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George Berkeley Through History: Multimodal Perception from the 1700s to Present.

George Berkeley, British empiricist philosopher of the 1700s, embraced an idealist-monist perspective, sometimes called "immaterialism," in which he rejected the notion of matter itself and argued that reality consisted only of that which is perceived. This perspective stood in stark contrast to the "new" philosophy of materialism that explained the world using natural laws and mathematical terms. The idealist-monist perspective informed his theory of perception, in which he unified primary and secondary qualities, known as his inseparability argument (Bracken, 1974).

In Berkeley's philosophy, objects do not exist as anything other than ideas, which we define as sense-impressions or sense experiences for the purposes of this argument as others have in previous works (Armstrong, 1965; Bracken, 1974; Urmson, 1982). According to Berkeley, each of these sense-impressions depends on the relative experience of the perceiver. The principle of ontological inherence states that reality is perceiver-dependent due to the nature of sensation. This means that each person creates their own mental reality as a function of their own personal experience. Anti-abstractionism complemented ontological inherence as both notions depend upon experience (Berkeley, 1710, P. (1) 1, 7, 77-81; Berkeley, 1713, pp. 111, 139-140; Bracken, 1974). While both anti-abstractionism and ontological inherence focus on perception as a function of previous experience, anti-abstractionism focused on calling forth individual ideas while ontological inherence focused on the formation of reality as a whole.

Berkeley's extreme empiricism reflects the unifying of these concepts. In stressing the importance of secondary qualities and perception above all else, Berkeley attempts to undermine skepticism. Skepticism, to Berkeley, was full of doubts and paradoxical arguments. However, pure empiricism in which only perception matters allows the perceiver to experience the true reality, and Berkeley believes this reality to be the sight of God.

Through his Principles of Human Knowledge and Three Dialogues, Berkeley's claims lead to the conclusion that through God, it is possible to know and understand the world in its entirety, something that cannot be accomplished by skeptics (Berkeley, 1710, Introduction 1, P. 147, 154; Berkeley, 1713, pp. 192, 202). Berkeley explained that all things cannot exist without perception, but they cannot be within his mind at all times, therefore, they must be in the mind of God (Berkeley, 1713, pp. 152-153, 155-156). Berkeley believed this understanding of the world leads to clarity within the realms of science, metaphysics, and mathematics (Berkeley, 1713, pp. 203-204). For Berkeley, extreme empiricism was the root of all understanding and the only source of knowledge from which to draw was sense-impressions.

Berkeley summarized his philosophy of existence as a whole in one Phrase--"to be is to be perceived" (Berkeley, 1710, P. 6; Berkeley, 1713, pp. 152). His emphasis on sensation led to a theory of distance perception remarkably similar to our modern-day understanding of how we perceive distance.

Berkeley's theory of distance perception

Projectionist theory, the prevailing theory of distance perception in Berkeley's time, held that the brain performed continuous, unconscious mathematical calculations to visually perceive stimuli and focus attention (Wolf-Devine, 2000). Specifically, the movement of the eyes created the input on which the brain computed the distance of the object. Berkeley took a decidedly different view--he stated that distance perception emerged from the background of multimodal sensory experience. Berkeley's perceptual theory systematically refuted the mechanistic arguments of projectionism and instead examined the association of sense-impressions as they co-occur (Berkeley, 1709, P. 12, 16-20, 29, 90).

Aside from the correlations between sight and touch, a key theme in Berkeley's An Essay Towards a New Theory of Vision, he explored experiences across other sensory modalities (Berkeley, 1709, P. 1, 46). In his stagecoach example, Berkeley detailed the visual sensory experience of seeing a stagecoach approach and the resulting increase in the apparent size of the coach during that approach. He also described associating the coach with an increase in the relative volume of the coach's approach and the act of entering the coach (Berkeley, 1709, P. 46). As each of these experiences co-occur, they are associated, forming an example of the Law of Contiguity in action.

Additionally, Berkeley explained the convergent and divergent movements of the eye itself as a function of previous experience and focused attention (Berkeley, 1709, P. 16-18; Berkeley, 1710, P. 43-44). When the eyes focused on objects close to the face, they crossed at the nose, known as a convergent movement. A divergent eye movement occurred when the eyes widened while focusing on an object further from the face. Berkeley specifically mentioned the "sensation from the turn of the eyes" and paired the sensation with the experience of perceiving an object at great distance (Berkeley, 1709, P. 18).

