Information transmission by stimulus-dependent modulation of noise correlation

Adaptation improves neural coding efficiency despite increasing correlations in variability

Exposure of cortical cells to sustained sensory stimuli results in changes in the neuronal response function. This phenomenon, known as adaptation, is a common feature across sensory modalities. Here, we quantified the functional effect of adaptation on the ensemble activity of cortical neurons in the rat whisker-barrel system. A multishank array of electrodes was used to allow simultaneous sampling of neuronal activity. We characterized the response of neurons to sinusoidal whisker vibrations of varying amplitude in three states of adaptation. The adaptors produced a systematic rightward shift in the neuronal response function. Consistently, mutual information revealed that peak discrimination performance was not aligned to the adaptor but to test amplitudes 3-9 μm higher. Stimulus presentation reduced single neuron trial-to-trial response variability (captured by Fano factor) and correlations in the population response variability (noise correlation). We found that these two types of variability were inversely proportional to the average firing rate regardless of the adaptation state. Adaptation transferred the neuronal operating regime to lower rates with higher Fano factor and noise correlations. Noise correlations were positive and in the direction of signal, and thus detrimental to coding efficiency. Interestingly, across all population sizes, the net effect of adaptation was to increase the total information despite increasing the noise correlation between neurons.

Stimulus-induced reduction of noise correlation in rat prefrontal cortex

We have shown previously that stimulus-induced modulation of noise correlation in rat somatosensory cortex conveys additional information about the delivery of tactile stimulation. Here we investigated whether noise correlation is also modulated by an external sensory stimulus in rat prefrontal cortex and, if so, whether such modulation conveys additional information on stimulus delivery. Noise correlation was significantly reduced after the onset of a conditional stimulus (auditory tone) that signaled an electric foot shock in the prefrontal cortex. However, noise correlation contributed little to the transmission of information on stimulus delivery. These results indicate that a meaningful sensory stimulus reduces noise correlation in rat prefrontal cortex, but such modulation does not play a significant role in conveying information on stimulus delivery.