The neural basis of conscious experience

by Bernard Baars



3.0 Introduction.

3.1 Neurophysiological fit with Model 1.

3.11 The nervous system as a parallel distributed system.
3.12 The reticular-thalamic activating system: Evidence for
a global workspace in the nervous system.
3.13 One possible scenario.

3.2 Extensions suggested by the neurophysiology.
3.21 Model 1A: Changes suggested by the neurophysiology.

3.3 Some afterthoughts.

3.31 Recent refinements of the neurophysiological evidence.
3.32 Control of access to the global activating system.
3.33 Some thoughts about the duality of the brain.

3.4 Summary.



3.0 Introduction.


In this chapter we apply the contrastive analysis strategy
to the neural basis of conscious experience. That is, we look for
populations of neurons that control the difference between
conscious and unconscious states --- most obviously sleep,
waking, and coma. These neural structures behave in several ways
like the global workspace model we have developed so far.

There is a curious traditional dichotomy between
psychologists and neuroscientists in the way they tend to regard
the nervous system. By and large, neuroscientists tend to see a
gigantic assemblage of complex neurons, extremely densely
interconnected, operating in parallel and at a fairly fast rate
(e.g. Thompson, 1967; Shepherd, 1983). Psychologists have
traditionally seen a very different system. Their nervous system
was slow, appeared to do comparatively simple tasks with high
error rates, and seemed to operate serially, performing only one
task at a time (e.g. Newell and Simon, 1974; Atkinson & Shiffrin,
19xx; Broadbent, 1958; Norman, 1976). Naturally there are
exceptions to these generalizations. Over the past decade‹j ‹
psychologists have increasingly explored parallel or parallel-
interactive processes, while some neuroscientists have been
studying relatively serial aspects such as event-related
potentials (e.g. Anderson, 1983; Hinton and Anderson, 1981;
Donchin, McCarthy, Kutas, & Ritter, 1983). Nevertheless, over
the broad sweep of the last hundred years of research, the
dichotomy between these two views of the nervous system appears
to hold to a remarkable degree.

In fact neither extreme is wrong, though both are
incomplete. Viewed at the level of neurons, a structure such as
the cerebral cortex is indeed immensely complex, containing by
recent estimates 55,000,000,000 neurons, each firing off an
electrochemical pulse 40 - 1000 times per second, with rich
subcortical and contralateral connections, and all apparently
active at the same time (Mountcastle, 1978). But when we look at
the same system functionally, through input and output
performance, it appears to solve simple problems (especially
novel ones) at a rate slower than 10 Hz, it makes numerous
errors, it tends to serialize even actions that seem
superficially executable in parallel, and its efficiency in
learning new facts and strategies seems relatively unimpressive.

The difference is, of course, that most psychologists work
with the limited capacity component of the nervous system, which
is associated with consciousness and voluntary control, while
neuroscientists work with the "wetware" of the nervous system,
enormous in size and complexity, and unconscious in its detailed
functioning. But what is the meaning of this dichotomy? How does
a serial, slow, and relatively awkward level of functioning
emerge from a system that is enormous in size, relatively fast-
acting, efficient, and parallel? That is the key question.

One guise in which this puzzle appears is the issue of
"attention" îvsï. "cortical arousal". Both of these concepts have
been associated with conscious processes, but in quite different
ways (Scheibel, 1980). The psychologist can easily find
îselectivityï in human information processing, so that the great
array of potential stimulation is reduced to just one stream of
information at a time. From William James to the present,
psychologists have thought of attention and consciousness in
terms of selectivity, a îreductionï in complexity. But the
neuroscientist, looking at the nervous system more directly,
finds plentiful evidence for system-wide îcortical arousalï
associated with wakefulness and orienting to novel stimuli, but
much less evidence for selectivity (Shepherd, 1983). Cortical
arousal involves widespread desynchronization in the EEG. That is
to say: when novel stimuli "catch the attention" of an animal,
regular, relatively slow brain waves are interrupted by fast,
irregular, low-voltage activity suggestive of increased
information processing. This implies not a reduction but an
îincreaseï in complexity at the neural level. Thus attention and
arousal seem to be quite different things, and tend to be treated
as separate though somehow related topics. ‹j ‹å
This chapter pursues the hypothesis that the split between
psychologists and neuroscientists in looking at the nervous
system reflects the global-workspace architecture. One advantage
of the GW model is that it predicts îbothï selectivity îandï
widespread activation, so that it reconciles these apparently
contradictory views within a single framework.