Berkeley's theory of distance perception was revolutionary (Atherton, 1990). His insistence on the importance of the correlation of multimodal sensory experiences was not well accepted by his peers and not recognized as revolutionary in his time. In the century following Berkeley's proposal, many in the fields of psychology and physiology, such as Broca, Wernicke, Ferrier, Fritsch and Hitzig, focused on exploring the concept of functional localization. However, moving through the 1800s, 1900s, and into present-day, some scientists, as delineated in the succeeding sections, explored ideas similar to those of Berkeley and, through their explorations, built upon his work.

Early post-Berkeley thinking

Immanuel Kant (1724-1804), philosopher of the German Enlightenment, had certain ideas about knowledge that harkened back to Berkeley's work. In his A Critique of Pure Reason (1781/1787), Kant outlined the necessity for experience in the gaining of knowledge. Kant believed understanding arose through synthesis, which he defined as the interaction between experience and the innate categories of thought (Kant, 1781, pp. 78-79).

Kant's theory of knowledge incorporates aspects of both rationalism and empiricism. His categories of thought, which he consistently refers to as a priori concepts, are the framework within which experiences are purely understood (Kant, 1787, pp. 106-108). Despite the fact that an innate process runs contrary to Berkeley's theory, there are facets of Kant's perspective that relate directly to Berkeley, specifically in terms of Kant's notions of the subjective nature of experience and the necessity of experience in the accumulation of knowledge.

Kant's version of subjective experience stated that self-consciousness, which he refers to as the empirical unity of apperception, also relied on the experiences and associations of the perceiver (Kant, 1781, pp. 105). Additionally, Kant believed that the manner in which we know things requires perception, which is inherently perceiver-dependent (Kant, 1787, pp. 219, 265-266, 272-274). Kant was an anti-abstractionist in certain regards as well. When referring to his innate categories of thought, Kant believed that the categories only functioned in conjunction with actual experiences and did not grant insight independent of these experiences (Kant, 1781, pp. xiii-xvi, 78-79, 157; Kant, 1787, pp. 145).

For Kant, experience was a necessity for creating knowledge and the innate categories served as the mechanisms to act on experiences and build our understanding (Kant, 1787, pp. 104-105). In a similar manner, the relative experiences of the perceiver were a key for Berkeley as shown in the unifying of the likeness principle, ontological inherence, and anti-abstractionism (Berkeley, 1710, P. 1, 7, 77-81; Berkeley, 1713, pp. 111, 139-140; Bracken, 1974; Urmson, 1982).

Ernst Mach (1838-1910), who contributed to the fields of physics, psychology, and physiology, among others, believed that the conceptual organization of sensory data was the key to the accumulation of knowledge. This theory aligns with Berkeley's in that knowledge emerges and organizes through repeated sensory experience. Mach also associated convergence and divergence with sensation and perception as Berkeley had.

Mach described numerous instances of sensory input that are individual sensations but part of a whole object represented in our minds, such as the sensation of heat and recognition of fire or the taste of fruit (Mach, 1914). For Mach, memory with association is a necessity for making sense of the world. If memory does not associate incoming sensations, life becomes a disjointed collection of meaningless events and humans would have no ability to ascertain cause and effect or avoid the sensation of pain. Mach explained that when children experience the pain of a wasp's sting, they modify their behavior to avoid the stimulus that elicited the sensation. Children who have not had the same experience do not behave in the same ways (Mach, 1914). Berkeley also believed in the importance of sensory experience in the development of cause and effect relationships (Berkeley, 1732, pp. 70). For Berkeley, correlation of experiences occurs simply by repeated contiguous occurrence, and this was how the mind perceived whole objects (Berkeley, 1732, pp. 68-69). In Berkeley's philosophy, this occurred through different organs working together to perceive objects through concurrent sensory input. Mach also believed that memory must be reliant upon the interaction between certain organs.

An example of this reciprocal interaction could be the sensation of the eyes moving and vision itself as described in Mach's explanation of convergence and divergence (Mach, 1914). Mach described the connection between motor movements of the eye and geometric space. He specifically mentioned the difference in sensation elicited by near-sight versus far-sight, and in fact, these sensations could not be mistaken for one another at all (Mach, 1914). Berkeley, too, made a distinction between the sensations of convergence and divergence. He explained visual perception of distance as the culmination of repeated sensory experiences arising from convergence and divergence in the context of focused attention (Berkeley, 1709, P. 16-18; Berkeley, 1710, P. 43-44).

