Session 2: The Many Dimensions of Subjectivity

Beyond Vision: Exploring Diverse Subjective Experiences

Overview

In Session 1, we explored how expectations shape perception. Now we turn to a more fundamental question: Do different people experience the same sensory input in the same way? This session surveys subjective experiences beyond visual perception, focusing on dimensions that present particular measurement challenges. We'll examine interoception (sensing internal body states), temporal perception (how we experience time), synesthesia (cross-modal sensory experiences), aphantasia (absence of mental imagery), and taste perception (genetic variation in gustatory experience). Each domain reveals substantial individual differences and raises the question: How do we study something as private as subjective experience? Through behavioral tasks and neuroimaging studies, we'll see how researchers attempt to map the structure of consciousness across these diverse dimensions.

Interoception: Sensing the Body from Within

What Is Interoception?

While we're familiar with exteroceptive senses (vision, hearing, touch, taste, smell) that tell us about the external world, interoception refers to sensing the body's internal state. This includes awareness of heartbeat, breathing, hunger, thirst, temperature, pain from internal organs, and the need to urinate.

Why Interoception Matters: Interoceptive signals form the basis of emotional feelings. Different theories propose different relationships between bodily sensations, cognitive appraisal, and emotional experience.
Theories of Emotion Processing

Figure: Different theoretical models of how interoceptive signals relate to emotional experience

Measuring Interoceptive Accuracy: The Heartbeat Detection Task

Schandry (1981) developed the most widely used measure of interoceptive accuracy:

Heartbeat Counting Task:
  • Purpose: The task is a simple and widely used way to assess an individual's ability to perceive their internal bodily signals, specifically their heart rate
  • Participants silently count their heartbeats over specific intervals (25, 35, 45 seconds) without taking their pulse
  • Their reported count is compared to actual heartbeats measured by ECG
  • Accuracy score = 1 - (|recorded beats - counted beats| / recorded beats)
Key Finding: People vary dramatically—from near-perfect accuracy (0.90+) to almost no correlation (0.10) between reported and actual heartbeats.
Methodological Concerns: Körmendi et al. (2021) demonstrated that the Schandry task is sensitive to non-interoceptive (top-down) influences. Performance-related expectation was strongly related to task performance (β = .595, p < .001) even after controlling for physiological factors. People with high expectations may categorize vague sensations (such as attention-evoked sensations) as heartbeats, leading to inflated scores. This raises questions: Does the task measure true interoceptive sensitivity, or partly cognitive beliefs about one's heartbeat?

Neural Basis of Interoception

Critchley et al. (2004) used fMRI to identify brain regions involved in interoceptive awareness:

  • Anterior insula: Shows activation during heartbeat detection; gray matter volume correlates with interoceptive accuracy
  • Anterior cingulate cortex: Integrates interoceptive signals with emotional and cognitive processing
  • Somatosensory cortex: Represents body state information
Threat Anticipation and Interoception: Liu et al. (2021) demonstrated a whole-brain signature that specifically predicts anxious anticipation under uncertainty. Importantly, this neural pattern is not sensitive to predicting pain, general anticipation, or unspecific emotional and autonomic arousal—suggesting that the subjective experience of threat anticipation has a distinct neural basis tied to interoceptive processing.
Patient Study: Patients with insular cortex damage show impaired interoceptive accuracy while maintaining normal exteroceptive perception. One patient could see, hear, and feel external touch perfectly but couldn't detect their own heartbeat or accurately judge their emotional arousal.

Interoception and Mental Health

Discussion Point: If two people have different interoceptive accuracy, do they experience emotions differently? Is someone with high interoceptive accuracy having "more emotional" experiences, or just noticing bodily signals that others miss? And critically—are they accurately perceiving their body, or do their beliefs shape what they think they perceive?

