Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

Appendix D: Approaches to Explore the Brain 347

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

fiber connections without providing information on activity. Variants

of this technology include diffusion tensor imaging (DTI), diffusion

spectrum imaging (DSI), and high-angular resolution diffusion

imaging (HARDI)

Positron emission tomography (PET) — Maps the metabolic activ-

ities of brain regions using a positron emitter such as radioactive flu-

oro-deoxyglucose, an analogue of glucose that will be taken up by a tis-

sue where glucose is actively utilized. The radioactive tracer has a short

half-life and has to be prepared locally in a cyclotron. The positron emit-

ted creates gamma radiation which is then detected by a machine. The

resolution is poorer that MRI.

Single photon emission computed tomography (SPECT) — Uses

an injected radioisotope to measure blood flow in different areas of

the brain. Like PET, SPECT detects functioning of a given area of the

brain. They both expose the subject to gamma ray radiation. Compared

to PET, the machine for SPECT is more affordable and the radioisotope

used is longer lasting and therefore far less expensive. The drawback is

that SPECT has poorer spatial resolution.

Genetic manipulations

Genetic modification by transgenic and knock-in/knockout tech-

niques — In this approach, specific genes can be added or deleted from

the genome of an animal. Depending on the promoter used in the added

DNA segment, a given protein can be expressed in a particular neuronal

type matching the specific promoter sequence, making the approach

highly cell-specific. However, the approach is tedious and requires the

raising of an animal (mice) colony.

Optogenetic localization and stimulation — Optogenetics applies

light technology to genetically modified brain cells. This method, which

detects the function of a single cell (or a group of similar cells), in cul-

tures or in the live brain, is of two approaches: (1) Optogenetic sensing

reveals the activation of a cell by incorporating a reporter fluorescent

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