Its too bad that false information was pointed out elsewhere and then repeated. There is no such thing as non-functional tissue in the brain. Neural function, esp. cortical function is progressively acquired. At any point in development, the tissues are functioning at their capacity. Development of the human brain follows a slow progression that peaks in the early teens. Fetal pain responses, speech, or the abilities of a 6 month old or 2 year old, are all milestones along that progression.
It is active, but it isn't "functioning" in any cognitive sense and to say so is misleading. Babies are born with very little myelin in the brain; this is most of the missing mass. Without myelin, there is very little cognitive function no matter how much activity there is. There may be electrical activity in the brain, but it is almost purely noise without myelin to control the cross-talk between axons. From the standpoint of cognitive capacity, this amounts to a reduction in actually cognitive function of orders of magnitude at the same neuron activity level as an adult human.
Yes, there is plenty of neural activity. But without myelin there is very little cognitive function possible. As myelin grows in the brain (taking a few years for the bulk of it), the SNR of the axons increases and the total cognitive capacity of the brain increases exponentially with it. The myelinization process is very analogous to linear increases in bit depth for digital systems.
In summary: Neural activity is not a sign of cognitive function. Cognitive function actually scales with myelinization, which takes a couple years after birth for the majority of it. In my book, having neurons that have negligible information carrying capacity compared to a mature human makes them "non-functional" regardless of how "active" they may be.