3.1 The neurophysiological fit with Model 1.


3.11 The nervous system as a parallel distributed system.


The various parts of the nervous system operate all at the
same time, and to a degree independently from each other
(Thompson, 1976). Further, there is extensive evidence that
anatomical structures in the brain often subserve very
specialized functions (e.g. Luria, 19xx; Geschwind, 1979). Under
these circumstances it is natural to think of the brain as a
parallel distributed system, and several interpreters of brain
function have done so. Arbib has for some years argued that motor
systems should be viewed as collections of multiple specialized
processors, operating independently of each other to a
considerable degree (e.g. Arbib, 1980). And recently a number of
neuroscientists have interpreted the columnar organization of the
cerebral cortex in terms of distributed "unit modules"
(Mountcastle, 1978; Edelman, 1978). Rozin (1976) has interpreted
the evolution of intelligence as an increase in the accessibility
of specialized functions, which originally developed as very
specific evolutionary adaptations. In more highly evolved nervous
systems, he suggests, specialized functions can become available
for new adaptive purposes. All these contributors support the
idea of the nervous system as a parallel distributed system. Thus
Mountcastle (1978) writes:

"The general proposition is that the large entities of the
nervous system which we know as the dorsal horn, reticular
formation, dorsal thalamus, neocortex, and so forth, are
themselves composed of local circuits. These circuits form
modules which vary from place to place ... but which are at the
first level of analysis similar within any large entity. ... The
closely linked subsets of several different large entities thus
form precisely connected, distributed systems; these distributed
systems are conceived as serving distributed functions." (p. 36).

Mountcastle also interprets the cerebral neocortex as such a
collection of specialized distributed processors. The cortex is
really a huge layered sheet folded into the upper cranium. Seen
in cross-section, this sheet consists of many microscopic columns
of cells: ‹j ‹å
"The basic unit of operation in the neocortex is a
vertically arranged group of cells heavily interconnected in the
vertical axis ... and sparsely connected horizontally.

"I define the basic modular unit of the neocortex as a
minicolumn. It is a vertically oriented cord of cells ... (which)
contains about 110 cells. This figure is almost invariant between
different neocortical areas and different species of mammals,
except for the striate cortex of primates, where it is 260. Such
a cord of cells occupies a gently curving, nearly vertical
cylinder of cortical space with a diameter of about 30 microns.
... the neocortex of the human brain ...contains about 600
million minicolumns and on the order of 50 billion neurons.

Next, Mountcastle suggests that these minicolumns of cells
are gathered together into îcortical columnsï, which constitute the
basic "unit modules" of the cerebral cortex:

"... it is possible to identify within the neocortex a much
larger processing unit than the minicolumn. The diameters or
widths of this larger unit have been given as 500 microns to
1,000 microns for different areas. ... this larger unit may vary
in its cross- sectional form, being round, or oval, or slablike
in shape. ... one can estimate that the human neocortex contains
about 600,000 of these larger (cortical columns), each packaging
several hundred minicolumns. The calculations ... are given to
indicate order of magnitude only.

" ... Thus a major problem for understanding the function of
the neocortex ... is to unravel the intrinsic structural and
functional organization of the neocortical module.

"That module is, I propose, what has come to be called the
îcortical columnï."

Unlike Mountcastle, who defines a module anatomically, I
would like to view the basic units as functional rather than
anatomical (Luria, 19xx). These approaches are not contradictory
of course, because functional units must ultimately make use of
anatomical units. But there is a difference of emphasis. To mark
the difference, I will call these specialized distributed units
"processors" rather than "modules".





3.12 The reticular-thalamic activating system: Evidence for a
global workspace in the nervous system.


What part of the brain could carry out the functions
described by Model 1? We can specify some of its properties:‹j ‹å
First, it should be associated with conscious functions like
wakefulness, focal attention, habituation, and indeed all the
facts described in the contrastive analyses in this book.

