howclose.to
WHOLE BRAIN CONNECTOMICS

The problemsIntelligence & MachinesBrain Connectomics

Updated · July 2026

How close are we to mapping every connection in a mammalian brain?Can we draw a complete wiring diagram of a brain?

Can we draw a complete wiring diagram of a brain?How close are we to mapping every connection in a mammalian brain?

A whole adult fly is mapped at synapse resolution and a mouse cortical fragment now contains more than half a billion detected synapses; the whole mouse remains an estimated three orders of magnitude larger.Researchers have mapped a whole fly brain and a tiny piece of mouse brain. The hard part is scaling that detail across an entire mammal without losing or inventing connections.

Current stageScalingScaling toward dependable, competitive operationMaking it work at useful scale
Read the evidence Where we stand
The path so far

Recorded milestones

  1. Adult worm reconstructed

  2. Mammal millimeter becomes the frontier

Open tests · conceptual branches

TODAY
  1. OPEN TEST04 ↗
    Wire-map an entire mouse brainWhole mouse synaptic connectomePartial volumes do not bank a percentage of whole-brain completion. The gate stays open until an anatomically complete mouse is traced with measured errors.Trace a complete adult mouse brain at synapse resolution with quantified acquisition, segmentation and synapse-detection errors.
TODAY
OPEN TEST04 ↗
Wire-map an entire mouse brainWhole mouse synaptic connectomePartial volumes do not bank a percentage of whole-brain completion. The gate stays open until an anatomically complete mouse is traced with measured errors.Trace a complete adult mouse brain at synapse resolution with quantified acquisition, segmentation and synapse-detection errors.
Milestones are in recorded order. Open tests are separate questions, not a forecast.Milestones are in date order. Open tests are separate tests, not a promise.
Brain ConnectomicsWhere we stand

The path shows Mammal millimeter becomes the frontier. Open thresholds are separate tests in a conceptual map, not a forecast. No single percentage represents progress.

01 · Where we standWhere we stand

Where we standWhere we stand

The current stage, the evidence recorded, and the single threshold that gates the next stage.The stage reached, the evidence recorded, and the one test still blocking the next step.

What the evidence has shownWhat the evidence has shown

Researchers have mapped a whole fly brain and a tiny piece of mouse brain. The hard part is scaling that detail across an entire mammal without losing or inventing connections.A whole adult fly is mapped at synapse resolution and a mouse cortical fragment now contains more than half a billion detected synapses; the whole mouse remains an estimated three orders of magnitude larger.

3 of 4 tests recorded3 of 4 thresholds achieved open
✓Map a whole wormComplete adult worm nervous systemAchieved
✓Map a whole adult fly brainWhole adult insect brain connectomeAchieved
✓Map and watch a mammal brain blockCubic-millimeter functional mammal connectomeAchieved
Wire-map an entire mouse brainWhole mouse synaptic connectomeOpen test

The next testBlocking threshold

Wire-map an entire mouse brainWhole mouse synaptic connectome

Demonstrate an end-to-end acquisition and proofreading pipeline across a whole mouse region that preserves both local synapses and long-range axons.Demonstrate an end-to-end acquisition and proofreading pipeline across a whole mouse region that preserves both local synapses and long-range axons.

02 · Thresholds & evidenceThresholds & evidence

What would count as proof?What would count as proof?

A clear test, the evidence behind it, and the limits of the claim.A clear test, the evidence behind it, and what it still leaves open.

✓Achieved · 1986

Map a whole wormComplete adult worm nervous system

The 1986 reconstruction covered 302 neurons, about 5,000 neuron-to-neuron chemical synapses and about 700 gap junctions.Reconstruct every neuron and the chemical and electrical connections in an adult C. elegans hermaphrodite.

Recorded result White, Southgate, Thomson and Brenner

SOURCE RECORD

C. elegans nervous-system reconstruction reference ↗

Still to demonstrateWhat this does not establishA separate system can still have different conditions and results.The named measurement has no wider implication until the supporting system test is also shown.

