Dual coding of knowledge in the human brain
Authors: Yanchao Bi
Journal: Trends in Cognitive Sciences (2021)

Neural systems have been assumed to represent knowledge through the embodiment of sensory experiences. This type of knowledge representation is considered fundamentally different from the artificial intelligence approach, which implements knowledge representation using the 'symbolic' approach.Recent behavioral and neuroimaging studies of color knowledge (and other visual knowledge) with and without visual experience provide empirical evidence for the neural coding of non-sensory, language-derived knowledge, along with the sensory-derived representation, in different brain systems.The proposed dual-coding neural framework motivated by these recent empirical findings offers nature's solution to the classical challenges of knowledge representation associated with either only the symbolic tradition in artificial intelligence or only the embodied approach in neuroscience. How does the human brain code knowledge about the world? While disciplines such as artificial intelligence represent world knowledge based on human language, neurocognitive models of knowledge have been dominated by sensory embodiment, in which knowledge is derived from sensory/motor experience and supported by high-level sensory/motor and association cortices. The neural correlates of an alternative disembodied symbolic system had previously been difficult to establish. A recent line of studies exploring knowledge about visual properties, such as color, in visually deprived individuals converge to provide positive, compelling evidence for non-sensory, language-derived, knowledge representation in dorsal anterior temporal lobe and extended language network, in addition to the sensory-derived representations, leading to a sketch of a dual-coding knowledge neural framework. How does the human brain code knowledge about the world? While disciplines such as artificial intelligence represent world knowledge based on human language, neurocognitive models of knowledge have been dominated by sensory embodiment, in which knowledge is derived from sensory/motor experience and supported by high-level sensory/motor and association cortices. The neural correlates of an alternative disembodied symbolic system had previously been difficult to establish. A recent line of studies exploring knowledge about visual properties, such as color, in visually deprived individuals converge to provide positive, compelling evidence for non-sensory, language-derived, knowledge representation in dorsal anterior temporal lobe and extended language network, in addition to the sensory-derived representations, leading to a sketch of a dual-coding knowledge neural framework. (A human to an android): There was a thought experiment they gave us. It's called 'Mary in a Black and White room.' Mary is a scientist, and her special subject is color. She knows everything there is to know about it […] But she lives in a black and white room. She was born there, raised there […] One day… Mary walks out and she sees a blue sky. And at that moment she learns something that all her studies couldn't tell her. She learns what it feels to see color. The thought experiment was to show the students the difference between a computer and a human mind. The computer is Mary in the black and white room. The human is when she walks out. – Ex Machina (a Movie, 2014), citing [1.Jackson F. Epiphenomenal qualia.Philos. Q. 1982; 32: 127-136Crossref Google Scholar]. What does it mean, to know? Is knowing supported by symbolic relations versus knowing through sensory experience (see Glossary) the difference between (current) machines and humans, as suggested in the earlier movie dialogue? The human brain stores vast amounts of knowledge about the world, such as the color of roses, the shape of the earth, the mechanisms of evolution, and the functions of a jury. Various sorts of knowledge guide a broad range of behaviors, from recognizing objects to understanding words, reasoning, and decision making. Understanding how world knowledge is represented is one of the oldest and most-debated questions in any discipline interested in the mind and brain. The discussions in every field, including philosophy, psychology, and cognitive science, center on two broad notions of representing knowledge: symbolic versus embodied (sensory/motor-experience-derived). The symbolic representation, granted by Mary in the black and white room in the thought experiment earlier [1.Jackson F. Epiphenomenal qualia.Philos. Q. 1982; 32: 127-136Crossref Google Scholar], is the prevalent way to represent knowledge in artificial intelligence. Beyond computers, similar notions have also been assumed in various cognitive (e.g., dual coding by both verbal