The resulting data can be visualized as a picture of the brain with colored representations of activation. Although this may be seen as similar to neuroimaging techniques, molecular measures help to enhance areas of activation that would otherwise be indecipherable to neuroimaging. One such technique that aids in both the temporal and visual resolution of fMRI is the blood-oxygen-level dependent (BOLD) response. Changes in the BOLD response can be seen when there is differing levels of activation in suspected areas of functioning. Energy is supplied to the brain in the form glucose and oxygen (which is transferred by hemoglobin). The blood supply is consistently regulated so that areas of activation receive higher amounts of energy compared to areas that are less activated. In positron emission tomography, the use of radionuclides (isotopes with short half lives) facilitates visual resolution. These radionuclides are attached to glucose, water and ammonia so that easy absorption into the activated brain areas is accomplished. Once these radioactive tracers are injected into the bloodstream, the efficiency and location of chemical processes can be observed using PET. The main method of measuring sleep in humans is polysomnography (PSG). For this method, participants often must come into a lab where researchers can use PSG to measure things such as total sleep time, sleep efficiency, wake after sleep onset, and sleep fragmentation. PSG can monitor various body functions including brain activity (electroencephalography), eye movement (electrooculography), muscle movement (electromyography), and heart rhythm (electrocardiography). Actigraphy is a common and minimally invasive way to measure sleep architecture. Actigraphy has only one method of recording, movement. This movement can be analyzed using different actigraphic programs. As such, an actigraph can often be worn similarly to a watch, or around the waist as a belt. Because it is minimally invase and relatively inexpensive, this method allows for recordings outside of a lab setting and for many days at a time. But, actigraphy often over estimates sleep time (de Souza 2003 and Kanady 2011). Most studies point to the specific deficits in declarative memories that form pre or post REM sleep deprivation. Conversely, deficits in non-declarative memory occur pre or post NREM sleep deprivation. This is the stage specific enhancement theory. There is also a proposed dual-step memory hypothesis suggesting that optimal learning occurs when the memory trace is initially processed in SWS and then REM sleep.