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
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.
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.
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
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.
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.
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 general arousal 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.
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.
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
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?
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.
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.
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.
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.
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
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.
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.
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 explicitly expressing the perceived
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
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 specific neuronal correlate of consciousness.
firing of two neurons are correlated if the time at which action potentials occur 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
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.
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.
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
Vivid and conscious hallucinations that
feel as real as life itself. They primarily occur during rapid-eye-movement sleep.
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.
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.
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 the local 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.
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
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.
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.
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.
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.
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.
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
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.
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.
The unique view-point of a conscious being,
experiencing and perceiving events in the world. One of the two
principal problems in the mind-body debate is
how a subjective, first-person perspective is compatible with, and can
be explained in terms of, an objective, third-person account.
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
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.
The fusiform gyrus lies on the
inferior surface of cortex, extending from the occipital to the
temporal lobe (see inferior temporal cortex).
Principal form of fast synaptic
inhibition in the forebrain on the basis of the neurotransmitter
gamma-amino-butyric acid (GABA).
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.
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.
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.
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
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.
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.
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
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
Intermediate-level theory of consciousness
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.
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
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.
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.
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 memory,
short-term (or immediate) memory, and
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.
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.
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).
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.
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.
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
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.
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.
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
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.
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
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.
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. The NCC would have to reflect such a quasi-periodic
dynamics. The duration of such episodes are quite variable,
distributed between 20 and 200 msec.
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.
If a patient, following profound brain damage, remains in a vegetative state for 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 and minimally conscious
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.
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.
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 employs sparse representations.
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.
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.
Conscious percepts or memories are only ever directly experienced
by the subject. In this sense they are deeply private. The content of consciousness cannot 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
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.
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.
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.
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
Saccade or saccadic eye movement
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.
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.
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 thalamic matrix neurons.
These latter two place the computations carried out in this patch of
cortex into a more global context.
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.
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
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 the neocortex. 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.
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.
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 also
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.
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
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 and minimally conscious
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.
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
The anatomical hierarchy found in the visual cortex.
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