1. Comparing Single and Multiple Neuron Simulations of Integrated Dorsal and Ventral Striatal Pathway Models of Action Initiation
Selin Metin1, Neslihan Serap Şengör1, Koray Çiftçi2
1Electronics Engineering Department, Istanbul Technical University
2 Biomedical Engineering, Namık Kemal University
Striatum (Str) is the critical structure in various cognitive processes. The direct and indirect pathways of dorsal Str determine the actions in performing goal directed behaviors. Ventral Str (nucleus accumbens-NAc) calculates the reward value of tasks and the error in expectation. This reward value may alter an action decision even when the dorsal striatal output is against it, especially for effort-related decision making processes. Recent research shows that both ventral and dorsal Str take part in decision making processes. Striatal processing of a stimulus under the presence of dopamine (DA) signaling from midbrain neurons produces the stimulation to the motor circuits of the brain.
We compare the results of two computational models to study the effects of neural synchronization in action initiation. Both models integrate the outputs of dorsal and ventral striatal structures to show that through the striato-nigro-striatal pathway, limbic regions have impact on the motor regions of the basal ganglia.
Introduction
Neural Brain Structures Modeled
The physiological properties of neurons in each basal ganglia structure are reflected on Hodgkin-Huxley type conductance based neuron models to demonstrate the effects of ion channel currents on neural processing. Striatal neurons are modeled as two groups which have D1 or D2 type dopamine receptors. Dopamine input from midbrain neurons acts on the ion channels and stimulates the D1 neurons while inhibiting the D2 neurons. By changing the β quotient in the connection dynamics between VTA and NAc, the amount of DA secretion from the VTA to the NAc can be modulated. This modulation allows to obtain different levels of activation on NAc neurons and changes the output of the ventral loop.
Conductance Based Neuron Model
Limbic structures taking part in goal-directed behavior (ventral striatal pathway is colored). Aml, amigdala; DA, dopamine, D1/D2, neurons with D1/D2 type DA receptor; GABA, gamma-aminobutyric acid; GPe, globus pallidus externus, GPi, globus pallidus internus; LDTg, laterodorsal tegmental nucleus; LH, lateral hypotalamus; NAc, nucleus accumbens; PPTg, pedunculopontine tegmental nucleus; SNc, substantia nigra pars compacta; SNr, substantia nigra pars reticulata, STN, subthalamic nucleus; Str, striatum (caudate and putamen); THL, thalamus, VTA, ventral tegmental area.
For each neuron equation, stimulating input currents are used besides afferent connection currents and parameters are changed to obtain random spikes specific to each brain structure. v shows membrane potential, u is the neuron repolarization parameter, and VThr in the last resetting equation is the threshold at which a spike is generated. There are 100 neurons in each neuron group and these are randomly connected with a sparseness factor of 0.1.
AdEx Neuron Model
The first model is studied with different levels of external inputs and burst, tonic, phasic types of action potentials are observed. Also the output potential of each neuron is calculated by the input currents of its afferent projections and this is consistent with the addition of pre-synaptic potentials to constitute the post-synaptic potential.
In the second model it can be observed that the stimulation of the striatal structures and total activation of the neuron group directly effects Thl activity and also the firing frequency. With the contribution from the ventral pathway the firing synchronization increases and fine tunes the selectivity of a behavior.
The proposed network is the first example which contains both the dorsal and ventral action selection circuits and correctly reflects their interaction. Our results support the hypothesis that even though a behavior is unwanted (through the dorsal striatal pathway), it can be preferred due to its rewarding or pleasing quality (through the ventral striatal pathway).
The results of both models show that ventral striatal structures can modify the action decisions even when connections are sparse and random and neural firing frequencies vary.
The dynamics of the designed neural circuitry can be efficiently used in computational neuroscience to understand the cognitive processes.
Discussion
1. DeLong MR, Wichmann T: Circuits and Circuit Disorders of the Basal Ganglia. Arch Neurol. 2007, 64(1),
2. Haber SN, Knutson B: The reward circuit: linking primate anatomy and human imaging. Neuropsychopharmacology 2010, 35(1):4-26.
3. Salamone JD, Correa M, Farrar A, Mingote SM: Effort-related functions of nucleus accumbens dopamine and associated forebrain circuits. Psychopharmacology 2007, 191: /s
4. Nicola SM: The nucleus accumbens as part of a basal ganglia action selection circuit. Psychopharmacology 2007, 191:521–550.
This work was supported by TUBITAK (Project No: 111E264)
References
Ad-Ex Model Results
Conductance Based Model Results