by a decrease in cerebral blood flow in frontal and parietal association cortex and an increase in midline structures such as the mediodorsal thalamus. relatively local bilateral injuries to midline (paramedian) subcortical structures can also cause a complete loss of awareness. these structures therefore enable and control brain arousal (as determined by metabolic or electrical activity) and are necessary neural correlates. one such example is the heterogeneous collection of more than two dozen nuclei on each side of the upper brainstem (pons, midbrain and in the posterior hypothalamus), collectively referred to as the reticular activating system (ras). their axons project widely throughout the brain. these nuclei – three-dimensional collections of neurons with their own cyto-architecture and neurochemical identity – release distinct neuromodulators such as acetylcholine, noradrenaline/norepinephrine, serotonin, histamine and orexin/hypocretin to control the excitability of the thalamus and forebrain, mediating alternation between wakefulness and sleep as well as general level of behavioral and brain arousal. after such trauma, however, eventually the excitability of the thalamus and forebrain can recover and consciousness can return. another enabling factor for consciousness are the five or more intralaminar nuclei (iln) of the thalamus. these receive input from many brainstem nuclei and project strongly, directly to the basal ganglia and, in a more distributed manner, into layer i of much of the neocortex. comparatively small (1 cm3 or less) bilateral lesions in the thalamic iln completely knock out all awareness. forward versus feedback projections many actions in response to sensory inputs are rapid, transient, stereotyped, and unconscious. they could be thought of as cortical reflexes and are characterized by rapid and somewhat stereotyped responses that can take the form of rather complex automated behavior as seen, e.g., in complex partial epileptic seizures. these automated responses, sometimes called zombie behaviors, could be contrasted by a slower, all-purpose conscious mode that deals more slowly with broader, less stereotyped aspects of the sensory inputs (or a reflection of these, as in imagery) and takes time to decide on appropriate thoughts and responses. without such a consciousness mode, a vast number of different zombie modes would be required to react to unusual events. a feature that distinguishes humans from most animals is that we are not born with an extensive repertoire of behavioral programs that would enable us to survive on our own ("physiological prematurity"). to compensate for this, we have an unmatched ability to learn, i.e., to consciously acquire such programs by imitation or exploration. once consciously acquired and sufficiently exercised, these programs can become automated to the extent that their execution happens beyond the realms of our awareness. take, as an example, the incredible fine motor skills exerted in playing a beethoven piano sonata or the sensorimotor coordination required to ride a motorcycle along a curvy mountain road. such complex behaviors are possible only because a sufficient number of the subprograms involved can be executed with minimal or even suspended conscious control. in fact, the conscious system may actually interfere somewhat with these automated programs. from an evolutionary standpoint it clearly makes sense to have both automated behavioral programs that can be executed rapidly in a stereotyped and automated manner, and a slightly slower system that allows time for thinking and planning more complex behavior. this latter aspect may be one of the principal functions of consciousness. other philosophers, however, have suggested that consciousness would not be necessary for any functional advantage in evolutionary processes. no one has given a causal explanation, they argue, of why it would not be possible for a functionally equivalent non-conscious organism (i.e., a philosophical zombie) to achieve the very same survival advantages as a conscious organism. if evolutionary processes are blind to the difference between function f being performed by conscious organism o and non-conscious organism o*, it is unclear what adaptive advantage consciousness could provide. as a result, an exaptive explanation of consciousness has gained favor with some theorists that posit consciousness did not evolve as an adaptation but was an exaptation arising as a consequence of other developments such as increases in brain size or cortical rearrangement. consciousness in this sense has been compared to the blind spot in the retina where it is not an adaption of the retina, but instead just a by-product of the way the retinal axons were wired. several scholars including pinker, chomsky, edelman, and luria have indicated the importance of the emergence of human language as an important regulative mechanism of learning and memory in the context of the development of higher-order consciousness. it seems possible that visual zombie modes in the cortex mainly use the dorsal stream in the parietal region. however, parietal activity can affect consciousness by producing attentional effects on the ventral stream, at least under some circumstances. the conscious mode for vision depends largely on the early visual areas (beyond v1) and especially on the ventral stream. seemingly complex visual processing (such as detecting animals in natural, cluttered scenes) can be accomplished by the human cortex within 130–150 ms, far too brief for eye movements and conscious perception to occur. furthermore, reflexes such as the oculovestibular reflex take place at even more rapid time-scales. it is quite plausible that such behaviors are mediated by a purely feed-forward moving wave of spiking activity that passes from the retina through v1, into v4, it and prefrontal cortex, until it affects motorneurons in the spinal cord that control the finger press (as in a typical laboratory experiment). the hypothesis that the basic processing of information is feedforward is supported most directly by the short times (approx. 