Failure in identification of overlapping spikes from multiple neuron activity causes artificial correlations

Recording of multiple neurons from a single electrode is common practice du ring extra-cellular recordings. Separation and sorting of spikes originatin g from the different neurons can be performed either on-line or off-line us ing multiple methods for pattern matching. However, all spike sorting techn iques fail either fully or partially in identifying spikes from multiple ne urons when they overlap due to occurrence within a short time interval. Thi s failure, that we termed the 'shadowing effect', causes the well-known phe nomenon of decreased cross-correlation at zero offset. However, the shadowi ng effect also causes other artifacts in the auto and cross-correlation of the recorded neurons. These artifacts are significant mainly in brain areas with high firing rate or increased firing synchrony leading to a high prob ability of spike overlap. Cross correlation of cells recorded from the same electrodes tends to reflect the autocorrelation functions of the two cells , even when there are no functional interactions between the cells. Therefo re, the cross-correlation function tends to have a short-term (about the le ngth of the refractory period) peak. A long-term (hundreds of milliseconds to a few seconds) trough in the cross-correlation can be seen in cells with bursting and pausing activities recorded from the same electrode. Even the autocorrelation functions of the recorded neurons feature firing propertie s of other neurons recorded from the same electrode. Examples of these effe cts are given from our recordings in the globus pallidus of behaving primat es and from the literature. Results of simulations of independent simple mo del neurons exhibit the same properties as the recorded neurons. The effect is analyzed and can be estimated to enable better evaluation of the underl ying firing patterns and the actual synchronization of neighboring neurons recorded by a single electrode. (C) 2001 Elsevier Science B.V. All rights r eserved.