Synaesthesia: a Conceptualization (‘Synthesis’-) Phenomenon. Philosophical and Neurobiological Aspects

Abstract

Neurobiological aspects of synaesthesia are discussed from the perspective of the broader philosophical topic of “syn-aisthesis” and the basic fundamentals of a neuropsychological understanding of perceptual inter-modal integration. Herein, the predominance of conceptualization processes in regard to top-down functions of the brain appears as a prerequisite for perception. Functional Magnet Resonance Imaging (fMRI) data of synaesthetes compared to controls are discussed, providing evidence for the theory that prefrontal and parietal conceptualization processes by themselves exert transmodal functions and thus contain properties of “binding”. A partial hyperactivity of such processes in synaesthesia may thus be a causal factor of this condition.

Full text

Christopher Sinke*, Janina Neufeld*, Markus Zedler,
and Hinderk M. Emrich
Hannover Medical School; Clinic for Psychiatry, Social Psychiatry and Psychotherapy,
Germany
* C. Sinke and J. Neufeld contributed equally to this paper
Correspondence to:
Janina Neufeld
Hannover Medical School;
Clinic for Psychiatry, Social Psychiatry and Psychotherapy
Carl-Neuberg-Straße 1
30625 Hannover
Germany
Email: neufeld.janina@mh-hannover.de
Phone: 0049 511 532 6658
Synaesthesia: a Conceptualization (‘Synthesis’-)
Phenomenon.
Philosophical and Neurobiological Aspects
Abstract. Neurobiological aspects of synaesthesia are discussed from the perspective
of the broader philosophical topic of “syn-aisthesis” and the basic fundamentals
of a neuropsychological understanding of perceptual inter-modal integration. Herein,
the predominance of conceptualization processes in regard to top-down functions
of the brain appears as a prerequisite for perception. Functional Magnet Resonance
Imaging (fMRI) data of synaesthetes compared to controls are discussed, providing
evidence for the theory that prefrontal and parietal conceptualization processes by
themselves exert transmodal functions and thus contain properties of “binding”.
A partial hyperactivity of such processes in synaesthesia may thus be a causal factor
of this condition.
Keywords: synaesthesia; syn-aisthesis; unitarity of consciousness; binding;
Immanuel Kant; bottom-up and top-down functions; prefrontal and parietal
conceptualization.
THEORIA ET HISTORIA SCIENTIARUM, VOL. X
Ed. Nicolaus Copernicus University 2013
http://dx.doi.org/10.2478/ths-2013-0003

38 Christopher Sinke, Janina Neufeld, Markus Zedler, Hinderk M. Emrich
Aisthesis and synthesis
Neurobiology of consciousness represents one of the most challenging
research topics in natural science – as regards the relationship between
neuronal realities and mental life which takes place within our minds. Herein,
one central question is how the different modalities are integrated during
the process of “aisthesis” (Greek = perception) in order to give rise to the
unied percepts we experience in our mind. This unication process is also
termed “synthesis” or “binding”. For example, we do not see a car and hear
the sound of its engine separately but we perceive a car as an integral object
(“Gestalt”) (Wertheimer 1938) with visual and acoustical qualities.
At the neuronal level, we know since the elegant pioneering work
of Hubel and Wiesel (Hubel and Wiesel 1979) that the primary visual data
are analysed step by step by highly specialized neuronal assemblies which
detect, e.g., the direction of a movement, a contour, the colour or another
quality of retinal excitation/inhibition patterns. This parallel processing
of different aspects of sensory data also takes place in other modalities, e.g.,
in audition (Merzenich and Brugge 1973). While it enables us to analyse
parts of the signal, this process results in fragmentation of the sensory
data, leading to the famous computer-brain analogy which ts in with our
everyday understanding of perception: From a naive point of view, one
assumes that outer reality is strictly constituted in precisely that fashion
in which it appears to the subject. This is as if it is sufcient to photograph
or lm outer objects; and subjective experience represents nothing else than
the function of a computer which calculates from sensory data outer reality
(Fig. 1a). The brain would – in this metaphor – be a pure ‘sink’, no ‘source’
of information (and meaning).
Here, one main question arises which has to do with the problem
of ‘unitarity of consciousness’, namely the problem of the so called
‘intermodal integration’, the ‘binding’ problem: How does it come that
we observe in perception not an addition of elements of percepts but
a type of holisms, i.e., ‘perceptual gestalts’? The mechanism by which our
consciousness integrates different aspects of a perceived object is, as a matter
of fact, not yet elucidated.
However, as Immanuel Kant already pointed out, an interpretation
of reality is possible only if a leading conceptualization (a ‘world model’)
is applied to the raw material of sensory inputs (Fig. 1b). Before sensory
data can be calculated and interpreted, a set of working hypotheses about
possible outer realities is required. This is presented philosophically –

