Study claims that human brain does not produce new brain cells?

Question

Neurogenesis is one of the most fascinating subjects to discuss. However, it seems like scientific community has difference of opinion about whether neurogenesis is possible or not in adult humans. The aim of my question is to gain insights about whether neurogenesis in adult humans is possible or not.

According to an article presented in Scientific American "The Birth of a Brain Cell: Scientists Witness Neurogenesis", Mirjana Maletic-Savatic, an assistant professor of neurology at Stony Brook University in New York State, claims that neurogenesis is possible. Her study is based on brain imaging and suggests that scientists "witnessed" neurogenesis and the birth of new brain cells in adult human brain.

However, on the other hand, there is a Californian study, reported on in ScienceNews "Study casts doubt on whether adult brain’s memory-forming region makes new cells", that claims that neurogenesis in adult humans does not exist.

Analyzing the study contradiction

These two studies contradict each other. One may try to reconcile these two studies by claiming that humans only produces new cells in only certain parts of the brain but not in other parts of the brain, including hippocampus. Hippocampus may not be able to produce more brain cells, but other parts of the brain may generate new brain cells. However, this explanation and the above mentioned study directly contradicts another study that was published by Berg D.A. et al., 2013:

active adult neurogenesis is confined to two distinct locations: the subventricular zone (SVZ) of the lateral ventricles in the forebrain; and the subgranular zone (SGZ) of the dentate gyrus (DG) in the hippocampus

We can see that the Californian study contradicts the other study clearly when it comes to studying and analyzing neurogenesis in the hippocampus.

The question

This theory and the Californian study does not explain why people who suffer from memory loss in some parts of their lives regain memory and perform much better on memory tests.

If the hippocampus does not make new brain cells, then how do people regain memory by eating the right diet and exercising? Is neurogenesis possible in the hippocampus region of the brain?

_{Berg, D. A., Belnoue, L., Song, H., & Simon, A. (2013). Neurotransmitter-mediated control of neurogenesis in the adult vertebrate brain. Development, 140(12), 2548-2561.}

Answer 1

The hippocampus does make new brain cells, especially after aerobic exercise; there is an overwhelming amount of evidence for this claim (Erickson, Voss, et al., 2011; Firth, Stubbs, et al., 2017; Patten, Sickmann, et al., 2013). I was unable to find the California study published or peer-reviewed anywhere, and the abstract presented at the conference contained no indication of the sample size used in the study or a justification for why those specific types of neurons were studied. Therefore, it is unclear whether this single study was correctly conducted, and the article discussing the abstract is appropriately cautious about interpreting the results.

There's many different reasons why this study might contradict the scientific literature on hippocampus. One might have to do with a neurogenesis marker they used, doublecortin (DCX). One study notes that DCX levels may be dependent on specific brain regions and can be affected by age, although this is uncertain (Balthazart and Ball, 2014). Another study also warns that DCX reading levels should be interpreted with caution because they can exist at relatively high levels outside of neurogenesis (Kremer, Jagasia, et al., 2013).