Neuroscience: Each synapse to its own

A neuron can receive thousands of inputs that, together, tell it when to fire. New techniques can image the activity of many inputs, and shed light on how single neurons perform computations in response. Many sensory neurons in the mammalian cortex are tuned to specific stimulus features — for example, some will fire exclusively when horizontal bars move vertically from top to bottom in the visual field. Whether such tuning is already encoded in a neuron's dendritic inputs or whether the neuron itself computes its selective response has been unclear. Jia et al. have now succeeded in imaging dendritic calcium signals that precede action potentials in mouse visual cortical neurons in vivo, with high resolution in time and space. They discover that, while all neurons receive distributed input signals coding for multiple stimulus orientations, each neuron makes its own 'decision' as to the orientation preference of its firing output. These fundamental results and the authors' new technique — visualizing receptive fields of individual synapses — have major implications for the future of functional mapping of the brain.