diff --git a/applications/visualizer/backend/ingestion/validator.py b/applications/visualizer/backend/ingestion/validator.py
index 7b8ed2de..9a59afe5 100644
--- a/applications/visualizer/backend/ingestion/validator.py
+++ b/applications/visualizer/backend/ingestion/validator.py
@@ -1,17 +1,19 @@
 from __future__ import annotations
 
-from enum import IntEnum
-from typing import Any, Dict, List, Literal, Optional, Tuple
+from enum import IntEnum, StrEnum
+from typing import Dict, List, Literal, Optional, Tuple
 
-from pydantic import BaseModel, Field
-from pydantic.alias_generators import to_camel
+from pydantic import BaseModel, Field, RootModel, model_validator
 
 
 class Data(BaseModel):
     neurons: List[Neuron]
     datasets: List[Dataset]
-    connections: Dict[str, Connection]
-    annotations: Dict[str, Annotation]
+    connections: Dict[str, List[Connection]]
+    annotations: Dict[
+        Literal["head", "complete"], # TODO: should 'tail' be included
+        Annotation
+    ]
 
 
 class Neuron(BaseModel):
@@ -24,6 +26,12 @@ class Neuron(BaseModel):
     typ: str  # type of the neuron
 
 
+class DatasetType(StrEnum):
+    COMPLETE = "complete"
+    HEAD = "head"
+    TAIL = "tail"
+
+
 class Axe(BaseModel):
     face: str
     axisIndex: int
@@ -33,7 +41,7 @@ class Axe(BaseModel):
 class Dataset(BaseModel):
     id: str
     name: str
-    type: Literal["complete", "head", "tail"]
+    type: DatasetType
     time: int  # TODO: should be gte than 0?
     visualTime: float  # TODO: should be gte than 0?
     description: str
@@ -67,11 +75,25 @@ class Connection(BaseModel):
     )
     typ: ConnectionType  # the type of connection ("electrical" or "chemical")
 
-
-class Annotation(BaseModel):
-    increase: List[
-        Tuple[  # the type of annotation
-            str,  # pre, the ID/name of a neuron from "neurons.json"
-            str,  # post, the ID/name of the other neuron from "neurons.json" that is part of the couple
-        ]
-    ]
+    @model_validator(mode="after")
+    def check_same_size_elements(self):
+        length = len(self.ids)
+        ok = all(
+            len(l) == length for l in iter([self.post_tid, self.pre_tid, self.syn])
+        )
+        assert (
+            ok
+        ), "ids, post_tid, pre_tid and syn must have the same number of elements"
+        return self
+
+
+class Annotation(RootModel):
+    root: Dict[
+        Literal["increase", "variable", "postembryonic", "decrease", "stable"],
+        List[
+            Tuple[  # the type of annotation
+                str,  # pre, the ID/name of a neuron from "neurons.json"
+                str,  # post, the ID/name of the other neuron from "neurons.json" that is part of the couple
+            ]
+        ],
+    ] = {}
diff --git a/applications/visualizer/backend/tests/ingestion/test_validator.py b/applications/visualizer/backend/tests/ingestion/test_validator.py
index c9a5fa21..cbea9138 100644
--- a/applications/visualizer/backend/tests/ingestion/test_validator.py
+++ b/applications/visualizer/backend/tests/ingestion/test_validator.py
@@ -1,20 +1,29 @@
+import json
 from typing import Any, Dict, List, NamedTuple
 
 import pytest
 from pydantic import ValidationError
 
-from ingestion.validator import Axe, Dataset, Neuron
+from ingestion.validator import (
+    Annotation,
+    Axe,
+    Connection,
+    ConnectionType,
+    Dataset,
+    DatasetType,
+    Neuron,
+)
 
 JSON = Dict[str, Any]  # just for type checking; otherwise does nothing
 
 
-class NeuronValidTc(NamedTuple):
+class NeuronTc(NamedTuple):
     data: JSON
     expected: Neuron
 
 
-valid_neurons_tc: List[NeuronValidTc] = [
-    NeuronValidTc(
+valid_neurons_tc: List[NeuronTc] = [
+    NeuronTc(
         data={
             "inhead": 1,
             "name": "ADAL",
@@ -38,15 +47,11 @@ class NeuronValidTc(NamedTuple):
 
 
 @pytest.mark.parametrize("data, expected", valid_neurons_tc)
-def test__valida_neuron(data: JSON, expected: Neuron):
+def test__valid_neuron(data: JSON, expected: Neuron):
     neuron = Neuron.model_validate(data)
     assert neuron == expected
 
 
-class NeuronInvalidTc(NamedTuple):
-    data: JSON
-
-
 invalid_neurons_tc: List[JSON] = [
     {
         "inhead": 2,  # not valid bool interpretation
@@ -84,13 +89,13 @@ def test__invalid_neuron(data: JSON):
         Neuron.model_validate(data)
 
 
-class DatasetValidTc(NamedTuple):
+class DatasetTc(NamedTuple):
     data: JSON
     expected: Dataset
 
