
thalamic output and its coupling to cortical
activity can enable thalamocortical neurons to actively participate in the coding
and retrieval of cortical signals.
