The Neurobiological Mechanisms Underlying Extraordinary
Memory Skills in Savantism
Eva H.H. Albers University of Amsterdam Eva Albers Student ID: 10138677 Supervisor: mw. Drs. M. de Vries Co-assessor: dhr. Dr. P.C.M.P Koolschijn
MSc Brain and Cognitive Sciences, Cognitive Neuroscience Track Universiteit van Amsterdam
October 6th, 2013
Amount of words: 8735 (8000 – 10.000)
Abstract
Approximately ten per cent of people with Autism Spectrum Disorders (ASDs) exhibit extraordinary memory skills. This phenomenon is known as savantism. However, little is known about the neurobiological mechanisms underlying these extraordinary memory skills of autistic savants. The aim of this review is to examine what neurobiological differences can underlie enhanced memory skills in savant individuals. Three cognitive approaches that possibly can explain the emergence of enhanced memory skills in autistic savants are discussed, namely 1) repetition and preoccupation, 2) hyper-attention and sensory hyper-sensitivity and 3) Thinking in Pictures. Neurological and neurobiological evidence for either cognitive theory or hypothesis is examined. Most neurological evidence point towards the thinking in pictures hypothesis. The most plausible neurobiological mechanism underlying differences memory encoding is overexpression of consolidation proteins. Nevertheless, hyper-attention and repetition might also play a crucial but supporting role in the development of extraordinary memory skills in autistic savants.
Keywords: savantism, enhanced memory, neurobiological mechanisms,
Probably the most exemplary case of savantism is Laurence Kim Peek, who was the inspiration for the movie Rain Man. He has an extraordinary memory for city maps,
exceptional lightning calculation and calendar-calculation abilities (Peek, 1996). However, in the Diagnostic and Statistical Manual for Mental Disorders (5th ed.; DSM-5, American
Psychiatric Association, 2013) savantism is not included as a distinct diagnosis, so it is not clearly defined what the criteria for savantism are. Most definitions used in literature include the sharp contrast between exceptional cognitive abilities in one or more specific domain(s) and the lower overall cognitive level of these individuals (Bókkon, Salari, Scholkmann, Dai, & Grass, 2013; Bolte, 2004; Corrigan, Richards, Treffert, & Dager, 2012; Dawson, Mottron, & Gernsbacher, 2008; González-Garrido, Ruiz-Sandoval, Gómez-Velázquez, De Alba, & Villaseñor-Cabrera, 2002; Happé & Frith, 2009; Snyder, 2009). However, it is not clear whether ‘high abilities’ are measured relative to the general cognitive level of savants or that those high abilities should be extraordinary even for the typically developing population. For example, Murray (2010) also incorporated the possibility that savants have an average general intelligence combined with exceptional abilities and Bolte (2004) argued that an average ability relative to typically developing individuals in a specific domain can be seen as savant ability relative to the overall cognitive level of the savant individual. Finally, Mitterauer (2013) defined savants as individuals with autism who show high abilities in specific domains.
There is a spectrum of savant skills that starts with the most common form, ‘splinter skills’, and progresses through ‘talented savants’ to the extraordinarily rare ‘prodigious savants’ (Treffert, 2009). Splinter skills savants and talented savant show outstanding performance relative to their other skills. Prodigious savants show skills that are exceptional in any context, relative to typically developing individuals (Happe & Frith, 2009). Savant
skills often occur in a narrow range of five domains: music, art, calendar calculating, mathematics, mechanical, and spatial skills (Treffert, 2000). Besides those domains of expertise, savantism is also associated with synaesthesia (Johnson, Allison, & Baron-Cohen, 2011), which refers to the phenomenon that a stimulus in one sensory domain leads to an automatic, involuntary experience of a percept in another sensory domain (Cytowic & Eagleman, 2009). Together with those five domains of specialties, prodigious memory is commonly present among all savants (Treffert, 2009). However, what underlies this prodigious memory remains unclear.
Individuals with savantism seem to remember information ‘instantaneously’, like city maps and phonebooks, and they also seem less susceptible to ‘forget’ this acquired factual knowledge. These characteristics of prodigious memory seem qualitatively different
compared to typical learning and forgetting. As first described by Ebbinghaus (1885/1964), typically developing individuals needed several presentations with stimuli before those items were remembered. Ebbinghaus (1885/1964) found that the amount of words that can be recalled increased after each subsequent presentation. Also, he found that the longer the time was between learning and recalling, the more items were forgotten. Only a minor part of the information was consolidated for long term (Ebbinghaus, 1885/1964). For these behavioral observations Donald Hebb (1949) found evidence on a neuronal level, namely that ‘neurons that fire together, wire together’. This means that neuronal connections are strengthened when those neurons are simultaneously activated multiple times. Similarly, neuronal connections that are not activated frequently are weakened over time. The neurochemical mechanisms underlying synaptic strengthening and weakening were later discovered by Lømo (1966). Long-Term Potentiation (LTP) reflects an increased receptor density and susceptibility for
decrease in receptor density leading to less susceptibility for synaptic transmission. The prodigious memory skills of savants do not seem to follow these aforementioned principles of learning and forgetting. This raises the question whether there might be different neuronal and neurochemical processes involved in the memory skills of savant individuals.
