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, GarcĂa-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)