Paulus & Stein (2010) proposed a model integrating interoception with mental health, though the relationships remain debated:

  • Anxiety disorders:People with anxiety might get confusing signals from their body (like a racing heart) and interpret them as dangerous. The problem isn't necessarily that they feel their body "too much," but that they expect threat and their beliefs amplify ambiguous body sensations.
  • Depression: Some research finds that people with depression are less accurate at sensing their body's signals, though studies show mixed results. It's possible that their beliefs about themselves interfere with how they interpret what their body is telling them.
  • Alexithymia: "No words for feelings"—people who struggle to identify their emotions might process body signals differently. But we don't know if they actually feel less, or if they just have trouble interpreting what they feel.
  • Eating disorders: People with eating disorders may process hunger and fullness signals differently and feel disconnected from their body. Again, we can't tell if their body sends different signals, or if their beliefs about food and their body change how they interpret normal signals.
A More Nuanced View: Depression and anxiety may not simply be problems with sensing the body accurately. Instead, they might involve a complicated interaction between: (1) what your body is actually signaling, (2) your beliefs about what those signals mean, and (3) your brain trying to make sense of confusing or inconsistent body signals. The key challenge is figuring out whether someone truly feels their heartbeat differently, or whether they just interpret the same sensations differently based on their beliefs and expectations.

Critchley, H. D., et al. (2004). Neural systems supporting interoceptive awareness. Nature Neuroscience, 7(2), 189-195.

Subjective Time: The Flexibility of Temporal Experience

Why Time Perception Matters

Our experience of time passing is fundamental to consciousness yet highly variable. The same objective duration can feel dramatically different depending on context, attention, and emotional state. Unlike space, which we can measure with rulers, time exists only as subjective experience in the present moment.

Duration Estimation: Prospective vs. Retrospective

Block & Zakay (1997) distinguished two fundamentally different ways we judge time:

Prospective Timing: You know in advance you'll need to judge duration
  • "Tell me when 30 seconds has passed"
  • Paying attention to time makes it feel longer
  • Depends on attention and working memory
Retrospective Timing: You're asked after the fact how long something lasted
  • "How long was that movie?"
  • More memorable/eventful periods feel longer in retrospect
  • Depends on memory encoding

The Oddball Effect and Time Dilation

Stetson et al. (2007) investigated whether fear actually slows down time perception:

The Free-Fall Experiment:
  • Participants free-fell 150 feet into a safety net
  • Wore a device displaying rapidly flickering numbers
  • Asked to report: (1) How long the fall lasted, (2) Could they read the numbers?
Results:
  • Participants estimated the fall lasted 36% longer than it actually did
  • BUT they could NOT read numbers during the fall (no enhanced temporal resolution)
  • Interpretation: As the authors state: "Therefore, at this stage there is no evidence to support the hypothesis that subjective time as a whole runs in slow motion during frightening events. Rather, we speculate that the involvement of the amygdala in emotional memory may lead to dilated duration judgments retrospectively, due to a richer, and perhaps secondary encoding of the memories. Upon later readout, such highly salient events may be erroneously interpreted to have spanned a greater period of time." Fear doesn't create "slow motion" experience in the moment—it creates richer memories that retrospectively feel longer.

Neural Mechanisms of Time Perception

Unlike 'true' senses (vision has eyes, hearing has ears, touch has skin receptors), there's no single "time organ." Instead, timing is distributed across brain regions:

  • Striatum & dopamine: Coull et al. (2011) showed dopamine levels affect internal clock speed. Drugs that increase dopamine (like methamphetamine) make time feel slower; dopamine blockers speed it up.
  • Cerebellum: Critical for millisecond-level timing (motor coordination, speech)
  • Prefrontal cortex: Important for working memory-based timing and attention to duration
  • Parietal cortex: Integrates temporal and spatial information
The Internal Clock Model (Treisman et al., 1990):
  • A pacemaker generates pulses at a steady rate
  • An accumulator counts pulses during timed intervals
  • Arousal speeds the pacemaker → more pulses → time feels longer
  • Attention acts as a gate controlling how many pulses get counted
This explains why time flies when you're absorbed (gate closed, fewer pulses counted) and drags when bored and watching the clock (gate open, all pulses counted).

