Sensory syncrasis
McLuhan's sense ratios as a physiological hypothesis, and the neuroscience that can finally test it.
Marshall McLuhan claimed that every medium alters "sense ratios," the balance among the senses, and does so "steadily and without any resistance."1 For sixty years this has been read as metaphor. This paper argues McLuhan meant it literally.
He stated it as a formal hypothesis in 1960, framed his account of media effects in the stress physiology of Hans Selye, and his circle ran media-comparison experiments in the 1950s. His sharpest critics, the physician Jonathan Miller and the psychologist Michael Bross, attacked it as a claim about the nervous system. The neuroscience McLuhan later reached for, left and right hemisphere theory, did not survive, and the claim was quietly reclassified as poetry.
Predictive-processing neuroscience now supplies a mechanism the idea maps onto. The brain weights each sensory channel, including signals from inside the body, by how far it trusts it. It can also turn every channel up or down together, fuse or separate them, and hold a setting long after its cause is gone. We call this whole configuration sensory syncrasis, from the Greek root of idiosyncrasy, and keep McLuhan's sense ratio for its one truly proportional part. Read this way, McLuhan's best-known claim becomes testable: media change us by re-tempering what we perceive, before any content reaches us. The paper sets out the history, the translation, five predictions, and the conditions under which the thesis would fail.
1A claim filed as poetry
McLuhan's most physiological idea is now mostly taught as a figure of speech. This paper argues that the filing was a mistake, and explains how it happened.
A small episode shows what McLuhan thought he was describing. The people translating Understanding Media into Braille wrote to him about another book they had translated: And There Was Light, the memoir of the French resistance leader Jacques Lusseyran, who went blind as a child. Lusseyran described his other senses sharpening as sight disappeared. The memoir states the sense-ratio thesis in a single life: alter one channel and the whole balance resets. Neuroscience later documented exactly this in blind and deaf brains, where cortex normally given to the lost sense is taken over by the remaining ones (Bavelier & Neville, 2002).
The paper makes three claims, in order of confidence:
- 1Historical (documented). McLuhan stated sense ratios as a physiological hypothesis, sought empirical tests, and was read literally by critics who attacked his physiology.
- 2Diagnostic (interpretation). The claim came to be read as metaphor largely because the neuroscience McLuhan reached for, hemispheric lateralization, failed.
- 3Theoretical (proposal). Predictive processing provides a mechanism the claim translates into. The paper names the resulting configuration sensory syncrasis, and the translation generates testable predictions.
McLuhan has been joined to neuroscience before. Nicholas Carr's The Shallows (2010) paired McLuhan with neuroplasticity to argue that the internet reshapes attention, and Derrick de Kerckhove's "psychotechnologies" extended McLuhan toward cognition (de Kerckhove, 1995). Neither treats the sense ratio itself as a measurable quantity, or connects it to precision weighting or interoception. That is the gap this paper addresses.
The paper does not argue that McLuhan anticipated modern neuroscience, or that neuroscience has confirmed him. It argues something narrower: that his claim was a hypothesis about the nervous system, and that it can now be tested as one.
2The literal claim
By 1960 McLuhan was stating sense ratios as a formal hypothesis, not an aphorism. The 1960 Report on Project in Understanding New Media, written for the National Association of Educational Broadcasters, opens its key section with a "basic hypothesis": any medium that codes experience through one sense transforms the ratio among the other senses, and with it patterns of thought, feeling and action. The same report puts it bluntly: "It is the ratio among our senses which is violently disturbed by media technology."1
The core statements
| Year | Source | What it claims |
|---|---|---|
| 1960 | Report on Project in Understanding New Media (NAEB) | The "basic hypothesis": coding experience through one sense resets the ratio among all of them |
| 1962 | The Gutenberg Galaxy | Quoting Aquinas: the senses delight in things "duly proportioned"; sense is "a kind of reason"2 |
| 1964 | Understanding Media, ch. 1 | Media "alter sense ratios or patterns of perception steadily and without any resistance"3 |
| 1964 | Understanding Media | Media as extensions of the senses "institute new ratios" among the senses and among media themselves |
| 1964 | Understanding Media, ch. 4 | Citing Selye and Jonas: every extension is an attempt by the nervous system to maintain equilibrium |
Physiological in reference
McLuhan tied his account of media to the physiology of his day. In Understanding Media he compares his method directly to Hans Selye's stress theory of disease, which studied the whole environmental situation rather than a single cause. His account of "autoamputation" rests on Selye and Adolphe Jonas: the central nervous system, overstimulated, protects itself by numbing the offending function. Sense ratios belong to this picture. A medium that intensifies one channel forces the nervous system to rebalance the rest.
