glossary of terms


of terms relating to the study of consciousness and the brain


Professor Christof Koch

This is a useful compendium of definitions of terms and expressions that arise in the study of the brain, the mind and the relationship among these two. It should be of interest to biologists, psychologists, clinicians, philosophers and other scholars of consciousness. Many of the terms relate in particular to vision and visual perception.

The text is an updated version of the glossary in the book The Quest for Consciousness that I wrote in 2004, and that was published by Roberts & Company. For more information, references, figures and so on illustrating these terms, please consult that book.

40 Hz Oscillations
Periodic variation in the local field potential or in the timing of action potentials from one or many neurons. The frequency can be anywhere between 30 and 70 Hz. See oscillations. 
Access consciousness
The philosopher Ned Block distinguishes, on conceptual grounds, access consciousness from phenomenal consciousness (Block, 1999, 2005). Phenomenal consciousness corresponds to the subjective feeling of seeing red (as compared to the feeling of seeing green), while access consciousness is what is made accessible to multiple cognitive processes, including memory, language, and other behaviors. Phenomenal consciousness in isolation may correspond to consciousness without top-down attention, while the confluence of access and phenomenal consciousness occurs when the subject is attending to an object or event and is consciousness of it. Access consciousness is usually what is studied in the laboratory, while phenomenal consciousness encompasses experiences difficult to quantify. 
A very important neurotransmitter. In the peripheral nervous system, acetylcholine (ACh) transduces action potentials in motorneurons into muscular action. In the brain itself, acetylcholine acts both rapidly to excite its postsynaptic targets as well as more slowly, to up- or down-regulate the gains of neuronal populations. Activity in the neurons that release ACh, called cholinergic cells, correlates with increasing arousal levels. 
A specific deficit in the perception of colors due to cortical lesions in the fusiform gyrus. 
Action Potential
All-or-none, pulse-like change in the membrane potential, about 100 mV in amplitude and 0.5-1 msec in width. Action potentials, or spikes, (also referred to as spiking discharge or firing activity) are the primary means to rapidly communicate information between neurons and from neurons to muscles. 
Activity Principle
A hypothesis according to which underlying every direct percept—seeing red, smelling wet moss, the feeling of initiating an action–there will be one or more groups of neurons that explicitly represent the different attributes of this percept (see explicit coding). 
Prolonged exposure to a stimulus attribute causes a short lived deficit in the ability to detect that attribute (as in the orientation-dependent aftereffect). In some cases, the opposite attribute is seen, as in the motion aftereffect where the observer sees upward motion after being habituated to downward motion (also known as the waterfall illusion) or in color afterimages. Aftereffects are thought to be caused by a recalibration or adaptation of the underlying neurons. 
A specific deficit in the perception of visual motion due to bilateral lesions in and around cortical area MT
Anterior cingulate cortex (ACC)
Part of the central executive in the frontal lobe that may be key to the NCC. The ACC monitors complex behaviors and is particularly active when incorrect behavior or errors occur. 
Ascending reticular activation system or ARAS
A set of about 40 (on each side) anatomical and functional distinct nuclei in the midbrain, pons and upper medulla. Collectively, they ennervate the cortex, thalamus and related structures and are responsible for arousal, the sleep-wake cycle and other household functions. If some or more of these nuclei are damaged on both sides consciousness can be transiently or permanently impaired; in other words, they are part of the enabeling factor for consciousness or NCCe
Arousal or gating system
A set of upper brainstem (mesencephalic reticular formation), hypothalamic and midline thalamic structures (intralaminar nuclei and reticular nuclei) that mediate arousal states (wakefulness, REM sleep and deep sleep). Bilateral damage to these causes coma. A functioning arousal system is a necessary prerequisite for any conscious content to occur. The brainstem part of this system is known as the the ascending reticular activation system. All of these structures constitute enabeling factors for consciousness, or NCCe
The ability to concentrate on a particular stimulus, event, or thought while excluding competing stimuli. Selective attention is necessary for many forms of conscious perception and must be distinguished from the more generalarousal and alertness. Historically, two broad forms of selective attention have been distinguished, top-down and bottom-up attention. The function of attention is to select a subset of information from the constant stream of information (on the order of one megabyte per second per eye) impinging onto the brain proper. Such a selection enables the organism to respond in real-time to important events. Non-attended events are processed at a reduced bandwidth. At the neuronal level, one important manifestation of attention is to bias the coalitions that encode these objects. Selective attention is distinct from consciousness. 
Attentional searchlight
See top-down attention
I use this term interchangeably with consciousness. Some scholars distinguish between these two on ontological (Chalmers, 1996), conceptual (Block, 1995), or psychological (Tulving, 1993) grounds. 
Basal ganglia
A collection of nuclei buried below the cerebral cortex that are involved in the regulation of voluntary movement, motor learning and related behaviors. They receive input from throughout cortex and the intralaminar nuclei of the thalamus and project back, via the thalamus, to the frontal lobes. Many neurodegenerative diseases, such as Huntington’s or Parkinson’s disease, attack neurons in the basal ganglia. 
Back of the cortex
A shorthand for all cortical regions that lie behind the central sulcus, including all purely sensory regions (with the notable exception of olfaction). This definition is the complement of the front of the cortex
Binding problem
How distinct attributes of one or several objects in the world, represented by neural activity at many distributed sites, are combined into unitary percepts is known as the binding problem. For instance, how are the color, motion, and sounds of a red Ferrari, zooming past at high speed, combined into a single percept when their underlying neural activity is distributed at many sites? And how is this kept apart from the neural representation of a simultaneously perceived motorcycle? 
