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Recently, the laboratory has been developing a quantitative understanding of how neurons at multiple levels of the sensory pathway encode dynamic, complex whisker stimuli in their spike trains. This involves analyses of neuronal activity during computer-controlled "noisy" stimuli (Lak et al., 2008) as well as with natural textural stimuli. We have monitored the vibrations produced in the shaft as the whisker sweeps across different surfaces and have identified distinctive patterns of neuronal activity induced by such vibrations. These patterns seem to be the neuronal "signature" for textures (Arabzadeh et al., 2005;Hipp et al., 2006;Arabzadeh et al., 2006).
Having generated precise hypotheses from studies of anesthetized animals, we set out to study the nature of the cortical representations underlying judgments of texture in awake, behaving rats (von Heimendahl et al., 2007). In the behaving animal, the whisker sensory system is ‘‘active’’: the animal generates sensory signals by palpating objects through self-controlled whisker motion (just as we move our fingertips along surfaces to measure their tactile features). Active sensing (Kleinfeld et al.,2006;Diamond et al.,2008a)endows the animals with the capacity for fast and accurate texture discrimination (although, from the experimenter’s point of view, the animal's own control over whisking is an enormous complication!). We recorded neuronal activity from barrel cortex while rats used their whiskers to discriminate between rough and smooth textures. On whisker contact with either texture, firing rate increased by a factor of two to ten. Average firing rate was significantly higher for rough than for smooth textures, and we therefore propose firing rate as the fundamental coding mechanism. The rat, however, cannot take an average across trials, but must make an immediate decision using the signals generated on each trial. To estimate single-trial signals, we calculated the mutual information between stimulus and firing rate in the time window leading to the rat’s observed choice. Activity during the last 75 ms before choice transmitted the most informative signal; in this window, neuronal clusters carried, on average, 0.03 bits of information about the stimulus on trials in which the rat’s behavioral response was correct. To understand how cortical activity guides behavior, we examined responses in incorrect trials and found that, in contrast to correct trials, neuronal firing rate was higher for smooth than for rough textures. Analysis of high-speed films suggested that the inappropriate signal on incorrect trials was due, at least in part, to nonoptimal whisker contact. In conclusion, this experiment suggests that barrel cortex firing rate on each trial leads directly to the animal’s judgment of texture, as discussed in Diamond et al.,2008a, b. This work provides us with many new insights about how to "decode" what the rat has touched only by seeing its neuronal activity. This, after all, is exactly what the rat's brain must accomplish! We are now particularly interested in higher-level representations of tactile stimuli – how the physical parameters of whisker motion become transformed to a representation of object identity.