Empirical evidence for the causal or correlational connection between the mind and brain has been elusively sought by many neuroscientists and philosophers for over a hundred years. The question is: How, exactly, do each person’s phenomenal and subjective experiences arise from neuronal firings and physiological processes in the brain? Many neuroscientists have sought to answer this question by trying to find the “neural correlates of consciousness,” meaning, the exact areas of the brain which give rise to conscious experiences. For this research paper, I will go over some current (from earliest to most recent) biological studies and research into the neural correlates of consciousness. Also, I will outline how each study seems to correlate not only with one another, but also with one viable biological theory into the nature of phenomenal and subjective consciousness, namely, the global workspace model of consciousness.
In “Neural Correlates of Consciousness in Humans,” Geraint Rees et al offers an overall review of several research studies into the neural correlates of consciousness, specifically focusing on the sense of vision as the starting point for conscious experience. First, Geraint et al offers two important distinctions regarding consciousness. The first distinction is between the neural correlates of being awake (as opposed to being asleep) versus the neural correlates of being phenomenally aware of one’s experiences (meaning, what it is like for one as an individual to have an experience). The second distinction is between neural activity correlated to a phenomenal experience versus neural activity correlated to underlying, unconscious processes or sensory inputs related to those phenomenal experiences.
These distinctions are important because visual processes can cause activity in the brain on many different levels, however, this activity does not necessarily reflect consciousness in the sense of being phenomenally aware. For example, sleepwalkers can see and subsequently interact with their environments in complex ways, but have no actual conscious, phenomenal awareness of what they are seeing or doing. Additionally, it is important to understand the visual pathway starts at the V1 in the back of the brain and moves through many other areas of the brain. Rees et al describe how damage to the primary visual cortex (V1) results in a condition known as blindsight, where a person can seemingly perceive of objects they report being unable to actually see. Additionally, Rees et al describe how blinking and microsaccades (slight, quick eye movements which occur normally and frequently) both do not interrupt our conscious perceptions, but both register changes in activity in the V1. Blindsight, blinking, and microsaccades, are just a few examples offered by Rees et al which demonstrate how the V1, even though necessary for normal eyesight, is not a neural correlate of consciousness because the V1 can be damaged resulting in one having no visual perception but having some conscious perception of objects or because the V1 can undergo fluctuations in activity without altering phenomenal experience.
Rees et al offers other evidence in support of another area of the visual system, the ventral visual cortex, as playing more of a role in conscious phenomenal experiences. Rees et al note how damage to specific areas of the visual system result in the subject being unable to consciously perceive of the specific type of visual features associated with this area. Rees et al explain specific areas of the visual system are specialized to analyze visual input for specific types of visual stimuli, and activity in these areas is required for one to have consciousness of that specific type of visual stimuli. As examples, Rees et al notes damage to the middle temporal area of the ventral visual cortex leads to one being unable to perceive of movement (a condition known as akinetopsia) or damage to areas in the ventral occipitotemporal cortex leads to one being unable to perceive of colors (a condition known as achromatopsia). In both circumstances, the individual is seeing the stimuli, but not consciously perceiving of it. Further, Rees et al notes neuroimaging studies have shown the ventral visual cortex is active during schizophrenic hallucinations, where the patients are having a phenomenal experience with no corresponding visual stimuli.
Additionally, as subjects are flashed “masked” (or hidden by flashing symbols before and after) and “unmasked” (or unhidden by not flashing symbols after) words within their field of vision, areas of the brain which are connected to an individual thinking about the meaning of a word only show activation when the individual reports consciously perceiving the unmasked word. The area previously activated becomes deactivated when the individual’s phenomenal experience changes by no longer perceiving the masked word, even though the word flashing in their field of vision remains the same. Finally, Rees et al notes how electrophysiological research using epileptic patients with embedded electrodes shows how neurons in the anterior areas of the medial temporal lobe fire in response to a specific type of visual stimuli and, in other neuroimaging studies, these neurons also fire when the individual is asked to imagine the specific type of visual stimuli.