Second, it should fit the model developed in Chapter 2. On
the îinput sideï, many systems should have access to the presumed
global workspace, and incompatible inputs should compete for
access. On the îoutput sideï, it should be able to distribute
information to many specialized parts of the nervous system.
Since a great many parts of the nervous system seem to be
specialized in some way, GW output should be able to reach
essentially everywhere.

There is an anatomical and functional system in the brain
stem and forebrain that is known to have close relationships with
consciousness, in the sense that people gain or lose
consciousness when it is activated (Magoun, 1963; Scheibel &
Scheibel, 1967; Dixon, 1971; Hobson & Brazier, 1982). This
structure includes the classic Reticular Formation discovered by
Moruzzi and Magoun (1949), which receives information from all
major structures within the brain, including all sensory and
motor tracts, and permits very close interaction between all
these sources of information. It extends well upward to include
the non-specific nuclei of the thalamus. It makes functional
sense to include in this larger system the Diffuse Thalamic
Projection System, which sends numerous fibers to all parts of
the cortex (Figure 3.12). It is possible that cortico- cortical
connections should also be included. We will refer to this whole
set of anatomical structures as the îExtended Reticular-Thalamic
Activating Systemï (ERTAS).

------------------------------
Insert Figure 3.12 about here.
------------------------------

We can summarize the results of a great deal of research
since the later 1940s in the following contrastive table:










==============================================================

---------------------------------------------------------------

Neural Contrasts.

---------------------------------------------------------------

îConsciousï îUnconsciousï


Stimulation of the Rapid lesioning of the
reticular formation reticular formation and the
and outer thalamus. outer thalamus, and of the
thalamo-cortical projection
system.


===============================================================



The lower component of this system, the Reticular Formation
of the brainstem and midbrain, was described by one of its co-
discoverers as follows:

"Within the brain, a central transactional core has been
identified between the strictly sensory or motor systems of
classical neurology. This central reticular mechanism has been
found îcapable of grading the activity of most other parts of the
brainï ... it is proposed to be subdivided into a grosser and more
tonically operating component in the lower brain stem, subserving
global alterations in excitability, as distinguished from a more
cephalic, thalamic component with greater capacities for
fractionated, shifting influences upon focal regions of the
brain.

"In its ascending and descending relations with the cerebral
cortex, the reticular system is îintimately bound up with and
contributes to most areas of nervous activity.ï It has to do
significantly with the îinitiation and maintenance of wakefulnessï;
with the îorienting reflexï and îfocus of attentionï; with îsensory
control processesï including habituation ...; with îconditional
learningï; through its functional relations with the hippocampus
and temporal cortex, with îmemory functionsï; and through its
relations with the midline thalamus and pontile tegmentum, with
the cortex and îmost of the central integrative processesï of
the brain." (Italics added.) (Magoun, 1964)

The fact that the Reticular Formation involves wakefulness,
the orienting response, focus of attention, and "most of the
central integrative processes of the brain" certainly suggests
that it may be a part of what we are looking for. Other‹j ‹
neuroscientists associate parts of this system with the
capability of "altering the content of consciousness"
(Livingston, 1969), and with "general alerting" and "focused
attention" (Lindsley, 1969). The Reticular Formation, which is
part of the larger Reticular-Thalamic System we are considering
here, thus easily meets our first criterion, that our neuronal
candidate should be closely associated with conscious experience.



îNeurophysiological evidence that specialists can cooperate
and compete for access to a central integrative "blackboard".ï

The Reticular Formation is called "reticular" (i.e. network-
like) because the neuronal axons in this system are usually very
short, suggesting a great amount of interaction between adjacent
neurons. Further, it receives input from all sensory and motor
systems, as well as from other major structures in the brain.
Through its connections with the thalamus, it can send
information to, and receive it from, all areas of the cortex. If
the Extended Reticular-Thalamic System corresponds to our
"blackboard", different specialized systems can have access to
it.

