56 Chapter 2 Biology and Behavior FIGURE 2 .6 A Reﬂex Pathway Sit on a chair, cross one leg over the other, and then use by the handle of a butter knife or some other solid object to gently tap your top knee, just below the joint, until you get a “knee jerk” reaction. Tapping your knee at just the right spot sets off an almost instantaneous sequence of events that begins with stimulation of sensory neurons that respond to stretch. When those neurons ﬁre, their axons, which end within the spinal cord, cause spinal neurons to ﬁre. This, in turn, stimulates the ﬁring of motor neurons with axons ending in your thigh muscles. The result is a contraction of those muscles and a kicking of the lower leg and foot. Information about the knee tap and about what the leg has done also goes to your cerebral cortex, but the reﬂex is completed without waiting for guidance from the brain. Spinal cord To brain Sensory neuron from muscle doing 2 learn Knee tap Reflex motor output, causing thigh muscle to contract Single synapse in reflex To other leg consequences of an action goes back to the source of the action for further adjustment. That is a feedback system. The Brain When pain messages from that hot burner reach your brain, you don’t become aware just of being burned. You might also realize that you have burned yourself twice before in the past week and get annoyed at your own carelessness. The brain is the most complex element in the central nervous system, and it is your brain’s astonishing capacity for information processing that allows you to have these thoughts and feelings. A variety of new brain-scanning techniques, combined with some older measures, are giving scientists ever better views of the workings of the human brain (Amaro & Barker, 2006; Miller, 2003; see Table 2.1). Each technique can indirectly measure the activity of neurons ﬁring, and each has different advantages and disadvantages. One of the earliest of these techniques, called the electroencephalograph (EEG), measures general electrical activity of the brain. Electrodes are pasted on the scalp to detect the electrical ﬁelds resulting from the activity of billions of neurons (Figure 4.3 in the consciousness chapter shows how EEG can be used to record brain activity during sleep). Although this tool can associate rapidly changing electrical activity with changes in the activity of the brain, it cannot tell us exactly where the active cells are. A newer technique, called the PET scan, can locate brain cell activity by recording where radioactive substances become concentrated when injected into the bloodstream. PET stands for positron emission tomography. It records images from the brain that indicate the location of the radioactivity as the brain performs various tasks. For instance, PET studies have revealed that speciﬁc brain regions are activated when we look at fearful facial expressions or engage in certain kinds of thoughts (Morris et al., 1998; Wharton et al., 2000). PET scans can tell us a lot about where changes in brain activity occur, but they can’t reveal details of the brain’s physical structure. A detailed structural picture of the brain can be seen, however, using magnetic resonance imaging, or MRI. MRI exposes the brain to a magnetic ﬁeld and measures the resulting radiofrequency waves to get amazingly clear pictures of the brain’s anatomical details (see Figure 2.7). Functional MRI, or fMRI, combines the advantages of PET and MRI and is capable of detecting changes in blood ﬂow and blood oxygen that reﬂect ongoing changes in the activity of neurons—providing a sort of “moving picture” of the brain (e.g., Shu et al., 2002). The newest techniques offer even deeper insight into brain activity, structure, and functioning. These techniques include a variant on fMRI called diffusion tensor imaging (DTI), as well as a procedure called transcranial magnetic stimulation (TMS). 57 The Central Nervous System: Making Sense of the World TA B L E 2.1 Techniques for Studying Human Brain Function and Structure Technique What It Shows Advantages (ⴙ) and Disadvantages (ⴚ) EEG (electroencephalograph): Multiple electrodes are pasted to the outside of the head Lines that chart the summated electrical ﬁelds resulting from the activity of billions of neurons Detects very rapid changes in electrical activity, allowing analysis of stages of cognitive processing Provides poor spatial resolution of the source of electrical activity; EEG is sometimes combined with magnetoencephalography (MEG), which localizes electrical activity by measuring magnetic ﬁelds associated with it. PET (positron emission tomography) and SPECT (single-photon emission computed tomography): Positrons and photons are emissions from radioactive substances An image of the amount and localization of any molecule that can be injected in radioactive form, such as neurotransmitters, drugs, or tracers for blood ﬂow or glucose use (which indicates speciﬁc changes in neuronal activity) Allows functional and biochemical studies Provides visual image corresponding to anatomy Requires exposure to low levels of radioactivity Provides spatial resolution better than that of EEG but poorer than that of MRI Cannot follow rapid changes (faster than 30 seconds) MRI (magnetic resonance imaging): Exposes the brain to a magnetic ﬁeld and measures radiofrequency waves TMS (transcranial magnetic stimulation): Temporarily disrupts electrical activity of a small region of brain by exposing it to an intense magnetic ﬁeld. Traditional MRI provides high-resolution image of brain anatomy. Functional MRI (fMRI) provides images of changes in blood ﬂow (which indicate speciﬁc changes in neural activity). A new variant, diffusion tensor imaging (DTI), shows water ﬂow in neural ﬁbers, thus revealing the “wiring diagram” of neural connections in the brain. Requires no exposure to radioactivity Normal function of a particular brain region can be studied by observing changes after TMS is applied to a speciﬁc location. Shows which brain regions are necessary for given tasks. Provides high spatial resolution of anatomical details ( 1 mm) Provides high temporal resolution 1 ( 10 second) Long-term safety not well established.