While neither Mach nor Kant believed we are born tabula rasa as Berkeley did, each of them held beliefs that mirrored Berkeley in many ways. All three believed the key to knowledge was the association and organization of repeated sensory experiences. Their theories join two other theories that would appear, at first glance, to be at odds with one another. However, each emphasizes multimodal sensory experience in its own way. In doing so, they contribute to the body of work that extended Berkeley's theory.

Early mechanistic versus holistic perspectives on multimodal sensory experience

Hermann von Helmholtz (1821-1894), German physician and physicist, had a distinctly mechanistic perspective of multimodal experience, specifically focusing on visual experience. He developed a groundbreaking means of analyzing the visual system by dividing studies of the eye into physical studies which focused on the structures of the eye as a functional optical instrument; physiological studies which examined how the retina transmitted information resulting in the conscious sensory experience of sight; and psychological studies which revealed how light was processed into meaningful perceptions of objects and events by the brain (Helmholtz, 1909). Helmholtz showed that the effects of sensation, or the sensory input received by afferent neurons, and perception, or the interpretation of that information into meaningful experience, could be measured and observed empirically, which was not previously thought to be the case. Despite his mechanistic and materialistic emphasis, Helmholtz's perspective correlates to Berkeley's idealistic theory in surprising ways.

Helmholtz's psychological studies examined much of his previous work within the context of empiricism. He believed that experience mediated the mechanistic operations of the eye. His research led him to conclude that, after sensory organs transduced light, perception occurred as a function of previous experience. Helmholtz felt that sight was the most important sense for ascertaining distance, stating that touch and sight performed the same function but sight had greater range, which established the argument for the correlation of sight and touch (Helmholtz, 1895). He developed an empirical theory of vision, determined in part by sensory experience on the retina. Helmholtz argued that all sensory experience, while transduced by different nerves, was one whole experience by the perceiver, not many different experiences (Helmholtz, 1895). This links directly to Berkeley's theory that all sensory experience is correlated (Berkeley, 1709, P. 46). Berkeley's idealism contradicted the mechanistic specificity of Helmholtz, but both men arrived at similar conclusions when discussing multimodal perception.

Helmholtz postulated that some perceptions are the result of unconscious assumptions we make about the environment, known as the theory of unconscious inference, in which individuals draw reflexive conclusions, based on prior experiences, about incoming sensory signals (Turner, 1993). Additionally, Helmholtz also discovered the concept of perceptual adaptation, where participants have their perception distorted and the perceptual system affected shifts to compensate for the distortion after a period of time (Helmholtz, 1909). These are examples of repeated experience mediating sensory perception. In both the theory of unconscious inference and the concept of perceptual adaptation, the brain corrects sensory perception to what it anticipates from previous experience.

Helmholtz ascribed to a highly mechanistic perspective, but holistic perspectives borrow from Berkeley's theory as well. Wolfgang Kohler, one of the founders of Gestalt psychology, described the phenomenon of the self-organizing brain in his Dynamics of Psychology (1940). His perspective, which supported the idea that "the whole is different from the sum of its parts," stressed the interaction of environmental stimuli and the brain to create perception.

This interaction, however, was not pinpointed to a certain area of the brain. Kohler believed cortical processes to be far more distributed than theorized in his time (Kohler, 1940). He theorized that the brain had the ability to view events holistically without the conscious knowledge of the perceiver. He called this approach a field theory, where the facets of one sense have the ability to influence another sense. In other words, there is a great connectivity across the senses, an idea that foreshadowed ideas of degeneracy and redundancy (Edelman, 1987; Sporns and Edelman, 1993). This field theory gave rise to a series of cognitive organizational principles (e.g. principle of likeness), presumably emerging out of the activity of a holistically-functioning brain, which related to perception and explained various perceptual phenomena, such as visual illusions and tricks of the eye (Kohler, 1940). A holistic view of perception links directly back to Berkeley's theory of distance perception. His theory was a holistic view of perception as well, examining how seemingly different modalities function as a whole, out of which perception emerged (Berkeley, 1709, P. 12, 16-20, 29, 90).

