Please explain the following terms:

• Voxel – minimum 3D unit in the brain, activated as a result of certain activity. It is the place where BOLD located. Refers to statistical analysis of fMri and structural data.

• BOLD –см ниже

What is the bold signal? How is the bold signal related to actions potentials and to fMri method?

Blood Oxygen Level-Dependent (BOLD) signal

BOLD takes advantage of the different magnetic properties of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) Hb is paramagnetic and introduces an inhomogeneity into the nearby magnetic field, whereas HbO2 is weakly diamagnetic and has little effect.

The change in the MR signal from neuronal activity is called the hemodynamic response (HDR). It lags the neuronal events triggering it by 1 to 2 seconds, since it takes that long for the vascular system to respond to the brain's need for glucose. From this point it typically rises to a peak at about 5 seconds after the stimulus. If the neurons keep firing, say from a continuous stimulus, the peak spreads to a flat plateau while the neurons stay active. After activity stops, the BOLD signal falls below the original level, the baseline, a phenomenon called the undershoot. Over time the signal recovers to the baseline. There is some evidence continuous metabolic requirements in a brain region contribute to the undershoot

Activation potential need energy which provided by oxyhemoglobin with blood. Thus Bold signal reflect neural activity. However neural activity is faster than BOLD because it takes 2 second to deliver blood to this the needed brain region. Thus BOLD reflect neural activity but indirectly and with delay.

Explain the ‘diffusion’ model of decision making

Schematic representation of decision making:

(a) Detection of sensory evidence to support the alternatives.

(b) Integration of sensory evidence over time, because the

incoming evidence is noisy.

(c) Checking whether a certain criterion has been satisfied

(e.g pass a threshold). It indicates if the action can be

executed or if it is better to wait and continue the integration

process.

Firstly sensory information reach motion detector neurons, then it is proceeded to LIP neuron which integrates information. This continuous integration lead to accumulation of information which contributed certain decisions A or B. this neurons are also influenced by each other attempting to inhibit activity of another. When the threshold is reached the activity performed.

The diffusion model is usually includes just one abstract integrator that accumulates the difference between the evidence for the two alternatives; the choice is made when the level of the activity of this integrator exceeds a threshold

In the ‘diffusion’ model – a choice should be made as soon as the difference between the evidence supporting the winning alternative and the evidence supporting the losing alternative exceeds a threshold

The diffusion model reacts adaptively to the levels of evidence supporting the losing alternative: the diffusion model will integrate for a shorter time if the evidence supporting the losing alternative is weak relative to the winning alternative. • This adaptive ability is not present in the race model. If the thresholds in the two models are chosen to give the same accuracy, the diffusion model, on average, will be faster than the race model.

Perceptual decision-making in posterior DLPFC. The DLPFC shows both a higher response to clear images of faces and houses relative to noise images, and a correlation with Face(t ) – House(t ), suggesting that this brain region integrates sensory evidence from sensory processing areas to make a perceptual decision

The ‘diffusion’ model: choice should be made as soon as the difference between the evidence supporting the winning alternative and the evidence supporting the losing alternative exceeds a threshold.

Nervous system can actually perform ‘diffusion’ model-like calculations.