100 ms) required for a selective response to appear in it cells. conversely, conscious perception is believed to require more sustained, reverberatory neural activity, most likely via global feedback from frontal regions of neocortex back to sensory cortical areas that builds up over time until it exceeds a critical threshold. at this point, the sustained neural activity rapidly propagates to parietal, prefrontal and anterior cingulate cortical regions, thalamus, claustrum and related structures that support short-term memory, multi-modality integration, planning, speech, and other processes intimately related to consciousness. competition prevents more than one or a very small number of percepts to be simultaneously and actively represented. this is the core hypothesis of the global workspace theory of consciousness. in brief, while rapid but transient neural activity in the thalamo-cortical system can mediate complex behavior without conscious sensation, it is surmised that consciousness requires sustained but well-organized neural activity dependent on long-range cortico-cortical feedback. history the neurobiologist christfried jakob (1866-1956) argued that the only conditions which must have neural correlates are direct sensations and reactions; these are called "intonations". neurophysiological studies in animals provided some insights on the neural correlates of conscious behavior. vernon mountcastle, in the early 1960s, set up to study this set of problems, which he termed "the mind/brain problem", by studying the neural basis of perception in the somatic sensory system. his labs at johns hopkins were among the first, along with edward v.evarts at nih, to record neural activity from behaving monkeys. struck with the elegance of ss stevens approach of magnitude estimation, mountcastle's group discovered three different modalities of somatic sensation shared one cognitive attribute: in all cases the firing rate of peripheral neurons was linearly related to the strength of the percept elicited. more recently, ken h. britten, william t. newsome, and c. daniel salzman have shown that in area mt of monkeys, neurons respond with variability that suggests they are the basis of decision making about direction of motion. they first showed that neuronal rates are predictive of decisions using signal detection theory, and then that stimulation of these neurons could predictably bias the decision. such studies were followed by ranulfo romo in the somatic sensory system, to confirm, using a different percept and brain area, that a small number of neurons in one brain area underlie perceptual decisions. other lab groups have followed mountcastle's seminal work relating cognitive variables to neuronal activity with more complex cognitive tasks. although monkeys cannot talk about their perceptions, behavioral tasks have been created in which animals made nonverbal reports, for example by producing hand movements. many of these studies employ perceptual illusions as a way to dissociate sensations (i.e., the sensory information that the brain receives) from perceptions (i.e., how the consciousness interprets them). neuronal patterns that represent perceptions rather than merely sensory input are interpreted as reflecting the neuronal correlate of consciousness. using such design, nikos logothetis and colleagues discovered perception-reflecting neurons in the temporal lobe. they created an experimental situation in which conflicting images were presented to different eyes (i.e., binocular rivalry). under such conditions, human subjects report bistable percepts: they perceive alternatively one or the other image. logothetis and colleagues trained the monkeys to report with their arm movements which image they perceived. temporal lobe neurons in logothetis experiments often reflected what the monkeys' perceived. neurons with such properties were less frequently observed in the primary visual cortex that corresponds to relatively early stages of visual processing. another set of experiments using binocular rivalry in humans showed that certain layers of the cortex can be excluded as candidates of the neural correlate of consciousness. logothetis and colleagues switched the images between eyes during the percept of one of the images. surprisingly the percept stayed stable. this means that the conscious percept stayed stable and at the same time the primary input to layer 4, which is the input layer, in the visual cortex changed. therefore, layer 4 can not be a part of the neural correlate of consciousness. mikhail lebedev and their colleagues observed a similar phenomenon in monkey prefrontal cortex. in their experiments monkeys reported the perceived direction of visual stimulus movement (which could be an illusion) by making eye movements. some prefrontal cortex neurons represented actual and some represented perceived displacements of the stimulus. observation of perception related neurons in prefrontal cortex is consistent with the theory of christof koch and francis crick who postulated that neural correlate of consciousness resides in prefrontal cortex. proponents of distributed neuronal processing may likely dispute the view that consciousness has a precise localization in the brain. francis crick wrote a popular book, "the astonishing hypothesis", whose thesis is that the neural correlate for consciousness lies in our nerve cells and their associated molecules. crick and his collaborator christof koch have sought to avoid philosophical debates that are associated with the study of consciousness, by emphasizing the search for "correlation" and not "causation". there is much room for disagreement about the nature of this correlate (e.g., does it require synchronous spikes of neurons in different regions of the brain? is the co-activation of frontal or parietal areas necessary?). the philosopher david chalmers maintains that a neural correlate of consciousness, unlike other correlates such as for memory, will fail to offer a satisfactory explanation of the phenomenon; he calls this the hard problem of consciousness.