Figure 2.
Membrane potential dynamics in auditory cortex do not resemble a random walk. a1, This 4 s example of a whole-cell patch-clamp recording from an auditory cortical neuron in vivo clearly exhibits the bumpy appearance ubiquitous in our dataset; QX-314, an intracellular fast sodium-channel blocker, was included in the patch pipette to prevent spiking as well as some other nonlinearities that can distort the relationship between synaptic activity and membrane potential. Throughout the trace, steeply rising peaks in the postsynaptic potentials follow most of the 65 dB, 25 ms tone pips (gray hash marks below the voltage trace; for stimulus protocol, see Materials and Methods), consistent with the occurrence of synchronous volleys of synaptic input. Aside from these narrow bumps, the membrane voltage remained close to the resting potential of the neuron. a2, These large excursions from rest were not restricted to stimulus transients. The stimulus here consisted of a 4-s-duration tone (gray bar beneath voltage trace), which began 15 ms following the far left of the trace. a3, Even in the absence of any auditory stimulation, the membrane potential displayed the same bumpy appearance, even long after the onset of the tone. b, As a control, we repeated these experiments after the topical application of TTX, a fast sodium channel blocker, to the cortical surface so as to abolish all presynaptic spiking and thus ensure the independence of synaptic events (see Materials and Methods, TTX application). As the expanded view of the trace demonstrates (bottom), the membrane potential does resemble a random walk in the absence of input correlations. We indicated two putative mEPSPs with asterisks. c, An example trace from an unanesthetized, head-restrained rat shows the same bumpy appearance as the records from anesthetized animals. The stimulus consisted of 100-ms-duration pure tones of 65 dB presented every 500 ms (gray hash marks below the voltage trace). Because QX-314 was not included in the patch pipette for this recording, the neuron sometimes fired action potentials. To allow comparison with the previous figures, we therefore median filtered [filter duration of 3 ms (Jagadeesh et al., 1997)] the trace to remove three spikes.