The Brain’s Online Contrast Hook

In The Shallows, Nicholas Carr discusses the possible — and documented — brain effects of web surfing, for example compared with deep reading of a traditional text-on-paper book. While it’s not my purpose to discuss his broad findings and ideas about the neuropsychology and neurobiology of the web, his themes seem relevant to the fundamental contrast-enhancing functionality of brain sensory systems.

Carr’s overarching theme is about distraction, or in other words, diversion of attention from one cognitive task to another available through following links or via the various feeds that Google et al. will gladly deliver to our attention. Anyone online for more than 10 minutes comes to realize the reality of this phenomenon. But as Carr tries to remind us, we seem to conveniently forget the findings of 20th-Century neuroscience regarding the incredible synaptic flexibility of our brains. Simply stated, repeated inputs, especially when they are paired in certain ways will, over time, change the wiring strengths in brain pathways. Changing our minds, our mode of cognitive processing.

For better? For worse? While I agree with Carr’s major cautionary conclusion, I think perhaps his analysis should perhaps also include one feature of our brains that may help attract (and possibly addict) us to the multimedia distraction machine that is the internet. That feature: our brains are naturally wired for contrast. Said another way, our brain’s sensory-perception systems are wired to perform as contrast detectors.

Musical Hooks

On a psychological level, think about the auditory contrast among the notes of a syncopated melody or of different harmonies or chord progressions. Musical hooks use contrast, and they can be both persistent and insistent, as the songs constantly playing in some of our brains attest.

Perception of Light Touch: Ordered Maps

As a relatively straightforward neurobiological example, let’s take a look at our perception of light touch. And please be aware that I’m oversimplifying to flesh out the idea of the contrast-detection inherent in the wiring of brain sensory systems. It’s usually characterized as center-surround organization, with contrast achieved by lateral inhibition. The wiring is highly ordered so that each touch receptor in the skin ultimately sends its signal to a specific place in the cerebral cortex. Hence, an ordered map of the skin exists in the brain.

This ordering is often represented as a sensory homunculus, a map of body parts on the surface of the somatosensory cortex.

Contrast in Perception of Light Touch: Center-Surround Organization

And while it may come as little surprise that this brain system is highly organized, our interest here is to ask the question: how does the brain use contrast to determine the exact location of a touch to the skin. For instance, how do I accurately perceive that my thumb was touched at a specific spot rather than a centimeter to the right or left of that place?

The answer is that center-surround organization makes that precise localization of the point of touch far more likely. As shown by my homemade diagram below, each point on the skin sends information to the thalamus and subsequently to the postcentral gyrus of the cerebral cortex where perception of that touch occurs. Again, the wiring is highly ordered so that each touch receptor (R) in the skin sends its signal to a specific place in the cortex, so that an ordered map of the skin exists in the brain. Beyond the lower resolution homunculus that shows the thumb projecting to a different place in the sensory cortex than the index finger, a higher resolution map would show us that adjacent points on the thumb project to adjacent places in the sensory cerebral cortex.

To improve perception/localization of touch, the brain creates contrast as shown schematically in my diagram.

Touch the skin and a receptor (red R) is activated and nerve impulses are sent in a sensory neuron (yellow/orange) to a relay neuron (purple triangle) in the Thalamus, exciting that cell (represented by the green +). But it also excites (+) the pink hexagonal neurons slightly to its left and right. In turn those pink neurons send inhibitory (red –) signals to the relay (purple) neurons close by on either side. So a touch to the skin not only stimulates the relay neuron in the direct transmission line to the cerebral cortex, but also inhibits transmission from the surrounding areas of skin (lateral inhibition). This essentially creates contrast — the central touch signal goes through the Thalamus to the cortex while the surrounding areas of the skin are in effect temporarily desensitized (center-surround organization). This represents enhanced contrast and makes it much more likely that our brains can establish the exact location of the touch to the skin of our thumb.

Vision: More Complicated, But Same Idea

More complex sensory surfaces, such as the retinas of our eyes use similar center-surround contrast strategies. (For a video explaining some of this wiring, see for example, .) And while characterizing the retina as a contrast detector might not seem as obvious, the same ideas about lateral inhibition apply. Suffice it to say that contrast creation through center-surround organization of neuron wiring appears to be a fundamental aspect of brain processing of sensory information.

Brain as Contrast Detector or Contrast Junkie?

As we’ve seen, one conclusion often expressed about the sensory brain is that it can be characterized as a contrast detector. So it seems reasonable to ask whether the brain might be predisposed by its neuronal wiring to also be a contrast seeker, even perhaps a contrast junkie in extreme instances. In other words, does the structure of the internet, replete with contrast opportunities — embedded or linked text, images, music and video — represent an irresistible inducement to a contrast-detecting brain to become strongly attached? As some folks clearly seem to be in their relationship with their digital devices.

So beyond Carr’s premise of the web as “distraction machine,” is it also an addiction machine? Are the contrasts of mixed media and the attention-shifting contrasts of multimedia surfing addictive in their own right? In the context of what our modern corporately entrained brains have become, of course.

Revisiting music — the songs in our heads — may be an indicator of the addictive nature of sensory contrast. Perhaps the double dose of pitch/harmony contrast in music superimposed onto the intrinsic contrast producing mechanisms in our brains is irresistible. Hell, occasionally playing in my brain are commercial jingles from my childhood. I wish I could shoo them away, but they’re amazingly resistant to dismissal.

And while I don’t have definitive answers, I think that against the backdrop of Carr’s revelations concerning the distracting and deep attention-disrupting aspects of web surfing, to consider this question of contrast attachment is necessary. It has implications for human cognitive structure and evolution, as well as for how educators employ the web in developing curricula for their students.

And despite the enthusiasm of advertisers for such attraction/addiction, such issues may have relevance to the ability of our species to adjudicate productive approaches to the existential challenges facing us. Hence they require significantly more discourse than they seem to receive. Such discussions are likely not the preference of Big Tech corporate oligarchs, particularly in light of their recently demonstrated propensities toward censorship. But that shouldn’t stop us.


Carr, Nicholas (2020, updated edition). The Shallows, Atlantic Books, London. ISBN 978–1–83895–268–7

Ideas about bio-medicine and environmentalism. Vin holds a PhD from Columbia U. in Cell and Molecular Biology & worked as a college prof. & science writer.

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