 What is plotted is the activity of an MT neuron, in spikes per second, to a specific type of motion stimulus placed within its receptive field. How to relate a tuning curve to a determinate content is complicated. Since the neural response is not simply to one stimulus value, it is not obvious that the neuron should be taken to represent 0 degrees of motion, namely the value at its peak response. Indeed, theorists have noted that the tuning curve looks like a probability density function, and many now take neurons to have probabilistic content  (section 5.4).