39
Synaesthesia: a Conceptualization (‘Synthesis’-) Phenomenon
in Kant’s ‘Critique of Pure Reason’ (Kant 1974) – as conceptualization and
in psychological terms of the ‘accompanying world model’ by Prinz (Prinz
2006). Wolfgang Welsch (Welsch 1992) has formulated this referring to
Immanuel Kant enunciating “we can cognize of things a priori only what
we ourselves have put into them” (Kant, ‘Kritik der reinen Vernunft’; quote
translation as per Allen W. Wood (1998)) and these are primarily aesthetic
preconditions of, namely, the recognition-categories of space and time.’
Figure 1. The computer-brain analogy
a) An object (tree) is lmed by a camera and the data are transferred to a computer
where they are processed. b) In contrast to a computer, the human brain needs
concepts to explain the sensory data, coming from sensory receptors, e.g., the eyes
(camera).
This leads to the comparison between expected and real reality and
includes the subjective experience which may be described in the terms:
‘This occurs actually now’. In his Historical Anthropology Carl Friedrich
von Weizsäcker claims, under the title ‘As to the biology of subjects’: “From
empiricism one accepts sensory data as given [. . .] it is not recognized that
sensory data, already according to our biological constitution, can only
be given under the prerequisite of a simultaneously perceived concept”
(Weizsäcker 1982).

40 Christopher Sinke, Janina Neufeld, Markus Zedler, Hinderk M. Emrich
The thinking of constructivism has – in opposition to empiricism – led
to the concept that the term ‘reality’ has lost tremendously in its impact and
unity, leading to a scientic climate in which we nd ourselves in a ‘reality-
relativism’ which may be characterized in terms like ‘ctional reality’ and
internal ‘reality-censorship’.
As a consequence, we can construe an interactive circuit in which
interpretation of sensory data is possible only on the basis of conceptualizing
hypotheses about possible realities; so to speak, nuclei of pre-categories which
are – in a probational fashion – used to test the applicability of hypothetical
world-models. The ‘top-down components’ might also be interpreted as the
manifestation of a conctructivistic part within the neuronal basis of aisthesis.
Thus, aisthesis incorporates at least two components: bottom-up analysis
of data with top-down categorization.
The neurobiology of aisthesis, so to speak, the neuroscience of ‘sensory
cognition’, represents an enterprise which has not yet reached a stable position
of knowledge. It rather tries, within an iterative asymptotic approximation,
to establish some innovative ideas how – due to a complex interaction
of subcomponents within the plumbable brain – the subjective experiences
in sensory cognition are generated (cf. Gregory 1970; 1980).
The interaction of top-down and bottom-up components of aisthesis
under normal and altered circumstances is the main subject of our group’s
basic research in (psychiatric) cognitive neurosciences. Desimone has
published a diagram (Fig. 2) in which the superimposition of bottom-up and
top-down components is conceptualized as a manifestation of two streams
of excitation, antagonizing each other (Gazzaniga et al. 2002). This function
is related to a third component, relevant in processes of conceptualisation
of ‘reality’, namely internal ‘censorship’. This component is also attributed to
plausibility control, to stabilization of internal data processing, if ambiguity
and instability of perception occur, e.g., in emergency situations. It is attributed
to a component which may work over incongruences in perception and thus
may induce a predominance of ctional internal reality in comparison to the
intricate, un-assimilated raw material of information about outer reality. This
censorship is comparable to Gazzaniga´s ‘interpreter’ (Gazzaniga 2000),
a module which coordinates and integrates the computations (the ‘votes’)
of other modules, and the result of such an ‘interpreter’-related computation
apparently has the subjective quality of being ‘granted’, being ‘true’, being
‘real’. The function of this module is to stabilize ‘reality ctions’ and one may
anticipate that especially this system is disturbed or, to an increased extent,
‘vulnerable’ in psychosis. To be more precise, in hallucinations, one may
argue that it is the equilibrium between ‘conceptualizations’ and ‘censorship’

