Paired-pulse transcranial magnetic stimulation protocol applied to visual cortex of anaesthetized cat: effects on visually evoked single-unit activity

Abstract

In this study, we tested the paired-pulse transcranial magnetic stimulation (ppTMS) protocol - a conditioning stimulus (CS) given at variable intervals prior to a test stimulus (TS) - for visually evoked single-unit activity in cat primary visual cortex. We defined the TS as being supra-threshold when it caused a significant increase or decrease in the visually evoked activity. By systematically varying the interstimulus interval (ISI) between 2 and 30 ms and the strength of CS within the range 15-130% of TS, we found a clear dependence of the ppTMS effect on CS strength but little relation to ISI. The CS effect was strongest with an ISI of 3 ms and steadily declined for longer ISIs. A switch from enhancement of intracortical inhibition at short ISIs (2-5 ms, SICI) to intracortical facilitation (ICF) at longer ISIs (7-30 ms), as demonstrated for human motor cortex, was not evident. Whether the CS caused facilitation or suppression of the TS effect mainly depended on the strength of CS and the polarity of the TS effect: within a range of 60-130% a positive correlation between ppTMS and TS effect was evident, resulting in a stronger facilitation if the TS caused facilitation of visual activity, and more suppression if the TS was suppressive by itself. The correlation inverted when CS was reduced to 15-30%. The ppTMS effect was not simply the sum of the CS and TS effect, it was much smaller at weak CS strength (15-50%) but stronger than the sum of CS and TS effects at CS strength 60-100%. Differences in the physiological state between sensory and motor cortices and the interactions of paired synaptic inputs are discussed as possible reasons for the partly different effects of ppTMS in cat visual cortex and human motor cortex.

Figure 1. Analysis of paired-pulse transcranial magnetic stimulation (ppTMS) effect on visually evoked activity

A, peristimulus time histogram (PSTH) showing single-unit activity evoked by an optimally orientated bright bar moving back and forth across the receptive field of the unit as indicated by the drawing below the diagram. TMS was given close to visually enhanced activity (see arrow). B, mean activity within a time window of 500 ms following TMS (time of suprathreshold test stimulus (TS), the second stimulus in the case of ppTMS). Curves 1–3 are: (1) activity after TS only, (2) activity following ppTMS (in this case a conditioning stimulus (CS) of 60% strength of TS, given 4 ms prior to TS), (3) difference between curves 1 and 2 (2 minus 1). Activity rates were calculated as the mean of 20 identical trials. Three different time windows following TMS were analysed (as indicated by the dashed lines): 30–100 ms, 100–200 ms and 200–500 ms. For further analysis, the activity within these time windows was averaged for each condition.

Figure 2. Correlation between ppTMS effect and TS effect

Scatter plots show the relationship between changes in visual activity resulting from single-pulse TMS (only the test stimulus (TS), abscissa) and additional changes achieved with ppTMS (ordinate). The effect of ppTMS was measured as the difference from the TS effect (ppTMS-elicited activity minus TS-elicited activity). Separate diagrams are shown for each ISI between CS and TS tested (2–30 ms), with CS strength in the range of 60–90% of TS strength. Data points give mean activity in spikes s⁻¹ within 30–100 ms after TMS (time window 1) for 51 measurements. A clear correlation between ppTMS and TS effect is evident for all ISIs. Slopes of the regression lines and Pearson correlation values are given in Fig. 3. Pearson correlation was significant in all cases with α < 0.001.

Figure 3. CS-dependent change in correlation between ppTMS effect and TS effect

A, as in Fig. 2, scatter plots show the relationship between ppTMS- and TS-induced changes in visual activity. Each of the scatter plots shows data for ppTMS with ISI of 3 ms only, but for 5 different ranges of CS strength. Changing the ISI did not affect the correlation as shown in Fig. 2 and panel B of this figure. B, diagrams showing the dependence of slope of the regression line (black curves) and the dependence of Pearson correlation (grey curves) on ISI for each of the 5 ranges of CS strength. Pearson correlation was significant for CS range 15–30% (α < 0.01) and for the range 60–130% (α < 0.001).

Figure 4. ISI- and CS-dependent mean change in visual activity with ppTMS

For the 5 different ranges of CS strength analysed, mean change in visual activity caused by ppTMS is plotted versus the ISI used in ppTMS. Mean rates of activity were calculated separately for cases in which visual activity was either facilitated (black curve) or suppressed (grey curve) by the TS alone. Mean change in visual activity caused by TS only is indicated by the large dots (labelled TS at abscissa). Number of measurements averaged for each condition are indicated close to the curves. Potentiation of TS facilitation is present with CS strength of 60–130% and most pronounced around an ISI of 3 ms, while diminution of TS facilitation is found for CS 15–30%. Suppression of visual activity by TS is less enhanced by a conditioning stimulus (grey curve). Data points labelled by asterisks are significantly different from TS values given by the large dots (P < 0.05, one-sided paired t test). Error bars give s.d. of differences between ppTMS and TS effect.

Figure 5. ppTMS effect versus the sum of CS and TS effect

For 25 neurones in which at least 2 CS strength effects have been recorded, the mean effects of single CS of different strength are compared with the mean effects of TS alone, ppTMS with 3 ms and the difference between ppTMS effect and the sum of the individual CS and TS effects (D_pp =Δ_vis, _ppTMS– (Δ_vis, _CS+Δ_vis, _TS)). The upper diagram shows the mean values for those 16 cells in which the TS caused facilitation of visual activity; the lower diagram shows the means of 9 cells with TS suppression.

Figure 6. TMS effect declines with time

The data shown in Figs 2–5 were obtained from a time window of 30–100 ms after TMS. The same analyses were carried out for time windows 2 and 3, 100–200 ms and 200–500 ms after TMS, respectively. ppTMS effect on the facilitation (black continuous curve) or suppression (grey continuous curve) obtained with TS, and the characteristics of correlation between ppTMS and TS effect, slope of regression lines (black dashed) and Pearson correlation coefficients (grey dashed) are plotted versus CS strength separately for the 3 different time windows. Data obtained with different ISIs were averaged because of the small quantitative and absent qualitative differences. Thus, curves for time window 30–100 ms give the averaged values of the curves of Figs 3B and 4. Asterisks indicate data points with a significant difference compared to TS control values (black: P < 0.05 with paired t test; α < 0.05 for Pearson coefficient).