Individual and Cultural Differences

Wittmann (2009) reviewed factors affecting time perception:

  • Age: Older adults report time passing faster—possibly because each year is a smaller proportion of total life
  • Culture: "Clock time" cultures (Western industrial) vs. "event time" cultures (many traditional societies)
  • Personality: Impulsivity correlates with overestimating short durations; patience with underestimating
  • Body temperature: Fever makes subjective time pass faster (external world seems slower)

Wittmann, M. (2009). The inner sense of time. Philosophical Transactions of the Royal Society B, 364, 1955-1967.

Synesthesia: When Senses Blend

Defining Synesthesia

Synesthesia occurs when stimulation of one sensory pathway automatically triggers experiences in another sensory pathway. About 4% of people have some form of synesthesia, though many don't realize they're unusual until adulthood.

Types of Synesthesia

Common Forms:
  • Grapheme-color: Letters/numbers evoke colors ("A is red, B is blue")
  • Sound-color (chromesthesia): Sounds trigger visual colors
  • Lexical-gustatory: Words evoke tastes ("Derek tastes like earwax")
  • Number-form: Numbers occupy specific spatial locations
  • Mirror-touch: Seeing someone touched triggers tactile sensation on own body

Behavioral Validation

Ramachandran & Hubbard (2001) demonstrated that synesthesia is perceptual, not just associative:

The Embedded Figure Test (click to see the figures in the paper):
  • Display of many 5s with a few 2s scattered among them, forming a triangle
  • Non-synesthetes: Hard to spot the triangle quickly
  • Grapheme-color synesthetes: Immediately see the triangle because 2s and 5s are different colors
Implication: The synesthetic color experience happens automatically and early enough in processing to aid perceptual grouping—it's not just imagination or learned association.

Neural Basis: Cross-Activation Theory

Hubbard & Ramachandran (2005) proposed that synesthesia results from excess connectivity between normally separate brain regions:

  • Grapheme-color synesthesia: The brain area representing letter shapes (fusiform gyrus) is adjacent to color processing area (V4). Extra connections between them cause letters to activate color areas.
  • Developmental hypothesis: All infants may have dense cross-modal connections that are pruned during development. Synesthetes retain more connections.
  • Genetic component: Runs in families, suggesting hereditary factors in pruning process. Brang & Ramachandran (2011) discuss why the synesthesia gene might have survived evolution—synesthetes may have advantages in cognitive processing such as creativity, metaphorical thinking, and cross-domain problem solving.

What Synesthesia Teaches Us

Implications for Consciousness:
  • Sensory processing is more interconnected than traditional models suggest
  • Conscious experience can include genuine perceptual qualities without external stimulation
  • There's substantial variation in how different brains construct sensory experience
  • The line between perception and imagination is blurrier than we assume
  • Everyone may have somewhat different "perceptual worlds" due to neural wiring differences

Hubbard, E. M., & Ramachandran, V. S. (2005). Neurocognitive mechanisms of synesthesia. Neuron, 48(3), 509-520.

Aphantasia: The Absence of Mental Imagery

Discovering "Blind Imagination"

Zeman et al. (2015) described a remarkable case that revealed how differently people experience mental imagery:

Patient MX:
  • 65-year-old man who lost ability to visualize after cardiac surgery
  • Could still recognize faces, navigate, draw from memory—all visual tasks intact
  • But couldn't voluntarily summon visual images: "I know what my wife looks like, but I can't see her in my mind"
  • This wasn't a memory problem—he knew what things looked like, he just couldn't visualize them
  • Tells us that successful performance in visual imagery and visual memory tasks can be dissociated from the phenomenal experience of visual imagery

Aphantasia in the General Population

  • Prevalence: About 2-5% of population reports little to no visual imagery
  • Discovery: Many don't realize they're different until adulthood—they assume "picture this" is metaphorical
  • Spectrum: Imagery vividness exists on a continuum from aphantasia (none) to hyperphantasia (extremely vivid)
  • Multi-sensory: Some aphantasics also lack auditory, gustatory, or other imagery modalities

Measuring Mental Imagery

Vividness of Visual Imagery Questionnaire (VVIQ):
  • Participants rate vividness of imagined scenes (1 = no image, 5 = perfectly clear and vivid)
  • Example: "Visualize a friend or relative. Consider the exact contour of their face, head, shoulders, and body."
  • Aphantasics consistently score 16 (minimum possible); hyperphantasics score 75-80 (maximum 80)

Zeman, A., et al. (2010). Loss of imagery phenomenology with intact visuo-spatial task performance: A case of 'blind imagination'.