 
-valid_datasets_tc: List[DatasetValidTc] = [
-    DatasetValidTc(
+valid_datasets_tc: List[DatasetTc] = [
+    DatasetTc(
         data={
             "id": "white_1986_jse",
             "name": "White et al., 1986, JSE (adult)",
@@ -102,13 +107,13 @@ class DatasetValidTc(NamedTuple):
         expected=Dataset(
             id="white_1986_jse",
             name="White et al., 1986, JSE (adult)",
-            type="tail",
+            type=DatasetType.TAIL,
             time=60,
             visualTime=50,
             description="Adult legacy tail with pre-anal ganglion",
         ),
     ),
-    DatasetValidTc(
+    DatasetTc(
         data={
             "id": "witvliet_2020_1",
             "name": "Witvliet et al., 2020, Dataset 1 (L1)",
@@ -125,7 +130,7 @@ class DatasetValidTc(NamedTuple):
         expected=Dataset(
             id="witvliet_2020_1",
             name="Witvliet et al., 2020, Dataset 1 (L1)",
-            type="head",
+            type=DatasetType.HEAD,
             time=0,
             visualTime=0.5,
             description="~0 hours after birth",
@@ -145,10 +150,6 @@ def test__valid_dataset(data: JSON, expected: Dataset):
     assert dataset == expected
 
 
-class DatasetInvalidTc(NamedTuple):
-    data: JSON
-
-
 invalid_datasets_tc: List[JSON] = [
     {
         "id": "white_1986_jse",
@@ -165,3 +166,135 @@ class DatasetInvalidTc(NamedTuple):
 def test__invalid_dataset(data: JSON):
     with pytest.raises(ValidationError):
         Dataset.model_validate(data)
+
+
+class ConnectionTc(NamedTuple):
+    data: JSON
+    expected: Connection
+
+
+valid_connections_tc: List[ConnectionTc] = [
+    ConnectionTc(
+        data={
+            "ids": [9583833],
+            "post": "ADAR",
+            "post_tid": [9576727],
+            "pre": "ADAL",
+            "pre_tid": [9577831],
+            "syn": [1],
+            "typ": 2,
+        },
+        expected=Connection(
+            ids=[9583833],
+            post="ADAR",
+            post_tid=[9576727],
+            pre="ADAL",
+            pre_tid=[9577831],
+            syn=[1],
+            typ=ConnectionType.CHEMICAL,
+        ),
+    )
+]
+
+
+@pytest.mark.parametrize("data, expected", valid_connections_tc)
+def test__valid_connection(data: JSON, expected: Connection):
+    conn = Connection.model_validate(data)
+    assert conn == expected
+
+
+invalid_connections_tc: List[JSON] = [
+    {
+        "ids": [9583833],
+        "post": "ADAR",
+        "post_tid": [9576727],
+        "pre": "ADAL",
+        "pre_tid": [9577831],
+        "syn": [1],
+        "typ": 1,  # invalid connection type
+    },
+    {
+        "ids": [9583833, 9583834],  # not same length
+        "post": "ADAR",
+        "post_tid": [9576727],
+        "pre": "ADAL",
+        "pre_tid": [9577831],
+        "syn": [1],
+        "typ": 2,
+    },
+    {
+        "ids": [9583833],
+        "post": "ADAR",
+        "post_tid": [9576727, 9583834, 9583834],  # not same length
+        "pre": "ADAL",
+        "pre_tid": [9577831],
+        "syn": [1],
+        "typ": 2,
+    },
+]
+
+
+@pytest.mark.parametrize("data", invalid_connections_tc)
+def test__invalid_connection(data: JSON):
+    with pytest.raises(ValidationError):
+        Connection.model_validate(data)
+
+
+class AnnotationTc(NamedTuple):
+    data: JSON
+    expected: Annotation
+
+
+valid_annotations_tc: List[AnnotationTc] = [
+    AnnotationTc(
+        data={
+            "increase": [
+                ["ADAL", "RIPL"],
+                ["ADAR", "RIPR"],
+                ["ADEL", "AVKR"],
+            ]
+        },
+        expected=Annotation(
+            root={
+                "increase": [
+                    ("ADAL", "RIPL"),
+                    ("ADAR", "RIPR"),
+                    ("ADEL", "AVKR"),
+                ]
+            }
+        ),
+    ),
+    AnnotationTc(
+        data=json.loads("{}"),  # empty annotation seems to be valid
+        expected=Annotation(),
+    ),
+    AnnotationTc(  # multiple annotation types
+        data={"increase": [["ADAL", "RIPL"]], "postembryonic": [["ADAL", "RIPL"]]},
+        expected=Annotation(
+            root={"increase": [("ADAL", "RIPL")], "postembryonic": [("ADAL", "RIPL")]}
+        ),
+    ),
+]
+
+
+@pytest.mark.parametrize("data, expected", valid_annotations_tc)
+def test__valid_annotation(data: JSON, expected: Annotation):
+    annotation = Annotation.model_validate(data)
+    assert annotation == expected
+
+
+invalid_annotations_tc: List[JSON] = [
+    {"inexistent": [["ADAL", "RIPL"]]},  # inexistent is not an annotation type
+    {
+        "increase": [
+            ["ADAL", "RIPL", "CEPDL"],  # not a tuple of only pre and post
+            ["ADAR", "RIPR"],
+        ]
+    },
+]
+
+
+@pytest.mark.parametrize("data", invalid_annotations_tc)
+def test__invalid_annotation(data: JSON):
+    with pytest.raises(ValidationError):
+        Annotation.model_validate(data)
diff --git a/format-ingestion.md b/format-ingestion.md
index 2a6d7e35..5c691f87 100644
--- a/format-ingestion.md
+++ b/format-ingestion.md
@@ -89,7 +89,7 @@ The schema is the following:
     "pre": string,        // the name of a neuron as defined in "neurons.json"
     "pre_tid": [ ... ],   // a list of int where each int represents the ID of a pre synapse for a dedicated pre neuron
     "syn": [ ... ],       // a list of int where each int represents the weight of a post or pre synapses (indice matches the neuron in pre/post_tid)
-    "typ": int            // the type of connection ("electrical" or "chemical")
+    "typ": int            // the type of connection ("electrical" (0) or "chemical" (2))
 }
 ```