41
Synaesthesia: a Conceptualization (‘Synthesis’-) Phenomenon
which is disturbed by relative censorship impairment. It appears to be possible
to perform a type of illusion research on the basis of constructivism in which
the ctional character of subjective reality can be elucidated and especially
the perceptual disturbances in schizophrenic psychoses can be characterized.
The basic concept herein is the contention, that perception does not represent
one single procedure but is due to a complex interactive internal ‘dialogue’
between partial components of the system. And, interestingly, our research
group has recently succeeded in demonstrating in such experiments that
schizophrenia seems to be a result of a reduced ability to perform integrative
aisthesis processes due to an inhibited connectivity between neuronal
assemblies (Dima et al. 2011; Dima et al. 2010).
Concerning censorship, it has certainly great demands regarding
long-term memory, since the question of what is ‘plausible’, ‘sensible’,
‘biologically meaningful at present’ can only be resolved on the basis
of experience of failures and successes which are stored over longer periods.
Thus, hippocampal structures may play a prominent role herein. Hippocampal
structures may work as ‘comparators’ (Gray and Rawlins 1986) and calculate
meaningful expectations in relation to sensory input.
Thus, the censorship systems described above may be characterized
as such ‘comparators’. This concept ts in with the idea that to have
‘consciousness’ means to ‘compare’ world models with actual (sensory or
imagined) data (Emrich 1998) and further to have the opportunity to correct
an obviously wrong intention – by a type of servo-mechanism – within the
“last moment” (Gray 2004). Thus, an adequate denition of censorship may
be the comparison of actual data with sets of acquired world models as the
function of stabilizing ‘reality ctions’: and it is this that appears to be so
highly developed in human mental life and which also appears to be so
vulnerable and obviously impaired during psychotic states.
The scheme (adapted from Desimone 1995) shows the direction
of (visual) bottom-up processing (right to left) from sensory cortex areas
to higher associative cortex areas, especially in the temporal and prefrontal
cortex. In contrast, top-down processing (left to right) is initiated by
associative cortex areas, owing in the opposite direction.

42 Christopher Sinke, Janina Neufeld, Markus Zedler, Hinderk M. Emrich
Figure 2. Top-down and bottom-up processing in the human brain
Three-Component Model of Perception / Aisthesis
Taking into account all the considerations advanced above, one may propose
a model describing three hypothetical components of aisthesis:
1. Sensuality (‘bottom-up’)
2. Constructivity (‘top-down’)
3. Censorship (‘interpreter’)
The three-component model of perception assumes that perception
is principally made up of three functional domains: rstly, sensory input
(‘sensualistic’ bottom-up component); secondly, the internal production
of concepts (‘constructivistic’ top-down component); and thirdly, control
(‘censor’ component). It also assumes that special interaction between these
three components is responsible for a biologically fruitful and efcacious,
conscious internal representation of the external world during perception
and that the equilibrium between these three components may be disturbed
in psychosis.
The constructivistic component can also be termed ‘phantasy component’,
‘hypothesis-generating component’, or ‘conceptualization component’.
Its representation in the present model takes the fact into account that
processing of data is possible only on the basis of conceptualization which
has to be applied to sensory data, before successful interpretation is possible.