Metaphysical in form
The word "ratio" also carries a second meaning. McLuhan took it from Thomas Aquinas, for whom ratio meant both proportion and reason (Friesen). Consciousness, on this view, is a proportion among the senses. This double meaning is why the claim can be read two ways. It is also why the reading matters: McLuhan used the Thomist word to describe what he took to be a physiological process.
Tested, not only asserted
McLuhan's circle tried to measure media effects. Around 1954, the anthropologist Edmund Carpenter and the psychologist D. C. Williams ran an experiment in Toronto, funded by the same Ford Foundation grant that supported McLuhan and Carpenter's journal Explorations (1953–59). Students received the same lecture through speech, print, radio or television, and were tested on what they retained. The published study reported that the medium affected retention, both immediately and months later (Williams, Paul & Ogilvie, 1957). The 1960 NAEB project then piloted a media curriculum with Toronto high-school teachers.
These were crude studies, and they measured learning, not sensory balance. But they show the claim was treated as something that could be tested.
3Read as physiology
The strongest evidence that McLuhan's claim was literal comes from the people who attacked it. His two most serious critics both treated sense ratios as a claim about the nervous system, and judged it on those terms.
A critic's reading cannot prove an author's intent. What these readings show is narrower: the literal reading was the natural one at the time, held by readers equipped to test it.
Jonathan Miller (1971)
Miller was a physician by training. His McLuhan (Fontana Modern Masters, 1971) charged, in Frank Kermode's summary, that McLuhan dealt in gross historical, philosophical and physiological simplifications. He found no evidence that hearing is privileged over sight, or that reading is less simultaneous than listening. He concluded that McLuhan's psychology owed more to Aquinas than to the scientific sources he cited, and called the whole a "gigantic system of lies"7 that nonetheless made truth possible by shocking its readers.
The objection only makes sense if the claim was literal. Miller had earlier praised McLuhan as the first to bring the nervous system into the centre of discussions of ordinary communication (Katz & Katz, 1998). He was not complaining that McLuhan's metaphor was imprecise. He was complaining that his physiology was wrong.
Michael Bross (1992)
Bross, a psychologist, tested the theory against psychophysics and sensory-deprivation research. He found no evidence that the nervous system can "amputate" any of its functions. Yet he concluded that McLuhan was right to put sensory functions at the centre of media effects, and he rebuilt the theory in the language of cybernetics and control systems (Bross, 1992).
Bross matters here for a second reason. A control-systems account of the senses is a direct ancestor of the predictive-processing account used in section 5. He began the translation this paper attempts, three decades before the neuroscience could carry it.
4The wrong neuroscience
In the last phase of his work, McLuhan anchored sense ratios to the brain science of the 1970s, and that science did not hold. From the mid-1970s through Laws of Media (1988, with Eric McLuhan) and The Global Village (1989, with Bruce Powers), he mapped "visual space" onto the left hemisphere and "acoustic space" onto the right. He cited the neurosurgeon Joseph Bogen, whose split-brain work with Roger Sperry was then being popularised as a theory of two minds. McLuhan extended the mapping to whole cultures.
The narrow findings behind that work were real. Language is left-dominant in most right-handed people, and some visuospatial attention is right-biased. The broad version, that people or cultures are "left-brained" or "right-brained," has no support. A connectivity study of 1,011 people aged 7 to 29 found no whole-brain pattern of stronger left or right networks across individuals (Nielsen et al., 2013).
How a hypothesis became a metaphor (interpretation)
The metaphorical reading did not begin with this failure. McLuhan's style invited it from the start, and by 1965 he was describing his statements as exploratory probes. What the collapse of lateralization removed was the last physiological anchor. Once the brain science McLuhan had chosen was discredited, the literal reading had no evidence left, and the metaphorical reading became the default. Readers who still valued the insight could keep it as a probe, or as Thomist proportion. Both let the idea survive as poetry.
The timing was unlucky. McLuhan bet on a dichotomy between two hemispheres. The research that would have suited his claim better studies how the brain reallocates its resources across the senses through experience, and it was only beginning. Its mature form is the subject of the next section.
5The translation: from sense ratio to syncrasis
In predictive-processing terms, a sense ratio is the relative trust the brain places in each of its sensory channels. This section sets out that translation and the evidence that such a ratio exists, varies, and is shaped by experience.
Precision
Predictive-processing theories hold that the brain continuously predicts its own sensory input and updates on the errors. Not every error counts equally. Each is weighted by its precision, the brain's estimate of how reliable that signal is. Attention, on this account, is the optimisation of precision: turning up the gain on the channels that currently matter (Feldman & Friston, 2010). The weighting runs across outward senses, the body's position, and signals from inside the body (Clark, 2013; Seth, 2013).