View every evidence record

Achieved

Map a whole wormComplete adult worm nervous system

The 1986 reconstruction covered 302 neurons, about 5,000 neuron-to-neuron chemical synapses and about 700 gap junctions.Reconstruct every neuron and the chemical and electrical connections in an adult C. elegans hermaphrodite.

Recorded result: 1986 · White, Southgate, Thomson and Brenner

C. elegans nervous-system reconstruction reference ↗

Achieved

Map a whole adult fly brainWhole adult insect brain connectome

FlyWire reconstructed 139,255 neurons and 54.5 million chemical synapses in one adult female fly brain.Reconstruct and annotate a complete adult insect brain at synapse resolution.

Recorded result: Oct 2024 · FlyWire Consortium, version 783

Next test: Map additional adult flies to measure individual variation and the errors that remain.

FlyWire whole adult brain ↗

Achieved

Map and watch a mammal brain blockCubic-millimeter functional mammal connectome

MICrONS paired a roughly 1 mm³ mouse visual-cortex reconstruction with activity recordings; it is a fragment, not a whole brain.Reconstruct approximately one cubic millimeter of mammalian cortex and register its synapses to functional recordings from the same animal.

Recorded result: Apr 2025 · More than 200,000 cells and about 524 million detected synapses

Next test: Release a substantially larger contiguous mammalian volume with published reconstruction-error measurements.

One cubic millimeter of human cortex ↗

Open test

Wire-map an entire mouse brainWhole mouse synaptic connectome

Partial volumes do not bank a percentage of whole-brain completion. The gate stays open until an anatomically complete mouse is traced with measured errors.Trace a complete adult mouse brain at synapse resolution with quantified acquisition, segmentation and synapse-detection errors.

Next test: Demonstrate an end-to-end acquisition and proofreading pipeline across a whole mouse region that preserves both local synapses and long-range axons.

03 · The recordThe record

The record behind the verdictThe record behind the verdict

Major milestones and smaller context events share one source-linked record. Scheduled targets are marked separately from recorded events.Milestones, smaller events and setbacks share one sourced record. Scheduled targets are marked separately from recorded events.

Event key
TheoryExperimentFundingSetbackPolicyDeployment Future / scheduled
1956-19851 event1 shown

Synapses enter the microscope

Recorded event: “Synaptic cleft resolved by electron microscopy”.

1956
Synaptic cleft resolved by electron microscopyExperiment
Electron micrographs described the separated pre- and postsynaptic membranes, cleft and vesicle-rich terminals of vertebrate central synapses.
Sanford Palay
1986-20041 event1 shown

The first complete network

Recorded event: “First complete animal wiring diagram”.

1986
First complete animal wiring diagramDeployment
Serial electron micrographs reconstructed the adult C. elegans hermaphrodite nervous system: 302 neurons, about 5,000 chemical synapses between neurons and about 700 gap junctions.
White, Southgate, Thomson and Brenner
2005-20192 events0 shown

Connectomes split by scale

This record spans “The human connectome is defined” to “Human Connectome Project announced”.

2009
Human Connectome Project announced
NIH announced up to $30 million over five years for non-invasive maps of healthy adult human brain circuitry. These MRI-scale networks do not enumerate individual synapses.
US National Institutes of Health
2020-20244 events1 shown

Automation completes an insect brain

This record spans “Adult fly hemibrain released” to “Whole mouse cell atlas reaches 32 million cells”.

2020
Adult fly hemibrain released
A dense reconstruction of part of the adult fly central brain contained roughly 25,000 neurons and more than 20 million detected synapses; boundaries cropped many neurons.
Janelia FlyEM and collaborators
2023
NIH backs connectivity across scales
The first BRAIN CONNECTS round supported 11 grants projected to total $150 million over five years, including electron-microscopy pipelines for mouse-brain mapping.
US National Institutes of Health
2023
Whole mouse cell atlas reaches 32 million cells
A molecular atlas described the type, location and gene activity of more than 32 million mouse-brain cells. It was a whole-brain parts list, not a synapse-by-synapse connectome.
BRAIN Initiative Cell Census Network
2025-20281 event1 shown

The mammalian scaling test

Recorded event: “MICrONS maps a mammal millimeter”.