43
Synaesthesia: a Conceptualization (‘Synthesis’-) Phenomenon
The ‘censor’ function can also be termed a ‘correcting’ function and may
be qualied as a partially ‘erasing’ and partially ‘suppressing’ or ‘rejecting’
system. In the aisthesis process, a great difculty arises: The more elaborate
and complicated a ‘private conscious world’ is, the more prominent two
problems become, which interact in an intrinsic antinomy: on the one hand,
the ctional reality has to be so exible that it can be adapted to the changing
requirements of the external world; and, on the other hand, this private world
has to be stabilized so far that it is not continuously in danger of losing
its stability, i.e., to ‘decompensate’. This means that both the exibility
of creativity and, on the other hand, the stability of the subjective world
are required. To full these two opposing requirements, intricate equilibria
between neuronal networks have to be established. Disequilibrium of these
neuronal components of the aisthesis-networks has been observed by our
group in patients with schizophrenia and subjects under the condition of sleep
deprivation and cannabinoid intoxication (Emrich et al. 1991; Emrich et al.
1997; Schneider et al. 1996; Schneider et al. 2002; Sternemann et al. 1997).
Our group further demonstrated that, in schizophrenic patients, this is due
to disturbed top-down connectivity, especially between the inferior parietal
cortex and earlier sensory areas (Dima et al. 2009).
Synaesthesia
Synaesthesia appears to be a special case of aisthesis, which makes
it possible to investigate the phenomenon more precisely in healthy subjects:
normally synaesthesia does not impair every day life and therefore is not
listed in common classication systems for neurological disorders (DSM-IV
or ICD-10; Hubbard 2007).
Synaesthesia (Greek: syn = together; aesthesis = perception) represents,
for everybody who comes in contact with this phenomenon for the rst
time, a very peculiar, astonishing and challenging process of intermingling
of sensual percepts in which stimulation of one sensory modality or
cognitive event leads to another internally generated perceptual experience.
For example, hearing a tone or word may result in a subjective experience
of seeing coloured shapes.
Synaesthetic experience can formally be divided into an inducing stimulus
(‘inducer’) and an elicited perception (‘concurrent’) (Grossenbacher and
Lovelace 2001). Most synaesthetes report to perceive synaesthesia for ‘as long
as they can remember’ (Cytowic 2002). Its main characteristics are consistency
(Baron-Cohen et al. 1987; Simner and Logie 2007) and automaticity (Mills et

44 Christopher Sinke, Janina Neufeld, Markus Zedler, Hinderk M. Emrich
al. 1999). Consistency is actually used as a ‘gold standard’ in order to determine
whether someone is a synaesthete or not (Baron-Cohen et al. 1987; Eagleman
et al. 2007). There is evidence that conscious perception is necessary to elicit
synaesthesia: Mattingley and colleagues showed that colour priming with
letters is only possible when the letter is consciously perceived (Mattingley
et al. 2001). Another study from Johnson et al. used the ‘attentional blink’
paradigm in which they found that some synaesthetes are able to report the
colour of unconsciously perceived letters, whereas others are not (Johnson et
al. 2007), underlining the importance of individual differences in synaesthesia
(see also Laeng et al. 2004).
Although synaesthesia is believed to be a highly idiosyncratic
phenomenon, there seem to be some general rules in synaesthetic perceptions,
at least in grapheme-colour synaesthesia: frequently used letters are often
coupled with frequently used colours (Simner et al. 2005) and tend to be
brighter and more saturated (Beeli et al. 2007). These ndings indicate that
implicit learning mechanisms play a role in the development of inducer-
concurrent couplings.
Recent eld studies concerning the prevalence of synaesthesia revealed
a prevalence of 4% (Simner et al. 2006). As synaesthesia runs more often within
families (Baron-Cohen et al. 1996), there is evidence for a genetic component
of synaesthesia and some gene regions have been recently identied to be
involved in synaesthesia (Asher et al. 2009; Tomson et al. 2011).
Models of synaesthesia
In order to explain the neural mechanisms of synaesthesia particularly, two
models gained recognition within the last years, which are not necessarily
mutually exclusive. The model of ‘local cross-activation’, which is for
instance supported by Hubbard and colleagues, proposes a direct linkage
between an area referred to as ‘visual word form area’ and an adjacent
region which has been shown to be involved in colour processing (hV4)
in grapheme-colour synaesthesia (Hubbard et al. 2005). A possible reason
for this linkage could be a failure of pruning in prenatal pathways (Hubbard
and Ramachandran 2005; Maurer and Mondloch 2004). Other researchers
suggested that synaesthesia might be due to a `long-range disinhibited
feedback´ from a ‘multisensory nexus’ such as the temporo-parietal-occipital
junction, so feedback connection that are usually inhibited (Grossenbacher
and Lovelace 2001).

