Array-wide responses to individual stimuli in an experiment from protocol II.3. Spikes recorded on any electrode, 10–50 ms after a stimulus, are counted together and represented on a gray scale. Each pixel is one stimulus response; stimulation order is top to bottom, then left to right. Separating white bars indicate division into 1-hr windows (corresponding to blocks of 20 trials each per stimulation electrode); the dashed line represents the tetanus. Stimuli that evoked bursts show up as black pixels. The red arrow points to a burst that, on its own, was responsible for the top-most horizontal blue stripe in Figure 4A, right sub-panel.

Results of Series III: Changes induced in specific responses by paired-pulse tetanization. A Changes induced by tetanization using protocols III.1 (left) and III.4 (right) in responses to electrodes S₁, S₂ and the control electrodes, compared to spontaneous drift. Plot symbols indicate stimulation electrode; key applies to both panels. Each point corresponds to one recording electrode in one of 8 experiments. All responses are averaged over 120 presentations of the stimulus. In protocol III.4 only, responses to electrodes S₁, S₂ were significantly affected by tetanization. B Summary of all experiments. Shown are mean and SEM across all electrode pairs in all experiments for a given protocol (N = 67, 67, 142, and 167 for protocols III.1, III.2, III.3, and III.4). Large bars for spontaneous changes in the panel for protocol III.1 indicate ongoing drift. Asterisks above bars for protocol III.4 mark significant differences between changes concomitant with tetanization and spontaneous changes: p < 0.001 (***), paired t-test. Asterisks below bars indicate significant differences between changes in responses to tetanized electrodes and control electrodes: p < 0.001 (***), unpaired t-test.

Reactions to pharmacological manipulations. A Adding 1 mM Mg²⁺ (to the baseline of 0.8 mM) stopped spontaneous bursts, and reduced the array-wide spike detection rate (ASDR) outside of bursts slightly. B Adding 1 or 3 mM K⁺ (to the baseline of 5.4 mM) increased burst rates and inter-burst firing rates. The fraction of spikes that occurred inside bursts (as opposed to between bursts) remained similar. C CNQX, an AMPA channel blocker, inhibited bursting and reduced baseline ASDR. D AP5, an NMDA channel blocker, inhibited bursting for a limited period of time. E Bicuculline methiodide (BMI), a GABA channel blocker, increased burstiness. (Data for A–E were obtained from different cultures, N = 1 for each substance. Baselines were recorded immediately prior to adding drugs. Since the results were fully consistent with expectations, a more in-depth investigation was deemed unnecessary.)

Confirmation of the elasticity results of Eytan et al. (2003) [34]. One electrode was initially stimulated at 1 Hz for one hour (solid symbols), while another was stimulated at 1/60 Hz (open symbols). Then, the roles were reversed. The graph shows the number of spikes recorded array-wide, 15–30 ms after a stimulus, normalized to the value at the beginning of the experiment. 'Start' refers to the first stimulus to the 'slow' electrode, or the average of the first 5 stimuli to the 'fast' electrode; 'Early' refers to the average of the first 5 stimuli to the 'slow' electrode, or the average of the 5 × 4 surrounding stimuli to the 'fast' electrode; 'Late' refers to average of the last 20 stimuli to the 'slow' electrode, or the average of the last 1200 stimuli to the 'fast' electrode. (This slightly unusual way of organizing the data was used to balance the need to collect sufficient statistics with the desire to measure as close as possible to the beginning of the experiment.) Data are mean ± SEM (in log-space) from 16 experiments on 4 cultures. The sequence of open and closed symbols near the top of the graph are a cartoon of the stimulation sequence; the actual number of stimuli was much greater.

Results of Series I: Changes induced in spontaneous bursting by strong stimulation through several electrodes. A An exceptional example from protocol I.1, where the induction sequence resulted in reduced burst rates and sizes. Note though, that spontaneous drift in the burst rate before the analyzed portion of the recording was of comparable magnitude. B A typical example from protocol I.2, showing no effect. Induction sequences in A and B are marked by gray bars. Top to bottom: number of spikes in individual bursts; number of bursts in successive one-hour time windows (with error bars based on assumed Poisson statistics); total number of spikes in bursts in successive hours. C A summary of all experiments in Series I shows that changes concomitant with induction were no larger than spontaneous changes. D Comparison of spontaneous changes and changes concomitant with induction in hourly burst rates. Unlike in C, all changes were normalized to the hourly burst rate before the induction sequence. Data are mean ± SEM of absolute values of changes; N = 4, 16, 8 for protocols I.1, I.2, I.3 respectively. Paired t-tests revealed no significant effects of the induction sequence.

Results of Series II: Experiments on plasticity expressed in stimulus–response maps. A An example from protocol II.3. Colored pixels represent changes in the average number of spikes on a given recording electrode 10–50 ms after a test pulse to a given stimulation electrode. The horizontal stripes of similar coloration reveal stimulation-site-specific changes. However, spontaneous changes (right) were comparable in magnitude to changes concomitant with tetani (left). B A direct comparison between stimulation-site-specific changes and recording-site-specific changes across tetani reveals that stimulation-site-specific changes were dominant in all experiments. Each point corresponds to one experiment. Plot symbols indicate tetanization protocols; arrows mark data points that fell outside the plot limits. C Direct comparison between stimulation-site-specific changes concomitant with tetanization and due to spontaneous drift reveals that tetanization does not cause enhanced change compared to drift. D Summary of data in B. All values were normalized by . Asterisks indicate significance: p < 0.05 (*) or p < 0.001 (***), two-tailed t-test, N = 8, 6, 6, 16, 16 for protocols II.1, II.2, II.5. E Summary of data in C, same normalization as in D. T-tests revealed no significant effects of tetanization.