and nonverbal [2.Paivio A. Mental Representations: A Dual Coding Approach. Oxford University Press, 1986Google Scholar,3.Paivio A. Mind and Its Evolution: A Dual Coding Theoretical Approach. Lawrence Erlbaum Associates Publishers, 2007Google Scholar]) and philosophical theories (e.g., dualism [4.Papineau D. Thinking About Consciousness. Oxford University Press, 2002Crossref Google Scholar]). Despite this popularity in some areas, neurocognitive (biological) models of knowledge implementation are more strongly influenced by the notion of 'sensory embodiment', which start with the assumptions that knowledge, at least about the physical world (e.g., the shape of an apple), is acquired through and grounded in the sensory experiences stored in the higher-order sensory/motor and association cortices [5.Binder J.R. Desai R.H. The neurobiology of semantic memory.Trends Cogn. 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The 'sensory embodiment' notion in neurocognitive models of knowledge stems from a combination of considerations: biological entities acquire information through sensory/motor interaction with the world; the neural mechanisms of these sensory/motor systems have long been identified in the brain; and numerous neuroimaging studies during over two decades have shown that retrieving knowledge activates brain regions in the territory of the corresponding sensory/motor (primary and associative) cortices. For instance, retrieving color knowledge about objects (e.g., the color of roses) induces activation close to human V4, a region critical for color perception [13.Martin A. et al.Discrete cortical regions associated with knowledge of color and knowledge of action.Science. 1995; 270: 102-105Crossref PubMed Scopus (833) Google Scholar,14.Simmons W.K. et al.A common neural substrate for perceiving and knowing about color.Neuropsychologia. 2007; 45: 2802-2810Crossref PubMed Scopus (280) Google Scholar], and processing words whose meanings have strong associations with color tends to modulate activities in this territory [15.Fernandino L. et al.Concept representation reflects multimodal abstraction: a framework for embodied semantics.Cereb. Cortex. 2016; 26: 2018-2034Crossref PubMed Scopus (125) Google Scholar]. Thus, current neurocognitive theories often assume, to various extents, that object knowledge, at least that can be perceived through sensory modalities, is grounded in and abstracted from sensory/motor experiences/systems and their connections through some set of combinational mechanisms [5.Binder J.R. Desai R.H. The neurobiology of semantic memory.Trends Cogn. Sci. 2011; 15: 527-536Abstract Full Text Full Text PDF PubMed Scopus (1067) Google Scholar, 6.Martin A. GRAPES—grounding representations in action, perception, and emotion systems: how object properties and categories are represented in the human brain.Psychon. Bull. Rev. 2016; 23: 979-990Crossref PubMed Scopus (140) Google Scholar, 7.Pulvermüller F. How neurons make meaning: Brain mechanisms for embodied and abstract-symbolic semantics.Trends Cogn. Sci. 2013; 17: 458-470Abstract Full Text Full Text PDF PubMed Scopus (305) Google Scholar, 8.Barsalou L.W. On staying grounded and avoiding quixotic dead ends.Psychon. Bull. 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It is axiomatic that humans can learn via language experience about things they have not personally experienced sensorily (e.g., someone can learn that 'roses are red' by reading/hearing language without seeing red roses), yet, quoting [16.Binder J.R. et al.Toward a brain-based componential semantic representation.Cogn. Neuropsychol. 2016; 33: 130-174Crossref PubMed Scopus (126) Google Scholar] 'In common with other embodied cognition theories, however, our model maintains that all concepts are ultimately grounded in experience, whether directly or indirectly through language that refers to experience'. While cognitive research has shown that representations abstracted away from specific sensory experience is a given for human knowledge (e.g., [17.Mahon B.Z. Caramazza A. A critical look at the embodied cognition hypothesis and a new proposal for grounding conceptual content.J. Physiol. Paris. 