In a different approach to estimate the minimum number of spiking neurons to effectively substitute the effect of a sensory input, channelrhodopsin-2 (ChR2)-expressing neurons in the motor cortex were directly stimulated by light. Mice could detect the occurrence of single action potentials in approximately 300 synchronously active neurons. Even fewer neurons (~ 60) were required when the light induced a train of spikes (Huber et al., 2008). Under special conditions, stimulation of a single pyramidal cell or interneuron can recruit a large fraction of neurons in the circuit (Miles, 1990; Bonifazi et al., 2009; Ellender et al., 2010). Intense trains of intracellularly evoked spikes in a single motor cortex neuron were sufficient to evoke or reset whisking movement in the rat (Brecht et al., 2004). However, in these studies the directly discharged neurons likely activated an unknown number of other cells, and without monitoring the entire population the number of neurons that generated the desired behaviors has remained unknown.