Binocular neurons
Visual neurons that can be driven by an input from either eye. Binocular neurons first occur in primary visual cortex. Monocular neurons only respond to input from one eye. 
Binocular disparity
The relative separation of the image of an object in the two eyes. Disparity can be used to extract the distance between this object and the head, its depth. 
Binocular rivalry
One example of a perceptual stimulus, in which two different pictures, projected onto corresponding locations in the left and right eyes, are not seen superimposed, but are seen one after the other. This provides a vivid illustration of the winner-take-all dynamics of coalitions of neurons, suppressing competing percepts. 
Bistable illusions
A constant sensory input that can be perceived in one of two, mutually incompatible, ways. Two examples are the Necker cube and binocular rivalry. See perceptual stimuli
Residual visual-motor behavior within a scotoma. Patients profess to be blind in this part of their field of view yet can respond appropriately to simple stimuli. This is but one example of a selective dissociation between behavior and consciousness. 
BOLD signal
See fMRI
Bottom-up attention
A rapid and automatic form of selective attention, that only depends on intrinsic qualities in the input (exogenous attention). In the visual domain, it is known as saliency-based attention. The more salient a location or object in the image, the more likely it will be noticed. See also attention
A division of the brain that includes the midbrain, pons, and medulla. 
An event A can be said to cause another event B if (i) the onset of A precedes the onset of B and (ii) preventing A eliminates B. This definition must be suitably extended if either A or C can cause B. Given the highly interwoven, redundant and adaptive networks in molecular-, cell- and neuro-biology, moving from correlation to causation is not easy. 
Center-surround organization
The receptive field of a visual neuron, that is, the region in visual space that excites the cell (colloquially, “that it can see”), includes a quasi-circular region at the center, surrounded by an annulus. The response profile of this surrounding region opposes that of the center. For example, an on-cell will respond vigorously if a spot of light falls onto its central region and is inhibited when an annulus of light stimulates its surround. 
Cerebral cortex
Often simply called cortex, the cerebral cortices are a pair of folded sheets of nervous tissue, a few millimeter in thickness and of variable extent. In humans, one cortical sheet has the size of a large pizza, about 1,000 square centimeter. Cortex is highly laminated and is subdivided into neocortex—characteristic of mammals—and older regions, such as olfactory cortex and the hippocampus. 
Change blindness
The inability to notice large changes in images or scenes. Thought to reflect an attentional limitation. Change blindness and related visual illusions (e.g., inattentional blindness, the attentional blink) demonstrates that our experience of the visual world is far more limited than our intuition suggests. 
Cholinergic transmission
Using the neurotransmitter acetylcholine
Classical receptive field
Part of the receptive field defined for any sensory neuron. 
Coalition of neurons
A group of mono- or poly-synaptically coupled forebrain neurons that encode one percept, event or concept. Coalitions are born and die at the time scale of fraction of a second or longer. Members of a coalition reinforce each other and suppress members of competing coalitions. Attention biases these competitive interactions. Synchronized and oscillatory firing might play an important role in strengthening one coalition at the expense of others. Underlying every conscious percept must be a coalition of neurons explicitlyexpressing the perceived attributes. 
Columnar organization
A common design feature of the cortex whereby most neurons under a patch of cortex (from layers 2 through 6, with a possible exception of the input layer 4), encode one or more features in common. Examples include columns for visual orientation in V1 and for the direction of motion in MT. I argue that the attribute represented in this columnar fashion is made explicit
A clinically defined condition in which the patient cannot be aroused, has no sleep-wake cycle, shows no evidence of conscious sensations, reflexes, or any significant movement. It is a state of profound unconsciousness. Coma rarely lasts more than 2 to 4 weeks and can lead to a vegetative state
At this point in the scientific exploration of this phenomena, it cannot be defined rigorously. Consciousness usually (but not always) involves some form of attentional selection and a rapidly decaying form of information storage. For strategic reasons, most of the empirical research has focused on the brain states underlying conscious sensory perception, the neuronal correlates of consciousness, or NCC. I avoid taking any particular ideological position in the debate concerning the exact relationship between the NCC and conscious experience. 
A device that would measure the conscious state (or absence of thereof) of humans or animals. No such reliable method exists today. Indeed, many philosophers consider the very idea to be foolish. An alternative is a battery of experiments, including the delay test, that identify behaviors that require consciousness. 
Content of consciousness
Specific conscious percept or memory that forms part of the stream of consciousness (as in seeing a “red apple”, “having a toothache in my left molar” or “feeling sad”). Underlying any conscious content is a specificneuronal correlate of consciousness
Common anatomical term meaning on the opposite side; as in “The left primary visual cortex receives input from the right (contralateral) field of view.” Ipsilateral means on the same side. 
Core of the thalamus
One of two broad classes of thalamic relay cells (see matrix). Core neurons convey specific information to the intermediate layer of their cortical target area. 
Corpus callosum
About two hundred million fibers that keep the two cortical hemispheres in constant communication. These are cut in split-brain patients, creating two conscious minds in one skull. 
Correlated firing
The firing of two neurons are correlated if the time at which action potentialsoccur in one neuron is related to the time at which spikes occurred in the other one. For example, the two cells might spike more or less at the same time, or a spike in one is followed by a spike in the other neuron 10 msec later. A group of neurons whose firing is tightly synchronized is better at driving its target cells (their synaptic input will carry a stronger punch) than if the spiking activity is disorganized across the group. Spike synchrony is probably an important signal biasing competition among neurons. Synchrony can occur in the absence of oscillations. Sometimes synchrony is measured by evaluating the correlation between the spiking discharge of a neuron and the fluctuating local field potential (e.g. the cell fires when the local field increases). 
