Abstract
Rats and mice palpate objects with their whiskers to generate tactile sensations. This form of active sensing endows the animals with the capacity for fast and accurate texture discrimination. The present work is aimed at understanding the nature of the underlying cortical signals. 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, these data suggest that barrel cortex firing rate on each trial leads directly to the animal's judgment of texture.
Highlights
One aim in studies of sensory coding is to quantify how neuronal activity represents objects in the external world
Texture Discrimination Task and Cortical Spike Trains The purpose of this study was to identify the neuronal representation of texture in the barrel cortex of actively behaving rats
In the ‘‘3-arm task’’ (Figure 1B), they had to either cross to the opposite platform, or withdraw and proceed to a second gap at which the reward texture was present
Summary
One aim in studies of sensory coding is to quantify how neuronal activity represents objects in the external world. As in humans [1], tactile exploration entails the interplay of motor output and sensory input: Rats palpate objects by sweeping their whiskers in a rhythmic forward–backward cycle [2] This active sensing gives rise to a number of welldeveloped tactile capacities [3,4,5,6], including the sense of texture [4]. In the barrel cortex of anesthetized rats, the whisker vibrations associated with different textures evoke cortical responses that differ according to texture—coarser textures evoke more spikes per sweep [8,9] By extending this line of investigation to awake rats, we ask which features of sensory coding are conserved during active exploration of the environment, when stimuli are not imposed on the receptors, but are generated by the animal through its own motor program. Because the behaving animal makes choices based on the signals carried by its sensory neurons, we can ask how the neuronal code leads to the animal’s decisions
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