Contrarily, Rees et al also refers to fMRI studies which show how individuals with damage to the V1 area have shown activation in the extrastriate cortex of the ventral visual cortex, in an area specifically related to perceiving facial stimuli, when the facial stimuli is not presented nor initiated. Additionally, Rees et al mentions another study which found activity was still triggered in the extrastriate visual cortex even when individuals were unable to perceive a masked word, meaning, even when individuals were not consciously aware of the word. These studies seem to show activity in the ventral visual cortex which is not related to visually stimulated, conscious perception. However, Rees et al notes regarding the masked word study, while there is activity, it occurs at a much smaller frequency for masked words than for perceived words. Rees et al conclude even though the focused areas of the ventral visual cortex to specific visual types of stimuli is required for conscious, phenomenal experiences of stimuli related to those types, the ventral visual cortex in and of itself may not be sufficient to produce conscious, phenomenal experiences.
Rees et al offer other possible conditions necessary for individual phenomenal consciousness. Rees et al note how neuroimaging studies have shown when an individual is presented with two different images, one displayed in front of one eye while the other is displayed over the other eye, the individual’s attention and perception of the images switches back and forth between the two images. As the individual’s attention switches back and forth, activity is recorded in the individual’s extrastriate cortex, and parietal and frontal cortexes. The activity is similar to the activity observed in other studies as an individual views images such as Rubin’s face/vase figure. Rees et al assert this activity demonstrates a possible causal connection between the frontal and parietal cortexes and an individual’s phenomenal experience in focusing on images or objects competing visually. Additionally, Rees et al note how in other studies activity has been observed in the parietal and dorsolateral prefrontal regions when individuals go from being unaware of something changing in their field of vision and becoming aware of the change within their field of vision. Due to the findings of these studies, Rees et al assert it may be activity in the ventral visual pathway requires in connection additional activity from the parietal and prefrontal regions in order for awareness.
In “The Quest to Find Consciousness,” Gerhard Roth also discusses neural correlates of consciousness by explaining in more depth the neurological processes leading to activity in areas of the brain believed to correlate with consciousness. Researchers have discovered consciousness, according to Roth, requires the associative regions of the cerebral cortex. Roth explains the associative regions of the cerebral cortex are the occipital, parietal, temporal and frontal regions.
Roth asserts in the associative regions there exists “millions of nerve cells that are densely interconnected,” in which the “synapses can strengthen or weaken their connections for a short time”. Roth explains the ability of the synapses to strengthen or weaken their connections results in synchronized communication between the nerve cells so that all the nerve cells become focused together on a single experience, like picking out one object from several objects or understanding the meanings of words.
Furthermore, Roth explains, chemical neurotransmitters, such as glutamate or gamma-aminobutyric acid, are “messenger[s]” which signals the nerve cells and synapses to synchronize and modulate the changes in activity. Roth asserts the reticular formations and limbic centers control the release of chemical neurotransmitters. “Neuromodulating,” the controlled release of these neurotransmitters and the subsequent nerve cell and synaptic alternations in activity based on the individual’s circumstances, requires a lot of fuel, specifically oxygen and glucose, according to Roth. In order to accommodate the need for more oxygen and glucose, Roth notes how blood flow to the area increases almost immediately.
Roth explains the reason why the associative cortex allows for consciousness may be because of the vast number of nerve cell connections. When a stimulus is presented to an individual, basic sensory information is processed by the primary and secondary regions of the cortex then sent to the associative regions in the parietal and temporal lobes which then registers activity explains Roth. The specific areas in the parietal, occipital and temporal lobes which carry on higher order processing of various forms of sensory information have “reverse” connections back to the primary and secondary regions says Roth. Additionally, the associative cortex is also connected to the hippocampus (in the medial temporal lobe and is responsible for memory) and the amygdala in the limbic system (also in the medial temporal lobe and is responsible for “emotional memory”) explains Roth. Roth asserts the vast connections between the associative regions of the cerebral cortex and to other areas along with the interactive synchronization between nerve cells and synapses between the associative regions of the cerebral cortex and to other areas such as the thalamus, hippocampus and limbic systems, may be the answer to how consciousness arises from physical processes.
Similar to both Rees et al’s and Roth’s assertions, in “Conscious Awareness of Flicker in Humans Involves Frontal and Parietal Cortex,” David Carmel et al asserts the frontal and parietal areas (which are associative regions of the cerebral cortex) may contribute to conscious awareness of visual stimuli. Carmel et al used an LED flickering at the “critical flicker fusion (CFF) threshold,” meaning at the point where the flickering could just as likely be perceived as either flickering or as a steady stream of light. Carmel et al tested the flickering at the CFF threshold to determine whether subjects viewed the identical flickering of the stimulus as either flickering or a steady stream of light. As the subjects responded, Carmel et al monitored their brain activity using fMRIs in order to determine where the activity occurred based on perceptions of the light flickering versus perceptions of the light being a steady stream.
