Self Comes to Mind

promoting the superior colliculi to mind-contributing status. The superior colliculus produces electrical oscillations in the gamma range, a phenomenon that has been linked to synchronic activation of neurons and that has been proposed by the neurophysiologist Wolf Singer to be a correlate of coherent perception, possibly even of consciousness. To date, the superior colliculus is the only brain region outside the cerebral cortex known to exhibit gamma-range oscillations. 16
Closer to the Making of Mind?
     
    The picture that emerges from the foregoing indicates that mind-making is a highly selective business. It is not the case that the entire central nervous system is uniformly involved in the process. Certain regions are not involved, some are involved but are not principal players, and some carry out the bulk of the work. Among the last, some provide detailed images; others provide a simple but foundational kind of images such as bodily feelings. All regions involved in mind-making have highly differentiated patterns of interconnectivity, suggestive of very complex signal integration.
    Contrasting the set of regions that do and do not contribute to the mind-making effort does not tell us what kind of signals neurons must produce; it does not specify frequencies or intensities of neuron firing or patterns of coalition among neuron sets. But it tells about certain aspects of the wiring diagram that neurons require to be involved in mind-making. For example, the cortical mind-making sites are clusters of interlocked regions organized around the port of entry for inputs from peripheral sensory probes. The subcortical mind-making sites are also intensely interlocked clusters of regions, nuclei in this case, and they are also organized around inputs from another “periphery”—namely, the body itself.
    Another requirement, applying equally to cerebral cortex and subcortical nuclei: there must be massive interconnectivity among the mind-making regions so that recursiveness is prevalent and a high complexity of cross-signaling is achieved, a feature that in the case of the cortex is amplified by cortico thalamic interlocking. (The terms reentrant and recursive refer to signaling that, rather than merely going forward along a single chain, also returns to the origin, looping back to the ensemble of neurons where each element of the chain begins.) Mind-making regions in the cortex also receive numerous inputs from a variety of nuclei located underneath, some in the brain stem and some in the thalamus; they modulate cortical activity by way of neuromodulators (such as catecholamines) and neurotransmitters (such as glutamate).
    Finally, a certain timing of the signaling is necessary so that elements of a stimulus that arrive together at the peripheral sensory probe can stay together as the signals are being processed within the brain. For mind states to emerge, small circuits of neurons must behave in a very particular manner. For example, in small circuits whose activity signifies that a certain feature is present, neurons increase their firing rates. Ensembles of neurons that are working together to signify some combination of features must synchronize their firing rates. This was first demonstrated in the monkey by Wolf Singer and his colleagues (and also by R. Eckhorn), who found that separate regions of the visual cortex involved in processing the same object exhibited synchronized activity in the 40 Hz range. 17 The synchronization is probably achieved by oscillations of neuronal activity. When brains are forming perceptual images, the neurons of the separate regions that contribute to the percept exhibit synchronized oscillations in the high-frequency gamma range. This could be part of the secret behind the “binding” of separate regions by means of time; I will invoke this sort of mechanism to explain the operation of convergence-divergence zones ( Chapter 6 ) and the assembly of the self (Chapters 8, 9, and 10). 18 In other words, besides building rich maps at a variety of separate locations, the brain must relate the maps to one another, in coherent ensembles. Timing may well be the key to relating.
    In sum, the notion of a map as a discrete entity is merely a helpful abstraction. The abstraction hides the extremely large number of neuron interconnections that are involved in each separate region and that generate a huge degree of signaling complexity. What we experience as mental states corresponds not just