Density and numbers of neurons:
the number of neurons is an evolutionary factor which many of the brain reflexes depend on. (Elwood 2011) increasing the size or number of neurons can cause rising amounts of synaptic input to a particular nucleus or specific brain region (Rakic, 1975; Szaro and Tompkins, 1987; Tompkins et al., 1984); all these can be effective on the levels and complexity of cognitive abilities.
It is difficult to explain how neuronal segments control behavior because there are many levels of behavior and the complexity of each of these levels is different. However, in general, any invertebrate having more neurons has greater behavioral complexity. For example, among invertebrates octopus, cuttlefish and squid have the most advanced brain in terms of weight and cell number. The octopus with 500 million neurons is the best case study. (Hochner 2010)
Moreover, the high number and density of neurons in some areas of the telencephalon, besides the size of the brain, can explain the high cognitive abilities in avian such as parrots and corvids.(Shettleworth 2009, Karten 2015, Font, García-Roa et al. 2019). The level of intelligence in these birds is roughly equal to that of primates (Roth 2013, Dicke and Roth 2016) because of the high number of cortical neurons. It is found that avian brain has more neurons than mammals although the mass of their brain is equal to primates’ brain. These extra neurons are located in forebrain that is homologous to mammalian cortex. (Olkowicz, Kocourek et al. 2016)
However in some cases this asseveration which implies that high density and numbers of neurons cause the complexity behavior and cognition has been rejected (Iwaniuk 2017). For instance, despite the equal number of neurons in elephants, cetaceans and great apes, it is obvious that great apes display higher levels of intelligence (Manger 2013)