Localized lesion of caudal part of lobus parolfactorius caused impulsive choice in the domestic chick: evolutionarily conserved function of ventral striatum

The light and electron microscopic characterisation of identified striato-ventrotegmental projection neurons in the domestic chick (Gallus domesticus)

A major projection of the medial striatum (lobus parolfactorius, LPO) of birds is the striato-ventrotegmental pathway projecting to the area ventralis tegmentalis. In the present study, we investigated the morphology and connectivity of striato-ventrotegmental neurons in the medial LPO. The neurons were identified by injecting the fluorescent retrograde tracer fast blue (FB) into the area ventralis tegmentalis. FB-labelled neurons in the LPO were targeted and iontophoretically injected with lucifer yellow (LY) in paraformaldehyde fixed slices. The fluorescent LY label in the filled neurons was then photoconverted, and the ultrastructure of cells was investigated. According to our results, the soma of striato-ventrotegmental neurons is rich in organelles, in particular rough and smooth endoplasmic reticula and they possess a large, unindented and slightly eccentric nucleus. The LY-labelled cells possess relatively few, sparsely spiny dendrites, and represent a type of medium-sized spiny projection neuron characteristic of the striata of birds. Axospinous synapses on the labelled cells are asymmetric and correspond morphologically to the glutamatergic excitatory type of terminals described previously in the LPO. Both symmetric and asymmetric axodendritic and axosomatic synapses were detected. Some symmetric synapses were GABA immunolabelled, whereas some asymmetric synapses were immunopositive to glutamate. Axon collaterals of labelled cells formed symmetric or asymmetric axodendritic synapses. Direct contact without interposing glial processes was observed between one of the FB-labelled neurons and an adjacent neuronal soma. There was also a microneuron attached to one of the labelled cells, which we identified as a neurogliaform 'dwarf' cell.

Hippocampal lesion delays the acquisition of egocentric spatial memory in chicks

Effects of bilateral chemical lesion of the hippocampus was examined in 1- to 2-week-old domestic chicks. Chicks were trained and tested in an egocentric spatial task, in which subject chicks should memorize location of a rewarding object in reference to the subject's viewpoint. Two beads were simultaneously presented on a wall, and chicks pecked at one of them based on relative location (left-right or above-below) to gain a reward. Comparison of training curves revealed that the lesion significantly delayed, but did not impair, the acquisition. Recall of the spatial cue, as well as conditioning with color cues, was not impaired. Hippocampus could thus be involved in memory formation of spatial relationships between nearby objects.

Neural correlates of memorized associations and cued movements in archistriatum of the domestic chick

The archistriatum mediates a neural pathway from the medial part of intermediate hyperstriatum ventrale (in the dorsal pallium) to the lobus parolfactorius (in the medial striatum), thus is possibly involved in memory formation in the domestic chick. To elucidate the functional roles, we examined single neuron activities from archistriatum in unconstrained chicks during execution of a GO/NOGO task. In this task, a brief motor sound was given as initial cue, and immediately followed by presentation of a coloured bead. Chick was required to recall the memorized associations between the colour and reward, and pecked at the bead to gain food after a delay (GO trials) or stayed not pecking (NOGO trials). The ventral part of intermediate archistriatum proved to contain a group of neurons that selectively responded to the reward-associated colours before the reward was actually presented, possibly coding the memorized associations. Another group of neurons fired during the reward period, thus could code aspects of the food reward. Yet another group of neurons started to fire immediately on the cue sound and prior to the cued movements nonselectively in both GO and NOGO trials, thus could be involved in the sensori-motor link between the sound and the targeted body movements. It is concluded that even a subregion of archistriatum contains diverse neural codes for memorized associations and food rewards, and neural codes of movements cued by sounds, suggesting that archistriatum is a complex of different functional systems, possibly corresponding to striatum, limbic amygdala, and prefrontal cortex in mammals.

The mind through chick eyes: memory, cognition and anticipation

To understand the animal mind, we have to reconstruct how animals recognize the external world through their own eyes. For the reconstruction to be realistic, explanations must be made both in their proximate causes (brain mechanisms) as well as ultimate causes (evolutionary backgrounds). Here, we review recent advances in the behavioral, psychological, and system-neuroscience studies accomplished using the domestic chick as subjects. Diverse behavioral paradigms are compared (such as filial imprinting, sexual imprinting, one-trial passive avoidance learning, and reinforcement operant conditioning) in their behavioral characterizations (development, sensory and motor aspects of functions, fitness gains) and relevant brain mechanisms. We will stress that common brain regions are shared by these distinct paradigms, particularly those in the ventral telencephalic structures such as AIv (in the archistriatum) and LPO (in the medial striatum). Neuronal ensembles in these regions could code the chick's anticipation for forthcoming events, particularly the quality/quantity and the temporal proximity of rewards. Without the internal representation of the anticipated proximity in LPO, behavioral tolerance will be lost, and the chick makes impulsive choice for a less optimized option. Functional roles of these regions proved compatible with their anatomical counterparts in the mammalian brain, thus suggesting that the neural systems linking between the memorized past and the anticipated future have remained highly conservative through the evolution of the amniotic vertebrates during the last 300 million years. With the conservative nature in mind, research efforts should be oriented toward a unifying theory, which could explain behavioral deviations from optimized foraging, such as "naïve curiosity," "contra-freeloading," "Concorde fallacy," and "altruism."