We call the whole configuration sensory syncrasis. This is a new term, proposed here as the successor to McLuhan's sense ratio. It comes from the Greek synkrasis, "a mixing together, tempering."1 English renders the Greek k as c, as in its best-known descendant, idiosyncrasy. That word entered English in the 1600s as a medical term for a person's individual physical constitution (Collins). A person's sensory syncrasis is, literally, the idiosyncrasy of their senses. The adjective is syncratic. McLuhan's own term is kept for the narrowest part of it: the sense ratio, the relative weight the senses receive when they are combined.
The term is a simplification. A ratio captures only one of five separable dimensions of how the senses combine, set out below. When people judge an object's size by sight and touch together, each sense's weight tracks its reliability, and the weights sum to one (Ernst & Banks, 2002). Raising one lowers the other. But the same model shows the combined estimate is more reliable than either sense alone. The ratio is zero-sum; the whole is not. That is why the umbrella term cannot be "ratio."
| Dimension | What it is | Zero-sum? | Key evidence |
|---|---|---|---|
| 1. Sense ratio | Relative weight each sense gets in a combined judgement | Yes | Ernst & Banks 2002; Peterka 2002 |
| 2. Overall gain | How strongly all channels are amplified at once, set by arousal and neuromodulation | No | Aston-Jones & Cohen 2005 |
| 3. Integration mode | Whether signals are fused into one or kept apart | A switch | Körding et al. 2007 |
| 4. Inside–outside balance | Allocation between signals from the body and from the world | Partly | Ren et al. 2024; Suzuki et al. 2013 |
| 5. Persistence | How long a setting lasts after its cause is gone | Not applicable | Adams, Graf & Ernst 2004 |
In predictive-processing terms, all five are aspects of precision weighting. The name deliberately avoids that vocabulary. It names the configuration itself, so the concept survives if another theory turns out to explain it better.
| McLuhan's term | Translation | Status |
|---|---|---|
| Sense ratio | Relative precision across sensory channels | Proposed translation |
| A medium that intensifies one sense | Sustained up-weighting of one channel's precision | Proposed translation |
| Numbing, autoamputation | Down-weighting of the channels that lose out | Proposed translation |
| "Closure," a new equilibrium | Precision settings that persist after the medium is gone | Proposed; untested |
| The sensorium, "tactility" as interplay of all senses | Multisensory integration | Close fit; established field |
The ratio is real and shaped by experience
Three lines of evidence show the brain's weighting of the senses is plastic, not fixed. Only the second measures precision weighting directly; the other two show that the balance can move.
- 1Loss. In deaf and blind people, cortex normally given to the lost sense is recruited by the others (Bavelier & Neville, 2002). Cortical areas appear organised by the computation they perform rather than by a single sense, so senses compete for shared machinery (Pascual-Leone & Hamilton, 2001).
- 2Development. Children under about eight do not combine vision and touch in the reliability-weighted way adults do; one sense dominates instead (Gori et al., 2008). The weighting is learned.
- 3Visual dominance is acquired. Adults tend to miss a sound when it arrives with a flash, the Colavita effect. A meta-analysis found a medium-to-large effect in adults and none in children (Hirst, Cragg & Allen, 2018). The adult tilt toward vision is a ratio that develops.
The inside channel
The ratio includes the body's interior. In one EEG study, visual stimuli arriving with stronger heartbeat signals were selectively suppressed, and frequent overlap of heartbeat and visual timing reduced visual sensitivity. The authors conclude that internal cardiac processing trades off against external visual processing (Ren et al., 2024). The effects were modest and operate within a heartbeat, but they show a ratio between inside and outside in the physiology itself.
This matters because interoception is not a minor sense. Leading accounts treat it as the brain's means of regulating the body, predicting and meeting its needs before they arise (Barrett & Simmons, 2015; Kleckner et al., 2017), and as the basis of felt emotion (Seth, 2013). A shift in the balance between outside and inside is therefore never only perceptual. It touches regulation itself.
6What the translation buys
The translation does three things the original theory could not: it explains why media effects go unnoticed, it turns "the medium is the message"3 into a measurable claim, and it replaces a dead dichotomy with a continuous variable. All three are interpretation, offered for testing.
"Without any resistance"
McLuhan said media alter sense ratios "steadily and without any resistance."1 Precision explains why. We experience the world through the weighting of our senses, not the weighting itself. A change in which channel is trusted shows up as a change in what seems salient, real or urgent. It does not show up as an event. There is nothing to resist, because nothing is presented.
This is figure and ground at the level of mechanism. Content is figure: what we notice. Syncrasis is ground: the setting that decides what can be noticed at all. McLuhan compared content to the "juicy piece of meat"1 a burglar throws to distract the watchdog of the mind. On this reading, the content holds attention while the medium re-tunes the weights.
"The medium is the message" as a measurable claim
If a medium's effect is the change it makes to the syncrasis, the claim becomes concrete. Each medium has a characteristic pattern of channels it up-weights and down-weights, regardless of what it carries. Two media delivering identical content should therefore leave different traces in the syncrasis. That is a testable proposition, and it is roughly what the 1954 Toronto experiment was groping toward with the tools it had.