Cortical processing hierarchy
See hierarchy
Deep layers
Layers 5 and 6 of neocortex. Also called lower layers. Pyramidal neurons whose cell bodies are located in deep layers project to the thalamus, down to the superior colliculus and to targets beyond (e.g. to the spinal cord). Thus, these layers provide the output of neocortex. Feedback pathways to lower cortical regions also originate in deep layers. 
Delay test
An operational means, by training the subject to enforce a delay between stimulus and motor response, to test for the presence of conscious behaviors in animals, babies or patients who can’t talk. See also consciousness-ometer
Depth of computation
See logical depth of computation
See binocular disparity
Dorsal pathway
Massive anatomical stream that originates in primary visual cortex, projects through the middle temporal area into regions in the posterior parietal cortex. From there, it sends axons into the dorsolateral prefrontal cortex. Also known as the vision-for-action or where pathway. 
Vivid and conscious hallucinations that feel as real as life itself. They primarily occur during rapid-eye-movement sleep
Driving connections
Axons that originate in the thalamus or in a cortical region that primarily terminate in layer 4 of cortex or onto the proximal part of thalamic neurons, can directly drive, by themselves, their targets, leading to vigorous spiking activity. This may be partially due to temporal synchrony in the input. Driving connections are complemented by modulatory connections.Forward connections that ascend the cortical hierarchy are driving connections. Crick and Koch (1998) postulate that the thalamo-cortical system avoids loops made up entirely of strong connections. 
The metaphysical principle that the world and everything in it is made out of two domains, the mental and the physical, governed by two distinct sets of laws. Most people on the planet, whether or not they belong to a traditional religion, hold various forms of dualistic positions, in particular the belief in some sort of spirit or soul. A key problem for dualism is the nature of the interaction between the mental (i.e. phenomenal consciousness) and the physical (i.e., the brain). See also the hard problem
Easy Problem
A term used by some philosophers to describe the project at the heart of this book; to discover and characterize the neuronal and, more general, the material, basis of consciousness. To the extent that consciousness has one or more functions, understanding their mechanistic causes is conceptually and epistemologically straightforward, easy (even if difficult from a scientific and practical perspective). In this view, however, solving the easy problem will not explain the mystery of subjective experience. This is the hard problem. I suspect that the hard problem, like other questions that have occupied philosophers in the past – how is it that humans see the world upright, when the retinal image is inverted – will disappear once we understand the easy problem. 
See visual eccentricity
An electrical conductor, often simply a wire that is insulated everywhere except at its tip, coupled to an amplifier, to record changes in the electrical potential inside or outside nerve cells and/or to directly stimulate neurons. Two types of electrical signals are typically extracted from extracellular recordings: trains of action potentials from one or more nearby cells and thelocal field potential, the fluctuating electrical potential outside of neurons. Arrays of electrodes can eavesdrop on the simultaneous spiking activity of up to hundred neurons. Electrode recordings sample the activity of individual neurons with very high temporal (sub-millisecond) resolution. Their principal limitations are lack of coverage—only a tiny fraction of all neurons in any one area are picked up—and the anonymous nature of the recording. That it, it is very difficult to infer anything about the identity of the neuron being recorded from. 
Electroencephalogram (EEG)
Recordings of the electrical potential by attaching numerous electrodes to the surface of the skull. Oscillatory activity in different frequency bands (theta, alpha, beta, gamma, and so on) serve as a rough indicator of distinct cognitive states and as clinical diagnostic tool. The EEG’s high temporal (millisecond) but poor spatial (centimeter) resolution severely constrains its ability to identify discrete neuronal populations. 
Enabling factors
One or more biological mechanisms that need to be in place to be conscious at all (for instance, cholinergic and glutamergic synaptic transmission). These are the NCCe
Ephaptic interaction
Electrical interaction between neighbouring neuronal processes by way of the extracellular potential rather than by specific chemical or electrical synapses. The biophysics of neurons sharply limits the amplitude and specificity of such interactions. The extracellular potential probably only plays a minor role in the processes underlying consciousness. 
Essential node
A cortical region whose destruction causes the loss of a specific conscious attribute, such as seeing colors or motions. We argue that the NCC for this attribute must be located at these nodes. 
Evoked potential
Changes in the electrical potential on the surface of the scalp following presentation of an image (visually evoked potential), sound (auditory evoked potential), or internal cognitive event (e.g.; committing an error during some task; event-related potential). The evoked potential is obtained by averaging the EEG over hundreds of trials. 
Executive summary hypothesis
Francis Crick’s and mine proposal that a key function of the neural correlates of consciousness is to summarize the present state of affairs in the world and to make this brief summary available to the planning stages of the brain (somewhat similar to the summary demanded by a time-pressed executive or president who has to make a decision on a complicated topic). This is in contrast to the many sensory-motor agents or zombies who need no such summary, since they only deal with very restricted input and output domains. 
Explicit coding or explicit representation
A representation that allows the encoded attribute—orientation, color, or facial identity—to be easily extracted This notion could be formalized by demanding that the existence of the to-be-represented feature or object must be inferred from a suitably weighted linear or nonlinear combination of cells. Thus, an explicit face representation might be one in which a single-layered neural network can detect whether or not a face is present in the ring activity of a pool of neurons. In general, any explicit representation must be grounded in an earlier, implicit stage. An explicit coding has a larger logical depth of computation compared to an implicit coding of the same information. A population of neurons can represent one attribute in an explicit manner and another in an implicit one (for instance, V1 cells encode orientation in an explicit but facial identity in an implicit manner). In other words, an explicit representation is a necessary, but not sufficient condition for the NCC. See also activity principle
See neglect
Extrastriate cortex
A bevy of cortical areas surrounding primary (or striate) visual cortex in the occipital lobe, at the back of the cortex that subserve vision. 