Carmel et al deduced subjects perceiving of the light as flickering or a steady stream would be a result of activation of the higher cortical regions because the activation of neurons in the “early” visual cortex (see above regarding Rees et al’s research regarding the primary visual cortex) respond to flickering at a much higher rate than the CFF. In other words, Carmel et al reasoned higher cortical regions must be responsible for an individual’s perception of a light appearing to be flashing versus a steady stream because the primary visual cortex can see the flickering of a light source when the flickering is occurring at a much faster rate than the higher cortical regions can consciously perceive of it, which goes along with Rees et al’s findings. Further, Carmel et al hypothesized activity in the frontal and parietal regions would determine if the flickering of the LED would be perceived (meaning, as “conscious awareness”) as either a flicker or a steady stream of light.
Carmel et al’s results of the fMRIs showed more activity in the frontal and parietal cortexes when the subject reported perceiving of the light as a flicker. Carmel et al notes the results showed more activity in the occipital extrastriate cortex when the subjects reported perceiving of the light as a steady stream. Carmel et al’s results seem to corroborate with Rees et al’s research regarding the extrastriate cortex, the frontal and parietal cortexes showing activation in response to the display of two different images in subjects’ visual fields. In both Carmel et al’s and Rees et al’s scenarios, the subjects are consciously trying to determine what they are seeing. According to Carmel et al, their results demonstrate how important the activity in the higher cortical regions is for awareness in certain types of visual events. Furthermore, Carmel et al assert their results show how particularly the frontal and parietal regions may be involved in overall visual awareness.
In “Human Single-Neuron Responses at the Threshold of Conscious Recognition,” R. Quian Quiroga et al also examine neuronal activity. However, their research focuses on activity in the medial temporal lobe, which corresponds with Roth’s findings regarding consciousness requiring the associative areas of the brain and connections to the hippocampus and limbic systems (located in the medial temporal lobe). Quiroga et al explain the process of visual perception starts with the neurons in the primary visual cortex (“early visual areas”), which take in the visual stimuli, and then moves along the ventral visual pathway to neurons in the higher cortical regions which will organize and analyze the information to result in conscious recognition. According to Quiroga et al, the medial temporal lobe is the apex where perception takes place. While the medial temporal lobe, per Quiroga et al, does not directly affect conscious recognition, it plays a role in changing visual perceptions into long term memories. Therefore, according to Quiroga et al, activity in the medial temporal lobe is required to initiate “perception processes” so that the perceived images can be stored as long term memories. In other words, according to Quiroga et al, the medial temporal lobe “signals” other areas of the brain to perceive or recognize a visual stimulus so the stimulus can be committed to long term memory.
Quiroga et al sought to determine the response rate of neurons in the medial temporal lobe when subjects reported conscious recognition of images portraying familiar faces, landmarks and animals. To test the response rate, Quiroga et al flashed the images for successively shorter intervals in order to examine the activity of neurons in the medial temporal lobe. Quiroga et al assert they found activity in the medial temporal lobe was essentially the same for images flashed for durations of 33ms, 66ms, and 132ms. However, Quiroga et al state, images flashed for 264ms triggered a much greater amount of neuronal firings in the medial temporal lobe. According to Quiroga et al, the subjects’ reports of conscious perception of the images matched the activity in the medial temporal lobe. In other words, the activity in the medial temporal lobe increased when subjects reported consciously recognizing the images. Quiroga et al state the activity in the medial temporal lobe started at about 300ms, and in some cases lasted up to 1000ms, after the image was flashed in the subjects’ field of view. Additionally, Quiroga et al note, the neurons fired in an “all-or-none fashion,” meaning, before stimulus presentation there was practically no activity but once a familiar image was presented a great deal of activity occurred. Quiroga et al also state subjects displayed no activity when the image flashed was not familiar to them.
Quiroga et al’s findings regarding the activation of the medial temporal lobe and the continuation of activity even after the image is no longer in the field of sight seem to corroborate Rees et al’s findings regarding the medial temporal lobe showing activity in response to specific types of visual stimuli and this activity occurring even when the individual is asked to simply just imagine the specific type of visual stimuli. For Quiroga et al, these findings seem to indicate the “memory-like” nature of the medial temporal lobe. Ultimately, Quiroga et al assert the medial temporal lobe serves as a “link” between conscious recognition and long term memory because the neurons only become activated in response to recognized images, and because the neurons continue their activation long after the image is no longer visually seen.