2025
MICrONS maps a mammal millimeterExperimentWe are here
A roughly one-cubic-millimeter mouse visual-cortex volume contained more than 200,000 cells and about 524 million detected synapses, registered to activity from around 75,000 neurons.
MICrONS Consortium
2024-20284 events1 shown

Additional records

This record spans “Human cortical millimeter reconstructed” to “Mouse-EM center reaches scheduled award endpoint” and includes scheduled targets.

2024
Human cortical millimeter reconstructed
A surgical sample of about one cubic millimeter yielded 149,871,669 machine-identified synapses. It was a small cortical fragment from one person, not a whole human brain.
Harvard and Google Research
2024
Whole adult fly brain completedDeployment
FlyWire reconstructed 139,255 neurons and a graph containing 54.5 million chemical synapses in one adult female fruit-fly brain.
FlyWire Consortium
2024
Proofreading still consumes decades of labour
The FlyWire authors estimated about 33 person-years of manual proofreading even after automated alignment, segmentation and synapse detection.
FlyWire Consortium
2028
Mouse-EM center reaches scheduled award endpointFundingTarget
The current NIH award period for one whole-mouse electron-microscopy pipeline is scheduled to end. The award funds a pipeline; it does not promise a completed connectome by that date.
NIH BRAIN CONNECTS and Allen Institute
— end of record · 13 events shown —
04 · The dataThe data

The data behind the verdictThe data behind the verdict

Read each measure with its units, conditions, uncertainty and source; targets remain distinct from observations.Read each chart with its conditions, limits and source. Future targets remain distinct from recorded results.

Open figure in archive: Complete synapse-resolution animal maps climbed from an adult worm nervous system to a whole adult fly brain, though synapse definitions and anatomical scope differ.Open figure in archive: The number of reconstructed neurons in complete animal connectomes rose about 461-fold from the adult worm to the adult fly.
Synapses reconstructed in landmark mammalian EM volumesSynapses reconstructed in landmark mammalian EM volumes
2e+95e+91e+102e+105e+101e+112e+111
Partial mouse visual cortex · ~1 mm³5.240e+8 synapses · 2025partial mammalian volume; approximately 1 mm³
  1. 1Partial human cortex · ~1 mm³ · 1.499e+8 synapses · 2024 · partial mammalian volume; approximately 1 mm³
  1. AOrder-of-magnitude whole-mouse estimate · not a measured census · 5e+11 synapsesOrder-of-magnitude whole-mouse estimate · not a measured census · 5e+11 synapses

Records are separate measurements; no line joins them. The observed points are partial mammalian volumes from different species, not a whole-brain time series. The 500-billion goal is an order-of-magnitude mouse estimate inferred from brain scale and cortical synapse density; it is an estimated reference, never an observed endpoint.

View data table
All independent metric records
DateValueStatusCondition / labelVerificationCICensoringSource
2024149871669 synapsesobservedPartial human cortex · ~1 mm³
partial mammalian volume; approximately 1 mm³		unspecified—noneOne cubic millimeter of human cortex
2025524000000 synapsesobservedPartial mouse visual cortex · ~1 mm³
partial mammalian volume; approximately 1 mm³		unspecified—noneMICrONS functional connectomics
Metric context · these are authored references, not observed records
ContextValueLabelBasisSource
Goal500000000000 synapsesOrder-of-magnitude whole-mouse estimate · not a measured census—One cubic millimeter of human cortex · MICrONS functional connectomics
AS OF July 2026
View every data figure
Synapses reconstructed in landmark mammalian EM volumesSynapses reconstructed in landmark mammalian EM volumes
2e+95e+91e+102e+105e+101e+112e+111
Partial mouse visual cortex · ~1 mm³5.240e+8 synapses · 2025partial mammalian volume; approximately 1 mm³
  1. 1Partial human cortex · ~1 mm³ · 1.499e+8 synapses · 2024 · partial mammalian volume; approximately 1 mm³
  1. AOrder-of-magnitude whole-mouse estimate · not a measured census · 5e+11 synapsesOrder-of-magnitude whole-mouse estimate · not a measured census · 5e+11 synapses

Records are separate measurements; no line joins them. The observed points are partial mammalian volumes from different species, not a whole-brain time series. The 500-billion goal is an order-of-magnitude mouse estimate inferred from brain scale and cortical synapse density; it is an estimated reference, never an observed endpoint.

