Understanding the Complexities of Sensory Awareness
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In an insightful review featured in Frontiers in Psychology, Todd E. Feinberg and Jon Mallatt delve into the intricate challenges of comprehending conscious experience through scientific lenses. They argue that consciousness is not only rooted in the complex nature of life itself but also relies on the integration of various sensory inputs and their processing mechanisms. While I encourage readers to engage with their comprehensive review, this piece will focus on sensory experience, which is crucial to the study of consciousness.
To begin, Feinberg and Mallatt emphasize the essential role of embodiment in consciousness:
> "A fundamental aspect of understanding feelings is recognizing that both life and sensations are embodied. Each organism has a physical form that separates it from the external environment, and thus consciousness requires a body for a subject to experience a personal, first-person perspective."
We possess a singular body and life. How do we ensure its protection? By sensing and responding to environmental changes that could threaten our survival, a process facilitated by sensory consciousness.
Feinberg and Mallatt have meticulously compiled a list of unique neurobiological characteristics of consciousness in their review. Key features associated with sensory consciousness include:
- Specialized sensory organs.
- Neurons organized into maps of the external environment and bodily structures.
- Extensive reciprocal communication pathways among different sensory modalities.
Focusing exclusively on humans, we have distinct neural systems tailored for each sensory modality. Each sensory organ is attuned to a different aspect of the physical world: our eyes perceive light's intensity, wavelength, and movement; our tongues detect soluble chemicals from food mixed with saliva; our noses sense airborne molecules; our ears pick up vibrations in the air; and our skin responds to mechanical pressure, temperature, and harmful substances.
You might already be aware of these distinctions, but have you considered how these varied sensations are encoded in the brain? Each sensory modality is represented by its unique neural system, further refined into specific neuronal ensembles and microcircuits.
Researchers have been investigating the brain regions that become activated when individuals experience particular sensory inputs, such as visual stimuli, odors, or sounds. This research has led to the creation of brain maps, which depict the anatomical structure of the brain alongside the functions associated with neural activity in specific areas. A significant limitation of non-invasive brain imaging techniques is that they primarily yield reliable functional data from the cerebral cortex, the outermost layer of the brain. This predominance is due to the wealth of data available for the cortex, which is the most recent evolutionary development in the brain, believed to be essential for higher cognitive functions like goal-directed behavior, decision-making, and consciousness.
Feinberg and Mallatt summarize sensory input brain maps in their Figure 2. Current consensus suggests that sensory information is organized into structured maps, which show remarkable consistency across individuals (although more advanced analyses are beginning to reveal individual variations, as noted in a paper published in eLife).
What do these maps actually look like? The earliest examples were created for the visual system. Consider a scenario where you are the test subject peering through binocular eyepieces in a dark room, with intermittent flashes of light appearing at various locations in your visual field. By monitoring the activity in your cerebral cortex during this task, researchers have found that specific regions of your cortex correspond to certain areas of your visual field. Every time a flash occurs in a given location, the respective part of the cortex becomes active. When the entire visual field is tested, researchers can construct a retinotopic map, as illustrated below:
As technology advances, brain maps have become increasingly detailed.
The stimuli used in these studies have also evolved, with some researchers employing natural visual scenes. For an interactive brain map developed by Jack Gallant’s Lab, see their 2013 publication, “Attention during natural vision warps semantic representation across the human brain” (Cukur et al., Nature Neuroscience, 2013).
By synthesizing various sensations and functions encoded by the cerebral cortex, we can now produce standardized maps:
Nonetheless, the quest for understanding continues, with ongoing efforts to map deeper brain regions (refer to Jie Lisa Ji et al.'s paper in NeuroImage, published January 2019).
An interesting development is that the cerebellum—those unusual structures at the brain's base—also appears to have retinotopic maps of visual space (see Daniel M. van Es et al.'s study in Current Biology, May 2019). Although lesion studies suggest the cerebellum may not be critical for consciousness, our comprehension of its functions is rapidly evolving, leaving the matter open to further investigation.
Integrating Sensory Inputs
To encapsulate the preceding discussion, each sensation is encoded through distinct and organized neural systems. How does a living organism interpret this information? Feinberg and Mallatt assert that “consciousness efficiently organizes vast amounts of sensory data into a cohesive set of phenomenal properties for selecting among various active responses.” But how does this process unfold?
First, the brain must assimilate sensory data into a coherent representation of reality. This is where the extensive interconnections within and between sensory areas of the brain—one of Feinberg and Mallatt’s highlighted neurobiological features—become vital. This area remains an active field of research. For a thorough overview of a recognized sensory systems integrator, the thalamus, refer to “The Human Thalamus Is an Integrative Hub for Functional Brain Networks” by Kai Hwang et al.
Second, to determine an appropriate response or action, the brain must assign value to sensory inputs. Should I focus on this sensory stimulus? Does it pose a potential threat or benefit to my survival as an embodied being? The human brain assigns value using emotions, also referred to as affective states.
I plan to explore both of these research areas in future writings.
Feinberg and Mallatt identify three distinct types of sensory experience:
- Exteroceptive: mental models of the world created from integrating sensory maps.
- Affective: emotional states derived from interpreting sensory inputs as positive or negative.
- Interoceptive: a mental model of one’s body, encompassing awareness of whether incoming sensations are beneficial or detrimental to bodily functions.
They emphasize that these sensory experiences are underpinned by unique neural architectures, characterized by specific structural arrangements and neural connectivity. In humans, exteroceptive processing occurs within sensory-specific hubs of the cerebral cortex, while affective processing is distributed across various deeper brain structures.