At this point, the above mentioned articles have asserted the neural correlates of consciousness seem to exist in several areas of brain, as well as indicate it is not just activity in the brain, but the duration and type of activity in the brain which leads to consciousness. In “Converging Intracranial Markers of Conscious Access,” Raphael Gaillard et al researched overall locations, strengths and durations of neuronal activity. To do this, Gaillard et al briefly flashed masked and unmasked words in epileptic subjects’ (with embedded electrodes) field of view and measured brain activity using an iEEG.
Gaillard et al assert their experimental aim was to test assertions proposed by the global workspace model of consciousness and they begin by explaining the model. Per Gaillard et al, while many areas of the brain work at the same time to process information unconsciously, consciousness arises if and only if very specific conditions convene. First, according to Gaillard et al, activity in the visual cortical regions must occur in response to specific types of stimuli associated with those areas (seemingly corresponding with Rees et al’s, Roth’s, Carmel et al’s and Quiroga et al’s research regarding the ventral visual cortex). Secondly, per Gaillard et al, the activity beginning in the visual cortical regions must occur for a sufficient amount of time, and must be of a sufficient amount of strength, in order to proceed along the cortical pathway to the higher cortical areas of the parietal and prefrontal cortices (again, seemingly corresponding to Rees et al’s, Carmel et al’s and Roth’s findings regarding the roles of the parietal and prefrontal regions). Thirdly, states Gaillard et al, the activity must be amplified throughout several areas of the brain through forward and backward connections, “ignit[ing]” neurons throughout several areas in synchronous “self-sustained reverberation” (seemingly similar to Roth’s assertions regarding forward and backward synaptic connections temporarily strengthening their connections and working together, synchronously). Gaillard et al assert, according to the global workspace model, the experience of phenomenal and subjective consciousness is a result of stimuli being represented on so many different levels of the brain.
Gaillard et al explain their results for masked words showed activity in 43 of the 176 electrodes, primarily in the occipital lobe, and in order of extent, in the temporal lobe, parietal lobe and the frontal lobe. Conversely, for unmasked words, 121 of the 176 electrodes showed activity primarily in the frontal lobe, and in order of extent, in the occipital lobe, temporal lobe and parietal lobe. Per Gaillard et al, while the majority of activation for masked words occurred in just a few areas, activity was more equally distributed among more areas for unmasked words.
Regarding the duration of the activation, Gaillard et al note masked words, as a mean average, resulted in activity lasting approximately 60ms. Conversely, states Gaillard et al, unmasked words, as a mean average, resulted in activity lasting approximately 378ms. However, explains Gaillard et al, the activity from masked words occurred earlier, at 366ms (mean), than unmasked words, at 522ms (mean). Once activity started, however, the activity associated with unmasked word lasted longer and extended to more areas than did the activity for masked words, states Gaillard et al.
Additionally, explains Gaillard et al, while masked words showed gamma band oscillations (which are neuronal patterns of oscillating frequencies ranging from 20-100 Hz) initially peaking at 100-200ms after being presented then dropping off, unmasked words showed a peak and then continued activity for up to 800ms after being presented. Gaillard et al assert masked words showed an initial peak of gamma band activity which then quickly leveled off, while unmasked words showed more oscillations at a greater strength and for a longer period of time. Additionally, Gaillard et al used iERPs to measure the voltage strength and progression of activity. The activity, per Gaillard et al, for masked words showed a slight peak originating in the occipitotemporal areas then declining with almost no observable activity in the other areas. The activity for unmasked words was “stronger and lasted longer,” states Gaillard et al, and started in the posterior (visual cortical) regions with significant activity progressing to other areas. Gaillard et al assert the gamma band activity and the activity observed through the iERPs together are “correlated measures” which show conscious and unconscious brain processes. Gaillard et al’s findings seem to corroborate Quiroga et al’s findings which also showed activity starting at about 300ms and lasting up to 1000ms (specifically in the medial temporal lobe).
