How Nature & Other Physical Environments Impact Your Focus, Cognition & Health | Dr. Marc Berman

This stretch of the conversation is framed as a practical problem before it becomes a mechanistic one: attention is treated as a finite resource that the built world drains, and nature as one of the few environments that refills it. Huberman opens the episode by noting that the discussion will range over stress, rumination and the effect of the natural world on the brain, and he starts from what he calls the issue of recapturing attentional abilities, on the premise that almost everyone, diagnosed with ADHD or not, feels their attention pulled in directions they did not choose. Berman accepts the framing and widens it: attention is not merely the thing children need in a classroom, because elements of attention are involved in controlling all of our behaviors, and when attention is depleted there is less impulse control, possibly more aggressive behavior, and a reduced capacity to reach goals.

The theoretical scaffolding comes from Berman's mentor Steve Kaplan, credited here with the idea that humans possess two kinds of attention. Directed attention is the voluntary kind: the individual decides what to attend to, as Huberman is deciding to attend to Berman's speech rather than to anything more inherently interesting in the room. Berman calls this a distinctly human capability and notes that other species may share it, but humans are unusually good at sustaining focus on a lecture, a paper or a math problem. The limitation is duration. He describes the familiar late-afternoon state of staring at a screen without being able to focus, and the lecture-hall version in which students are engaged for the first five minutes and visibly fading forty-five minutes in. He names this directed attention fatigue.

Involuntary attention is the counterpart: attention captured automatically by interesting stimulation, such as bright lights or loud noises, over which the individual has little control. Berman's argument is that this second system is far less susceptible to depletion, and he offers an informal demonstration rather than a citation: people rarely say they must step away from a beautiful waterfall because it is too interesting, or stop watching a movie because the interest has exhausted them. The clinical implication follows directly, in his words, that maybe we can restore directed attention by going into environments that can softly capture our involuntary attention.

Huberman adds an evolutionary argument about the digital interface. If involuntary attention evolved to interrupt a person moving through space, alerting them to arrivals, danger or the smell of dinner, then a screen collapses the whole world into the central visual field, where the peripheral channel that would normally carry those interrupts is occupied by the same content one is trying to resist. His inference is that maintaining directed attention inside that narrow tunnel is therefore unusually hard, and Berman agrees with each step of the reasoning without adding data of his own. The exchange is a good example of how much of this episode's mechanistic talk is inference layered on a robust behavioral effect.

I guess if you felt like you had to create an isolation chamber sort of thing to focus your directed attention, that's where we would say it's probably time to take a break, that that's when we might recommend that you go for a walk in nature to recharge this precious directed attention resource.

That line converts a laboratory theory into a self-monitoring rule. Needing sensory isolation to work is read as a symptom rather than a strategy, and the prescribed response is a walk in nature rather than more effort. Berman is explicit about what does not count: "the break better not be scrolling on social media." He does not claim that powering through is impossible, only that it is probably not the most productive use of a depleted state.

Huberman probes for a physiological substrate, offering three candidates: catecholaminergic signaling in the noradrenergic and dopaminergic systems, fatigue of the visual or auditory machinery itself, and the difficulty of holding fixation on a target. He notes that letting the eyes rest makes it easier to return to a target, which would be consistent with a peripheral sensory contribution. Berman's answer is unusually candid for a podcast of this format.

I'm not sure I have a great answer yet.

What he offers instead is a puzzle and a level of description. The puzzle is metabolic: brain metabolism is roughly 20% of overall metabolism regardless of what a person is doing, apart from extreme exercise, where it declines slightly, so it is not obvious why a hard calculus problem should produce fatigue when sleeping does not. Because he cannot point to the neurological component, he deliberately shifts to the psychological level, where fatigue is the sensation of being unable to focus. His anatomical speculation is offered as speculation: directed attention is probably frontal cortex, while involuntary or exogenous attention, driven by external stimulation, probably recruits parietal, occipital or auditory cortex depending on the modality. Nothing in the surrounding discussion tests these assignments.

Berman identifies the 2008 study as the seminal experiment, and his rationale is methodological rather than theoretical. Earlier nature-walk studies asked participants how they felt afterwards, and people reliably reported feeling refreshed. He compares that to a pill that is supposed to make you stronger: if the only evidence were participants saying they felt able to lift more weight, the result would be unsatisfying, because the interesting question is whether they actually can. The study was therefore built as an experimentally controlled comparison with objective performance measures before and after a walk.

The cognitive probe was the backwards digit span task, with digits presented aloud at about one digit per second and repeated in reverse order, starting at three digits and ascending to about nine. Berman notes that five digits is already difficult. Participants then walked one of two routes of roughly 2.6 miles, taking about fifty minutes: through the Ann Arbor Arboretum beside the psychology building at the University of Michigan, or along busy Washtenaw street in downtown Ann Arbor. Cell phones were confiscated so that nobody texted or chatted, and a GPS watch served two purposes, confirming that participants actually walked rather than detouring to a coffee shop and revealing whether anyone got lost, since a lost participant might not be restored at all.

The design was within-subject and crossed over: participants repeated the whole procedure a week later on the other route, so each person served as their own control. The headline result was an improvement of about 20% in working memory capacity and in the ability to direct attention after the nature walk relative to the urban walk.

Two controls address the obvious confound that the nature walk was simply nicer. Participants did tend to prefer it, and mood did improve, but the correlation between mood improvement and cognitive improvement was not strong. The sharper test was seasonal. Some participants walked in June at 80 degrees Fahrenheit and told Berman they could not believe he was paying them to walk in nature; others walked in January at 25 degrees Fahrenheit and reported being miserable and not enjoying the walk at all. The January group obtained the same working memory and attention benefits as the June group. Berman treats this as the counterintuitive core of the finding: the benefit does not depend on liking the environment, which points toward something in the auditory, visual and possibly tactile processing of nature rather than toward pleasantness or mood.

Asked to explain the mechanism, Berman separates two contributions and insists that both are at play. The first is that the nature route simply demanded less directed attention. The urban route required crossing many streets, which enforces vigilance; it carried traffic noise at an estimated 40 to 45 miles per hour; and it passed shops and billboards that also make claims on directed attention, so the mind could not wander. The arboretum route involved crossing only one or two streets before entering, carried no advertising, and therefore left directed attention comparatively unexercised.

The second contribution is what Kaplan's framework calls soft fascination. A waterfall is the illustrative case: the rush of water, the bubbles and froth hold attention, but a person can still mind-wander and think about something else at the same time, so the stimulation does not consume the entire attentional budget. Times Square is offered as the opposite: also extremely interesting, but captivating in a way that leaves no room for reflection or mind wandering. Berman's formulation is that both capture involuntary attention, but the city does so harshly and the waterfall softly, and only the soft version is restful for directed attention. His summary of why the nature walk restored performance is that it did not place as many demands on directed attention and it supplied softly fascinating stimulation without being all-consuming.

Huberman's own hypothesis, which he presents as improvised, is structural: nature is irregular, with trees not spaced at engineered intervals and with features that break up regularity, whereas cities and interiors are dominated by right angles and fixed block sizes. Berman accepts this as plausible and connects it to the finding that the effects survive replacement of the environment by media.