Neuroanatomy of language 1 Sept 13, 2013 – DAY 8. Brain & Language LING 4110-4890-5110-7960 NSCI 4110-4891-6110 Harry Howard Tulane University. Course organization. The syllabus, these slides and my recordings are available at http://www.tulane.edu/~howard/LING4110/ .
Brain & Language
To produce a PET scan, a patient is administered a solution of a metabolically-active substance, such as glucose, tagged with a positron-emitting isotope. The substance eventually makes its way to the brain and concentrates in areas of high metabolism and blood flow, which are presumably triggered by increased neural activity. The positrons emitted by the isotopes are collected by detectors arrayed around the patients’ body and converted into signals which are amplified and sent to a computer for construction of an image.
All atoms spin on their axes. Nuclei have a positive electronic charge, and any spinning charged particle will act as a magnet with north and south poles located on the axis of spin. The spin-axes of the nuclei in the subject line up with the chamber’s field, with the north poles of the nuclei pointing in the ‘southward’ direction of the field. Then a radio pulse is broadcast toward the subject. The pulse causes the axes of the nuclei to tilt with respect to the chamber’s magnetic field, and as it wears off, the axes gradually return to their resting position (within the magnetic field). As they do so, each nucleus becomes a miniature radio transmitter, giving out a characteristic pulse that changes over time, depending on the microenvironment surrounding it. For example, hydrogen nuclei in fats have a different microenvironment than do those in water, and thus transmit different pulses. Due to such contrasts, different tissues transmit different radio signals. These radio transmissions can be coordinated by a computer into an image. This method is known as magnetic resonance imaging (MRI), and it can be used to scan the human body safely and accurately
An elaboration of MRI called functional MRI (fMRI) has become the dominant technique for the study of the functional organization of the human brain during cognitive, perceptual, sensory, and motor tasks. As Gregg (2002) explains it, the principle of fMRI imaging is to take a series of images in quick succession and then to analyze them statistically for differences. For example, in the blood-oxygen-level dependent (BOLD) method introduced by Ogawa et al. (1990), the fact that hemoglobin and deoxyhemoglobin are magnetically different is exploited. Hemoglobin shows up better on MRI images than deoxyhemoglobin, which is to say that oxygenated blood shows up better then blood whose oxygen has been depleted by neural metabolism. This has been exploited in the following type of procedure: a series of baseline images are taken of the brain region of interest when the subject is at rest. The subject then performs a task, and a second series is taken. The first set of images is subtracted from the second, and the areas that are most visible in the resulting image are presumed to have been activated by the task.
Ingram §3: Neuroanatomy of language, any leftovers
☞ Go over questions at end of chapter.