Aristotle's "common sense" was supposed to be a domain of
integration between the different senses. In fact, anatomists who
have studied the Reticular Formation have pointed to its
resemblance to Aristotle's concept. Scheibel and Scheibel (1967)
point out that "Anatomical studies of Kohnstamm and Quensel,
which suggested pooling of a number of afferent and efferent
systems upon the reticular core, led them to propose this area as
a 'centrum receptorium', or 'sensorium commune' --- a common
sensory pool for the neuraxis."

Moreover, and of great significance to our discussion, these
authors note that "... the reticular core mediates specific
delimitation of the focus of consciousness îwith concordant
suppression of those sensory inputs that have been temporarily
relegated to a sensory role"ï (p. 579). Along similar lines,
Gastaut (1969) describes the brain stem reticular formation as an
area of "convergence ... where signals are concentrated before
being redistributed in a divergent way to the cortex". Thus
different sensory contents can suppress each other, as we would
indeed expect of input to a global workspace. This meets our
second requirement, that different specialized processors can
compete for access to the ERTAS.

îNeurophysiological evidence that integrated, coherent
information can be broadcast by the Reticular-Thalamic System to
all parts of the nervous system.ï

As we noted above, we are including in the term îExtended
Reticular-Thalamic Systemï the diffuse thalamic projection
system, a bundle of neurons which projects upward like a fountain‹j ‹
From the thalamus to all parts of the cortex. It contains bo
specific and non-specific projections, and the specific ones
usually contain feedback loops going in the opposite direction as
well. The thalamic portion of this system may "broadcast"
information from the Reticular System to all parts of the brain.
We have already discussed evidence from evoked potentials which
indicates that non-habituated stimuli are indeed broadcast non-
specifically throughout the brain (Thatcher & John, 1977) (2.xx).


In one possible scenario, one sensory projection area of the
cortext provides input input to the Extended Reticular-Thalamic
Activating System. If this input prevails over competing inputs,
it becomes a global message which is widely distributed to other
areas of the brain, including the rest of the cortex. Thus one
selected input to the ERTAS is amplified and broadcast at the
expense of others.

We can therefore suggest that the ERTAS underlies the
"global broadcasting" function of consciousness, while a selected
perceptual "processor" in the cortex supplies the particular
îcontentsï of consciousness which are to be broadcast. (These are
typically perceptual contents, because the ERTAS receives
collateral pathways from all sensory tracts; and of course, we
have previously remarked on the favored relationship between
conscious experience and perception). These conscious contents,
in turn, when they are broadcast, can trigger motoric, memory,
and associative activities.

There is independent evidence that cortical activity îby
itselfï does not become conscious (Magoun, 1964; Libet, 1977,
1978, 1981; Shevrin and Dickman, 1980). We would suggest that any
cortical activity must trigger ERTAS "support" in a circulating
flow of information, before it can be broadcast globally and
become conscious (e.g. Shevrin and Dickman, 1980; Scheibel,
19xx). Dixon (1971) has also argued that a circulating flow of
information between the reticular formation and the sensory areas
of the cortex is required before sensory input becomes conscious.




3.13 A possible scenario.

There are probably several ways to gain access to the brain
equivalent of a global workspace. In one scenario, two perceptual
inputs arrive in the cortex at the same time through the direct
sensory pathways, and begin to compete for access to the limited-
capacity system --- presumably the thalamus and reticular
formation. Suppose the two events are auditory and visual, so
that we get stimulus competition (Figure 3.13). One may be a
speech sound in the left ear, and the other a falling glass in
the right visual field. It has been known for at least a century‹j ‹
that two simultaneous, incompatible events do not become
conscious at the same time (e.g. Wundt, 1912; Blumenthal, 1977).
In our scenario, only one of the two can be broadcast at any
moment, because they conflict in spatial location and content, so
that the two simultaneous cortical events cannot be fused into a
single, consistent conscious event. One of the two may be favored
because of readiness in the receiving specialized processors to
support it. For instance, we may be alert to the possibility of
the glass falling; in that case, the specialized processors
involved with moving a hand to catch the falling glass would
trigger quickly to help direct consciousness to the visual
stimulus, and away from the auditory input. Possibly there is
rapid alternation between the visual and auditory stimulus, so
that each is broadcast for 100 milliseconds to recruit additional
processors. Receiving processors may then support the visual
message over the auditory one. But glasses fall quickly; losing a
few hundred milliseconds will probably cause us to miss the
falling glass; and competition for access to consciousness
inevitably slows down effective action.

