The Center for Research of Higher Brain Functions,
The Weizmann Institute of Science
Cortical evoked responses exhibit a large variability to repeated presentations of the same stimulus. Here I report that activity evoked by a stimulus does NOT replace the pre-existing pattern, therefore the spatio-temporal patterns in the on-going activity generate the trial by trial fluctuations in the evoked activity. In spite of the large variability, evoked responses can be predicted in single trials by taking into account the preceding on-going activity. A linear model fits the data. Experimental evidence is presented, based on real-time optical imaging, local field potentials (LFP) and single-neurons discharges, all recorded simultaneously in visual cortex of anesthetized cat.
Recently we found that on-going activity does also affect the behavior of the awake macaque monkey. We analyzed LFP and multiple single-unit activity from up to 21 neurons recorded by up to 8 microelectrodes in the motor cortex (MI) during performance of a visually guided arm reaching task, examining the relations between the on-going and evoked activity. We defined on-going activity in a behaving animal as the activity that is not directly related to any stimulus or behavioral event, and which is therefore averaged out by conventional analysis of trial-averaged evoked responses. Also in a monkey performing an arm movement task there is a large trial by trial variability in the cortical activity evoked both by the presentation of a given visual cue, as well as by the initiation of the arm movement. We found that these variable evoked responses were well correlated with the instantaneous on-going activity preceding it. In addition, we found that the reaction time from visual cue onset to the initiation of arm movement was correlated with the amplitude of this preceding on-going activity.
In summary, both in anesthetized and behaving animals the source of the variability in the evoked response is not stochastic noise, but coherent on-going activity. Since these waves are highly structured and they are correlated to the state of the animal we would expect them to play a major role in cortical function.