=Paper= {{Paper |id=None |storemode=property |title=A new Molyneux's problem: Sounds, shapes and arbitrary crossmodal correspondences |pdfUrl=https://ceur-ws.org/Vol-1007/paper3.pdf |volume=Vol-1007 |dblpUrl=https://dblp.org/rec/conf/shapes/DeroyA13 }} ==A new Molyneux's problem: Sounds, shapes and arbitrary crossmodal correspondences== https://ceur-ws.org/Vol-1007/paper3.pdf
           A new Molyneux’s problem:
      Sounds, shapes and arbitrary crossmodal
                correspondences
                          Ophelia DEROY a1, and Malika AUVRAY b
                a
                    Centre for the Study of the Senses, Institute of Philosophy,
                                    University of London, UK
                                 b
                                   LIMSI, CNRS, Orsay, France



            Abstract. Several studies in cognitive sciences have highlighted the existence of
            privileged and universal psychological associations between shape attributes, such
            as angularity, and auditory dimensions, such as pitch. These results add a new
            puzzle to the list of arbitrary-looking crossmodal matching tendencies whose
            origin is hard to explain. The puzzle is all the more general in the case of shape
            that the shapes-sounds correspondences have a wide set of documented effects on
            perception and behaviour: Sounds can for instance influence the way a certain
            shape is perceived (Sweeny et al., 2012). In this talk, we suggest that the study of
            these crossmodal correspondences can be related to the more classical cases of
            crossmodal transfer of shape between vision and touch documented as part of
            Molyneux’s question, and reveal the role that movement plays as an amodal
            invariant in explaining the variety of multimodal associations around shape.

            Keywords. Crossmodal correspondences, Audition, Touch; Molyneux’s problem;
            Amodal invariants



Introduction: A contemporary version of Molyneux’s problem

     How do shapes sound? The question does not seem to make sense metaphysically:
Shapes are not endowed with auditory properties. In addition, similarities or differences
in shapes do not directly correlate with differences in sounds, given that crucial
elements such as density, size, and material properties will make similarly shaped
objects sound very differently when they are similarly struck. For instance, a small
dense sphere might have the same sound as a bigger and less dense cylinder when both
are struck in a similar way; and the rich repertoire of drums should convince us that
shape is not all that matters to determine how objects sound.
     If the question ‘how do shapes sound?’ needs to be dismissed then, a milder
version of the question might be more resistant: Supposing that shapes have sounds,
what would their sound be? Surprisingly, several studies in cognitive sciences have
highlighted convergent and stable responses to this question, and they have shown the
existence of privileged psychological associations between shape attributes and
auditory dimensions, such as pitch. When asked which of two shapes, one rounded and
the other one angular, should be called ‘Takete’ and which one should be called


    1
      Correspondence to: Ophelia Deroy, Centre for the Study of the Senses, Institute of Philosophy,
University of London, Malet Street, London, UK. ophelia.deroy@sas.ac.uk




                                                    61
‘Maluma’, most participants answer that the angular shape should be ‘Takete’ and the
rounded one, ‘Maluma’ (Kohler, 1929, 1947; see also Ramanchandran & Hubbard,
2001a and figure 1).




Figure 1. Three examples of crossmodal correspondences, documented between (a)
sounds and size by Sapir (1929); (b) sounds and shape (angularity) by Köhler (1929,
1947) and Ramachandran & Hubbard (2001); and (c) sounds and shape (aspect ratio)
by Sweeny et al. (2012).

      This crossmodal association between shapes and sounds might look surprising at
first, but a series of evidence shows it to be present across cultures (Bremner et
al.,2013) and from an early age (i.e., four months, see Orztuck et al., 2012, see also
Maurer et al., 2006, for evidence in 2 to 2,5 years old). While neurological
investigation starts to unveil a specific pattern of neurological activity in the superior /
intraparietal regions as well as in frontal areas corresponding to the shape-sound
associations (Kovic et al., 2009; Peiffer-Smadja, 2010; see also Bien et al., 2012 for a
EEG/ TMS study and Sadaghiani et al., 2009, for a fMRI study of related arbitrary
audio-visual correspondences), associations between shapes and sounds is absent in
individuals with damage to the angular gyrus (Ramachandran & Hubbard, 2001b),
suggesting that this is a robust neuropsychological phenomenon.
      What’s more, shapes-sounds correspondences have recently been shown to have
behavioural consequences, as the visual perception of briefly presented shapes can be
affected by certain types of sounds (Sweeny et al., 2012; see also Spence & Deroy,
2012a for a discussion). Sweeny and his colleagues have indeed shown that oval shapes,