Taste Perception: Genetic Variation Creates Different Gustatory Worlds

The Discovery of Supertasters

In the 1930s, a chemist accidentally discovered that people perceive the bitter compound PTC (phenylthiocarbamide) very differently—some found it intensely bitter, others tasteless. This led to decades of research on genetic variation in taste.

The Genetics of Bitter Taste

Kim et al. (2003) identified the genetic basis:

  • The TAS2R38 gene encodes a bitter taste receptor
  • Two main variants: PAV (taster) and AVI (non-taster)
  • Three groups:
    • PAV/PAV homozygotes: Supertasters (~25%)
    • PAV/AVI heterozygotes: Medium tasters (~50%)
    • AVI/AVI homozygotes: Non-tasters (~25%)

Broader Taste Differences

Supertasters don't just taste PROP differently—they experience many foods more intensely:

  • Bitter vegetables: Brussels sprouts, kale, broccoli taste more bitter (often avoided)
  • Sweetness: Sugar tastes sweeter
  • Capsaicin: Chili peppers cause more intense burning
  • Fats: Can detect lower concentrations of fatty acids
  • Alcohol: Tastes more bitter and harsh

Behavioral and Health Consequences

Duffy & Bartoshuk (2000) examined real-world implications:

Food Preferences and Diet:
  • Supertasters eat fewer vegetables (too bitter)
  • Non-tasters consume more alcohol and fatty foods
  • This affects health risks: cancer, heart disease, obesity
  • But effects are complex—supertasters also tend to avoid bitter coffee and dark chocolate (which have health benefits)

The Fundamental Question of Qualia

Comparing Subjective Experiences:

A supertaster's experience of coffee is qualitatively different from a non-taster's. This isn't just "stronger" in a quantitative sense—the actual quale (subjective quality) of bitterness may be entirely different. How can we compare subjective experiences across individuals when the underlying sensory apparatus is fundamentally different?

This is the problem of "inverted spectrum" made real: We can prove that supertasters and non-tasters have different neural responses and different behaviors, but can we ever know if the conscious experience of "bitterness" is the same quality, just more or less intense? Or are they experiencing entirely different qualities?

Methodological Challenges: How Do We Study Private Experience?

The Core Problem

Unlike measuring external stimuli (light wavelength, sound frequency), subjective experiences are private and directly accessible only to the person having them. This creates fundamental challenges for scientific investigation.

Between-Subject Comparability

The Rating Scale Problem:

When one person rates pain as "7/10" and another as "5/10," are they experiencing different intensities, or just using the scale differently? When a supertaster says coffee is "very bitter" and a non-taster says it's "slightly bitter," can we compare these reports?

Convergent Validation Approaches

Researchers use multiple methods to triangulate on subjective experience:

  • Consistency tests: Do people give the same reports across time? (Synesthetes show 90%+ consistency in their color-letter pairings over years)
  • Behavioral consequences: Does reported experience predict behavior? (Synesthetes are faster at tasks where synesthetic colors help)
  • Neural correlates: Do brain patterns support subjective reports? (Aphantasics show reduced visual cortex activity during imagery)
  • Psychophysical matching: Have participants adjust stimuli across modalities to "match" intensity

Within-Subject Designs

To avoid comparison problems, researchers often focus on within-person changes:

Example Studies:
  • Does this person's time perception change with arousal? (Compare same person in different states)
  • Does training improve interoceptive accuracy? (Compare before/after in same individuals)
  • Does context change taste perception? (Same person tasting same wine in different contexts)
This avoids the problem of comparing Person A's "7" to Person B's "7."

The Hard Problem of Other Minds

Philosophical Question: Even with perfect neural measurements, could we ever know if two people have the same subjective experience? If Person A's "red" neural pattern is identical to Person B's "red" neural pattern, does that guarantee they experience the same quale of redness? Or...?