Parallels between timing of onset responses of single neurons in cat and of evoked magnetic fields in human auditory cortex

Sound onsets constitute particularly salient transients and evoke strong re sponses from neurons of the auditory system, but in the past, such onset re sponses have often been analyzed with respect to steady-state features of s ounds, like the sound pressure level. Recent electrophysiological studies o f single neurons from the auditory cortex of anesthetized cats have reveale d that the timing and strength of onset responses are shaped by dynamic sti mulus properties at their very onsets. Here we demonstrate with magnetoence phalography that stimulus-response relationships very similar to those of t he single neurons are observed in two onset components, N100m and P50m, of auditory evoked magnetic fields (AEFs) from the auditory cortex of awake hu mans. In response to tones shaped with cosine-squared rise functions, N100m and P50m peak latencies vary systematically with tone level and rise time but form a rather invariant function of the acceleration of the envelope at tone onset. Hence N100m and P50m peak latencies, as well as peak amplitude s, are determined by dynamic properties of the stimuli within the first few milliseconds, though not necessarily by acceleration. The changes of N100m and P50m peak latencies with rise time and level are incompatible with a f ixed-amplitude threshold model. The direct comparison of the neuromagnetic and single-neuron data shows that, on average, the variance of the neuromag netic data is larger by one to two orders of magnitude but that favorable m easurements can yield variances as low as those derived from neurons with m ediocre precision of response timing. The striking parallels between the re sponse timing of single cortical neurons and of AEFs provides a stronger li nk between single neuron and population activity.