This scenario has the following features. First, there is
competition between perceptual systems for access to the global
workspace. Only one input can win, and it is the one that garners
most support from potential receiving systems, especially those
that are ready for and "interested in" the winning system.
"Winning" means that one system gains access to the thalamus and
perhaps reticular formation, allowing a general broadcasting of
at least some central aspects of the winning system --- perhaps
its spatiotemporal properties, its significance, its relevance to
current goals, etc. Probably some receiving processors gain more
information from the global message than others. There is
probably a circulating flow of information between the winning
input system, the global workspace, and the receiving processors,
each component feeding back to the others, so that there is for
some time a self-sustaining loop of activated systems (see Figure
3.21). Possibly this flow may allow more direct local channels to
be established between the perceptual input and some receiving
systems; over time, this local flow of information may allow the
creation of a new, efficient specialized "falling glass
detector," which operates independently of the global workspace.

3.2 Extensions suggested by the neurophysiology.
‹j While the neurophysiology seems compatible with the GW
model, it also suggests some additions to the model.


1. îThe outer thalamus as a common sensory mode.ï

The outer layer of the thalamus, the nucleus reticularis
thalami, is thought to contain a body-centered spatio-temporal
code, that can "gate" different inputs before cortical activation
occurs (Scheibel, 1980). Thus auditory signals to the right rear
of the body may be coded in one place, and visual signals in the
same location may converge on the same area. This suggests the
existence of a kind of lingua franca in which the outer thalamus
may thus act as a common sensory mode. The thalamic centers have
much more specificity in this sense than the lower reticular
centers.



2. îThe brainstem reticular formation as a mode switch. ï

What then, is the role of the Reticular Formation (RF) ---
especially the brain stem components that are known to be
involved in sleep, waking, and coma? The RF may act as a "mode
switch" on the system that does more specific selection. If we
use the search-light metaphor of consciousness, the RF nuclei may
act as a dimmer switch, to increase or decrease the amount of
light, but not to direct it to any particular object. In terms of
the GW model, the RF may control overall activation levels, while
the thalamic nuclei may modulate activation to and from specific
specialized processors.




3. îLocations of some specialized capacities.ï


îSensory/imaginal systems as GW input.ï
A large part of the cortex is devoted to perceptual
analysis, especially vision, and this may be one reason for the
predominance of perceptual/imaginal input to consciousness. It
seems likely that imagery also makes use of these perceptual
systems, with stimulation of internal origin. Thus some of the
input specialists would seem to be located in the sensory
projection areas of the cortext.

Clearly voluntary decisions can affect conscious contents,
and these are not perceptual for most people, so that it is
possible that non-perceptual events can gain global access.
Alternatively, it is possible that these non-perceptual systems‹j ‹
make use of perceptual/imaginal processors to gain access to the
system underlying consciousness.


îShort term memory and the hippocampus.ï

There is now good evidence that the hippocampus, a structure
that surrounds the thalamus, is closely associated with the
transfer of short-term memory information to long term memory
(e.g. Milner, 19xx). Clearly short-term memory is intimately
associated with consciousness, and if the hippocampus contains
such a system, it is presumably one of the recipients of global
broadcasting (Winson, 19xx).



îVoluntary speech control and the rehearsal component of
short term memory.ï

Similarly, voluntary control of speech is clearly involved
in short-term rehearsal, as in memorizing a telephone number.
Speech production is one of the few functions that is quite well
lateralized to the left hemisphere (Springer & Deutsch, 19xx), in
particular to Broca's area. It seems likely that this system is
involved in mental rehearsal, which is after all mental speaking;
rehearsal really acts to refresh conscious access to immediate
memory. Therefore this rehearsal system would also seem to
provide input to the GW. However, voluntary control in general is
more associated with the frontal cortex, so that this functional
system may include both frontal areas and Broca's area.