                                            62
whose aspect ratio (relating width to height) varied on a trial-by-trial basis, were rated
as looking wider when a /woo/ sound was presented at the same time, and as looking
taller when a /wee/ sound was presented instead. By contrast, the perceived shape was
not affected by other natural sounds such as birds or engine sounds, showing that a
specific crossmodal effect was at stake between these sounds and these shapes. On the
one hand, these findings add to a growing body of evidence demonstrating that audio-
visual correspondences can have perceptual (as well as decisional) effects (see Parise &
Spence, 2012; Deroy & Spence, 2013, for a review). On the other hand, the results
concerning sound-shape correspondences add a new puzzle to the list of arbitrary-
looking crossmodal matching tendencies whose origin is hard to explain.
     The puzzle is all the more general in the case of shape that the shape-sound
correspondences have a wide set of documented effects and applications. Besides the
aforementioned bias in shape perception, they are shown to facilitate language learning
(Imai et al., 2008) and to be exploited in various audio-visual mapping technologies
such as music visualization software representing sounds as shapes or sensory
substitution devices encoding shapes as sounds (see Deroy & Auvray, 2012).
     In the present paper, we suggest that the study of these multimodal associations
surrounding shape can be related to the more classical cases of crossmodal transfer of
shape between vision and touch documented as part of Molyneux’s question (part 2).
We review the dominant explanations offered to explain shape-sound correspondences,
in terms of conceptual mediation (part 3) and innate hyper-connectivity which is not
eliminated by perceptual learning (part 4), before arguing that the hypotheses of
associative learning and common neurological representations, proposed to explain the
tactile-visual Molyneux’s shape transfer can also explain the shape-sound crossmodal
matchings. In conclusion, we stress that the hypotheses currently investigated for shape
matchings in touch and vision benefit from being extended to the more arbitrary-
looking cases of matchings shapes between audition and vision, thereby stressing the
multimodal dimension of shape.



1. A new Molyneux problem


     Arbitrary-looking crossmodal matchings, as they are called (Maurer & Mondloch,
2005; Spence & Deroy, 2012b), can be defined as tendencies to associate distinct
sensory features that do not obviously co-occur in experience or in the environment.
For instance, moving away from sound-shape pairings for a moment, the tendency to
pair higher-pitched sounds with brighter visual surfaces is also shown to be present in
adults (Marks, 1974) and in infants (Maurer et al., 2006). So is the tendency to match
higher frequency sounds with higher visual locations (e.g. Evans & Treisman, 2010;
Spence, 2011 for a review). These pairings occur although brighter objects and animals
do not (at least straightforwardly) emit higher pitched sounds than their darker
counterparts, and although higher pitched sounds do not regularly come from higher
locations in space. The same lack of environmental grounding holds for the
correspondence between shapes and sounds: Unless it should turn out that angular
objects give rise to sounds that are relevantly different from rounded objects when, for
example, they are explored haptically (e.g. Guzman-Martinez et al., 2012), there seems




                                            63
to be no straightforward environmental correlation between shapes and sounds of
objects either.
     Crossmodal correspondences between sounds and shapes (or between pitch,
brightness and elevation) are difficult to square with the currently popular view that
crossmodal associations need to be learned out of the natural multisensory statistics of
the environment (see Spence, 2011). Their origin therefore prompts a series of
questions. How do such crossmodal correspondences come to be present in humans and
other animals? Do they have any ecological value? Determining here whether these
sound-shape associations are innate (Ludwig et al., 2011; Maurer & Mondloch, 2005;
Maurer et al., 2012) or acquired; and in this case, determining how they are acquired
(see Martino & Marks, 1999; Spence, 2011; Walker et al., in press) raise, as we shall
see, a new Molyneux’s problem, which teaches us new lessons on the multimodal
aspect of shapes.
     The core of Molyneux’s problem, raised initially by Molyneux back in the 17th
century, in the heat of the rationalist-empiricist controversies (Locke, 1690; see also
Morgan, 1977) is still very much relevant today (e.g., Held et al., 2011, Streri, 2012).
The question is to determine whether the crossmodal matching observed between felt
and seen shapes at a very early age is acquired through exposure and associative
learning, or whether it pre-exists exposure instead. To put it in a philosophical way, the
question consists in deciding whether the crossmodal matching of shapes is a priori or
a posteriori. To put it in a psychological way: is the tactile-visual connection for shapes
innate / hardwired or acquired?
     All past and current replies to Molyneux’s problem have been framed on the basis
that the matching between tactile and visual shapes targets one and the same
environmental property (that is , shape is viewed as an objective or primary quality.
Note that Berkeley (1948) is one of the rare philosophers who seems to have accepted
that tactile shapes and visual shapes can constitute different objective properties). This
objective grounding is what gives the crossmodal matching of tactile shapes and visual
shapes a form of necessity and rationality of interest to philosophers.
     Now, necessity, rationality and objectivity are what become problematic when we
turn to arbitrary crossmodal matchings between sounds and shapes; as they obviously
do not target one and the same environmental feature. Certain shapes do not necessarily
go with certain sounds. For instance, associating the sound ‘Bouba’ to a rounded rather
than to an angular shape looks irrational and this association does not seem to inform
us about an objective regularity. So why would we pair sounds to shapes? Due to these
key differences, the mainstream proposals developed for the Molyneux-type of
crossmodal associations have not been thought to be relevant to address this question.
     The very fact that the crossmodal corespondences between shapes and sounds is
called arbitrary comes from the fact that scientists have had a hard time pinning down a
regular environmental correlation between the property of being of a certain shape and
the property of emitting a certain sound’s pitch. Even harder to explain are crossmodal
correspondences between shapes and flavours (Deroy & Valentin, 2011) or between
symbolic shapes and smells (Seo et al., 2010) which also do not receive a
straightforward explanation as internalised statistics of the environments. These other
matchings might deserve a separate treatment, but they stress the crux of the problem:
If shapes and the other properties are not necessarily or regularly correlated, how could
these matchings be learned by association? And if they are not learned by exposure,
how could one make sense of the fact that we have evolved to have hard-wired or a