Savantism is associated with autistic spectrum disorders (ASDs). In the Diagnostic and
Statistical Manual for Mental Disorders (5th ed.; DSM-5, American Psychiatric Association,
2013) some savant skills, like calendar and lightning calculation, are mentioned as possibly co-occurring with autism spectrum disorders (ASDs). Autism spectrum disorders are
characterized by a triad of behavioral symptoms, namely restrictive and repetitive behaviors and interests (RRBI’s), social impairments, and deficits in language development (DSM-5, American Psychiatric Association, 2013). Moreover, the prevalence of savantism among individuals with ASDs is more than 60 times higher than among other cognitively disabled individuals. In general it is accepted that approximately ten per cent (Rimland, 1978; DSM-5, American Psychiatric Association, 2013) of the individuals with autism has savant abilities, although even higher numbers have been found (Howlin, Goode, Hutton, & Rutter, 2009). Ten per cent is a relatively high prevalence, compared to only 0.14 per cent of other
cognitively disabled individuals that show savant skills (Saloviita, Ruusila, & Ruusila, 2000). The high prevalence of savantism among ASDs could point to common underlying factors that predispose individuals to develop both savant skills and ASDs or that some characteristics of the ASD phenotype predispose to development of savant skills.
The goal of this review is to examine what cognitive theory related to autism can explain memory skills in autistic savants best and whether is neurological, neuroanatomical and neurobiological evidence for this cognitive theory regarding memory differences in autistic
savants. If there appear to be neurobiological differences between savantism and typically developing individuals, this might contribute to the diagnostic criteria of savantism.
Cognitive theories
In the following section, three theories are discussed that can explain the development of savantism within ASDs. Firstly, preoccupation and repetition within a narrow field of interest can result in extraordinary skills due to extensive practice in that specific field (Happé & Frith, 2009). Secondly, the hyper-attention and sensory hyper-sensitivity theory (Baron-Cohen, Ashwin, Ashwin, Tavassoli, & Chakrabarti, 2009) posed that attention to detail and urge to systemize, rather than excellent memory, is the key to savant abilities. Finally, the Thinking in Pictures hypothesis (Bókkon et al., 2013; Kunda & Goel, 2011) claimed that savant abilities depend on preferred visual information processing and enhanced mechanisms to store visual information in individuals with ASDs.
Preoccupation and repetition
Extensive practice, known as the 10.000 hours rule, can lead to extraordinary skills in a specific field of interest, for example in sports (Weinberg & Gould, 2011) or taxi drivers in London (Woollett, Spiers, & Maguire, 2009). Repetitive behavior and restricted interests are part of the autistic phenotype (DSM-5, American Psychiatric Association, 2013). A narrow field of interest and obsessive preoccupation with specific subjects can lead to extensive practice in that field, so this type of behavior can result in extraordinary memory skills (Happé & Frith, 2009). Similarly, autistic-like traits in general and especially restricted and repetitive
developing children (Happé & Vital, 2009). However, whether the RRBI’s are the cause for emergence of savant skills remains unclear and savant skills can also emerge without the presence of RRBI’s, for example in frontotemporal dementia patients (Miller et al., 1998). Thus, preoccupation and repetition could benefit the development of extraordinary memory skills in savant individuals, but it does not seem to be a complete explanation.
Hyper-attention, hyper-sensitivity and low-level processing
Another explanation for enhanced memory skills can be found in differences in a precursor for memory: attention. Attention has to be directed to stimuli in order for
information to enter conscious perception. This can happen either via bottom-up automatic attention or via voluntary top-down attention (Baars, 2007). Bottom-up attention is guided by salience of the stimuli (Baars, 2007), whereas top-down attention can be directed by the Central Executive and is strongly influenced by cognition, like expectations (Baddeley, 2003).
Compared to typically developing individuals, people with ASDs store sensory
information more veridically (O’Connor & Hermelin, 1988). Furthermore, autistic individuals show enhanced detection of sensory differences (Baron-Cohen et al., 2009). The veridical storage of sensory information and enhanced sensory sensitivity indicate that they are more sensitive for low-level perceptual information, in contrast to top-down information. This increased sensory sensitivity can lead to attention to detail (Baron-Cohen et al., 2009). Typically developing individuals tend to generalize perceptually similar information, due to top-down driven processes, like expectation and prior experience. However, the hyper-sensitive autistic individual views this perceptually similar information not as similar at all, due to the lack of top-down generalization. Therefore, an increased attention to detail emerges.
