High-frequency encoding in visual and auditory cortex neurons in mice and rats

Neocortical neurons can phase-lock their firing to high frequency fluctuations of membrane potential and thus encode these fluctuations in patterns of spikes with millisecond precision. However, for neurons from different areas, different cutoffs of the frequency transfer function were reported. Is encoding precision tuned for processing environmental signals in modality-specific frequency ranges, e.g. up to kHz range in the auditory, but only < 100 Hz in the visual system? Or do the reported discrepancies reflect methodological differences rather than true differences in encoding abilities of neurons across areas? To resolve this issue, we measured the transfer function of neurons from auditory and visual cortices of mouse and rat under the exact same experimental conditions. Direct quantitative comparison revealed no difference in the high frequency cutoff of the frequency response function, and consequently in spike encoding precision, between neurons from auditory and visual cortex within each species. Neurons from both cortical areas can encode the same range of frequencies and thus generate sequences of action potentials with the same millisecond-scale precision and use the same-scale high-precision temporal code. Our results support the idea that cortical networks are universal processing devices, which use a broad dynamic range of temporal coding for processing signals, irrespective of their specific sensory origin. Sensory identity and modality-specific processing are then determined by the connectivity. Uniform spike-encoding abilities of neurons allow cortical networks to use the same high-precision temporal code for communication within and between cortical regions, thus facilitating multi-sensory integration.