Fallacy of the homunculus
The compelling illusion that at the center of my mind is the conscious I that directs and looks out at the world and initiates all actions. Francis Crick and I speculate that this illusion is reflected in the neuroanatomy of the connections between the front and the back of the cortex. See also the unconscious homunculus
Feedback pathways
There exist higher and lower neocortical and thalamic regions in the mammalian brain, defining a hierarchy. Top-down or feedback pathways are made up of axons of pyramidal cells that originate in a higher level and make synaptic connections in one or more lower levels of cortex (for example, from area MT to V1) or the thalamus (for example, from area V1 to the LGN). I and others argue that conscious perception requires feedback connections, in particular from the front to the back of the cortex. If these were to be blocked, the subject would not be conscious, although trained behaviors, such as the rapid discrimination of images, would still be possible. 
Forward pathways
Axons that originate in the thalamus and project into layer 4 of their cortical target region, or cortico-cortical axons that originate in upper layer and that project into layer 4 are said to be forward, feed-forward or ascending. Such pathways are driving, that is, they can induce firing in their targets. Multi-layer forward processing is responsible for rapid behaviors, including complex object recognition, but in the absence of any conscious perception. The pattern of forward and feedback connections define a cortical hierarchy
Field theories of consciousness
These postulate the existence of some sort of field that is the physical carrier of conscious sensations. I have little sympathy for such theories, as the electro-magnetic field in the brain is too minute and far too unspecific to be able to mediate the specific content of consciousness. 
Filling-in is a catch-all term used for many distinct perceptual phenomena that include interpolation and completion in which an attribute that is not present is inferred from its context (in space or in time). Examples include illusory boundary completion, the retinal blind-spot, the apparent motion of a spot that disappears behind an occluding box or the shape of a partially hidden object and many other visual phenomena where you clearly see something that isn’t there. Filling-in and reinterpretation of incomplete or contradictory data makes human speech intelligible. The powerful, nonconscious biases that govern people’s social lives in the form of gender-, racial- or age-based prejudices, borne from the sum of life’s experiences, are a manifestation of filling-in operating at a cognitive level. None of these phenomena are a question of logically deducing the existence of something, akin to Sherlock Holmes chain of reasoning based on minute observations of the way people look and dress. Rather, the brain automatically infers aspects of the stimulus that are missing and presents these as a fully grown percept. 
First-person account or perspective
The unique view-point of a conscious being, experiencing and perceiving events in the world. One of the two principal problems in the mind-bodydebate is how a subjective, first-person perspective is compatible with, and can be explained in terms of, an objective, third-person account
Fleeting memory
Another name for iconic memory
A short (lasting 20-200 msec) and discrete unit of conscious experience, aperceptual moment
A division of the brain that includes the cerebral cortex, the basal ganglia, amygdala, olfactory bulb, and the thalamus. Forebrain neurons mediate the specific content of consciousness
Front of the cortex
A shorthand for all cortical regions that lie forward off the central sulcus, including motor, premotor, prefrontal and anterior cingulate cortex. A good operational definition is that the front includes all cortical regions that receive a significant input, via the thalamus, from the basal ganglia
Functional magnetic resonance imaging (fMRI)
A way to record brain signals in a noninvasive, safe and convenient manner from behaving and conscious subjects. A commonly used technique is blood-oxygenation level dependent (BOLD) contrast imaging, which measures localized changes in blood volume and flow in response to metabolic demand (due to synaptic and spiking activity). fMRI relies on the fact that deoxygenated blood has slightly different magnetic properties than oxygenated blood. fMRI does not directly measure the rapid (millisecond) synaptic and spiking events but a proxy, hemodynamic signals with a very sluggish time course lasting 5 to 10 seconds. Like its cousins – positron emission tomography and optical imaging of brain activity – fMRI measures changes in the local blood supply in response to the increased metabolic demand of active synapses, neurons, and glia cells. Technological considerations currently limit the spatial resolution for human imaging to around 2 mm. Indeed, the smallest volume element (voxel), about 2x2x2 mm3, includes about one million neurons. It is generally assumed that hemodynamic activity is directly proportional to spiking activity. However, the fMRI signal primarily reflects synaptic input to a region and local processing, rather than neuronal output (i.e., the trains of action potentials that are sent to more distant sites. 
Fusiform gyrus
The fusiform gyrus lies on the inferior surface of cortex, extending from the occipital to the temporal lobe (see inferior temporal cortex). 
GABA synapses
Principal form of fast synaptic inhibition in the forebrain on the basis of the neurotransmitter gamma-amino-butyric acid (GABA). 
Gamma oscillations
See oscillations
Very sparse, and semantic description of a visual scene (e.g., a dog runs after a squirrel). This makes change blindness so compelling: large changes in the scene are often completely missed since their gist remains the same. 
The principal form of fast, synaptic excitation in the forebrain is based on the neurotransmitter glutamate. Glutamate can act at a variety of postsynaptic receptors. One form acts within a few milliseconds; most of the ordinary synaptic traffic among forebrain neurons uses these glutamate receptors. Another type involves N-methyl-D-aspartate (NMDA) receptors that turn on and off more slowly (50-100 milliseconds). NMDA receptors are important for inducing synaptic plasticity
Hard Problem
A term popularized by the philosopher David Chalmers to express the grave conceptual difficulty of explaining, in a lawful and reductionist manner, how phenomenal sensations arise out of a physical system: “why is it that brain activity does not go on in the dark, without any subjective quality?” This stance assumes a form of dualism. In this view, discovering and characterizing the material correlates of consciousness in the brain constitutes the Easy Problem. Not easy from a practical or methodological point of view but from an epistemological one. While the question of how any physical system can posses phenomenal state is, indeed, very puzzling, it is not a given that it shall always remain so. Too often in the past have philosophers claimed a problem to be insoluble for such purely logical and semantic arguments to be fully convincing. 
