You are right that active adult neurogenesis is generally considered to be restricted to the dentate gyrus of the hippocampus, and the subventricular zone of the lateral ventricles. The latter generates neurons that subsequently migrate through the rostral migratory stream to the olfactory bulb to become interneurons (Ming & Song, 2011).
Although the hippocampus is critical for the formation of memory, hippocampal neurogenesis does not simply 'add memory' (Kemperman, 2002). Instead, hippocampal neurogenesis is believed to mediate the continuing modulation of cortical functions in response to exposure to novelty (Aimone et al., 2014). Specific examples are spatial-navigation learning and long-term spatial memory retention, spatial pattern discrimination and contextual fear conditioning. Adult olfactory bulb neurogenesis is associated with maintaining long-term structural integrity of the olfactory bulb, short-term olfactory memory, olfactory fear conditioning, and long-term associative olfactory memory involving active learning (Ming & Song, 2011).
Neurogenesis in other regions in the adult brain are generally believed to be limited under normal physiological conditions, but can be reproducibly induced after inflicting injury to the grain (Gould, 2007). These regions include the neocortex, amygdala, hypothalamus, substantia nigra and the brainstem, among other regions (see Fig. 1).
Fig. 1. Regions of neurogenesis in mammals, shown in a rat's brain. The red regions (hippocampus & subventricular zone of the lateral ventricles) are considered to be neurogenic. The other regions may also be neurogenic, but only reproducibly so in experiments after damage to these regions. Source: Gould (2007).
- Aimone et al., Physiol Rev (2014); 94: 991–1026
- Gould, Nature Rev Neurosci (2007); 8 481-8
- Kemperman, J Neurosci (2002); 22(3): 635-8
- Ming & Song, Neuron (2011); 70(4): 687–702
- What evidence is there that the adult brain can produce new neurons?