Enhanced attention to detail in information seems to play an important role in the
development of savant skills (Baron-Cohen et al., 2009; Happé & Vital, 2009; Mottron et al., 2013). Enhanced attention to detail enables detection of regularities and the rule-based structure of several aforementioned domains makes it possible for savants to excel in those domains relative to typically developing individuals. The domains of knowledge in which savants excel are, among others, numerical operations, drawing in perfect perspective, musical pieces and acquisition of a foreign language (Treffert, 2009). These domains depend on law-based regularities. For example, the ability to acquire a new language is predicted by the extent to which one is able to recognize patterns (Frost, Siegelman, Narkiss, & Afek, 2013). Also, calendar calculating abilities in savants depend on structural knowledge, rather than event related knowledge, which refers to episodic memory (Heavey, Hermelin, Crane, & Pring, 2012).
However, extraordinary abilities in domains that are not rule-based, like perfect
reproductions of city maps or literal knowledge from a phonebook, do seem to rely merely on instant memory, as the unique details of a city map or phonebook cannot be deducted from law-based regularities of familiar exemplars. Additionally, hyper-systemizing is not consistent with the self-reports of some savants, who mention that they ‘see’ answers to calendar
calculations (Murray, 2010). Thus, hyper-sensitivity and enhanced attention to detail can explain part of the savant skills, namely the domains of skills that are based on pattern recognition and regularities. However, the domains of skills that seem based on immediate storage of specific, unique information cannot be explained by hyper-sensitivity and enhanced attention.
Thinking in Pictures hypothesis focusses on differences in working memory. Items can be kept in working memory by means of auditive repetition, the phonological loop, and by means of visual representations, the visuospatial sketchpad (Baddeley, 2003). Some individuals with ASDs have a preference to use visual mental representations as opposed to verbal
representations, whereas typically developing individuals use both verbal and visual representations (Kunda & Goel, 2011).
Visual representations can store information in more detail than propositional, linguistic representations can (Bókkon et al., 2013). For example, chemical structures, crossings, city maps and paintings are representations that are almost impossible to remember by means of language only (Bókkon et al., 2013). Visual representations are closely related to concrete perceptual mechanisms, whereas verbal representations are merely propositional, which is more abstract than perceptual (Kunda & Goel, 2011). Concrete reasoning is acquired earlier in development than abstract reasoning. If savants have overdeveloped concrete reasoning skills, possibly due to impaired abstract reasoning (Treffert, 2000), they might be able to recall the concrete percept of an abstract concept, like time or a number sequence and hence ‘see’ the answer, rather than calculate it (Murray, 2010).
The mechanism of seeing abstract representations as concrete visual percepts is
comparable to the phenomenology of synaesthesia (Murray, 2010). Synaesthesia can occur in different combinations of sensory modalities, but the most well-known is number-color synaesthesia. In this variant of synaesthesia, individuals see the abstract concept of a number always in a specific color, that is, a concrete visual percept. Seeing abstract concepts as concrete percepts can contribute to extraordinary skills. For example, synaesthetes show cognitive benefits in that specific domain compared to non-synaesthetes (Simner, Mayo, & Spiller, 2009). Additionally, the fact that synaesthesia co-occurs with savantism (Johnson et
al., 2011), also indicates that concrete visual thinking can play a role in the memory skills of savants.
Furthermore, cognitive dual tasks have shown that some individuals with ASDs more often use a visual modality to solve tasks that are solved verbally by typically developing individuals (Kunda & Goel, 2011). Dual tasks consist of a primary task that draws upon one modality, for example the verbal phonological loop, and a secondary task that draws upon another modality, for example the visual sketchpad. When both tasks are solved by different modalities, no interference between tasks takes place. However, when both tasks are solved by means of the same modality, performance on the primary task drops due to interference with the secondary task on the same modality. In a recent review, Kunda and Goel (2011) indeed found that individuals with ASDs tend to solve the primary task visually. Moreover, autistic individuals outperform typically developing individuals on visual tasks, which can be an indication that they are more familiar with visual thinking due to extensive practice (Kunda & Goel, 2011).
In conclusion, both individuals with ASDs (Kunda & Goel, 2011) and savant individuals (Murray, 2010) seem to have a bias towards using visual representations. The presence of visual representations with more detailed, complex information than purely linguistic
representations can explain some savant skills, like perfectly identical drawings of city maps and detailed recall of a phonebook content, but also numerical operations like lightning calculations and prime number identification. However, visual thinking alone cannot explain the extraordinary nature of memory skills. Typically developing individuals also use visual working memory for certain tasks, but they do not exhibit such extraordinary memory for visually processed information. If visual thinking alone was the key to extraordinary memory,
typically developing individuals should also show enhanced memory in tasks that are solved by means of visual thinking.