View data table
All independent metric records
DateValueStatusCondition / labelVerificationCICensoringSource
2024149871669 synapsesobservedPartial human cortex · ~1 mm³
partial mammalian volume; approximately 1 mm³		unspecified—noneOne cubic millimeter of human cortex
2025524000000 synapsesobservedPartial mouse visual cortex · ~1 mm³
partial mammalian volume; approximately 1 mm³		unspecified—noneMICrONS functional connectomics
Metric context · these are authored references, not observed records
ContextValueLabelBasisSource
Goal500000000000 synapsesOrder-of-magnitude whole-mouse estimate · not a measured census—One cubic millimeter of human cortex · MICrONS functional connectomics
AS OF July 2026
Complete synapse-resolution animal maps climbed from an adult worm nervous system to a whole adult fly brain, though synapse definitions and anatomical scope differ.Complete synapse-resolution animal maps climbed from an adult worm nervous system to a whole adult fly brain, though synapse definitions and anatomical scope differ.

Independent records

Synapses in complete animal connectomes log scale · synapses

log scale · synapses
Adult worm · complete nervous system19865,000 synapses

Conditioncomplete-organism map; animal species and synapse definition vary

Larval fly · complete brain2023548,000 synapses

Conditioncomplete-organism map; animal species and synapse definition vary

Adult fly · complete brain graph20245.450e+7 synapses

Conditioncomplete-organism map; animal species and synapse definition vary

observed self-reported target / pathway right-censored lower bound
View data table
All plotted observations
SeriesDateValueStatusCondition / labelVerificationCICensoringSource
Synapses in complete animal connectomes19865000 synapsesobservedAdult worm · complete nervous system
complete-organism map; animal species and synapse definition vary		unspecified—noneC. elegans nervous-system reconstruction reference
Synapses in complete animal connectomes2023548000 synapsesobservedLarval fly · complete brain
complete-organism map; animal species and synapse definition vary		unspecified—noneComplete larval fly brain connectome
Synapses in complete animal connectomes202454500000 synapsesobservedAdult fly · complete brain graph
complete-organism map; animal species and synapse definition vary		unspecified—noneFlyWire whole adult brain
AS OF July 2026
The number of reconstructed neurons in complete animal connectomes rose about 461-fold from the adult worm to the adult fly.The number of reconstructed neurons in complete animal connectomes rose about 461-fold from the adult worm to the adult fly.

Independent records

Neurons in complete animal connectomes log scale · neurons

log scale · neurons
Adult worm1986302 neurons

Conditioncomplete-organism map; animal species and neuronal scope vary

Larval fly brain20233,016 neurons

Conditioncomplete-organism map; animal species and neuronal scope vary

Adult fly brain2024139,255 neurons

Conditioncomplete-organism map; animal species and neuronal scope vary

observed self-reported target / pathway right-censored lower bound
View data table
All plotted observations
SeriesDateValueStatusCondition / labelVerificationCICensoringSource
Neurons in complete animal connectomes1986302 neuronsobservedAdult worm
complete-organism map; animal species and neuronal scope vary		unspecified—noneC. elegans nervous-system reconstruction reference
Neurons in complete animal connectomes20233016 neuronsobservedLarval fly brain
complete-organism map; animal species and neuronal scope vary		unspecified—noneComplete larval fly brain connectome
Neurons in complete animal connectomes2024139255 neuronsobservedAdult fly brain
complete-organism map; animal species and neuronal scope vary		unspecified—noneFlyWire whole adult brain
AS OF July 2026

Methods & conditions

Under the measurement.

