variety of ways in spite of reporting a complete lack of awareness. studies of the neuroscience of free will have also shown that the experiences that people report when they behave purposefully sometimes do not correspond to their actual behaviors or to the patterns of electrical activity recorded from their brains. another approach applies specifically to the study of self-awareness, that is, the ability to distinguish oneself from others. in the 1970s gordon gallup developed an operational test for self-awareness, known as the mirror test. the test examines whether animals are able to differentiate between seeing themselves in a mirror versus seeing other animals. the classic example involves placing a spot of coloring on the skin or fur near the individual's forehead and seeing if they attempt to remove it or at least touch the spot, thus indicating that they recognize that the individual they are seeing in the mirror is themselves. humans (older than 18 months) and other great apes, bottlenose dolphins, orcas, pigeons, european magpies and elephants have all been observed to pass this test. neural correlates schema of the neural processes underlying consciousness, from christof koch a major part of the scientific literature on consciousness consists of studies that examine the relationship between the experiences reported by subjects and the activity that simultaneously takes place in their brains—that is, studies of the neural correlates of consciousness. the hope is to find that activity in a particular part of the brain, or a particular pattern of global brain activity, which will be strongly predictive of conscious awareness. several brain imaging techniques, such as eeg and fmri, have been used for physical measures of brain activity in these studies. another idea that has drawn attention for several decades is that consciousness is associated with high-frequency (gamma band) oscillations in brain activity. this idea arose from proposals in the 1980s, by christof von der malsburg and wolf singer, that gamma oscillations could solve the so-called binding problem, by linking information represented in different parts of the brain into a unified experience. rodolfo llinás, for example, proposed that consciousness results from recurrent thalamo-cortical resonance where the specific thalamocortical systems (content) and the non-specific (centromedial thalamus) thalamocortical systems (context) interact in the gamma band frequency via synchronous oscillations. a number of studies have shown that activity in primary sensory areas of the brain is not sufficient to produce consciousness: it is possible for subjects to report a lack of awareness even when areas such as the primary visual cortex (v1) show clear electrical responses to a stimulus. higher brain areas are seen as more promising, especially the prefrontal cortex, which is involved in a range of higher cognitive functions collectively known as executive functions. there is substantial evidence that a "top-down" flow of neural activity (i.e., activity propagating from the frontal cortex to sensory areas) is more predictive of conscious awareness than a "bottom-up" flow of activity. the prefrontal cortex is not the only candidate area, however: studies by nikos logothetis and his colleagues have shown, for example, that visually responsive neurons in parts of the temporal lobe reflect the visual perception in the situation when conflicting visual images are presented to different eyes (i.e., bistable percepts during binocular rivalry). furthermore, top-down feedback from higher to lower visual brain areas may be weaker or absent in the peripheral visual field, as suggested by some experimental data and theoretical arguments; nevertheless humans can perceive visual inputs in the peripheral visual field arising from bottom-up v1 neural activities. meanwhile, bottom-up v1 activities for the central visual fields can be vetoed, and thus made invisible to perception, by the top-down feedback, when these bottom-up signals are inconsistent with brain's internal model of the visual world. modulation of neural responses may correlate with phenomenal experiences. in contrast to the raw electrical responses that do not correlate with consciousness, the modulation of these responses by other stimuli correlates surprisingly well with an important aspect of consciousness: namely with the phenomenal experience of stimulus intensity (brightness, contrast). in the research group of danko nikolić it has been shown that some of the changes in the subjectively perceived brightness correlated with the modulation of firing rates while others correlated with the modulation of neural synchrony. an fmri investigation suggested that these findings were strictly limited to the primary visual areas. this indicates that, in the primary visual areas, changes in firing rates and synchrony can be considered as neural correlates of qualia—at least for some type of qualia. in 2013, the perturbational complexity index (pci) was proposed, a measure of the algorithmic complexity of the electrophysiological response of the cortex to transcranial magnetic stimulation. this measure was shown to be higher in individuals that are awake, in rem sleep or in a locked-in state than in those who are in deep sleep or in a vegetative state, making it potentially useful as a quantitative assessment of consciousness states. assuming that not only humans but even some non-mammalian species are conscious, a number of evolutionary approaches to the problem of neural correlates of consciousness open up. for example, assuming that birds are conscious—a common assumption among neuroscientists and ethologists due to the extensive cognitive repertoire of birds—there are comparative neuroanatomical ways to validate some of the principal, currently competing, mammalian consciousness–brain theories. the rationale for such a comparative study is that the avian brain deviates structurally from the mammalian brain. so how similar are they? what homologues can be identified? the general conclusion from the study by butler, et al., is that some of the major theories for the mammalian brain also appear to be valid for the avian brain. the structures assumed to be critical for consciousness in mammalian brains have homologous counterparts in avian brains. thus the main portions of the theories of crick and koch, edelman and tononi, and cotterill seem to be compatible with the assumption that birds are conscious. edelman also differentiates between what he calls primary consciousness (which is a trait shared by humans and non-human animals) and higher-order consciousness as it appears in humans alone along with human language capacity. certain aspects of the three theories, however, seem less easy to apply to the hypothesis