Economic essay nature science significance

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Article activity alert. JEL classification alert. Receive exclusive offers and updates from Oxford Academic. Related articles in Google Scholar. Citing articles via Google Scholar. Nudge versus boost: Agency dynamics under libertarian paternalism. Identity and Bias: Insights from Driving Tests. The powerfully rewarding effect of the electrical stimulation arises from a stream of action potentials triggered in an identifiable set of neurons. This should make the ICSS phenomenon an attractive entry point for efforts to map the structure of brain reward circuitry and to account for its functional properties in terms of neural signaling between its components.

In this section, I review some candidates for neural circuitry underlying BSR. This brief overview highlights some achievements of prior research as well as many challenges that have yet to be addressed in a satisfactory way. Kate Bielajew and I have provided evidence that the volley of action potentials elicited by stimulation at this site must propagate caudally in order the reach the efferent stages of the circuit responsible for the rewarding effect Bielajew and Shizgal, Selected descending pathways coursing through a lateral hypothalamic region of the medial forebrain bundle, where electrical stimulation is powerfully rewarding, and some associated neural circuitry.

The lefthand view is in the horizontal plane and the righthand view in the sagittal plane. Pursuit of BSR is attenuated by treatments that reduce dopaminergic neurotransmission Fouriezos et al. Nonetheless, these authors did express some reservations, which turn out to be well founded. The axons of dopamine neurons are fine, unmyelinated, and difficult to excite by means of extracellular stimulation Yeomans et al. The mere proximity of these axons to the electrode tip does not guarantee that a large proportion of them are excited directly under the conditions of ICSS experiments.

Indeed, the electrophysiological properties of these fibers provide a poor match to the properties inferred from behavioral studies of ICSS Yeomans, , ; Bielajew and Shizgal, , These behavioral data suggest that non-dopaminergic neurons with descending, myelinated axons, more excitable than those of the dopamine-containing cells, compose an important part of the directly stimulated stage Bielajew and Shizgal, ; Shizgal, Multiple components of the descending MFB provide monosynaptic input to dopamine cell bodies in the midbrain, and glutamatergic neurons are prominent among them You et al.

Blockade of glutamatergic receptors on midbrain dopamine neurons decreases transient release of dopamine by rewarding electrical stimulation Sombers et al. Cholinergic neurons in the pons constitute one limb of a disynaptic path that links MFB electrodes to activation of dopamine neurons Oakman et al. These cholinergic neurons are implicated in the rewarding effect of MFB stimulation Yeomans et al. It has been proposed, in the case of posterior mesencephalic stimulation, that the spatio-temporal integration of the reward intensity signal arises prior to, or with the participation of, midbrain dopamine neurons Moisan and Rompre, Given the important roles ascribed to dopaminergic neurons in the allocation of effort and in reward-related learning, it is important to understand how information is processed in their afferent network.

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The preceding paragraphs attest to the fact that it has proved simple neither to identify the neurons composing the most accessible stage of the circuitry underlying ICSS, the directly activated stage, nor to determine the precise role played by the neural population most extensively studied in the context of BSR, midbrain dopamine neurons. In the following section, I discuss in more general terms the requirements for establishing such linkages, and I argue that new research techniques provide grounds for optimism that long-standing obstacles can be overcome.

Multiple, converging, experimental approaches are required to link an identified neural population to a psychological process Conover and Shizgal, Each approach tests the linkage hypothesis in a different way, by assessing correlation, necessity, modulation, sufficiency or computational adequacy. All of these approaches have been applied in the search for the directly stimulated neurons underlying BSR and in efforts to determine the role played by midbrain dopamine neurons.

Nonetheless, the full promise of the convergent strategy has yet to be realized. An example of a correlational test has already been mentioned. Inferences are drawn from behavioral data about physiological properties of a neural population, such as the directly stimulated neurons that give rise to BSR. A method such as single-unit electrophysiology is used to measure neural properties, which are then compared to those inferred from the behavioral data.

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For example, the experimenter can ascertain, by means of collision between spontaneous and electrically triggered antidromic spikes, that the axon of a neuron from which action potentials are recorded is directly activated by rewarding stimulation. Properties of the stimulated axon, such as its refractory period and conduction velocity, are then measured and compared to those inferred from the behavioral data Shizgal et al.

This approach can provide supporting evidence for a linkage hypothesis, but it cannot prove it. The portrait assembled on the basis of the behavioral data is unlikely to be unique, and the neuron under electrophysiological observation may resemble those responsible for the behavior in question but, in fact, subserve another function. Tests of necessity entail silencing the activity of some population of neurons and then measuring any consequent changes in the behavior under study.

Traditional methods include lesions, cooling, and injection of local anesthetics.

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Although this approach can also provide supporting evidence, it is fraught with difficulties. Many neurons in addition to the intended targets may be affected, and the silencing may alter the behavior under study in unintended ways, for example, by reducing the capacity of the subject to perform the behavioral task rather than the subjective value of the goal. The typically employed silencing methods have durations of action far longer than those of the neural signals of interest. Tests of modulation are similar logically to tests of necessity but can entail either enhancement or suppression of neural signaling and are usually reversible.

Drug administration is typically employed for this purpose. This approach often achieves greater specificity than is afforded by conventional silencing methods. Nonetheless, it is difficult to control the distribution of a drug injected locally in the brain, and the duration of drug action often exceeds that of the neural signal of interest by many orders of magnitude. Tests of sufficiency entail exogenous activation of a neural population and determination of whether the artificially induced signal so produced affects the psychological process under study in the same way as a natural stimulus.

The demonstration that the rewarding effect of electrical stimulation of the MFB competes and summates with the rewarding effect of gustatory stimuli Conover and Shizgal, ; Conover et al. Traditional sufficiency tests, which often entail delivery of electrical brain stimulation, provide much better temporal control than local drug injection, but they too are plagued by major shortcomings: Many neurons in addition to the target population are typically activated, and the stimulation may produce undesirable behavioral side-effects.

Computational adequacy is another important criterion for establishing linkage. Simulations are then performed to determine whether the model can reproduce the behavioral data using the parameters derived from neural measurement. This is a demanding test, but it too is not decisive. There is no guarantee that any given model is unique or sufficiently inclusive. Although all its elements have shortcomings as well as virtues, the convergent strategy is nonetheless quite powerful. The virtues of some elements compensate for the shortcomings of others, and the likelihood of a false linkage decreases as more independent and complementary lines of evidence are brought to bear.

That said, one may well wonder why, if the convergent approach is so powerful, has it not yet generated clear answers to straightforward questions such as the identity of the directly stimulated neurons subserving BSR or the role played by midbrain dopamine neurons?

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As I argue in the following section, many of the problems are technical in nature, and recent developments suggest that they are in the process of being surmounted. The example of the correlational approach described above entails recording from individual neurons, one at a time, in anesthetized subjects, after the collection of the behavioral data.

Newer recording methods have now been developed that register the activity of dozens of neurons simultaneously while the behavior of interest is being performed. A lovely example of this substantial advance is a recent study carried out by a team led by David Redish van der Meer et al.

They recorded from ensembles of hippocampal neurons as rats learned to navigate a maze.