Complete blindness or loss of visual perception in half the field of view. Caused by a lesion in the pathway from LGN to V1 or upstream from here. 
Hemodynamic activity
Synaptic release, the generation and propagation of action potentials, and other neuronal and glial processes all require metabolic energy. The increased demand is supported by the rapid delivery of oxygen via hemoglobin molecules transported in the blood stream, caused by changes in the blood volume and flow in the highly elaborated vascular networks that pervade brain tissue. This hemodynamic activity is picked up by brain imaging techniques, including optical (intrinsic) imaging, positron emission tomography (PET), and functional magnetic resonance imaging (fMRI). Their spatio-temporal resolution lies in the millimeter-second range. 
Based on anatomical criteria, the 30 or more processing areas in the visual brain can be arranged in a hierarchy. A particular region receives forward input from an area at a lower level and sends, in turn, a < a href=”#forward”>forward projection to an area at a higher level or a sideway connection to a region at the same level of the hierarchy. Feedback pathways convey information from higher to lower regions. This hierarchy is neither strict nor unique. While similar hierarchical organizations have been reported for somatosensory and auditory regions, it is unclear to what extent regions in the front of the cortex can be ordered in this manner. 
The little man in the head. See fallacy of the homunculus
Iconic memory
A form of high-capacity, rapidly decaying (within a second or so) visual memory. It exists in other sensory modalities as well (e.g., echoic memory for sounds). Any stimulus presentation leaves a neuronal residue or afterglow in the form of briefly enhanced synaptic weights and spike discharge. This fleeting form of memory is necessary for consciousness. 
The metaphysical and ontological position that everything ultimately is mental, including the physical universe. Idealism is a form of monism. For example, in Eastern thought, Brahman is the unchanging, infinite, immanent, and transcendent reality. In Western thought, Schopenhauer conceives of Will as the underlying entity that determines everything. 
Implicit coding or implicit representation
The opposite of explicit coding
Inattentional blindness
Compelling psychophysical demonstration that unexpected stimuli, even when the subject is looking directly at them, may not be seen (Mack & Rock, 1998). Inattentional blindness highlights the crucial role of expectations in perception. 
Inferior temporal or inferotemporal cortex (IT)
In the monkey, the region starting just in front of V4 and continuing almost up to the temporal pole. Includes the dorsal and ventral divisions of PIT, CIT and AIT. Its human homologue are regions anterior to the occipito-temporal cortex, along the ventral surface of the temporal lobe (fusiform gyrus). This swath of neocortex is key to conscious, visual perception. 
Intermediate-level theory of consciousness
The hypothesis, put forth by Ray Jackendoff and others, that consciousness only has access to intermediate levels of representations. Neither primitive sensory representations nor the high-level, conceptual representations that underlie many cognitive operations are accessible. One surprising consequence is that thoughts are unconscious. What is conscious about them is their re-representation in terms of images, silent speech and other sensory qualities. 
Intralaminar nucleus of the thalamus (ILN)
A set of small, midline nuclei in the thalamus that receive input from the front of the cortex, and provide a strong output to the basal ganglia and a more diffuse output to much of the cortex. Their bilateral destruction results in loss of arousal and in a coma or in a vegetative state. The ILN are part of theNCCe
The opposite of contralateral
Laminar position
The layer of cortex the cell body of a neuron is found in. The laminar position is an important determinant of the cell’s morphology, input, output, and functional role. 
Lateral geniculate nucleus
Most retinal ganglion cells send their axons, making up the optic nerve, into the six-layered lateral geniculate nucleus (LGN), one of many thalamic nuclei. Geniculate neurons, in turn, project into primary visual cortex. Like all thalamic nuclei, the LGN receives massive feedback from cortex whose function is not known. 
See lateral geniculate nucleus
Local field potential (LFP)
The electrical potential recorded in neural tissue from the tip of an electrode. Neuronal processes within a millimeter or so contribute to the LFP. 
Logical depth of computation
A measure of the number of steps necessary for any one computation. The logical depth of a retinal ganglion cell, signaling the occurrence of a spot of light, is far less than that of a cell in inferior temporal cortex, representing a face. The shallower the logical depth of a neuron’s output, the more computations the postsynaptic circuitry has to perform to extract the relevant information. 
Long-term memory
A set of distinct, neuronal processes that retain information over days, months, and years. Long-term memories includes both implicit, sensory-motor skills as well as declarative, memories for autobiographical details and facts. 
Lower layers
Same as deep layers
When one stimulus eliminates the percept associated with another one, it is said to mask it. Masking visual or auditory stimuli is a sophisticated art. 
Matrix of the thalamus
One of two broad classes of thalamic relay cells (see also core). Matrix neurons project widely into the superficial layers of cortex
Mean rate code
The hypothesis that all of the information carried by a neuron is contained in the number of spikes triggered within a suitable interval (say on the order of a 100 msec or more). 
Conscious states mean something, they are about something, they are grounded in the past, in future plans and in related associations. I argue that this must be instantiated by the myriad of synaptic connections among the relevant essential nodes and neurons, the penumbra of any conscious percept. 
Medial temporal lobe (MTL)
Forebrain structure involved in the consolidation of conscious memory and in emotional processing. Includes the hippocampus, and surrounding entorhinal cortex (Brodmann’s area 28), perirhinal (Brodmann’s areas 35 and 36) and parahippocampal cortices (Brodmann’s area 37), and the amygdala. 