2008; 102: 59-70Crossref PubMed Scopus (841) Google Scholar]), it has been difficult to isolate, articulate, and identify neural correlates supporting non-sensory-derived knowledge. This is because in a typically developed brain, various sensory experiences can be evoked by even abstract concepts (e.g., justice) or objects one cannot directly sensorily perceive (e.g., quark). In cases of deprivation of input from one sensory modality, such as in congenital blindness, most knowledge about the physical properties can still be gained and represented through other sensory modalities (e.g., the shape of an apple can be obtained through touch without vision). This difficulty in cognitively excluding sensory-derived representations leads to a further mapping difficulty between brain activity and cognition. A clean test of how non-sensory knowledge is represented would involve testing what happens to information that can only be obtained through a particular modality when that sensory modality is absent. This is exactly why philosophers [1.Jackson F. Epiphenomenal qualia.Philos. Q. 1982; 32: 127-136Crossref Google Scholar,18.Locke J. An Essay Concerning Human Understanding. Oxford University Press, 1690Crossref Google Scholar] have long been fascinated by color knowledge. Color sensation is exclusively visual, driven by the wavelength of light, and for humans there are no other senses to experience it. Colors to a congenitally blind individual mirror Mary in the black and white room. What, and how, do the brains of individuals without sight represent about colors (and other visual-exclusive knowledge)? Answers to these questions are a special opportunity to isolate non-sensory knowledge in the human brain. In the following sections, I will review recent empirical findings that provided such answers and which converge to provide compelling, positive evidence for the existence of non-sensory representations, even for highly sensory-related knowledge, along with a sensory-derived form of representation. The first wave of studies empirically testing color knowledge in congenitally blind individuals started almost four decades ago. This first wave showed that congenitally blind people do have various types of knowledge about color and object color properties, as reviewed in [19.Bedny M. Saxe R. Insights into the origins of knowledge from the cognitive neuroscience of blindness.Cogn. Neuropsychol. 2012; 29: 56-84Crossref PubMed Scopus (31) Google Scholar]. Briefly, early work found that blind children use color adjectives and color names in their speech, know that color refers to perceptual qualities of objects that cannot be felt by touch, learn to name colors of common objects [20.Landau B. 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Age at onset of blindness and the development of the semantics of color names.J. Exp. Child Psychol. 1978; 25: 267-278Crossref PubMed Scopus (44) Google Scholar]. Given this work's relevance to the larger question about knowledge representation, color knowledge in the blind has recently been more rigorously investigated, with detailed benchmark comparisons, as reviewed in detail below. For detailed comparison with sighted individuals, multiple types of color words (basic and descriptive color terms) have been tested using a similarity rating task with paired color terms and multiple blind (congenital and noncongenital) and control conditions [26.Saysani A. et al.Colour envisioned: concepts of colour in the blind and sighted.Vis. Cogn. 2018; 26: 382-392Crossref Scopus (15) Google Scholar]. The color space of the blind individuals is overall strikingly similar to that of the sighted individuals (Figure 1A ), with some of the congenitally blind subjects indistinguishable from the sighted, and there was greater individual variation in the blind group. The high degree of correlation (r = 0.93) between the color term spaces of blind and sighted individuals has been replicated, however, this relationship was lower than results for action words (r = 0.99) [27.Bottini R. et al.Brain regions involved in conceptual retrieval in sighted and blind people.J. Cogn. Neurosci. 2020; 32: 1009-1025Crossref PubMed Scopus (8) Google Scholar]. Compared with the intriguing cross-modal associations that have been reported in sighted individuals (red is heavier than yellow), blind individuals as a group also have color-weight associations, but not as strongly as object-taste associations (boulder-sour, with association strength in the sighted individuals as benchmarks), suggesting that both sensory and language experience contributes to such cross-modal associations [28.Barilari M. et al.Is red heavier than yellow even for blind?.Iperception. 2018; 9: 1-4Google Scholar]. Another line of research has tested what blind individuals know about object colors. Blind and sighted individuals' knowledge about colors of common fruits and vegetables was probed using multiple behavioral tasks [29.Wang X. et al.Two forms of knowledge representations in the human brain.Neuron. 