While Helmholtz and Kohler each held opposing views of the ways we sense and perceive, each of their views harken back to Berkeley's theory. Their perspectives shared a common thread of multimodal interaction begetting perception. This thread can be seen in modern-day research, illustrating the impact Berkeley's work continues to have on multimodal sensory theory.

Modern perspectives on multimodal sensory perception

Research in multimodal sensory perception is evident in the work of researchers exploring the distributed coding hypothesis. In neuroscience the distributed coding hypothesis refers to the idea that neurons fire to stimuli of related concepts. This goes back to Hebbian theory (Hebb, 1949), which is adeptly summarized by the statement, "cells that fire together, wire together." As events co-occur, they correlate within the brain. These concepts echo Berkeley's Law of Contiguity (Armstrong, 1965). Over time, areas of the brain associated with contiguous experience fire together more and more automatically. Eventually, different versions of the same stimulus elicit similar neural activity (Quiroga, Reddy, Kreiman, Koch, & Fried, 2005). The correlational process of the neuronal activity is evident in a functional MRI (fMRI) and through electroencephalographic (EEG) information.

EEG data can be categorized by wavelength as it is elicited, and these wavelengths correspond to different sensory modalities. Multimodal sensory perception evokes intersensory components. The simple combination of unisensory evoked potentials does not explain these components--essentially, experiencing two co-occurring stimuli creates different EEG activity patterns when compared with data created via stimulating only a single sense. The stimuli create different patterns of activity within the brain despite being the same activity, simply occurring concurrently (Sakowitz, Quiroga, Schurmann, & Basar, 2004).

Additionally, research on intersensory components suggests that multisensory convergence occurs at or prior to cortical processing. Consequently, memory coding occurs across areas devoted to disparate processes (Sakowitz et al., 2004). For instance, visual and auditory information are coded in their respective cortices. This convergence and coding across multiple cortical areas results in different interpretations and memories of the same basic stimuli even if the attempt was made to only compare visual activity to visual activity and auditory activity to auditory activity.

The ideas Berkeley espoused, it could be argued, became the building blocks of the distributed coding theory of neuronal activity. The creation of memories involves the creation of new synaptic pathways. As a result, our visual experiences contribute to the molding of our perceptual systems. Densely interconnected cortices provide feedback loops of sensory processing, resulting in faster processing and more meaningful memories when multiple sensory modalities are engaged (Edelman, 1987; Edelman & Gally, 2001; Sporns, Tononi, & Edelman, 2000). These memories are then used to infer future experiences via top-down processing. Such concepts link directly to Berkeley's theory of distance perception; we have simply operationalized it to the very minutiae of neuronal activity across the brain.

Dynamic Field Theory, a theory adopted from the field of physics and grounded in contemporary understanding of neural dynamics, is another perspective that closely corresponds to Berkeley's theory of multimodal perception. Dynamic Field Theory takes human behavior and breaks it down into a complex mathematical formula. This formula is dependent on factors such as the strength of a stimulus and the duration of stimulus activation (Schoner & Thelen, 2005).

Dynamic Field Theory is based on the concept that environmental stimuli interact with the body's perceptual systems to inform behavior. This interaction is based on salient previous sensory experience. Whenever a variable is augmented in the formula or when the participant experiences something different from what they experienced previously, the interaction changes. The theory stresses the importance of salient, repetitive experience to drive learning. In this theory, systems self-organize through soft assembly, which permits flexibility to adapt perception and behavior to the ever-changing nature of the world (Kugler & Turvey, 1987; Thelen, 2000; Thelen & Smith, 1998). Perceptual systems constantly evolve with new sensory experiences, which serves to augment human behavior and understanding (Johnson, Spencer, & Schoner; Schoner & Thelen, 2005).

While the distributed coding hypothesis took Berkeley's theory and broke it down to the neural level, Dynamic Field Theory succeeded in breaking Berkeley's theory down to the mathematical level. Every variable represents a factor in multimodal sensory perception. We have operationalized multimodal sensory interaction to mathematics, where each variable is dependent upon the other for emergence of behavior (cognitive and physical). Berkeley, too, argued that the interaction of variables determined our understanding of events (Berkeley, 1709, P. 46). For Berkeley, event understanding develops as a function of repeated experience (Berkeley, 1709, P. 3, 16-18, 20).