45
Synaesthesia: a Conceptualization (‘Synthesis’-) Phenomenon
Interestingly, synaesthetes commonly report that there is an apparent non-
separability of the semantic content of an inducer and the concurrent percept.
This leads to the concept of ‘hyperbinding’ in synaesthesia (Esterman et al.
2006; Robertson 2003), which is in line with the concepts of cross-activation
or disinhibited feedback: concurrent representation areas could be activated
by cross-activation or disinhibited feedback mechanisms and additionally be
bound to the inducer representation as the next step (Hubbard 2007).
Neural correlates of synaesthesia: evidence
from neurophysiological and neuroimaging studies
Neuroimaging studies indicate that colour processing areas may play a role
(as concurrent representation areas) at least in grapheme-colour synaesthesia,
which is the most investigated form: Many studies revealed increased brain
activation in these areas (Brang et al. 2010; Hubbard et al. 2005; Nunn et al.
2002), while others did not (Paulesu et al. 1995; Weiss et al. 2005; Rich et
al. 2006). This has to be interpreted with caution. First, there is evidence that
individual differences between synaesthetes determine the areas involved:
A recent investigation in which synaesthetes were differentiated according
to where they perceive their synaesthesia – projected to the written letter
(projectors) or on an inner screen in their ‘mind’s eye’ (associators) – found
that visual areas showed increased activation only in projectors, while
associators had additional activation and structural differences in limbic
(especially hippocampal) brain areas (Rouw and Scholte 2010). Second,
most investigations had only a rather small number of subjects (<10) and
it has been shown that, in neuroimaging studies, the results strongly depend
on the number of investigated subjects (Thirion et al. 2007).
Further, there is strong evidence for an involvement of the parietal
cortex in synaesthesia: Studies concerning different types of synaesthesia
revealed increased parietal activation in synaesthetes compared to non-
synaesthetic controls when perceiving inducers (Rouw and Scholte 2010;
Tang et al. 2008; Weiss et al. 2005; van Leeuwen et al. 2010), structural
differences in the parietal cortex between synaesthetes and controls (Rouw
and Scholte 2007; Weiss and Fink 2009) and a disruption of synaesthesia by
TMS (Transcranial Magnetic Stimulation) over parietal sites (Esterman et al.
2006; Muggleton et al. 2007; Rothen et al. 2010). Recent investigations with
fMRI by our own group (unpublished data) also found increased activation
in the left inferior parietal cortex in grapheme-colour as well as auditory-
visual synaesthetes. As this area is a multimodal integration area which is also

46 Christopher Sinke, Janina Neufeld, Markus Zedler, Hinderk M. Emrich
involved in non-synaesthetic binding (Robertson 2003), these ndings speak
in favour of an enhanced binding mechanism in synaesthesia. The parietal
cortex may further be involved in synaesthesia as a sensory nexus area,
leading to disinhibited feedback (Grossenbacher and Lovelace 2001). As
this area is involved in auditory-visual, grapheme-colour and number-form
(Tang et al. 2008) synaesthesia, it is likely that all types of synaesthesia share
a common mechanism.
But also other multimodal areas have to be considered to be involved
in synaesthetic binding: A neurophysiological investigation by our group
in 1999 found in synaesthetes a more positive waveform of event related
potential (ERPs) evoked by visual presentation of graphemes, especially
over frontal and prefrontal sites (Schiltz et al. 1999), which was interpreted
as prefrontal deactivation in synaesthesia. This was discussed as leading
to increased distractibility, causing a leakage between modalities. Also,
increased frontal brain activation due to synaesthesia has been detected
using fMRI by other groups (Rouw and Scholte 2010; Tang et al. 2008;
Paulesu et al. 1995). Therefore, prefrontal cortex areas seem to play a role
in synaesthesia – although it still remains unclear which function they full
therein.
Is synaesthesia a top-down phenomenon? Is Immanuel Kant’s
concept of synthesis erroneous?
As we have pointed out above, synaesthesia can be regarded as a special
case of aisthesis, an essential question is at which stage synaesthetic effects
occur. Are they more bottom-up or top-down driven? At the rst glance,
synaesthesia seems to be a bottom-up phenomenon, as sensory perceptions
are involved and mingled. But there is growing evidence for the involvement
of top-down processes in synaesthesia. First, the synaesthetic concurrent
depends on the interpretation of the inducer (Dixon et al. 2006; Bargary et
al. 2009). Second, when grapheme-colour synaesthetes learn a new alphabet,
the colour of a corresponding letter in the old alphabet transfers to the new
letter (Mroczko et al. 2009) and is therefore rather concept-driven instead
of depending on the letter’s shape. Third, synaesthetic colours do not behave
like real colours (van Leeuwen et al. 2010). All these ndings speak in favour
of the idea that synaesthesia is more a top-down phenomenon than an altered
bottom-up processing. Further, the involvement of multimodal integration
areas such as the parietal and the frontal cortex in synaesthesia, mentioned
above, speaks against the idea of a direct (bottom-up) cross-activation