2020; 107: 383-393Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar]. The results again yielded a high degree of similarity between blind and sighted (Figure 1B) and a greater degree of individual variation in blind relative to sighted individuals. Furthermore, the correlation strengths between knowledge about color and other typical (nonvisual) sensory properties for fruits and vegetables (including shape, touch, taste), as well as general semantic relations (both continuously rated and categorically measured), were indistinguishable between the blind and sighted groups, indicating that the blind was not differently relying on associations with other sensory properties to derive color relations. Beyond color, investigation has been carried out on other visual-specific properties [30.Bedny M. et al.There's more to "sparkle" than meets the eye: knowledge of vision and light verbs among congenitally blind and sighted individuals.Cognition. 2019; 189: 105-115Crossref PubMed Scopus (15) Google Scholar,31.Bedny M. et al.Growing up blind does not change the neural bases of theory of mind.Proc. Natl. Acad. Sci. U. S. A. 2009; 106: 11312-11317Crossref PubMed Scopus (68) Google Scholar]. For vision and light verbs (e.g., peek versus feel/perceive; shimmer versus boom/roll), a group of congenitally blind individuals showed an indistinguishable knowledge space from sighted controls (by means of word similarity judgment). Multidimensional scaling analyses showed that blind individuals distinguish, like sighted individuals, verbs of light emission from verbs of sound emission (Figure 1C). This work suggests that not only color, but also other visually based concepts, can be acquired without typical visual input in congenital blindness. Over a dozen behavioral studies spanning over four decades, across multiple paradigms (generation, sorting, rating, cross-modal association) and languages (English, Italian, Mandarin Chinese) paint a highly consistent picture of color and other visual knowledge in people with complete visual deprivation. (i) Without actually sensorially experiencing color, humans can still acquire knowledge about color relational structures, in patterns that are highly similar to those of the sighted; (ii) at the group level, the similarity with sighted color knowledge tends to be lower than non-visual-specific properties (see also [32.Kim J.S. et al.Knowledge of animal appearance among sighted and blind adults.Proc. Natl. Acad. Sci. U. S. 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What is the brain basis of visual-concept knowledge acquired without vision? Recent fMRI studies of neural activity patterns in congenitally blind and sighted groups have provided important clues, as described below (Figure 2). Neural responses in sighted and congenitally blind individuals have been examined to words depicting visually dominant phenomena, including colors (e.g., 'red', 'rainbow', 'island'), words without specific sensory features (abstract concepts, e.g., 'freedom'), and words for non-visual sensorily accessible concepts (e.g., 'square', 'rain', 'cup') [35.Striem-Amit E. et al.Neural representation of visual concepts in people born blind.Nat. Commun. 2018; 9: 5250Crossref PubMed Scopus (24) Google Scholar]. A cluster in dorsal anterior temporal lobe territory (ATL), including anterior superior temporal gyrus/sulcus (STG/STS) and temporal pole (TP), showed a significant word type by group interaction, with stronger activity to 'rainbow' (or 'red') compared with 'rain' (or 'square') in the blind group ('rainbow'/'red' is not sensorily accessible but 'rain'/'square' is), but no difference in the sighted group (all sensorily accessible; Figure 2 [35.Striem-Amit E. et al.Neural representation of visual concepts in people born blind.Nat. Commun. 2018; 9: 5250Crossref PubMed Scopus (24) Google Scholar]). In another neuroimaging study using an adaptation paradigm, perceptual similarity judgments about paired color names produced repetition suppression effects in V4 for sighted individuals and STG (along with precentral cortex) for the blind group, with a peak in the anterior portion of STG [27.Bottini R. et al.Brain regions involved in conceptual retrieval in sighted and blind people.J. Cogn. Neurosci. 2020; 32: 1009-1025Crossref PubMed Scopus (8) Google Scholar] (Figure 2). These two studies convergently identified dorsal ATL to be sensitive to non-sensory-derived knowledge. The brain basis of object color knowledge [29.Wang X. et al.Two forms of knowledge representations in the human brain.Neuron. 2020; 107: 383-393Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar] has been uncovered using representational similarity analysis (RSA) [36.Kriegeskorte N. et al.Representational similarity analysis - connecting the branches of systems neuroscience.Front. Syst. Neurosci. 