Continuous variables, not two hemispheres
The lateralization version gave McLuhan two boxes, left and right, and a claim about cultures. Syncrasis is a set of continuous quantities that vary between people, across development, from moment to moment, and across channels, including the body's interior. They can be estimated with existing tasks, such as cue-combination weights between senses, heartbeat-discrimination tests and computational models of interoceptive precision. A claim that can be measured can also be wrong. The next two sections say how.
7Predictions
If syncrasis is what McLuhan was describing, five things should be true. Each can be tested with existing methods, and each has a result that would count against it. None has yet been tested as stated.
| # | Prediction | How to test it | What would count against it |
|---|---|---|---|
| 1 | Medium signatures. The same content delivered through different media shifts the syncrasis differently. | Within-subject crossover: identical material as text, audio and video, with cue-combination weights (Ernst & Banks, 2002) and heartbeat-discrimination tasks before and after. | No medium-specific differences once content and duration are matched. |
| 2 | Closure. Sustained exposure leaves shifts that outlast the exposure. | Repeat the same measures hours and days after a period of heavy use of one medium. | Shifts vanish as soon as exposure ends. |
| 3 | Development. Early, heavy exposure to visually intense media brings forward or strengthens the developmental tilt toward visual dominance. | Track the Colavita effect longitudinally in children with differing early media exposure, controlling for family factors. | Timing and size of visual dominance unrelated to exposure. |
| 4 | The interior cost. Sustained up-weighting of external channels lowers interoceptive precision, not only self-reported body awareness. | Heartbeat discrimination plus computational estimates of interoceptive precision under perturbation (the breath-hold paradigm of Smith et al., 2020). | Interoceptive precision unchanged or raised by heavy exteroceptive use. |
| 5 | Without resistance. People will not notice these shifts. Measured changes will outrun reported ones. | Compare measured precision shifts with participants' own ratings of change. | Self-reports track the measured shifts closely. |
One methodological note applies throughout. Heartbeat-counting tasks should be avoided, because their validity as accuracy measures is disputed (Zamariola et al., 2018). Discrimination tasks and computational estimates of precision are preferred.
The predictions are ordered from easiest to hardest to test. Prediction 1 needs a short laboratory study. Prediction 3 needs a cohort study measured over years. Prediction 5 is the most distinctively McLuhanian: it claims not only that media change us, but that the change is structurally invisible to the person undergoing it.
8Limits and objections
The thesis has five weak points. Each is stated here with the paper's response, or with an admission that there is none yet.
The Thomist objection. McLuhan's ratio comes from Aquinas, where it means proportion and reason, not a measurable quantity. Response: McLuhan used both senses at once. His claim was physiological in reference and metaphysical in form. The translation tests the reference. It takes no position on the metaphysics.
The probe objection. McLuhan called his statements probes, and is reported to have said in a 1965 interview, "I don't explain, I explore."1 Response: this describes his method of presentation, not a denial that the effects were real. His 1960 hypothesis and his critics' responses show the effects were meant as real.
The flexibility objection. Predictive processing is a broad framework, and critics argue it can describe almost anything after the fact. The predictive-processing description of syncrasis inherits this risk, though the term itself names the configuration, whatever theory explains it. The response is the specificity of section 7: named measures, named designs, named results that would count against the thesis.
The evidence on media is thin. Direct evidence that everyday media shift interoception is limited to one diary-style study linking screen time to lower self-reported body awareness (Wallman-Jones et al., 2023), one study linking smartphone-captured attention to lower interoceptive awareness (Haruki et al., 2025), and the laboratory trade-off in section 5. Large studies of screen use find average associations with well-being that are negative but very small (Orben & Przybylski, 2019). The predictions are untested, not supported.
Interoception is not one thing. Accuracy, self-reported sensibility and awareness of one's own accuracy are separable (Garfinkel et al., 2015), and they often disagree. Prediction 4 targets precision specifically, because the other measures could move for different reasons.
What would sink it
The core claim fails if media matched on content produce no distinct signatures in the syncrasis (prediction 1). The distinctively McLuhanian part fails separately if the shifts that do occur are readily noticed by the people undergoing them (prediction 5). Media would then still re-weight the senses, but not as an invisible ground. If both fail, McLuhan's sense ratios stay where they have been for decades: a striking metaphor.
9Conclusion: a lens, not a verdict
McLuhan's sense ratios were a hypothesis about the nervous system that lost its evidence and was demoted to metaphor. Translated as sensory syncrasis, the hypothesis can be tested again. This paper claims no more than that, and no less.
The lens opens three questions for further work:
If syncrasis holds, McLuhan's best-known sentence acquires a literal reading. The medium is the message because the medium changes the weights before the message arrives.
References
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