Figure 4

Cluster plot depicted changes of the temporal distribution patterns of neural activity before (A) and after morphine administration (B). A clustering analysis was performed to classify neuronal responses depending on the similarities in patterns of excitation or inhibition around stimulation events. The firing rates were transferred into z-scores and neurons with z-scores > 2 were accepted as significantly excited and < -2 as significantly inhibited. Colorbar indicates z-scores (light yellow for highest and light blue for lowest). Each line of the image represents normalized activity of one neuron. C1-C5 represent different categories according to the response patten. The cluster analysis revealed that the cortical and thalamic areas contain units with five clusters of coding patterns in response to the noxious radiant heat (A). All of the excitatory responses were weakened by systemic injection of morphine (B).