Neurological and neuroanatomical evidence
Several cognitive theories attempt to explain savant skills. Some stress differences in attention and early sensory processing (Baron-Cohen et al., 2009; Happé & Vital, 2009; Mottron et al., 2013), while others focus on differences in encoding and memory, namely a visual way of information storage (Kunda & Goel, 2011; Murray, 2010). This distinction between attentional processes and memory encoding will be leading structure in discussion of the neurological and neurobiological evidence. Both attention and visuospatial memory are processed in the non-dominant hemisphere (Kandel, Schwartz & Jessell, 2000), the parietal cortex (Corbetta, Miezin, Dobmeyer, Shulman, & Petersen, 1991; Kandel et al., 2000), and in the early visual areas. So regardless of which cognitive theory explains savantism best, neurological differences in structure or function are expected in the non-hemisphere, parietal cortex, and the primary visual areas of savant individuals. Contrary, if differences in memory are the ground for extraordinary memory skills in savant, neurological difference are expected in parts of limbic system that are involved in memory, that is, hippocampus and amygdala (Gluck, Mercado & Myers, 2008). If solely attentional processes enable savant memory, these regions should not be altered.
Right hemisphere dominance
Activity in the right hemisphere is associated with visual imagery tasks and numerical skills (Gardini et al., 2011). The domains of savant skills are mainly visual and numerical (Treffert, 2009). Therefore, left hemisphere dysfunction and right hemisphere compensatory
dominance might underlie the development of savant skills (Hou et al., 2000). It is indeed found that savants have an increased right hemisphere volume (González-Garrido et al., 2002; Corrigan et al., 2012) and increased blood flow in the right hemisphere (González-Garrido et al., 2002) compared to the left hemisphere. Furthermore, in an experimental study Snyder (2009) showed that low frequency repetitive transcranial magnetic stimulation (rTMS) on the left anterior temporal lobe (lATL) can induce savant-like skills in typically developing individuals. The lATL is involved in pattern search, matching these patterns to prior experience and computing meaning for this information (Wolford, Miller, & Gazzaniga, 2000). Disruption of this area leads to a decreased bias of prior expectation in information processing. Information is processed with less symbolic meaning and more alike its actual presence (Snyder, 2009). Thus, right hemisphere dominance seems essential for further processing of the primary detailed visual information that is integrated in the early visual areas (Bókkon et al., 2013). However, the effects of left hemisphere suppression were rather small, the improvement was not ubiquitous, and only present in a subset of savant skills, namely numeric and drawing skills (Snyder, 2009), so right hemisphere dominance only is not sufficient to explain savant skills.
Parietal and visual cortex
The parietal cortex plays an important role in processing attention (Corbetta et al., 1991). The superior parietal cortex of savant individual is thicker compared to controls (Wallace, Happé & Giedd, 2009) and increased perfusion in right parietal area is found (González-Garrido et al., 2002). However, pronounced activity in the parietal cortex is also related to aberrant sensory integration, which can lead to percepts that originate from different modalities, as is the case with synaesthesia (Jänke & Langer, 2011). Therefore, increased
activity in the parietal regions can point to differences in attention, but can also be a sign of seeing abstract concepts as concrete percepts.
Enhanced perceptual functioning and sensory hyper-sensitivity in autistic individuals can explain emergence and accessibility of low-level visual information (Baron-Cohen et al., 2009). Differences in primary visual areas are found (Neumann et al., 2010; Neumann et al., 2011). Savant individuals show activity in the primary visual cortex during a discrimination task, whereas matched controls exhibit activity outside the visual cortex (Neumann et al., 2011). So these differences do not support the enhanced perceptual functioning theory, but seem evidence for the notion that savants tend to think in visually. Furthermore, Neumann et al. (2010) found that the right visual areas of autistic savants were activated by word
recognition, whereas typically developing individuals showed left parietal activation. Both results indicate merely the presence of a visual working memory in savants, rather than enhanced perceptual skills.
Limbic system
Neuroanatomical regions that can sufficiently discriminate between the attentional and memory processes are the regions that are solely related to memory. Fiber tract bundle
volumes in the amygdala, hippocampus, frontal lobe, and occipital lobe are found to be larger in the right hemisphere of savants (Corrigan et al., 2012). Moreover, the right amygdala and caudate of a savant subject are found to be larger than in the left hemisphere and in normative adults (Corrigan et al., 2012). The caudate is involved in implicit or habit learning (Packard & Knowlton, 2002) and the amygdala regulates which pathway is used (Packard, 2009), so enlargement of these regions can lead to enhanced habit learning. These differences in memory pathways of savants indicate alterations in the way savant individuals processes and remember information.
Neuronal and neurochemical evidence
In order to further examine which cognitive theory explains the origins of savant memory skills best, some neurobiological mechanisms related to on one hand attention and on the other hand memory consolidation are discussed. Alterations in serotonin, an important neurotransmitter for attention (Kandel et al., 2000), memory consolidation proteins and a different mechanism for storage of visual information could reveal which of the possible mechanisms underlying extraordinary memory in prodigious savants is most plausible.
