The AIM model

As the reciprocal interaction and activation-synthesis hypotheses evolved, they metamorphosed into the AIM model based on findings in sleep and dream research [23]. Basic sleep research has identified three factors that interact to determine brain-mind state. Whether we are awake (with waking consciousness), in NREM asleep (with little or no consciousness), or in REM asleep (with dream consciousness) depends upon: (1) activation level (A) (which is high in wake and REM); (2) input-output gates (I) (which are open in wake but closed in REM); and (3) aminergic modu-latory ratio (M) (which is high in wake and low in REM). Thus, the AIM (Activation, Input Source, and Modulation) model proposes that conscious states can be defined and distinguished from one another by the values of these three parameters. The three factors can be used to construct a three-dimensional AIM state space as shown in Figure 1.3. Waking, NREM, and REM occupy discreetly different domains in the state space. The wake-NREM-REM sleep cycle is seen as an elliptical trajectory in the state space with time as a fourth dimension.

The basic neurophysiology that occurs during the three AIM domains of waking, NREM, and REM sleep is shown in Figure 1.4. During REM, not only do external input-output gates close, but REM is also characterized by very strong internal stimuli generated via ponto-geniculo-occipital (PGO) waves [24,25,26,27]. These electrical impulses arise in the pons, then travel to the lateral geniculate and to the visual cortex. It is this distinctive, internally generated pseudo-sensorimotor stimulation that most directly supports the hypothesis that REM sleep dreaming is a protoconscious rather than an unconscious state. Not only is the brain activated and kept offline [28,29], but it autoactivates in such a way as to impressively simulate waking. Although inhibited, this system is

Figure 1.3 AIM model. In this figure, the fully alert, wake state is depicted in the upper right corner of the back plane of the cube. This corresponds to maximallevels of brain activation (right surface of cube), maximalexternalinput sources with minimalinternal sources (back surface), and maximalaminergic and minimal cholinergic neuromodulation (top surface). Cognitively, this corresponds to alertness with attention focused on the outside world. In the center of the cube lies deep NREM sleep, with low levels of brain activation, intermediate levels of both aminergic and cholinergic neuromodulation, and minimallevels of both external and internalinput. In this state, the mind tends towards perseverative, non progressive thinking with minimalhallucinatory activity, and this is reflected in the brevity and poverty of NREM sleep reports. As cholinergic modulation increases and aminergic modulation decreases, the modulatory function falls to its low point. The brain mind, however, regains waking levels of activation and moves from NREM into REM sleep. AIM (here referring to the brain's location in the AIM state space) moves to the bottom front edge of the cube, with input now internally driven (front surface) and neuromodulation predominantly cholinergic (bottom surface). Note the paradox that instead of moving to the left surface of the cube to a position diametrically opposed to waking brain activation returns to waking level. This forces AIM to the right surface of the cube. As a result the mind is alert, but because it is demodulated and driven by powerful internalstimuli, it becomes both hallucinatory and unfocused. REM sleep's deviation from the main diagonalaxis provides a visualrepresentation of the distinctively unique phenomenology of REM sleep and shows why that state favors dreaming. Reprinted by permission from Macmillan Publishers Ltd: Nature Reviews Neuroscience (Hobson JA, Pace Schott EF. The cognitive neuroscience of sleep: neuronalsystems, consciousness and learning. 2002;3(9):679 693), copyright 2002.

also presumably used during waking to provide the brain with a model of the external world against which it compares inputs. I would hypothesize that waking provides this system with data, which it processes throughout life as dream content.

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