2008; 2: 1-28PubMed Google Scholar]. Participants listened to names of fruits and vegetables and retrieved their typical colors. The association between the neural representation similarity matrix (correlation pattern across items) and the object color behavioral judgments was computed and significant correlations were taken as evidence for neural representation of object color knowledge. Two types of regions were obtained (Figure 2): (i) in the sighted brain, but not the blind brain, a region in the posterior fusiform within the color-perception-sensitive territory (close to V4) yielded neural activity pattern associated with object color similarity; (ii) a region in the left dorsal ATL (anterior STG and TP), not sensitive to color perception in the sighted, supported object-color knowledge in both the blind and sighted groups, even after the effects of other sensory properties were controlled for. In the sighted, but not in the blind group, these two sets of brain regions are intrinsically, functionally connected [29.Wang X. et al.Two forms of knowledge representations in the human brain.Neuron. 2020; 107: 383-393Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar]. Note that the study also reported that under a more lenient threshold, object-color representation in the ventral/medial ATL was observed in the sighted and not in the blind, showing convergence with the findings that these regions are related to sensory-derived color processing (e.g., [9.Lambon Ralph M.A. et al.The neural and computational bases of semantic cognition.Nat. Rev. Neurosci. 2017; 18: 42-55Crossref PubMed Scopus (650) Google Scholar,35.Striem-Amit E. et al.Neural representation of visual concepts in people born blind.Nat. Commun. 2018; 9: 5250Crossref PubMed Scopus (24) Google Scholar]). Although only a handful of studies have addressed this question, the neural representations for visual knowledge with and without vision appear to be clear. These studies across multiple paradigms (univariate activity contrasts, adaptation, RSA) and languages (Italian, Mandarin Chinese) converge to provide positive evidence for neural coding of non-sensory knowledge representation (for at least color and other visual concepts) in dorsal ATL in the human brain, while also confirming the sensory-derived representation in the visual cortex in the sighted. Importantly, RSA revealed that such coding of color knowledge in dorsal ATL occurs not only in the blind but also the sighted brain, indicating that even when sensory-derived properties are available there are both sensory-derived and non-sensory representations. Are these findings reviewed earlier specific to color and the other specific visual concepts tested, or do they reflect a general mechanism of non-sensory knowledge coding in dorsal ATL? There has been a long tradition of studying the neural correlates supporting abstract concepts (i.e., those without specific sensory referents, such as 'justice' or 'rationality'). Brain imaging studies have consistently reported that these abstract words, compared with concrete words (e.g., 'cup'), or tasks requiring knowledge of verbal facts compared with perceptual facts, elicit stronger activation in the left ATL, centering around the dorsal-lateral aspects, along with other regions encompassing the language network [37.Fedorenko E. et al.New method for fMRI investigations of language: defining ROIs functionally in individual subjects.J. Neurophysiol. 2010; 104: 1177-1194Crossref PubMed Scopus (276) Google Scholar], including inferior frontal gyrus (IFG) and posterior middle temporal cortex (pMTG) [38.Wang J. et al.Neural representation of abstract and concrete concepts: a meta-analysis of neuroimaging studies.Hum. 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The interpretations of these findings have been controversial and researchers have debated whether these findings relate to different properties associated with abstract words, including verbal involvement [38.Wang J. et al.Neural representation of abstract and concrete concepts: a meta-analysis of neuroimaging studies.Hum. Brain Mapp. 2010; 31: 1459-1468Crossref PubMed Scopus (253) Google Scholar,42.Sabsevitz D.S. et al.Modulation of the semantic system by word imageability.Neuroimage. 2005; 27: 188-200Crossref PubMed Scopus (236) Google Scholar] and social or emotional associations [43.Zahn R. et al.Social concepts are represented in the superior anterior temporal cortex.Proc. Natl. Acad. Sci. U. S. 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This pattern might be because abstract knowledge is more richly derived from language and/or because perceptible knowledge has a network with additional sensory-derived representations across the distributed brain regions and thus the activation strength may be diluted across the network. The empirical review earlier, primarily focusing on visual knowledge in the blind and jointly considering studies of other types of broad semantic knowledge, allows us to sketch a dual-coding knowledge neural representation framework (Figure 3, Key figure ). The well-establ