Cerebral Imagery

The cerebral imagery (known as also neuro-imagery ) indicates the whole of the techniques resulting from the Medical imagery which make it possible to observe the brain, in particular when that an individual carries out a cognitive task .

History

The observation of the brain by autopsy was vague and incomplete, of the fact in particular of the observation postmortem of a solidified body, which could not give an account of the displacement of the cerebral tumors at the origin of cognitive disorders, like the Aphasie. The development of the techniques of Medical imagery coupled with the methods of cognitive and experimental psychology (for example, the Psycholinguistics) made it possible to in vivo observe the electric activity and blood flows in the brain, whose variations make it possible to determine the cerebral zones requested by various cognitive processes. Tools of the neuroimagery (IRM, Tomography with emission of positrons, electroencephalography, Magnétoencéphalographie,…) thus largely took part in progress of the cognitive Sciences since the Années 1990 (even front, as of the years 1950 for the electroencephalography), contributing so that one called the Décennie of the brain.

Various techniques of cerebral imagery

Structural Neuroimagery

Objectives of the structural neuroimagery

The structural imagery (known as also anatomical) seeks to identify, locate and measure the various parts of the anatomy of the central nervous system. In practice medical private clinic, it makes it possible to identify the localization and the extension of a cerebral lesion in a diagnostic aiming and/or of surgical operation.

Within the framework of research in cognitive neurosciences. The structural imagery brings elements to interpret the behavioral observations in Neuropsychologie. While determining with which lesions corresponds a cognitive deficit given, it is possible to establish that the injured cerebral area intervenes in the subjacent mechanism. Thus, it is by observing, postmortem, that the brain of a patient become unable to speak following an cerebral vascular accident presented a zone destroyed in the left frontal lobe, that Paul Broca deduced the role from this area in the processes of language.

More recently, with the increase in the precision of measurements, it became possible to correlate the measurement of the volume (or the density of Neuron S) of a cerebral area with behavioral results. Thus, a study showed that a cerebral structure implied in the space memory, the Hippocampe (brain), was developed in the London taxi drivers than in the average of the population, and this more especially as they led for a long time.

Tools of the structural neuroimagery

Histology and examination post-mortem
anatomical IRM
CT-scan
Tomography with mono-photonic emission (TEMP, or, SPECT)

Functional Neuroimagery

Objectives of the functional neuroimagery

The functional imagery seeks to characterize the brain in action. The traditional use of these methods consists in making carry out a cognitive task with an individual and measuring the signal produced by the cerebral activity. According to the used techniques and mathematical tools, it is possible to find, with more or less precision, which area of the brain was particularly active and at which time of the cognitive task.

Tools of the functional neuroimagery

the Imagerie by functional magnetic resonance (IRMf) consists in measuring the Signal BOLD which reflects the rate of oxygenation of the Sang in the brain. By a mechanism still badly explained, called hemodynamic Answer, the surge of oxygenated blood increases in the areas which consume energy. Thus, it is possible, by this method, to know with a high degree of accuracy which areas of the brain are especially active at the time of a given task. Since the years 2000, the technique of the event-driven functional IRM gives access to the dynamics of the Signal BOLD (with a temporal resolution of approximately a second) but that remains much slower than the dynamics of the cognitive processes.

the Tomographie by emission of positrons (Mtoe) consists in measuring the modifications of the blood flow by means of a radioactive tracer which it is necessary beforehand to inject by intravenous way. The diffusion of the tracer and the modulation of the blood flow being relatively slow phenomena, this technique does not give access to the dynamics of the neuronal mechanisms. This makes of it a technique today less and less used for the functional imagery. On the other hand by using radiotraceurs having a Affinity with some Neurorécepteur S, the Mtoe makes it possible to selectively measure the neuronal activity related to a precise physiological mechanism.

the electroencephalography (EEG) was the first method of noninvasive neuroimagery, developped at the point in 1929, by the neurologist Hans Berger. Contrary to the two methods known as metabolic, it is a direct measurement of the electric activity. The EEG is relatively not very precise spatially but it offers a temporal resolution only limited by the speed of the electronic of measurement. A first approach consists in measuring evoked Potentiels: by repeating the same stimulation a great number of times, it is possible to highlight positive and negative waves characteristic of the various stages of the process data processing (e.g., N100, P300, N400). Another approach consists in measuring the modifications of the rhythmic activities which seem to play an important functional part in cognition.

the Magnétoencéphalographie (MEG) offers information relatively similar to the EEG, but it measures the magnetic fields induced by the cerebral activity. The interest of the MEG lies in the fact that, contrary to the electric fields, the magnetic fields are almost not deformed by their passage through organic fabrics (in particular the interface between the céphalo-rachidian Liquide and the cranium). Just like with the EEG, it is possible, via a mathematical analysis of the signal to rebuild the sources of the electromagnetic signal. That makes it possible to identify with a more or less high degree of accuracy the areas from where are emitted the evoked potentials. However, these techniques of space localization lengthen the time of data processing considerably and remain still marginal.

the optical Imagery

the spectroscopic Imagery near infra-red

the measurement of the Optical signal evoked (EROS) is a relatively recent technique (developed at the end of the years 1990).

Comparison of the various methods

IRMf shares with the Mtoe the advantage of a space good resolution, and offers moreover a temporal good resolution since its use does not rest over the lifespan of a product. Nevertheless, IRMf also shares the disadvantages of the Mtoe: unknown harmlessness and invasive method, the patient having to be lengthened and the machine producing an infernal noise.

The EEG and the MEG are not invasive because it forces only very little the subject, the application of the electrodes being painless. They also offer a temporal good resolution. Nevertheless, the space resolution of these methods remains badly characterized.

An example of experiment psycholinguistics using the EEG

After affixing of the electrodes, the subjects are confronted with experiments of semantic and syntactic transgression. In the first case, a negative wave (called N400) is emitted approximately 400 ms after the transgressive stimulus corresponding to the semantic anomaly. In the second case, a positive wave (called P600) is emitted approximately 600 ms after the stimulus corresponding to the syntactic anomaly.

That indicates that the semantic activity precedes the syntactic activity, at least at the healthy subjects. Among patients aphasics, the N400 wave being later and of less amplitude, their access to semantic information would be slower.

References

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