Serotonin
Serotonin is involved in the regulation of attention (Kandel et al., 2000). Altered levels of serotonin (Kinast, Peeters, Kolk, Schubert, & Homberg, 2013) and the serotonin precursor tryptophan (Boccuto et al., 2013) are found in individuals with ASDs. However, these differences in serotonin are related to the social impairments that are part of the autistic phenotype (Kinast et al., 2013) and decreased tryptophan levels are associated with attention-deficit/hyperactivity disorder (ADHD). Hence, there is no evidence that altered serotonin and tryptophan levels could result in enhanced attention.
Protein synthesis
Altered synaptic plasticity related protein synthesis could explain extraordinary memory skills in savantism and the large variability in development of savant abilities, both among individuals with ASDs and among other clinical conditions. Deficiencies at different stages within the protein synthesis cascade can lead to a variety of outcomes, among which overexpression of consolidation proteins (Kelleher & Bear, 2008). Overexpression of
memory skills among ASDs individuals. Overexpression of consolidation proteins is only the result of specific, but possibly different combinations of deviant protein synthesis within the cascade. Alterations in the specific genes that code for these proteins are found in individuals with ASDs (Spooren, Lindemann, Ghosh, & Santarelli, 2013). This could explain the
relatively high prevalence of savantism among autism. Furthermore, the specificity in protein alterations that is necessary for overexpression of consolidation proteins implies that only a subset of protein synthesis alterations leads to increased availability of stabilization proteins (Kelleher & Bear, 2008). This can explain why only ten per cent of individuals with ASDs exhibit savant memory.
Firstly, short term encoding of information, known as early phase long term potentiation or depression (LTP/D), does not require protein synthesis. Contrary, long term storage of information, which is known as late phase LTP/D, involves gene expression (Kandel et al., 2000). The translational machinery that synthesizes the proteins necessary for consolidation consists of activation of the glutamatergic N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (Kandel et al., 2000), extracellular signal-regulated kinases (ERK), and mammalian target of rapamycin (mTOR) signaling pathways (Kelleher & Bear, 2008). These ERK and mTOR pathways are down regulated by among others, the fragile X mental retardation protein (FMRP) and the protein products of the PTEN gene, TSC1, and TSC2 genes. These genes and proteins are associated with autism (Spooren et al., 2012). If these negative regulators are genetically mutated, the decreased down regulation can lead to over activation of protein transcription. The
synthesized proteins facilitate consolidation of information. In order to stabilize newly formed synapses, those proteins are directed to active synapses. In a properly functioning translational machinery the supply of proteins is limited, which leads to competition among synapses for
consolidation proteins (Kandel et al., 2000). Consequently, only a selection of newly formed synapses stabilizes. Overexpression of proteins can lead to less competition among synapses and hence, less selectivity in consolidation. This excessive protein production and the subsequent lack of competition in autistic and savant individuals can lead to immediate capture and consolidation of newly acquired memory information, regardless of its salience (Kelleher & Bear, 2008). Additionally, the imbalance in neuronal excitation and inhibition resulting from mutations in the translational machinery (Kelleher & Bear, 2008) can lead to the cognitive deficits that are associated with the autistic phenotype (Casanova et al., 2003).
There is some evidence that neurochemical alterations in autism can lead to enhanced memory. Exposure to valproic acid (VPA) mimics the phenotype of ASDs in rats (for an extensive review, see: Markram & Markram, 2010). VPA exposed animals have a better memory than control animals (Markram, Kosten, Tate, Gervasoni, & Nicolelis, 2007). If exposure to VPA mimics the autistic phenotype, enhanced learning as the result of VPA exposure can reflect the extraordinary memory skills of the autistic savant. Increased levels of NMDA receptors and plasticity are found in rats after exposure to VPA (Rinaldi, Kulangara, Antoniello, & Markram, 2007). This can lead to pronounced post-synaptic LTP and,
therefore, increased learning and memory. NMDA is involved in learning (Nicoll & Malenka, 1999), so the underlying neurochemical components of enhanced memory in savants can be NMDA. AMPA is another receptor that is known to be important for synaptic plasticity and learning (Rumpel, LeDoux, Zador, & Malinow, 2005; Kessels & Manilow, 2009). However, no effects of VPA on the AMPA-R subunits are found (Rinaldi et al., 2007).
Further proteins that are associated with consolidation are cAMP-recognition element-binding protein (CREB; Debiec, Doyere, Nader, & LeDoux, 2006) and brain-derived
found in autism (Nishimura et al., 2007), but this overexpression of BDNF still leads to learning and memory impairments (Cunha et al., 2009), rather than enhanced declarative memory capacities as Kelleher and Bear (2008) suggested. Finally, Nuytens et al. (2013) did find increased BDNF expression and CREB phosphorylation related to facilitated LTP. However, they found learning and memory impairments on behavioral tests and therefore they suggested that the facilitation of LTP due to of BDNF and phosphorylated CREB leads to saturation of learning networks (Nuytens et al., 2013).