4. îSpatio-temporal coding as a lingua franca.ï


We have claimed that perception and consciousness have a
special relationship, in the sense that all qualitative
experiences are perceptual or quasi-perceptual (like imagery or
inner speech). All perceptual experiences involve spatio-temporal
information, of course, and the neurophysiology indicates that a
great many neural systems can process spatio-temporal
information. This suggests that spatio-temporal coding may be one
lingua franca that is broadcast through the neural equivalent of
a global workspace.



5. îGlobally broadcast information may feed back to its
sources.ï

‹j ‹å If broadcasting is truly global, the systems that provide
global input should also receive their own results, just as a
television playwright may watch his own play on television. Such
a circulating flow back to the source is postulated in certain
cognitive theories. It is known to have a number of useful
properties. For example, McClelland & Rumelhart (1981) have shown
that a circulating flow in an activation model of word
recognition helps to stabilize the representation of the word.





6. îReceivers of global information may feed back their
interest to the global workspace.ï


The physiological evidence discussed above suggests that
global îoutputï flows in two directions as well. There are
anatomical connections that allow feedback from cortex back to
the thalamus. Such feedback loops are extremely common in the
nervous system. Most sensory systems allow for a flow of
information "top-down" as well as "bottom up." This anatomical
evidence may mean that receiving systems, those that take in
globally broadcast information, may be able to feed back their
interest to the global workspace, thus strengthening or weakening
any particular global message. One can make an analogy to the
well-known Nielsen Ratings for television programs in the United
States. Each program is continuously sampled to see how many
viewers are watching it, and programs of low popularity are
quickly dropped. In a later chapter we will suggest that this
kind of popularity feedback may explain such phenomena as
habituation and the development of automaticity with practice
(Chapter 5).


7. îOther anatomical systems may facilitate global
broadcasting.ï


The diffuse thalamic projection system (Figure 3.12) is not
the only projection system that may be used to broadcast
information. There are long tertiary cortical neurons that
connect frontal to other areas of the cortex, and cross-
hemispheric fibers that connect the two halves of the cortex
through the corpus callosum. All such transmission pathways may
be involved in global broadcasting.



8. îCyclical snowballing rather than immediate broadcasting.ï


The neurophysiology suggests that broadcasting may not be an‹j ‹
instantaneous event, but a "snowballing" recruitment of global
activation, supported by many systems, that may feed back on
each other. For example, Libet's work indicates that for cortical
activity to become conscious may take as long as a half second
(Libet, 1978; 1981). This is much longer than a single broadcast
message would take, and suggests a circulating flow between
cortical and sub-cortical areas, building upon itself until it
reaches a threshold. Thus we must not take the broadcasting
metaphor too literally: a relatively slow accumulation would
accomplish much the same functional end. This kind of snowballing
would of course also explain the role of the anatomical feedback
loops described above.



9. îAttention: Control of access to the global activating
system.ï


Later in this book we will draw a distinction between
consciousness and îattentionï --- in which the latter serves to
control access to consciousness. Such attentional systems have
been found in the parietal and frontal cortext (e.g. Posner,
1982). Possibly the frontal components are involved in voluntary
control of attention, which can often override automatic
attentional mechanisms (see Chapter 8).





3.21 Changes suggested by the neurophysiology.


Figure 3.21 is a modified version of Model 1, with feedback
loops from the global message to its input sources, and from the
receiving processors back to the global message. We will find
additional evidence for these feedback loops later in this book.





3.3 Recent refinements of the neurophysiological evidence.

The above interpretation of the neurophysiology resembles
to‹j ‹
earlier models of the Reticular Formation (RF), which we treat
here as a subset of the more broadly defined ERTAS system
(Moruzzi & Magoun, 1949; Lindsley, 19xx; Magoun, 1964). Arguments
for a central role in conscious experience of the RF have come
under some criticism (e.g. Thompson, 1967; Brodal, 1981). Some of
these criticisms serve to qualify our conclusions, though they do
not contradict them decisively.