The boundary of a measurement changes what it means.

Technical note 1 of 3

A connectome is not one resolution

Method and conditions

The Human Connectome Project used diffusion and functional MRI to infer macro-scale links among brain regions. Dense EM connectomics instead traces membranes and synapses in fixed tissue. Whole-brain coverage in one does not imply synaptic resolution in the other.

Source notes
Human Connectome Project informatics overview MICrONS functional connectomics

Technical notes and sources

Each note keeps its authored method, conditions and citations available for review.

A connectome is not one resolution

The Human Connectome Project used diffusion and functional MRI to infer macro-scale links among brain regions. Dense EM connectomics instead traces membranes and synapses in fixed tissue. Whole-brain coverage in one does not imply synaptic resolution in the other.

Human Connectome Project informatics overview MICrONS functional connectomics
The target denominator is uncertain

No authoritative experiment has counted every mouse-brain synapse. The visual therefore plots sourced reconstructed numerators and treats 500 billion only as an order-of-magnitude target, without converting it into a completion percentage. Region, age, preparation and the definition of a detected synapse all affect comparisons.

NIH BRAIN 2.0 mouse-connectome assessment Mouse-brain volume and cortical synapse-density scaling MICrONS dataset paper
Segmentation is not verification

Machine learning now supplies most initial segmentation and synapse detection, but false splits, false merges and missed synapses remain. FlyWire estimated 33 person-years of manual proofreading and still describes its resource as correctable rather than error-free.

FlyWire adult brain reconstruction
05 · What it unlocksWhat it unlocks

If the remaining tests pass.If the remaining tests pass.

Downstream capabilities remain conditional on results that are not yet in.These are possible next capabilities, conditional on results that are not yet in.

TODAY
  1. PASSED · 198601 ↗
    Map a whole wormComplete adult worm nervous systemThe 1986 reconstruction covered 302 neurons, about 5,000 neuron-to-neuron chemical synapses and about 700 gap junctions.Reconstruct every neuron and the chemical and electrical connections in an adult C. elegans hermaphrodite.
  2. PASSED · Oct 202402 ↗
    Map a whole adult fly brainWhole adult insect brain connectomeFlyWire reconstructed 139,255 neurons and 54.5 million chemical synapses in one adult female fly brain.Reconstruct and annotate a complete adult insect brain at synapse resolution.
  3. PASSED · Apr 202503 ↗
    Map and watch a mammal brain blockCubic-millimeter functional mammal connectomeMICrONS paired a roughly 1 mm³ mouse visual-cortex reconstruction with activity recordings; it is a fragment, not a whole brain.Reconstruct approximately one cubic millimeter of mammalian cortex and register its synapses to functional recordings from the same animal.
  4. OPEN TEST04 ↗
    Wire-map an entire mouse brainWhole mouse synaptic connectomePartial volumes do not bank a percentage of whole-brain completion. The gate stays open until an anatomically complete mouse is traced with measured errors.Trace a complete adult mouse brain at synapse resolution with quantified acquisition, segmentation and synapse-detection errors.
PASSED · 198601 ↗
Map a whole wormComplete adult worm nervous systemThe 1986 reconstruction covered 302 neurons, about 5,000 neuron-to-neuron chemical synapses and about 700 gap junctions.Reconstruct every neuron and the chemical and electrical connections in an adult C. elegans hermaphrodite.
PASSED · Oct 202402 ↗
Map a whole adult fly brainWhole adult insect brain connectomeFlyWire reconstructed 139,255 neurons and 54.5 million chemical synapses in one adult female fly brain.Reconstruct and annotate a complete adult insect brain at synapse resolution.
PASSED · Apr 202503 ↗
Map and watch a mammal brain blockCubic-millimeter functional mammal connectomeMICrONS paired a roughly 1 mm³ mouse visual-cortex reconstruction with activity recordings; it is a fragment, not a whole brain.Reconstruct approximately one cubic millimeter of mammalian cortex and register its synapses to functional recordings from the same animal.
TODAY
OPEN TEST04 ↗
Wire-map an entire mouse brainWhole mouse synaptic connectomePartial volumes do not bank a percentage of whole-brain completion. The gate stays open until an anatomically complete mouse is traced with measured errors.Trace a complete adult mouse brain at synapse resolution with quantified acquisition, segmentation and synapse-detection errors.