                                            64
priori connections between the representations of shapes and these apparently
unrelated properties in our mind / brain?
     The competing options that have been recently proposed to explain arbitrary
crossmodal matchings between shapes and sounds, as we shall see below, recycle the
ones that were once proposed for Molyneux’s case but that were subsequently rejected.
On the one hand, the idea, initially proposed for Molyneux’s cases (see Locke, 1690; or
Morgan, 1977 for a review) is that matchings across sensory modalities take place
through an association made at the level of ideas or concepts and, on the other hand,
the idea, that they are fully present at birth (i.e. that they are a priori, see Kant, 1998).



2. Sounds-shapes correspondences as conceptually mediated


     The idea that crossmodal matchings require a conceptual mediation is very much
the way Molyneux’s cases were discussed at the times of Locke and Berkeley, when
the connection was supposed to be established between the ‘idea’ of shape prompted by
vision and the ‘idea’ of shape prompted by touch. The idea of a conceptual mediation is
however no longer considered appropriate in order to explain early crossmodal
matchings of visual and tactile shapes. However, in the case of arbitrary crossmodal
matchings, this hypothesis is pursued by a growing number of researchers:
Correspondences between pitch, brightness, and angularity, for instance, have been
explained by the cognitive capacity that observers have to represent various sensory
features, or dimensions, on a common scale (Martino & Marks, 1999; Walker et al.,
2012), to metaphorically map one conceptual domain onto another (Shen, 1997 ; Shen
& Eisemann, 2008; Williams, 1976) or to reason analogically (Premack & Premack,
2003; see also Deroy & Spence, 2013; Spence, 2011, for a discussion). Now, the main
problem for these conceptual solutions comes from explaining the presence of
crossmodal matchings at a very early age (e.g., as early as 4 months, for shapes and
sounds, see Orztuck et al., 2012) and the difference between the neurological
activations noticed for crossmodal correspondences and semantic or analogical
reasoning (Sadhigani et al., 2009).



3. Shape-sound correspondences as remnants of non-functional innate connections


     The nativist idea that crossmodal matchings could be present from birth has been
eliminated – at least in the case of non-arbitrary matchings –for a long time in favour of
the less radically nativist claim that they come from innate learning mechanisms guided
by amodal or redundant representations of time, space, and intensity in the brain (see
Bahrick & Lickiter, 2012, for a review). The strong nativist option is however still very
much present when it comes to explaining arbitrary crossmodal correspondences as
shown by the growing popularity of what is called the ‘neonatal synaesthesia
hypothesis’ (see Maurer et al., 2012, for a review). The idea here is that these
correspondences come from a lack of differentiation of the infant’s perceptual
apparatus, and persist into adulthood due to of a lack of pruning or inhibitory feedback




                                             65
of some of these non-functional connections (Maurer & Mondloch, 1995; Maurer et al.,
2012).
     Now, there are good reasons not to go back to strong nativist hypotheses, even to
explain the arbitrary crossmodal matchings evidenced in infants. The putative
functional role of arbitrary crossmodal matchings as coupling priors in multisensory
learning (Ernst, 2007; Spence, 2011) and multisensory integration (Parise & Spence,
2012), or as a kind of crossmodal Gestalt grouping principle (namely, a kind of
crossmodal grouping by similarity; see Spence, submitted), together with neurological
differences (Sadaghiani et al., 2009; Spence & Parise, in press), are sufficient to
distinguish them from non-functional associations that can exist in synaesthetes (no
matter whether they are adults or children; see Ward, 2012). This adds to the fact that
nativist explanations in general are now hard to support in face of the demand that
innate traits are traced back to their genetic encoding (a demand which is not easy to
meet for most nativist hypotheses, see Lewkowicz, 2011).