îFirstï, as more detailed studies have been performed using
long©term implanted electrodes, a number of specific components
have been found in the RF, so that the bald statement that the RF
is nonspecific is not quite true (Hobson & Brazier, 19xx). We
should be careful not to refer to the whole RF and thalamus as
subserving these functions, but only to nuclei and networks
within these larger anatomical structures. îSecondï, under some
circumstances one can show that lesioned animals with little or
no surviving RF tissue show relatively normal waking, sleeping,
orienting and conditioning. It is possible that the outer layer
of the thalamus may be able to replace RF functions, especially
if the lesions are made gradually, so that there is time for
adaptation to take place. îThirdï, it is clear that a number of
other parts of the brain are involved in functions closely
related to conscious experience, such as voluntary attention; the
sense of self; voluntary control of inner speech, imagery, and
skeletal musculature; and control of sleep and waking. We must be
careful therefore not to limit our consideration to just the
extended reticular-thalamic system; surely many other systems act
to contribute to, control, and interact with any neural
equivalent of a global workspace.


îBrain duality.ï

Before concluding this chapter, we should mention the
puzzling role of brain duality. The human brain has a major
division down the midline, extending far below the great cortical
hemispheres into most subcortical structures, including the
thalamus and even the brainstem reticular formation. This
suggests that duality may be an "architectural" feature of the
nervous system. But Model 1 has no place for duality; it
emphasizes unity rather than duality.

Brain duality is a fundamental fact of nervous system
functioning. In the intact brain, it is not clear that it has
major functional implications; most of the evidence for brain
lateralization in normal people shows only very short time delays
between left and right-sided functioning. The corpus callosum,
which connects the two hemispheres, is estimated to add perhaps 3
milliseconds of transmission time to interactions between the two
sides --- not enough to make much of a difference (D. Galin,
personal communication, 1986). Still, this massive anatomical
feature must be functional in some sense, and it is curious that
our architectural approach to the nervous system has no obvious
role for it. It is possible that its role is primarily‹j ‹
developmental, and that in the intact adult brain its effects are
more difficult to observe (e.g. Galin, 1977).



îSummaryï


Even with these qualifications, the evidence is strong that
parts of the Extended Reticular-Thalamic System serve as major
facilitators of conscious experience, while the cortex and
perhaps other parts of the brain provide the îcontentï of conscious
experience. This evidence can be naturally interpreted in terms
of the GW model, derived from purely cognitive evidence.
Contributions from both the ERTAS and cortex are presumably
required to create a stable conscious content. The evidence comes
From numerous studies showing a direct relationship between t
ERTAS and known conscious functions like sleep and waking,
alertness, the Orienting Response, focal attention, sharpening of
perceptual discriminations, habituation of orienting,
conditioning, and perceptual learning (see references cited
above). Further, there is evidence consistent with the three
major properties of Model 1: first, that major brain structures,
especially the cortex, can be viewed as collections of
distributed specialized modules; second, that some of these
modules can cooperate and compete for access to the ERTAS; and
third, that information which gains access may be broadcast
globally to other parts of the nervous system, especially the
huge cortical mantle of the brain.

Thus substantial neurophysiological evidence seems to be
consistent with Model 1, with one addition: there is evidence of
a feedback flow from cortical modules îtoï the ERTAS, suggesting
that a circulating flow of information may be necessary to keep
some content in consciousness. In addition, global information
may well feed back to its own input sources. Both kinds of
feedback may serve to strengthen and stabilize a coalition of
systems that work to keep a certain content on the global
workspace. These modifications have has been incorporated into
Model 1A (Figure 3.21).


3.4 Overall Summary.


Let us review where we have been. First, many
neuroscientists suggest that the nervous system is a distributed
parallel system, with many different specialized processors. A
constrastive analysis of neurophysiological evidence about
conscious vs. unconscious phenomena focused on the well-known
reticular formation of the brainstem and midbrain, on the outer
layer of the thalamus, and on the diffusely projecting fibers
From the thalamus to the cortex. Several established facts abo
the nervous system suggest that we may take the notion of îglobalï‹j ‹
broadcasting quite seriously, that conscious information is
indeed very widely distributed in the central nervous system. At
least parts of the reticular-thalamic system bear out our
expectations regarding a system that can take input from
specialized modules in the brain and broadcast this information
globally to the nervous system as a whole.