DISCUSSION

In this paper, we connected the philosophy of George Berkeley to multiple figures and theories throughout psychology and neuroscience. Berkeley's theory of distance perception focused on the correlation of multimodal sensory information to previous experience. Immanuel Kant also emphasized the importance of subjective experience, as these experiences and their interaction with the innate categories of thought served as the catalyst for the creation of knowledge. Kant's concept of the empirical unity of apperception also emphasized the subjectivity of the experiences and associations of the perceiver (Kant, 1787). Ernst Mach believed sensory experience elicited the recognition of certain objects, such as the specific heat of fire (Mach, 1914). This recognition emerged from previous repeated sensory experience, which links directly to Berkeley's theory of multimodal perception. Mach, too, associated convergence and divergence with near-sight and far-sight, as Berkeley had previously.

In spite of his strict materialism, Hermann von Helmholtz came to many of the same conclusions as Berkeley. Both men postulated a unified multimodal experience and correlated the relationship between sight and touch. Helmholtz's theory of unconscious inference and the concept of perceptual adaptation each further operationalize and support Berkeley's theory by elucidating the importance of previous experience in perceptual phenomena. Wolfgang Kohler (1940), whose approach to Gestalt psychology was in line with the famous statement that "the whole is different from the sum of its parts," developed a field theory where one sense can influence the sensory experience of another sense. His perspective emphasized the importance of a holistically-functioning brain in the perception of stimuli. This also supports Berkeley's multimodal theory of perception as the correlation and cooperation of the senses with previous experiences is what informs perception, as in the case of the stagecoach example (Berkeley, 1709, P. 46).

Modern neuroscientific theory echoes Berkeley's theory as well. The distributed coding hypothesis aligns with Berkeley's theory as it states that multiple versions of the same stimulus can elicit activity from the same parts of the brain. However, the distributed coding hypothesis also states that the activity elicited by multiple sensory modalities differs from the activity elicited by one sense, reflecting the potential for enhanced memory encoding. This further supports Berkeley's notion that repeated multimodal sensory experience creates meaningful associations. In Dynamic Field Theory, another modern perspective, we see Berkeley's theory operationalized all the way to a mathematical formula where each facet of multimodal experience is represented by a variable that can predict one's behavior in a certain situation (Schoner & Thelen, 2005). In that perspective, we see the full modern application of Berkeley's theory, as it emphasizes the flexibility of soft assembly and the specificity of mathematical modeling. This specificity and flexibility are hallmarks of Berkeley's theory of perception, as seen in the specificity of his explanation of convergence/divergence and the flexibility of the correlation of experience simply by contiguous occurrence (Berkeley, 1709, P. 18; Berkeley, 1732, pp. 68-69).

Berkeley's impact on the fields of psychology and neuroscience is undeniable. Enacting a simple Google Scholar search beginning in 1879, when the beginning of experimental psychology can be traced to the founding of Wilhelm Wundt's lab in Leipzig, Germany, Berkeley's An Essay Towards a New Theory of Vision has been cited in 721 separate works, indicating the impact of his theory from the beginning of experimental psychology to modern times. His theory can be clearly seen in the work of other philosophical and scientific giants throughout history. As we continue to research new theories and perspectives, we should remember the quote commonly attributed to Ivan Pavlov, "If you want new ideas, read old books." Berkeley's theory continues to resonate through history due to validation via experimental methods not available during his own time. As we explore the principles that underlie sensation and perception, modern researchers come to conclusions that support multimodal perceptual theory, further reinforcing Berkeley's perspective.

Before the capability to empirically observe and measure it, Berkeley inferred the relationship between sight and touch in his 1709 essay. In 2019, over 300 years later, we are still studying this concept, albeit with sophisticated technology, equipment, and experimental protocols (Carducci, Schwing, Huber, & Truppa, 2018). Between 1709 and today, there is a wealth of experimentation that examines similar relationships, and there will certainly be many more publications in this field in the future. Berkeley's theory sparked a revolution in the study of perception, and we may never have the ability to unravel all the ramifications of his philosophy. However, we can honor the contributions he and others made to the discipline before us, and, in addition to reading new articles as they are published, remember to reread our old books.

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(1) P represents Principles as stated in Berkeley's works

Mary Kate Moore

Joshua L. Williams

Nancy G. McCarley

Georgia Southern University

Author info: Correspondence should be sent to: Mary Kate Moore, Department of Psychology, Georgia Southern University, mm39622@georgiasouthern.edu
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