> "Exteroceptive circuits are designed to encode spatial representations, while affective circuits encode positive or negative values. The reticular formation, which extends throughout much of the vertebrate brainstem, plays a significant role in controlling the attention and arousal aspects of consciousness."
Another crucial neurobiological feature of consciousness, as noted by Feinberg and Mallatt, pertains to the brain mechanisms of selective attention and arousal. You can learn more about the reticular formation’s role in attention and arousal here.
The significance of these systems lies in their existence as specialized neurobiological features, which are fundamental to the evolution of consciousness. Feinberg and Mallatt contend that the rise of animal predators approximately 540 million years ago propelled the evolution of new sensory systems. For animals to thrive and survive as embodied entities, they needed to develop new neurobiological features capable of integrating sensory data into a functional model of the world—thus, perception evolved.
In essence, our consciousness arises from our diverse sensory systems.
While my focus is on humans, Feinberg and Mallatt illustrate that the neurobiological aspects of consciousness have manifested in various configurations throughout evolutionary history, complicating our understanding of consciousness across species. Nonetheless, despite the challenges posed by the diverse mechanisms of consciousness, Feinberg and Mallatt express optimism that we can unravel these complexities through scientific exploration.
> "Although this complexity complicates our understanding of how brains generate experience, it does not imply that the process is enigmatic or unexplainable. Numerous complex biological functions—including life itself—are aggregates influenced by multiple factors, yet they remain scientifically explicable."
Feinberg and Mallatt conclude that the intricate neurobiological uniqueness inherent in personal embodiment renders consciousness appear ‘mysterious’ and unexplainable by established physical laws. However, they assert that we need not resort to a ‘mysterious’ explanation to account for the individual and unique aspects of subjectivity.
Addressing Chalmers and Levine
In their concluding remarks, Feinberg and Mallatt tackle philosopher David Chalmers’ inquiry regarding the nature of conscious experience (“Why do individual experiences have their particular nature?” from Chalmers, 1996). They argue that the mechanisms underlying consciousness can be elucidated through the diverse, specialized neurobiological features that constitute it.
> "The answer lies in the varied neurobiology that underpins these unique subjective experiences. It is evident that the neural pathways for processing color, pain, and emotional responses exhibit significant neurobiological differences. Electromagnetic waves of light possess distinct physical properties from the mechanical forces of touch, and both diverge from chemical stimuli like odors. Thus, translating these three sensory inputs into similar experiences would overlook the unique characteristics that render each sense particularly informative. Consequently, these varied sensations should not—and indeed cannot—share the same subjective ‘feel.’ It is unsurprising that the phenomenal experiences produced by these different neural architectures are experienced subjectively in distinct ways. In other words, the qualitative features of phenomenal properties reside in the neural states themselves; they are not an ‘additional feature’ of the neural states that generate them."
Although numerous theories exist regarding the mechanisms of consciousness (which Feinberg and Mallatt categorize as “major-mechanism theories”), it remains uncertain whether any of these accurately capture the essence of subjective experience. Inspired by engaged readers and their feedback, I’ve begun to reconsider consciousness mechanisms in a new light. I believe Feinberg and Mallatt are correct—our subjective experiences can indeed be interpreted as a neural systems mechanism. However, I have two caveats for your consideration.
First, it is conceivable that focusing solely on electrical signaling within these neural systems may not encompass the entirety of the picture. There may be other signaling modalities underpinning consciousness that we currently lack the technology to measure.
Second, I agree that the mechanisms of experience and the individual sensations themselves are distinct phenomena (as discussed in a conversation with Paul Austin Murphy).
I do not consciously perceive my neural systems collaborating to attribute value to sensory inputs, just as I do not sense my kidneys filtering blood.
Is this a controversial stance? A hint of mysticism within my physicalist beliefs? No, rather, it reflects two different scales of consideration. On one scale, we examine the neural system; on another, we consider the individual person within a broader cultural context.
The neural states that constitute subjective experience can and will be elucidated through systems neuroscience. However, understanding the mechanisms behind subjective experience may not provide practical guidance for individuals seeking to lead fulfilling lives. While it may aid in treating mental disorders, it won’t answer the profound questions people seek. Thus, it is imperative to approach consciousness from multiple angles, including analysis of personal experiences, self-experimentation, philosophy, psychology, sociology, and art.
This nuance is subtle and challenging to articulate without appearing dualistic. I believe it underpins much of the discourse between neuroscientists and philosophers regarding consciousness.
Feinberg and Mallatt assert that we can elucidate the origins of subjective experiences through scientific inquiry and established physical laws. I concur, yet I maintain that there is always room for deeper understanding. Perhaps emerging theoretical frameworks from physics or other fields will one day enhance our comprehension of consciousness.
In their discussion, they suggest that integrating both the embodied organism and the diverse, specialized neurobiological features bridges the explanatory gap between the physical brain and subjective experiences. This remains a topic open to debate and largely depends on interpretations of philosopher Joseph Levine’s work (see Levine, 1983).
For instance, in a recent publication by Jussi Jylkkä and Henry Railo, they argue that a gap will always exist between our mechanistic models of experience and the experience itself. This does not imply that the experience is non-physical; rather, it indicates that the model and the phenomenon itself will invariably differ, as is true for any representation of the physical world.
Todd Feinberg serves as a Clinical Professor of Psychiatry and Neurology at Icahn School of Medicine at Mount Sinai, New York. Jon Mallatt is an Associate Professor at Washington State University, specializing in molecular and evolutionary biology. Over the past seven years, they have collaborated extensively to deepen our understanding of consciousness, resulting in three research papers and two books on the subject. Their most recent paper, which has been partially discussed here, is commendable for its depth and breadth; I highly recommend reading it.