2023 commentary · Current Biology
the brain can build a map from smells
Raithel and colleagues found that people can build a mental map using smells as landmarks, with map-like brain activity in both a navigation region and a smell region. in our commentary, we discussed what this means for how the brain builds maps in general.
the smell of warm bread means a bakery is nearby; coffee means a café. once you know where smells come from, they can work as landmarks. Raithel and colleagues had participants navigate a virtual environment using smells as landmarks while their brain activity was measured with fMRI, which tracks changes in blood flow as a stand-in for brain activity.
they found a ‘grid-like’ pattern, a signature of the brain’s map-making system, in two places: entorhinal cortex, a navigation hub, and piriform cortex, a main smell-processing area. in our commentary, we put forth the idea that this points to two kinds of systems working together: a general one that maps relationships no matter what they’re about, and more specialized ones based on the type of stimulus being mapped, like scent or a social hierarchy.
finding places by smell
you arrive in a neighborhood with no signs. you smell warm bread in one spot, coffee in another, and flowers and cut wood somewhere else. where would you look for the bakery?
try finding all four places from their smells. once you know which smell goes with which place, you can use it to find your way.
tap a smell on the map on the right to make a guess.
taking a shortcut
now you’ll start at one smell and point toward a place you’ve never walked to from there. if you remember where the smells were, you can work out which way to go.
this is what a ‘cognitive map’, an internal map of how places relate to one another, lets you do: find routes you’ve never taken. Raithel and colleagues tested this kind of navigation in a virtual environment with smells as landmarks, while participants were in the scanner. they wanted to know whether smells could engage the same mapping system that’s usually studied with visual landmarks.
a grid-like pattern in two regions
smells did engage it! while participants navigated by smell, entorhinal cortex showed a ‘grid-like’ pattern, the signature of cells the brain uses to map space (the details tab explains how this pattern works). so did anterior piriform cortex, part of the brain’s smell system.
the patterns in the two regions were also aligned, pointing in the same direction. this suggests that the navigation system and the smell system were organizing the same space together. the 3D map on the right shows their result in each region.
what’s shared and what’s specific
this is the point we raised in our commentary. some brain regions show map-like patterns in almost every mapping study, whatever is being mapped. others depend on what is being mapped.
our figure on the right lays this out. entorhinal and medial prefrontal cortex show map-like patterns across many kinds of tasks. when the map is of visual objects, a visual region also shows the pattern, and when the map is built from smells, piriform cortex does. we proposed that the general regions provide the structure of the map, while the specialized regions keep the details that make each kind of information useful.
‘grid-like’ refers to grid cells, neurons in entorhinal cortex that fire at locations arranged in a repeating hexagonal grid, like a honeycomb, as an animal moves around. because many grid cells share the same orientation, fMRI can pick up a signal that rises and falls every 60 degrees of movement direction. this is the signature Raithel and colleagues measured, and the orientation of that signal was aligned between entorhinal and piriform cortex. fMRI measures this at the level of large groups of cells, rather than recording individual grid cells.
our figure puts this study next to others. Constantinescu and colleagues (2016) found grid-like patterns when people moved through a “space” of bird drawings that varied in neck and leg length, including in lateral occipital cortex, a visual object region. Park and colleagues (2021) found them for a social hierarchy. Raithel and colleagues add smell and piriform cortex. across all of them, entorhinal and medial prefrontal cortex keep showing up, while the other regions depend on what’s being mapped.
read the full commentarysame places, different cues
choose a scent on the map. which one would lead you there?
click or tap in the direction you’d head. arrow keys and enter work too.
the garden and its floral smell locate the same place in this example.
the study discussed in Orloff & Boorman, 2023 used odor landmarks in a virtual environment.
the brain evidence
a grid-like pattern while navigating by smell
green marks where activity followed the grid pattern measured in each region.
bringing the brain into view…
the 3D map could not load. the original figure is below.
where activity rose and fell every 60 degrees as participants navigated by smell, in line with the grid orientation measured in each region. shown at P < 0.005, uncorrected.
their group maps on a standard brain surface, showing only the clusters that touch entorhinal or anterior piriform cortex (the full maps are on NeuroVault). lighter means stronger evidence (the bar shows the t-statistic scale). the threshold is uncorrected and for display only; the paper’s main tests are within anatomically defined regions.
drag to rotate
fMRI · Raithel and colleagues’ study
a hub for navigation and mental maps.
a main smell-processing area.
what we found here
Raithel and colleagues found grid-like patterns in both regions, lined up in the same direction (figures 4 to 6 of their paper). the 3D map shows the parts of their whole-brain maps that touch each region.
