Figure 6.
Firing frequency-intensity curves (f-I curves) measured in the same cell as in Figure 4A-C. First, the cell was adapted to some EOD amplitude Ipre higher, lower, or equal to the EOD baseline amplitude (2.43 mV/cm) to prepare the cell in different states of adaptation. Then the stimulus was stepped to various amplitudes I at time t = 0. Three examples of the firing-frequency response to the second part of the stimulus are shown in A-C (black solid lines). An exponential fit (dotted line; clipped at the maximum response) measured the effective time constant of adaptation τeff, and a fit of the computed response of the adaptation model Equations 1 and 4 (gray line) yields the adaptation time constant τ. Note that fitting an exponential to the step responses describes the data sufficiently well but, in contrast to the adaptation model Equation 1, does not define a dynamics for arbitrary stimuli. A, Stimulus amplitudes I larger than Ipre evoke a strong onset response f0, which rapidly adapts down to a steady-state value f∞ slightly above baseline activity (357 Hz). B, For very large intensities, the response saturates at the EOD frequency.C, Decreasing the EOD amplitude may even cause cessation of firing. After a while, the cell recovers from adaptation and starts firing again, leveling out below the baseline activity. D, Onset f-I curve f0(I) (triangles) and steady-state f-I curve f∞(I) (circles) constructed from onset and steady-state responses for a particular preadaptation stimulus Ipre (here the EOD baseline amplitude; vertical dotted line). Fitting Boltzmann functions to both f-I curves describes the data reasonably well (solid and dashed lines; see Materials and Methods). The baseline firing frequency (triangles) as the response to the fish's own EOD was measured during 100 ms before applying the preadapting stimulus. E, The effective time constant τeff (+) and the adaptation time constant τ (*) obtained from the measurement shown in D are independent of stimulus intensity I. The averaged time constants from all 18 cells are τeff = 7 ms (dashed dotted line) and τ = 42 ms (dashed line). F, Comparison of f-I curves measured for three values of Ipre (vertical dotted lines) of the preadaptation stimulus and thus three different states of adaptation. By definition, the steady-state f-I curves (solid lines) and the baseline firing frequencies (dashed dotted line) are independent of Ipre. In contrast, the onset f-I curves (triangles, data; dashed lines, fit) are shifted by adaptation along the intensity axis while keeping their shape.