A set of distinct psychological processes that operate with different representations, and physiological mechanisms to retain information over variable intervals. Important categories include long-term memoryshort-term(or immediate) memory, and iconic memory
Term introduced by Semir Zeki to denote consciousness for individual attributes of any one percept, the associated NCC. Microconsciousness for the motion of an object may be perceived at a slightly different time than the microconsciousness of its color. This would make the idea of the unity of consciousness difficult to sustain. 
Another term for a small electrode listening in on the way neurons talk to each other by generating trains of action potentials
Direct electrical stimulation of a minute region of the brain by an electrode inserted into grey matter (as in deep brain stimulators inserted into the subthalamic nucleus to ameliorate the tremor of patients with Parkinsons disease) or resting on top of cortex (as in a neurosurgical context to probe the nature of the underlying cortex). This can evoke elemental or, on occasion, more complex, percepts, memories and motor actions. 
Middle temporal area (MT)
A small but very popular cortical region involved in motion perception. Also referred to as V5. 
Mind-body problem
What is the relationship between the immaterial mind and the material body (or, more specifically, the brain)? The two most important questions in this debate are (i) What is the nature of consciousness and its relationship to the brain? and (ii) Is the mind truly free; that is, does free will exist? I take the following questions to be the charter for my quest: To understand how and why the neural basis of a specific conscious sensation is the basis of that sensation rather than another, and rather than a nonconscious state; why sensations are structured the way they are, how they acquire meaning, and why they are private; and, finally, how and why so many behaviors occur in the absence of consciousness (see zombie agents). 
Minimally conscious state
Patients with severe alteration in consciousness who do not meet diagnostic criteria for coma or the vegetative state (PVS). They demonstrate inconsistent but discernible evidence – behavioral or only accessible via brain imaging – of willful actions generally associated with consciousness. Patients may evolve to MCS from coma or vegetative state after acute brain injury. MCS is often transient but may also be permanent. 
Modulatory connections
Axons from the thalamus or from a cortical region that terminate in the superficial layers of cortex or onto the distal dendrites of thalamic neurons. Modulatory connections, by themselves, cannot make the target neurons fire strongly, but can modify the firing produced by driving connections.Feedback pathways are probably modulatory. 
The metaphysical position that all is the manifestation of a single underlying reality, principle, essence, or substance, and is governed by a universal set of laws. The two traditional forms of monism are physicalism – everything is matter and energy and is governed by physical laws and idealism – everything is mental. Monism is opposed to dualism, the belief that there are two domains, the physical and the mental. 
They are, like apes and humans, primates. Macaque monkeys are not endangered, and can easily be bred and trained in captivity. Although a monkey brain is much smaller than that of a human, its overall organization and processing elements are very similar, making it the most popular model organism to explore the neural basis of perception and cognition. There are two superfamilies of monkeys, which have distinct geographical distributions, New World and Old World monkeys. Old World monkeys, which include baboons and macaques. They have larger and more convoluted brains than New World monkeys. 
See neuronal correlates of consciousness
NCCe The neuronal correlates underlying any conscious state; the enabling factors underlying consciousness per se. See arousal
Neurological syndrome—often involving damage to the right parietal cortex—in which patients show a marked difficulty in responding to information in the affected field of view. Yet their early visual pathways, including retina and V1, are intact. Known more properly as visuo-spatial hemi-neglect. In the related syndrome of extinction, the patient can see isolated objects in the affected field, but not if presented simultaneously with a stimulus in the opposite, unaffected hemifield. 
The neocortex is the newest part of the cerebral cortex that is a unique hallmark of mammals. In general, neocortex consists of six layers. The neocortical neurons are essential for the content of conscious perception. 
Wave front of spiking activity, triggered by sensory input, that propagates in a rapid and predictable manner, by leaps and bounds, from the sensory periphery through the various stages of the cortical processing hierarchy
Neuronal correlates of consciousness (NCC)
The minimal set of neuronal mechanisms or events jointly sufficient for any one specific conscious percept or experience. Discovering and characterizing them in normal human subjects as well as in in patients, newborns, animals, and other non-linguistic competent individuals would represent great progress toward an scientific theory of consciousness. 
NMDA receptor
See glutamate
Nonclassical receptive field
The region outside the classical receptive field of a neuron from which its discharge can be modulated. This usually requires an appropriate stimulus inside the receptive field. 
Operations or computations that do not (directly) give rise to conscious feelings, sensations, or memories. Subliminal perception is an example of nonconscious processing. The term “unconscious” is neutral with regard to the still popular but ill-defined Freudian notions of the pre-, sub- and un-conscious that I avoid. 
A nucleus (plural, nuclei) is a three-dimensional collection of neurons with a prevailing neurochemical and/or neuroanatomical identity. For instance, they may all release the same neurotransmitter or project to a common destination. 
Optical flow field
Two-dimensional vector field on the retinae that is induced by changing image intensities. This occurs either during ego-motion (e.g., during an eye or head movement) or when an external object moves. 
Regular or semi-regular bouts of periodic activity in the EEG, local field potential or spiking discharge in a variety of frequency bands (e.g., theta, alpha, beta) Of particular note are oscillations in the 30 to 70 Hz domain, often referred to as 40 Hz or gamma waves, probably linked to competition in attentional selection. Found throughout the brain, the role of the distinct oscillations (brain waves) remain controversial. Francis Crick and I had postulated in 1990 that gamma oscillations could be one of the necessary correlates of consciousness. However, the current evidence suggests instead that their function is probably related to attentional selection and switching. 