4. Updating the associative learning and common coding hypotheses to explain
     sound-shape correspondences.

     In this section, we want to argue that the alternative to explain arbitrary
crossmodal correspondences either by late conceptual mediation or as being innate is
wrongly limited. A first step here consists in stressing that explanations in terms of
statistical learning and/or common neural coding have been too swiftly excluded.
     The assumption that pairings – between, for instance, angularity and high-pitched
sounds – are not regularly experienced by infants is more of an ungrounded assumption.
It rather appears to be the default conclusion once one cannot come up with a plausible
environmental source for the correlation. It should be more thoroughly investigated by
taking into account precise measurements of exposure. Audiovisual correspondences
between shapes and sounds might also come from a specific domain, namely speech.
The mouth movements observed when someone utters speech sounds like ‘Takete’ or
‘wee’ are more stretched (angular / narrow) than the wider rounded movements
observed when one utters ‘Maluma‘ or ‘woo’; suggesting a regular correlation between
pitch and shapes. This restores the plausibility of an associative learning account,
especially compatible with the idea that infants are particularly attentive to face / voice
or mouth / sounds in the first months of their life (see the perceptual narrowing
hypothesis, Lewkowicz, 2002).
     The second assumption that the neurological representation of visual brightness
and auditory pitch cannot have anything in common also appears to rely on a
predetermined view of what the legitimate common amodal representations in the brain
are (i.e., space, time – plus or minus number / magnitude and quantity / intensity; see
Marks, 1978). This assumption does not consider other possibilities which are getting
investigated in recent work in cognitive neurosciences, that movement (Held et al.,
2011) and embodiment could act as common sensibles (note that movement was
considered as such by Aristole and Locke).
     Once related to speech, the correspondences between sound and shape can also be
explained not merely in terms of audiovisual associations, but also in terms of audio-
motor associations, linking the sounds that one hears to the automatic articulatory




                                            66
movements generated when listening to speech (Galantucci, Fowler, & Turvey, 2006).
If the latter account were to be correct, this crossmodal correspondence would then
become embodied (Pezzulo et al., 2011), grounded in sensorimotor associations, rather
than being based on an external association between two sensory experiences, whose
resemblance would be processed in an amodal manner.
      One way to distinguish between the statistical and embodied accounts here would
be to test whether this correspondence exists only in cases or in species where the
vocalising follows the takete-sharp mouth movements rule. Note that this can be
contrasted with the correspondence between the sound-size of the source which can be
found across species, independently of their rules of vocalization (see Ludwig et al.,
2011).
      It will further be interesting to determine whether the sound-shape and sound-size
crossmodal correspondences are related, and whether the latter has multiple origins
(perhaps originating both in external and embodied underlying factors). Understanding
the role of embodied vs. external associations would certainly help to link Sweeny et
al.'s (2012) results to others showing that the shapes we see - and respond to - can also
influence the pitch (or fundamental frequency) of the speech sounds we utter (Parise &
Pavani, 2011) or that making a mouth movement (consistent with ‘ba’ or ‘da’) can give
rise to a McGurk effect (McGurk & MacDonald, 1976) when listening to
speech sounds, just as when actually viewing someone else’s mouth movements
uttering those sounds (see Sams, Mottonen, & Sihvonen, 2005).



5. Conclusion


     In this concluding section, we want to insist on the importance of focusing on
shape-sound correspondences when thinking about shapes, especially in a multi-
disciplinary approach. From a global / philosophical perspective, these
correspondences encourage a broadening of the investigation of Molyneux’s problem,
initially focused on tactile and visual shapes, to more contingent associations which can
come to matter as much for linguistic and perceptual behaviour. Interestingly, the
associative and commonality hypotheses framed here to account for correspondences
between shapes and auditory attributes are also at the moment pursued for ‘non-
arbitrary’ matchings of visual and tactile shapes, raising important questions as to how
these two shapes might interact, and how situations of single vs. distinct properties can
come to differ.
     From a more specific and empirical perspective, crossmodal correspondences
between shapes and sounds have a role in language acquisition and linguistic intuitions
(Imai et al., 2008). They can also explain the use of crossmodal adjectives to talk about
sounds (e.g., sharp sounds). But mostly, as we want to highlight, they show all their
importance when thinking about the optimization of auditory-visual translations, be it
the ‘auditory’ translation of visual shapes – as in sensory substitution through devices
which aim at compensating the loss of sight through a coding / decoding device, such
as the vOICe (Meijer, 1992) or the Vibe (Hanneton et al., 2010; see also Auvray &
Myin, 2009, for a review) or the visual translation of sounds; for instance as in musical
composition software.




                                           67
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