Shifting and scaling adaptation to dynamic stimuli in somatosensory cortex

Recent reports have shown that responses of midbrain neurons in the guinea pig rapidly shift the dynamic range of their responses to track changes in the statistics of ongoing sound-level distributions. This results in an increased coding accuracy for the most commonly occurring stimulus intensities. To investigate whether this type of adaptation might also be found in other sensory modalities, we characterized the intensity-response functions of neurons in rat primary somatosensory cortex (S1) to continuous sinusoidal vibration of the whiskers with amplitudes that were changed every 40 ms. Vibration amplitudes were selected randomly such that there was an 80% chance for the amplitude to be drawn from a relatively narrow 'high-probability region' (HPR). Stimulus mean and variance were then manipulated by shifting or widening the HPR. We found that rat S1 neurons adapt to shifts of the HPR mainly by shifting their thresholds, and to changes in HPR width by changing the slope of their rate-level curves. Using realistic single-neuron models, we go on to show that after-hyperpolarizing currents, such as those carried by K(Ca)(2+) channels, may be responsible for the threshold shifts, but not the slope changes.