A term that Francis and I introduced for the neuronal processes that receive synaptic input from the NCC, without being themselves part of it. The penumbra includes the neural substrate of past associations, the expected consequences, and the cognitive background of the conscious percept. The penumbra provides the meaning, the aboutness of the percept. Qualia come to symbolize all of this vast, explicit or implicit, information contained in the penumbra. 
Perceptual moment
The hypothesis that perception occurs in discrete processing episodes, called frames or snapshots. Each snapshot is associated with a constant percept of color, motion, sound and so on. The stream of consciousness consists of an endless sequence of such frames, not unlike a movie. The attributes within a frame, including the perception of motion, are experienced as constant. TheNCC would have to reflect such a quasi-periodic dynamics. The duration of such episodes are quite variable, distributed between 20 and 200 msec. 
Perceptual stimuli
Sensory input whose interpretation is ambiguous. Examples include bistable illusions—such as the Necker cube and binocular rivalry—motion-induced-blindness, and flash suppression. In each case, the same retinal input can give rise to different percepts. Tracking down the NCC associated with perceptual stimuli currently offers the most promising means to identify potential NCC. 
Permanent vegetative state (PVS)
If a patient, following profound brain damage, remains in a vegetative statefor more 3 to 12 months (depending on the etiology), the patient is considered to be in a permanent vegetative state (PVS). With proper medical care, such patients can be kept alive for decades (15 years in the case of Terri Schiavo of Florida), yet without any conscious sensation. See also coma andminimally conscious state
Phenomenal consciousness
The subjective, feeling part of any conscious sensation or quale. What can be reported, signaled or said about such a sensation is termed access consciousness
The metaphysical position that everything that exists has a physical property; there is nothing outside of physics. Physicalism is a more sophisticated form of materialism since it incorporates the wave/particle duality, dark matter and energy and so on. Most brain scientist take a physicalist position when it comes to explaining consciousness. A form of monism
Pineal gland
Also called pineal body or epiphysis, it is a pea-sized endocrine gland in the middle of the brain. The pineal produces melatonin, regulated in circadian rhythm, and plays a role in sexual development, hibernation and metabolism. Rene Descartes famously located the “seat of the soul” to the pineal gland. 
Population coding
A coding scheme whereby the information is distributed across a population of neurons, each of which is relatively broadly tuned (as, for instance, in visual cortex). By combining different subsets of them, information can be represented in a robust and efficient manner. An alternative strategy employssparse representations
Population sparseness
A coding scheme whereby information is expressed by a small fraction of a population of neurons firing. This notion of population sparseness must be distinguished from lifetime sparseness, although they often occur together. The advantage of sparse coding over population coding is that the information is represented in an explicit manner. Furthermore, it may be much quicker to learn new concepts with a sparse representation. In the limit of very sparse coding, a neuron may only respond to a few discrete stimuli, events or concepts. This occurs in the medial temporal lobe
Primary visual cortex
Cortical terminus of the visual input from the retina by way of the lateral geniculate nucleus. Also called V1, striate cortex or Brodmann area 17. 
There are approximately 200 primate species, of which humans are but one member. The order of primates is divided into two suborders, prosimians (literally, “before monkeys”) and anthropoids, encompassing monkeys, apes and humans. Gorillas orangutans and the two species of chimpanzees constitute the great apes. Given their highly developed cognitive abilities and kinship to humans, little invasive research is carried out on apes. Most of what is known about their brains derives from postmortem studies. 
If the processing of one stimulus affects the processing of a much later input, psychologists talk of priming. This is likely to involve changes in synaptic weights. The first input does not even have to be consciously perceived in order for it to increase the detection probability of a later stimulus. 
Privacy of consciousness
Conscious percepts or memories are only ever directly experienced by the subject. In this sense they are deeply private. The content of consciousnesscannot be directly communicated, except by way of example or comparison (“this red looks like the red of the Chinese flag”). 
An inability to recognize faces (also known as face blindness). In aperceptive prosopagnosia, the patient can’t consciously perceive a face as a face, seeing only individual features (eyes, ears, nose, mouth), but not the whole. She may also be unable to infer the age or the gender of the face. In associative prosopagnosia, the patient is unable to recognize highly familiar faces (e.g. their spouse). 
The elemental feelings and sensations making up conscious experience (seeing a face, hearing a tone, feeling angry). Qualia (singular quale) are at the very heart of the mind-body problem. I argue that qualia symbolize, in a compact manner, the vast amount of explicit and implicit information that is contained in the penumbra of the winning coalition sufficient for one particular conscious percept. 
Rapid-eye-movement sleep (REM)
A recurrent part of the sleep cycle, lasting 10-20 minutes. REM sleep is characterized by rapid eye movements, paralysis of other voluntary muscles, and vivid dream activity, in particular visual imagery. This is a form of consciousness, distinct from waking consciousness. Its counterpart is deep sleep where little dreaming occurs. 
Rate code
See mean rate code
Receptive field
The classical receptive field of a visual neuron is the location and shape of the visual field from which a stimulus can, by itself, directly excite the cell. For instance, while retinal and LGN neurons possess a center-surround organization, cells in primary visual cortex prefer elongated stimuli of a particular orientation. The much larger region from which the cell’s response can be up- or down-regulated is its nonclassical receptive field. For instance, if bars in the nonclassical receptive field have the same orientation as the bar in its center, creating a homogeneous texture, the cell might cease to respond while bars turned at right angle to the central bar evoke a frenzy of spikes. The nonclassical receptive field places the cell’s primary response into a larger context. 
REM sleep
See rapid-eye-movement sleep
Retinal ganglion cells
Over one million neurons in the retina summarize all of the optic information extracted by photoreceptors, horizontal, bipolar and amacrine cells and communicate this, in the form of action potentials, to the rest of the brain. Their axons make up the optic nerve. Their activity does not correspond to conscious, visual perception. 