Possible, if the open tests pass.

  1. Mechanistic circuit models

    This is a possible benefit only if mechanistic circuit models works safely, reliably and at useful scale.Potential benefit only if the stated use (Mechanistic circuit models) is demonstrated with evidence for safety, reliability, affordability and scale.

  2. Circuit-level disease hypotheses

    This is a possible benefit only if circuit-level disease hypotheses works safely, reliably and at useful scale.Potential benefit only if the stated use (Circuit-level disease hypotheses) is demonstrated with evidence for safety, reliability, affordability and scale.

  3. Biology-grounded intelligence

    This is a possible benefit only if biology-grounded intelligence works safely, reliably and at useful scale.Potential benefit only if the stated use (Biology-grounded intelligence) is demonstrated with evidence for safety, reliability, affordability and scale.

These are possible outcomes, if the open tests pass. They are conditional, not promises.
06 · SourcesSources

Follow the evidence.Follow the evidence.

15 sources — every figure on this page traces to one.15 sources — every figure on this page traces to one.

Source · what this page citesScope and limits
  1. 01

    C. elegans nervous-system reconstruction reference

    Read source ↗ncbi.nlm.nih.gov
    Used on this page
    All 8 uses across 6 destinations

    Scope: Not recorded. Limit: Not recorded.

  2. 02

    The human connectome: structural description

    Read source ↗pubmed.ncbi.nlm.nih.gov
    Used on this page
    All 2 uses across 2 destinations

    Scope: Not recorded. Limit: Not recorded.

  3. 03

    Scope: Not recorded. Limit: Not recorded.

  4. 04

    Scope: Not recorded. Limit: Not recorded.

  5. 05

    FlyWire whole adult brain

    Read source ↗nature.com
    Used on this page
    All 11 uses across 9 destinations

    Scope: Primary publication named by FlyWire whole adult brain. Limit: The publication's measured scope does not establish downstream engineering or deployment claims.

  6. 06

    Scope: Primary publication named by One cubic millimeter of human cortex. Limit: The publication's measured scope does not establish downstream engineering or deployment claims.

  7. 07

    Scope: Not recorded. Limit: Not recorded.

  8. 08

    NIH BRAIN CONNECTS awards

    Read source ↗braininitiative.nih.gov
    Used on this page
    All 2 uses across 2 destinations

    Scope: Not recorded. Limit: Not recorded.

  9. 09

    NIH BRAIN 2.0 mouse-connectome assessment

    Read source ↗nih.gov

    Scope: Not recorded. Limit: Not recorded.

  10. 10

    Mouse-brain volume and synapse-density scaling

    Read source ↗pmc.ncbi.nlm.nih.gov

    Scope: Not recorded. Limit: Not recorded.

  11. 11

    Timeline · Synaptic cleft resolved by electron microscopy

    Read source ↗pubmed.ncbi.nlm.nih.gov
    Used on this page
    All 2 uses across 1 destination

    Scope: Not recorded. Limit: Not recorded.

  12. 12

    Timeline · Human Connectome Project announced

    Read source ↗nimh.nih.gov
    Used on this page
    All 2 uses across 2 destinations

    Scope: Not recorded. Limit: Not recorded.

  13. 13

    Timeline · Whole mouse cell atlas reaches 32 million cells

    Read source ↗nimh.nih.gov
    Used on this page
    All 2 uses across 2 destinations

    Scope: Not recorded. Limit: Not recorded.

  14. 14

    Scope: Not recorded. Limit: Not recorded.

  15. 15

    Engineer note · A connectome is not one resolution

    Read source ↗pmc.ncbi.nlm.nih.gov

    Scope: Not recorded. Limit: Not recorded.

Sources describe evidence. Inclusion is not endorsement.