Hippocampus
Anatomical and electrophysiological information show there is some structure in most invertebrate, especially insects, that is homologous to the hippocampus of vertebrates and mammals called the mushroom bodies. MB is a morphological system that is also known as the center of memory and brain learning in some invertebrates. It also affects the olfactory and the most complete structure of it exist in insects.(Hochner 2010)
In insects, MB has become a source of cellular and molecular information. MB has great morphological diversity in insects and in some cases; it is related to social and ecological behavior. While MB evolves, flexibility and behavioral complexity increase. MB is similar to the cerebral cortex of vertebrates in terms of function. Its structure consists of hundreds to thousands of small cells. (Farris 2005)
There is a relationship between MB size and social behavior. As the brain evolves, flexibility and behavioral complexity increase. (Farris 2008)
The MB in insects is homolog of to the vertical octopuses’ lobes. MB in insects and vertical lobes of octopus are folded like vertebrates’ brain and are homologous of the hippocampus. (Hochner 2010)
Since hippocampus is located at the dorsomedial part of telencephalon in mammals, it is predicted that the homolog of this structure in nonmammalian taxa (reptiles, amphibians, avians and cartilaginous fishes) is occupying the same region. In ray finned fishes the homologous structure placed in lateral part of pallium because of everted process in their hemispheres instead of evagination. (Holmgren 1922, Nieuwenhuys 1962, Northcutt and Braford 1980, Butler 2017)
Despite of some variations in formation of hippocampus in amniotes, the function of this structure in attending to novel stimuli, allocentric spatial mapping, and memory is well known. Even in anamniotes, including amphibians and ray-finned fishes which have greater deviation in its morphological features (in homologous structure) , the allocentric spatial navigation and memory have been found to be preserved. However these similarities in functional roles cannot be used as evidence of structural homology.(Striedter 2002, Butler 2017)
It is suggested that the medial and dorsal cortex in reptiles almost certainly are the homologous of hippocampus based on their similar functions. Also The enlargement in medial cortex of reptiles is likely to provide them with better memory and spatial learning ability.(Amiel, Tingley et al. 2011) In many researches in is shown that the ability to learn mazes in reptiles is roughly equivalent to birds and mammals and they use hippocampal spatial map for navigating as well as other amniotes.(Holding, Frazier et al. 2012, Northcutt 2013)
The hippocampus in avian and mammals originate from the medial pallium.(Székely 1999, Atoji and Wild 2006, Medina and Abellán 2009, Allen and Fortin 2013) Avian and non-human mammalian hippocampus play a role in spatial memory(Colombo, Broadbent et al. 1997, Bingman, Gagliardo et al. 2005), episodic memory(Clayton and Dickinson 1998, Salwiczek, Watanabe et al. 2010), spatial navigation, and feedback onto the hypothalamic-pituitary-adrenal axis.(Kahn and Bingman 2009, Mayer, Watanabe et al. 2013, Herold, Coppola et al. 2015, Smulders 2017). The pallial area medial to the paraventricular sulcus (Atoji and Wild 2006) of avian is known as their hippocampus which includes medial parts of the parahippocampal area (Karten and Hodos 1967), but in exchange also includes possible homologs of CA1, CA3, and the dentate gyrus (Rattenborg and Martinez-Gonzalez 2011). The hippocampus proper is defined by the dentate gyrus and Cornu Ammonis (CA). While the dentate gyrus contains the fascia dentata and the hilus, the CA is anatomically and functionally differentiated into distinct subfields named CA1, CA2, CA3, and CA4. The CA3 region role in memory processes, susceptibility to seizures and neurodegeneration(Cherubini and Miles 2015).
The dorsolateral hippocampus is suggested to be a homolog of the mammalian entorhinal cortex (Rattenborg and Martinez-Gonzalez 2011), since it also functions as the main input structure to the hippocampus.(Atoji and Wild 2006)
Categorical processing is one of the differences between avian and mammalian hippocampus. Hippocampus of avian is involved in categorical processing by receiving input from telencephalon and nidopallium caudolateral (NCL) (Veit and Nieder 2013, Veit, Hartmann et al. 2014, Moll and Nieder 2015, Ditz and Nieder 2016, Ditz and Nieder 2016, Güntürkün and Bugnyar 2016, Nieder 2017). Recordings indicated that for variety of executive processes (working memory(Veit, Hartmann et al. 2014), rules (Veit and Nieder 2013), cross-modal associations (Moll and Nieder 2015) and numerical competence processing of visually presented items in a set, i.e., numerosities) NCL neurons are involved (Ditz and Nieder 2015, Ditz and Nieder 2016). Although it is not observed how hippocampus receive information from the NCL yet. There is not a direct connection between the hippocampus and the NCL (Leutgeb, Husband et al. 1996, Kröner and Güntürkün 1999, Kreiman, Koch et al. 2000). (Ditz, Kupferman et al. 2018)
Hippocampus formation (HF) in mammalian is discrete cell layers due to function (Amaral, Andersen et al. 2007) , although the avian HF can perform the same functions without the clearly-delineated cell layers.(Striedter 2016) HF is giving the birds the ability to find their home even if they are far apart. We also see this ability in mice, although there are a lot of differences between the HF of homing birds and mammals.(Bingman, Gagliardo et al. 2005) Generating discrete forms of neuronal oscillations is one of the capabilities of the mammalian hippocampus. Also gamma rhythms (30 – 80 Hz) are another feature of this structure which is important in memory processing.(Traub, Whittington et al. 1996)
Recently it is found that, the avian hippocampal formation is capable of endogenously generating gamma oscillations when stimulated with a cholinergic agonist like mammals hippocampus.(Dheerendra, Lynch et al. 2018)