Retinotopic organization
An example of a topographic organization. Nearby points in visual space are mapped to neighbouring neurons in visual cortex, with the representation of the fovea greatly expanded compared to the visual periphery. 
Saccade or saccadic eye movement
A very rapid, yet directed eye movement. Humans and other primates typically inspect and explore the world by executing a few saccades every second of waking life. 
Loss of visual perception in a spatially proscribed area of the visual field. Can be due to a lesion anywhere along the retino-cortical pathway. 
See top-down attention
Sensory-motor agents
See zombie agents
Short-term memory
A catch-all term for the temporary storage of information over tens of seconds. Working memory is one form of such immediate memory. 
A short (lasting 20-200 msec) and discrete unit of conscious experience, aperceptual moment
Sparse representation
Another term for population sparseness
Sparse temporal coding
The information is represented by a handful of spikes, triggered at a particular point in time (like a note of music) rather than by an elevated firing rate over hundreds of msec. This lifetime sparseness is different from population sparseness
Another term for action potential
Spike synchrony
See correlation
Striate cortex
Anatomical term for primary visual cortex
Strong connections
Another name for a driving pathway between the thalamus and cortex or among cortical areas. 
Superficial layers
Layers 1, 2, and 3 of neocortex. Also called upper layers. The forward projection from one cortical region into a region at a higher level originates from superficial layers. These layers receive massive intracolumnar input from layer 4 neurons, from cortical feedback pathways, and from thalamicmatrix neurons. These latter two place the computations carried out in this patch of cortex into a more global context. 
A philosophical notion that formalizes the intuitive idea that one set of facts can fully determine another set of facts. For example, the laws of physics determine biology: biological properties supervene on physical properties (even though the former can’t be predicted from the latter). Another way of putting is that any two systems that are physically identical must be biologically identical. Some philosophers, such as David Chalmers, assert that the phenomenal aspects of consciousness are not logically supervenient – do not follow – from physics. This is controversial. 
Synaptic plasticity
Biophysical and biochemical changes that modulate the effective connection strength of synapses. Different biophysical and biochemical mechanisms underly such changes that can last anywhere from seconds to days and longer. Functionally, some forms of synaptic plasticity follow Hebb rule and change weight as a function of both pre- and post-synaptic activity. Synaptic plasticity is thought to be key to long-term memory
Temporal code
The hypothesis that the time of occurrence of action potentials within one neuron and among groups of cells contains relevant information. Oscillatory discharges in the 40 Hz range and synchronization are the two most prominent examples of such codes. We think it is likely that such coding is important as the neuronal expression of selective attention. 
A structure situated on top of the midbrain that regulates all inputs into theneocortex. In its absence, no mental life is possible. The thalamus is divided into forty or more nuclei that don’t talk much to each other. These receive massive feedback from cortex. I consider the thalamus to be an organ of attention. 
Third-person account or perspective
The point of view of an external observer, having access to the behavior and brain states (e.g., by observing individual neurons) of a conscious subject, but not to his or her experiences. Throughout most of history, biology and psychology adopted a purely third-person perspective (as in the Vienna Circle or Behaviorism). This is in contrast to a first-person account
Top-down attention
A volitional-controlled, task-dependent or endogenous selection mechanism operating in vision and other sensory modalities. A popular metaphor for top-down spatial (or focal) visual attention is the searchlight of attention that illuminates objects in the field of view, enhancing their processing. See alsoattention
Topographic organization
The observation that two nearby points in space are represented by neighboring neurons. The LGN, and the early visual cortical and somatosensory cortices have such map-like representations. Topography is only weakly present in the higher regions of the ventral pathway
Unconscious homunculus
A speculation, according to which networks in part of the front of the cortex look at the back of the cortex, and use this processed sensory information to plan, make decisions and feed these to the relevant motor stages. Much of this neural activity does not contribute to the content of consciousness. These networks act as an nonconscious homunculus. 
Upper layers
See superficial layers
Another name for the primary visual cortex
Vegetative state (VS)
Following profound brain damage, the patient may be in a vegetative state with cyclical arousal (e.g. eye openings alternating with periods of closed eyes), breathing, spontaneous movements, and reflexes (including grimacing, crying, eye tracking for brief periods) but no evidence of awareness and without any purposeful response to commands. If these symptoms remain unchanged beyond 3-12 months (depending on the etiology), the patient is considered to be in a permanent vegetative state. VS usually involves widespread damage to the cortico-thalamic system. See also coma andminimally conscious state
Ventral pathway
Massive anatomical stream that originates in primary visual cortex, projects into V4 and inferior temporal cortex. From there, it sends afferents into the ventrolateral prefrontal cortex. Also known as the vision-for-perception or what pathway. 
Vision-for-action pathway
Another name for the dorsal pathway
Vision-for-perception pathway
Another name fr the ventral pathway
Visual eccentricity
The angle relative to the point of sharpest seeing, the fovea, is referred to as eccentricity. The more eccentric an object, the more difficult it is to see sharply. The fovea occupies the central 1 degree of vision. 
Visual hierarchy
The anatomical hierarchy found in the visual cortex. 
What pathway
Another name for the ventral pathway
Where pathway
Another name for the dorsal pathway
Working memory
One well studied memory module that stores information needed for ongoing tasks over tens of seconds (such as a phone number). It is not clear whether this is required for perceptual consciousness. 
Zombie agent
A stereotyped, rapid, and effortless sensory-motor behavior that does not give rise to a conscious sensation. Consciousness for this behavior may come later or not at all. Examples include many reflexes, but also most forms of eye movements, posture adjustments, walking, running, cycling, dancing, driving, climbing, and other highly trained athletic activities.