Gaillard et al also tested the synchronization of beta wave activity (12-30Hz) by testing to see if, after many observations, areas of the brain showed a “systematic phase relationship.” In other words, they tested to see if the activity in different areas of the brain showed to occur repeatedly together, each time in the same ways. Overall, Gaillard et al assert beta wave activity across all electrodes for unmasked words showed to be synchronous during the time frame of 300-500ms after exposure to the stimulus, meaning the beta waves in one area of the brain appeared to be in synch with the beta waves in other areas of the brain. According to Gaillard et al, masked words resulted in no changes to the synchrony of beta wave activity. Additionally, Gaillard et al tested sets of electrodes, to see if an electrode in one hemisphere of the brain showed beta wave activity in conjunction with an electrode in another hemisphere of the brain. Gaillard et al assert unmasked words showed beta wave activity increased across the sets while masked words actually showed a decrease in beta wave activity. Additionally, Gaillard et al assert they observed a small increase in beta activity in the visual cortex area for masked words, but unmasked words triggered synchronous activity across not only the visual cortex, but many other areas as well. These findings explain Rees et al’s findings regarding the activity triggered by masked words in the ventral visual cortex, in that the activity occurs but only marginally.
Gaillard et al state their final examination entailed the use of a mathematical program to check for Granger Causality. Per Gaillard et al, the beta wave testing and Granger Causality testing were similar because they both checked for correlation. However, as Gaillard et al assert, “it is possible for electrode j to causally influence i without i causally influencing j” and “it is also possible for two signals to exert mutual causal influences on each other.” The Granger Causality test, in other words, was done to determine the probabilities of the activity in different areas of the brain actually having a causal effect on each other, and in which direction (from j to i or from i to j) the causal effect occurred. Gaillard et al explain the program showed a large increase in probability of causation beginning with the posterior regions (i.e. the regions of the ventral visual cortex) and moving toward the occipitofrontal regions. The “evolution” of the causality showed a peak at 146ms when the masked words were presented and another peak at 325ms, which only occurred when the unmasked words were presented. Additionally, unmasked words showed a “causal gain” at 300ms-500ms after presentation, notes Gaillard et al. A probability gain which was not observed in the masked words, which shows a strong causal gain at times of “conscious processing” according to Gaillard et al.
According to Gaillard et al, their results all lend support to the global workspace model of consciousness. Additionally, Gaillard et al’s results seem to share similarities with Rees et al’s, Roth’s, Carmel et al’s and Quiroga et al’s findings. First, Gaillard et al’s results showed activity originating from the ventral visual cortex, an area which, as Rees et al had shown, is responsible for conscious perception of specific types of visual stimuli, including the processing of words. Secondly, Gaillard et al’s results showed the duration and strength of gamma band and beta wave activity was significant up to 300-500ms (in some cases up to 800ms) after the stimuli was no longer present, durations very similar to Quiroga et al’s findings regarding the durations of activity in the medial temporal lobe. The strength and duration of the activity is sufficient for the activity to proceed from the ventral visual cortex to higher order regions like the parietal and prefrontal regions. The parietal and prefrontal regions, per Rees et al and Carmel et al, are activated during conscious recognition and awareness, which requires, per Quiroga et al, activity in the medial temporal lobe. Thirdly, Gaillard et al’s results showed the activity occurred in several areas of the brain synchronously, with a strong probability of causation from posterior areas of the brain to the frontal regions, moving across distant parts of the brain. Per Roth, the associative regions of the parietal, frontal and temporal lobes (due to having a huge number of neurons tightly packed together) have forward and backward connections which facilitate synchronous activities and communications across distant parts of the brain. If these scientists’ findings are true, and if I am correct in correlating the findings, then it seems “consciousness,” from the scientific viewpoint, arises from a complex interaction of sustained synchronous gamma and beta wave events between many areas of the brain.
Carmel, David, Nilli Lavie and Geraint Rees. “Conscious Awareness of Flicker in Humans Involves Frontal and Parietal Cortex.” Current Biology (2006): 907-911.
Gaillard, Raphael, Dehaene Stanislas, Claude Adam, Stephane Clemenceau, Dominique Hasboun, Michel Baulac, Laurent Cohen, Lionel Naccache. “Converging Intracranial Markers of Conscious Access.” PLoS Biology (2009): 0472-0492.
Quiroga, R. Quian, R Mukamel, E.A Isham, R. Malach, I. Fried. “Human Single-Neuron Responses at the Threshold of Conscious Recognition.” Proceedings of the National Academy of Sciences (2008): 3599-3604.
Rees, Geraint, Gabriel Kreiman and Christof Koch. “Neural Correlates of Consciousness in Humans.” Nature Reviews (2002): 261-270.
Roth, Gerhard. “The Quest to Find Consciousness.” Scientific American Special Edition (2004): 32-39.