of avian consciousness. for instance, the suggestion by crick and koch that layer 5 neurons of the mammalian brain have a special role, seems difficult to apply to the avian brain, since the avian homologues have a different morphology. likewise, the theory of eccles seems incompatible, since a structural homologue/analogue to the dendron has not been found in avian brains. the assumption of an avian consciousness also brings the reptilian brain into focus. the reason is the structural continuity between avian and reptilian brains, meaning that the phylogenetic origin of consciousness may be earlier than suggested by many leading neuroscientists. joaquin fuster of ucla has advocated the position of the importance of the prefrontal cortex in humans, along with the areas of wernicke and broca, as being of particular importance to the development of human language capacities neuro-anatomically necessary for the emergence of higher-order consciousness in humans. a study in 2016 looked at lesions in specific areas of the brainstem that were associated with coma and vegetative states. a small region of the rostral dorsolateral pontine tegmentum in the brainstem was suggested to drive consciousness through functional connectivity with two cortical regions, the left ventral anterior insular cortex, and the pregenual anterior cingulate cortex. these three regions may work together as a triad to maintain consciousness. models a wide range of empirical theories of consciousness have been proposed. adrian doerig and colleagues list 13 notable theories, while anil seth and tim bayne list 22 notable theories. integrated information theory (iit) postulates that consciousness resides in the information being processed and arises once the information reaches a certain level of complexity. orchestrated objective reduction (orch or) postulates that consciousness originates at the quantum level inside neurons. the mechanism is held to be a quantum process called objective reduction that is orchestrated by cellular structures called microtubules. however the details of the mechanism would go beyond current quantum theory. in 2011, graziano and kastner proposed the "attention schema" theory of awareness. in that theory, specific cortical areas, notably in the superior temporal sulcus and the temporo-parietal junction, are used to build the construct of awareness and attribute it to other people. the same cortical machinery is also used to attribute awareness to oneself. damage to these cortical regions can lead to deficits in consciousness such as hemispatial neglect. in the attention schema theory, the value of explaining the feature of awareness and attributing it to a person is to gain a useful predictive model of that person's attentional processing. attention is a style of information processing in which a brain focuses its resources on a limited set of interrelated signals. awareness, in this theory, is a useful, simplified schema that represents attentional states. to be aware of x is explained by constructing a model of one's attentional focus on x. the entropic brain is a theory of conscious states informed by neuroimaging research with psychedelic drugs. the theory suggests that the brain in primary states such as rapid eye movement (rem) sleep, early psychosis and under the influence of psychedelic drugs, is in a disordered state; normal waking consciousness constrains some of this freedom and makes possible metacognitive functions such as internal self-administered reality testing and self-awareness. criticism has included questioning whether the theory has been adequately tested. in 2017, work by david rudrauf and colleagues, including karl friston, applied the active inference paradigm to consciousness, a model of how sensory data is integrated with priors in a process of projective transformation. the authors argue that, while their model identifies a key relationship between computation and phenomenology, it does not completely solve the hard problem of consciousness or completely close the explanatory gap. biological function and evolution opinions are divided as to where in biological evolution consciousness emerged and about whether or not consciousness has any survival value. some argue that consciousness is a byproduct of evolution. it has been argued that consciousness emerged (i) exclusively with the first humans, (ii) exclusively with the first mammals, (iii) independently in mammals and birds, or (iv) with the first reptiles. other authors date the origins of consciousness to the first animals with nervous systems or early vertebrates in the cambrian over 500 million years ago. donald griffin suggests in his book animal minds a gradual evolution of consciousness. each of these scenarios raises the question of the possible survival value of consciousness. thomas henry huxley defends in an essay titled on the hypothesis that animals are automata, and its history an epiphenomenalist theory of consciousness according to which consciousness is a causally inert effect of neural activity—"as the steam-whistle which accompanies the work of a locomotive engine is without influence upon its machinery". to this william james objects in his essay are we automata? by stating an evolutionary argument for mind-brain interaction implying that if the preservation and development of consciousness in the biological evolution is a result of natural selection, it is plausible that consciousness has not only been influenced by neural processes, but has had a survival value itself; and it could only have had this if it had been efficacious. karl popper develops in the book the self and its brain a similar evolutionary argument. regarding the primary function of conscious processing, a recurring idea in recent theories is that phenomenal states somehow integrate neural activities and information-processing that would otherwise be independent. this has been called the integration consensus. another example has been proposed by gerald edelman called dynamic core hypothesis which puts emphasis on reentrant connections that reciprocally link areas of the brain in a massively parallel manner. edelman also stresses the importance of the evolutionary emergence of higher-order consciousness in humans from the historically older trait of primary consciousness which humans share with non-human animals (see neural correlates section above). these theories of integrative function present solutions to two classic problems associated with consciousness: differentiation and unity. they show how our conscious experience can discriminate between a virtually unlimited number of different possible scenes and details (differentiation) because it integrates those details from our sensory systems, while the integrative nature of consciousness in this