In a neural network, the summation happens in the ______ layer and the activation takes place in the ______ layer.

Explanation: A neuron takes a group of weighted inputs, applies an activation function, and returns an output. Inputs to a neuron can either be features from a training set or outputs from a previous layer's neurons. Weights are applied to the inputs as they travel along synapses to reach the neuron. The neuron then applies an activation function to the "sum of weighted inputs" from each incoming synapse and passes the result on to all the neurons in the next layer.