In summary, to date there is some evidence that elevated levels of NMDA receptors can play a role in the development of extraordinary memory skills in autistic savants. However, it is not plausible that altered levels of consolidation proteins CREB and BDNF or AMPA receptor subunits have a beneficial effect on memory.
Biophotons
If visual thinking is qualitatively different than propositional thinking, there should be a distinctive mechanism of storage of information between abstract propositional information and concrete visual information. The bias towards using this visual storage mechanism can explain the memory advantages that are seen in autistic savants.
Bókkon and colleagues (2013) proposed a biophysical mechanism that could underlie storage of the detailed visual information. In contrast to the aforementioned classical synaptic transmission, the biophysical picture representation hypothesis consists of the idea that endogenous ultra-weak photon emission in retinotopic neurons represent highly detailed visual information (Bókkon et al., 2013). Living cells spontaneously emit weak light, which is a process known as biophoton emission (Kobayashi et al., 2012). Biophoton emission is correlated with neuronal metabolism (Kobayashi et al., 2012) and associated with cognitive
processes (Dotta, Saroka, & Persinger, 2012). Bókkon et al. (2013) proposed that the specificity of biophoton emission patterns represents detailed visual information.
In addition to classical interneuronal synaptic transmission (Kandel et al., 2000), epigenetic changes that take place within neurons also attribute to memory processes and plasticity (Arshavsky, 2006). According to the biophysical picture representation hypothesis, visual information is stored in long-term memory by means of changes in DNA molecules that are paired with specific patterns of photon emission (Bókkon et al., 2013). With object recognition the same molecular activity pattern and subsequent photon emission is activated and hence, a specific visual memory is activated. This way of information storage enables visual information, as opposed to linguistic or propositional representations, to be stored in greater detail than with classical synaptic transmission only (Bókkon et al., 2013).
Additionally, Dotta et al. (2012) found that biophoton emission during visual imagery was increased in the right but not the left hemisphere. This either implies that biophoton emission can be a storage medium for highly detailed visual information which is processed in the right but not the left hemisphere (Bókkon et al., 2013) or that there is metabolism in the right hemisphere during visual imagery, since biophoton emission is related to metabolism (Kobayashi et al., 2012).
In summary, the biophysical picture representation hypothesis might explain
extraordinary memory skills in savantism, because firstly, if autistic savants store information by means of biophotons, this hypothesis explains that savants can, for example, draw city maps from memory in greater detail than typically developing individuals can (Bókkon et al., 2013; Treffert, 2000). Secondly, the biophoton emission stores visual representations rather than propositional linguistic information (Bókkon et al., 2013). This is in accordance with the
(Kunda & Goel, 2011). Finally, biophotons are observed in the right, but not the left
hemisphere during visual imagery (Dotta et al., 2012). The right hemisphere dominance that is observed in savants (Treffert, 2009; Corrigan et al., 2012), may enable this detailed
epigenetically encoded visual information to be processed further and induce conscious experience of the visual information (Bókkon et al., 2013).
Astrocytes
Astrocytal mega-domains can also reflect a neuronal mechanism underlying memory skills in savantism. Astrocytal processes are indirectly related to white matter volume
(Mitterauer, 2013) and there is evidence that acquiring new skills is related to increased white matter volume in the associated brain region (Fields, 2010). Therefore, increased
computational complexity in astrocyte mega-domains can lead to enhanced memory skills. Astrocytes are glial cells that support neurons with nutrition, ionic balance,
neurotransmitter uptake, and play a role in synaptic information transmission (Kandel et al., 2000). Individual astrocytes have connections to m-neurons with n-synapses (Mitterauer, 2013). Each astrocyte connection to a synapse transmits signals by means of a unique
combination of receptor qualities. The amount of possible connections exponentially follows the amount of receptor qualities, so the amount of possible processes rapidly increases with the amount of receptor qualities. The transition from five to six receptor qualities already enables 203 processes. This amount of receptor qualities and up can be defined as mega-domain (Mitterauer, 2013). Astrocyte-astrocyte connections, so called gap-junctions, are activated by neuronal activation and activation of these pathways may represent memory traces. If there are more connections, larger computational complexity can be exhibited.
Notably, the astrocytal mega-domain organization, containing increased amount of processes and related connections to synapses, requires more synaptic space than typical organization. As a result of the extended synaptic space, the neuronal connectivity could become atypical (Mitterauer, 2013). Since atypical neuronal connectivity can lead to cognitive impairment (Ecker et al., 2012), this can also explain the cognitive deficits that are seen in autistic savants (Treffert, 2009).