47
Synaesthesia: a Conceptualization (‘Synthesis’-) Phenomenon
of visual areas of concurrent representation. The parietal cortex is known
to be involved in cross-modal integration (Calvert 2001), spatial processing
(Colby and Goldberg 1999) and non-synaesthetic feature-binding (Robertson
2003), while the prefrontal cortex is thought to be the highest instance for
executive functions, decision making and cognitive control (Miller 2000;
Tanji and Hoshi 2001; Fuster 2001; Sakagami and Pan 2007). Further, both
brain structures are part of a network involved in attention guidance and
visuo-motor control (Grefkes et al. 2004; Astaev et al. 2003; Corbetta and
Shulman 2002). Therefore, both structures might work together during the
formation of synaesthetic experience.
How might the concept of aisthesis help us to understand synaesthesia?
If synaesthesia is a top-down phenomenon, it involves conceptualization.
In synaesthetes, the concept of the inducer incorporates the concurrent
dimension in addition to the normal dimensions of the concept. The bottom-
up analysis of the incoming signals activates the concept (top-down).
The interpreter recognises the match, and the perception is formed through
mingling the concept with the signals. As the concept incorporates the
concurrent in addition, the overall perception is the fusion of both components
(inducer and concurrent); the concurrent is additionally perceived.
The additional quality (e.g., colour) may be bound to the concept via
a hyperbinding process (Esterman et al. 2006). Alternatively, it may be that
synaesthetes afford additional complex categories (e.g., “A-red” in addition
to “A” and “red”), forming an additional “reality-hypothesis”. This would be
in contrast to Kant’s theory of synthesis, as it would mean that besides the
pure concepts of reason there are also complex ones, as Hermann Schmitz has
pointed out (Schmitz 1994) (personal communication, 2011). From this point
of view, depicted within the scheme (Fig. 3), a modication of the concept
of Desimone, categories in the sense of Kant appear, neurobiologically, to
be not primary conditions of mind, but secondary conditions. Consequently,
reality-hypotheses, appearing within the prefrontal cortex, are generated
not as categorically partialized entities but primarily holistic ones (integral
conceptual world), whereas the ‘simple’ categories might rather be represented
by parietal cortex areas. One might further assume that, if a complex concept
is activated, the corresponding ‘simple’ contexts are recalled via top-down
processes.
But here the question remains, why synaesthetes have these enriched
concepts. One explanation would be that it is a strategy to cope with problems
with abstract thinking by concretizing abstract concepts with sensorial
content like, for example colours, shapes, sounds. It is also thinkable that it is
not a problem-solving strategy but a style of thinking: synaesthetes may be

48 Christopher Sinke, Janina Neufeld, Markus Zedler, Hinderk M. Emrich
more sensorially orientated than the general population and use their senses
to a higher degree in order to contextualize/make ‘sense’ of the world.
What does synaesthesia research tell us about aisthesis? Synaesthesia
makes it very clear that aisthesis is not just a process of analysing sensory
data and nding the corresponding concept to be able to interpret them but
that concepts are also able to shape our perception.
Figure 3. Advanced scheme of top-down and bottom-up processing
Sensory data (blue) coming from primary sensory cortex areas (here
exemplarily from the occipital cortex) are processed to associative cortex
regions, e.g., in the temporal cortex (TC), until they nally reach the prefrontal
cortex (PFC). This leads to an excitation of complex concept-representations
within the PFC, which are compared via top-down connections (red) to
simple concepts, represented, e.g., by the inferior parietal cortex (IPC),
and nally to sensory information. The world-model is represented by
a network including PFC and IPC, which is related to sensory information
by comparator functions, e.g., represented by limbic (temporal) structures.

49
Synaesthesia: a Conceptualization (‘Synthesis’-) Phenomenon
Conclusion
As a consequence of the current examinations, aisthesis appears as the result
of complex, dynamical non-linear interaction of subcomponents of the
brain, in which ‘bottom-up’ and ‘top-down’ components interact in a fashion
yielding another component, a component of ‘internal censorship’, which
apparently is partially hindered in conditions of psychoses and altered
states of consciousness induced by drug intoxication or sleep deprivation.
In contrast, in synaesthesia the top-down component may partially be
enhanced, leading to additional complex context-categories and therefore
to additional sensual experiences. The top-down component may be
represented by a network of prefrontal and parietal structures which is also
involved in attention guidance. This component may further represent
a neuropsychological feature which also implies ‘binding’ properties.
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