Thus, the astrocytal mega-domain hypothesis explains the apparent contradiction between cognitive impairments and the ‘islands of genius’ that is seen in savants. However, the
hypothesized causal relationship between astrocytal mega-domains and subsequent atypical neuronal connectivity does not hold for the majority of individuals with autism that do not possess savant memory skills. If aberrant astrocytal organization leads to atypical neuronal connectivity and the astrocytal mega-domains enable savant skills, the prevalence of savant skills among autistic individuals should be higher than approximately ten per cent.
Conclusions and discussion
The goal of this paper was to examine based on three cognitive approaches what
neurobiological mechanisms could underlie extraordinary memory skills in autistic savants. The first stated that the traits preoccupation and repetition, which are part of autistic
phenotype, can lead to extensive practice in a specific field of interest and hence to the development of excellence in this specific field (Happé & Vital, 2009). Practice is associated with implicit habitual memory (Gluck, Mercado & Myers, 2008). The amygdala (Packard, 2009) and the caudate (Packard & Knowlton, 2002) are involved in regulation of habitual
individuals (Corrigan et al., 2012). This indicates involvement of habitual memory in savant individuals, possibly due to excessive repetition. Implicit, or habitual, memory is not
consciously accessible, whereas explicit memory is consciously accessible (Baars, 2007). If savants would have a bias towards implicit learning, this could explain why savant individuals often report that they do not know how they came to their answers to numerical operations (Murray, 2010). Thus, repetitive behavior and restricted interests can be related to changes in habitual memory pathways and consequently to enhanced implicit memory. However, since savantism also occurs in patients without repetitive and restricted behaviors and interests (Miller et al., 1998) and not all individuals with repetitive and restricted behaviors and interests exhibit savant memory, it is not likely that savant memory emerges solely from extensive practice.
The second cognitive theory focusses on hyper-attention and enhanced perceptual functioning (Baron-Cohen et al., 2009). Enhanced attention to detail and perceptual functioning can explain how information gets in. That is, the difference in attentional
processes compared to typically developing individuals can be the precursor for extraordinary memory in savants. If information comes in veridically, this enables further processing and storage of this detailed information. Snyder (2009) has shown that absence of left hemisphere top-down inhibition can induce savant-like skills, so on a neurological level there is some evidence that differences in perceptual functioning can enhance memory. However, the remainder of the neurological evidence is inconclusive regarding the distinction between on one hand enhanced perceptual and attentional processes and on the other hand different visual storage of memory. Increased volume of the right-hemisphere (González-Garrido et al., 2002) and differences in perfusion (González-Garrido et al., 2002) and neuronal volume (Wallace et al., 2009) of the parietal cortex could point to both enhanced perceptual functioning and to
visual thinking. Some differences in serotonin levels (Kinast et al., 2013) and serotonin precursor levels (Boccuto et al., 2013) are found in individuals with ASDs, but there is no evidence that these alterations in serotonin can lead to enhanced memory. Thus, the
neurochemical evidence for differences in enhanced attentional and perceptual processes is scarce.
Thirdly, the Thinking in Pictures hypothesis can explain how detailed information is stored in savants (Kunda & Goel, 2011). Visual representations can store information in more detail than propositional representations can (Bókkon et al., 2013). Savants tend to solve cognitive problems in a visual way, rather than linguistically (Kunda & Goel, 2011). Increased activity in visual areas indicates the use of visual strategies to solve linguistic problems (Neumann et al., 2010). Also, the right hemisphere is important for visual imagery (Gardini et al., 2011), so right hemisphere dominance can enable increased visual processing. Thus, on a neurological level there is evidence for Thinking in Pictures as mechanism
underlying savant memory. On a neurochemical level there are three possible mechanisms underlying the storage of highly detailed information, that is, extraordinary memory. In a rat model of ASDs, increased levels of NMDA receptors are found (Rinaldi et al., 2007), which could underlie enhanced memory consolidation. However, altered levels of two other memory consolidation proteins, CREB and BDNF, are not associated with increased memory
capacities. Alternatively, the storage of visual information, contrary to propositional
information, can be enabled by activation of specific patterns of biophoton emission (Bókkon et al., 2013). Finally, alterations in organizational structure of astrocytes can encounter for the increased computational capacities that are associated with savant memory (Mitterauer, 2013). Overexpression of consolidation proteins is the most plausible neurobiological mechanism
memory that savants exhibit. Furthermore, it explains the high prevalence of savantism among individuals with ASDs, because the genes related to protein synthesis (Kelleher & Bear, 2008) are also genes related to autism (Spooren et al., 2012). Aberrant protein synthesis also
explains the selectivity of savantism among ASDs, because only specific combinations of protein alterations leads to overexpression of consolidation proteins. Moreover, this
hypothesis can account for the cognitive deficits that are part of the savant phenotype. These cognitive deficits can result from an imbalance between inhibitory excitatory processes. Last but not least, there is evidence for NMDA-mediated enhanced memory in a rat model of autism (Rinaldi et al., 2007).
In contrast, biophoton emission and astrocytal organization are purely hypothetical, that is, there is direct nor indirect experimental evidence relating biophoton emission or aberrant astrocytal organization to ASDs or savantism. Although biophoton emission nicely relates to the preference for visual thinking among savants and thereby fits with the domains of skills that are often seen in savantism, it does not explain the extraordinary nature of savant
memory. If biophoton emission is the storage mechanism underlying visual thinking, typically developing individuals should also show enhanced memory in visual tasks. Furthermore, biophoton emission does not explain why not all individuals with ASDs, that have a preference for visual thinking, show savant memory. Finally, biophoton emission cannot account for the cognitive deficits that are seen among savant individuals.
Astrocytal mega-domains can provide an explanation for the sharp contrast between cognitive disabilities and enhanced computational capacity in savantism. The increased space necessary for astrocytal connections is at expense of typical neuronal organization, which can lead to cognitive disabilities (Mitterauer, 2013). The astrocytal mega-domain hypothesis does explain the high prevalence of savantism among individuals with ASDs, but if differences in
astrocytal organization underlie the cognitive disabilities seen in ASDs, all autistic individuals should show increased computational capacity and, hence, some sort of savant skills.
Therefore it is less likely that astrocytal structure or biophoton emission are the mechanisms underlying extraordinary memory in savantism.
In addition to a different mechanism of memory storage, enhanced perceptual functioning and repetition and preoccupation might play a crucial role in savant memory too. Enhanced perception and attention seem required for the detailed information to enter the nervous system in a veridical way. This detailed information enables the autistic savant to deduce patterns and regularities in the information, that might be generalized and thus missed by typically developing individuals. If this systematic abstract information is subsequently processed in parietal lobe it might be cross-modulatory stored in a visual way, alike is the case in synaesthesia, this enables highly complex chunks of systemized information to be stored. Next, the right hemisphere dominance allows the visual concrete information to be processed further, where an atypical neurobiological mechanism of storage is responsible for the
information to be encoded in great detail. Interestingly, if the deduction of law-based regularities happens in an implicit, habitual way and only becomes conscious after it is visually represented in the parietal cortex and further upstream, this can explain that savant claim to ‘see’ the answers to numerical computations in the form landscapes of numbers. Additionally, if repetition and practice underlie this implicit memory, practice indeed can play a supporting role in the development of savant skills.
In order to further investigate which of the three neurobiological memory storage mechanisms is most plausibly involved in savant memory, future research could focus on possible genetic alteration that could play a role in the predisposition to store information in
(Walsh, Morrow, & Rubenstein, 2008) and on NLGN3 and NLGN4, PTEN and SHANK3 (Geschwind, 2008), which are on one hand associated with autism symptoms (Walsh et al., 2008) and on the other hand related to the protein synthesis cascade that is important for memory consolidation (Kelleher & Bear, 2008). Alternatively, CNTNAP2 polymorphisms are associated with language delays specifically in children with autism (Vernes et al., 2008). This gene is focally expressed in neurological areas that later in development represent social and language skills (Geschwind, 2008). If abnormal expression of this gene early in
development leads to deficits in language acquisition later on, this language deficit might predispose autistic individuals to maintain a preference for visual thinking. Finally, if genes related to astrocyte organization can be associated with ASDs, this could strengthen
Mitterauer’s (2013) hypothesis. However, to date no genetic studies are done yet with specifically savant individuals, a lot of different genes and chromosomes are identified in relation to autism (Casanova et al., 2003; Coghlan et al., 2012; Geschwind, 2008; Morrow et al., 2008; Voineagu et al., 2011; Weiss, Arking, Daly, & Chakravarti, 2009), but there is little to no consensus across studies what genetic mutations predispose to ASDs. Both genetic studies as well as, for example, thorough investigation of the VPA rat model for ASDs might elucidate the possible neurobiological mechanisms underlying enhanced memory skills in savantism, thereby serving diagnostic purposes in the future.
Taken together, this paper aimed to examine which neurobiological mechanisms could underlie memory skills in savantism. On a cognitive level both hyper-attention and visual thinking seem to contribute to differences in information processing style of savant individuals that lead to enhanced memory and extraordinary skills in the five domains of expertise. Also on a neurological level, increased right hemisphere volume, pronounced parietal activity, and increased neuronal activity in the primary visual areas correspond to
differences in attentional and perceptual processes and visual working memory. Increased connectivity and volume amygdala and caudate indicate differences in habitual learning in savants (Corrigan et al., 2012) and might be related to restrictive and repetitive behaviors and interests. On a neurobiological level there is scarce evidence for attentional processes as the ground for savant memory skills. Differences in memory encoding mechanisms are more plausible, specifically aberrant protein synthesis leading to overexpression of consolidation proteins. Astrocytal mega-domains and biophoton emission as storage mechanisms underlying extraordinary memory cannot explain all facets of savantism, whereas overexpression of consolidation proteins can.
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