eeg_signal_classification.ipynb

Kernel: Python 3

Electroencephalogram Signal Classification for action identification

Author: Suvaditya Mukherjee
Date created: 2022/11/03
Last modified: 2022/11/05
Description: Training a Convolutional model to classify EEG signals produced by exposure to certain stimuli.

Introduction

The following example explores how we can make a Convolution-based Neural Network to perform classification on Electroencephalogram signals captured when subjects were exposed to different stimuli. We train a model from scratch since such signal-classification models are fairly scarce in pre-trained format. The data we use is sourced from the UC Berkeley-Biosense Lab where the data was collected from 15 subjects at the same time. Our process is as follows:

• Load the UC Berkeley-Biosense Synchronized Brainwave Dataset

• Visualize random samples from the data

• Pre-process, collate and scale the data to finally make a tf.data.Dataset

• Prepare class weights in order to tackle major imbalances

• Create a Conv1D and Dense-based model to perform classification

• Define callbacks and hyperparameters

• Train the model

• Plot metrics from History and perform evaluation

This example needs the following external dependencies (Gdown, Scikit-learn, Pandas, Numpy, Matplotlib). You can install it via the following commands.

Gdown is an external package used to download large files from Google Drive. To know more, you can refer to its PyPi page here

Setup and Data Downloads

First, lets install our dependencies:

!pip install gdown -q
!pip install sklearn -q
!pip install pandas -q
!pip install numpy -q
!pip install matplotlib -q

Next, lets download our dataset. The gdown package makes it easy to download the data from Google Drive:

!gdown 1V5B7Bt6aJm0UHbR7cRKBEK8jx7lYPVuX
!# gdown will download eeg-data.csv onto the local drive for use. Total size of
!# eeg-data.csv is 105.7 MB

import pandas as pd
import matplotlib.pyplot as plt
import json
import numpy as np
import keras
from keras import layers
import tensorflow as tf
from sklearn import preprocessing, model_selection
import random

QUALITY_THRESHOLD = 128
BATCH_SIZE = 64
SHUFFLE_BUFFER_SIZE = BATCH_SIZE * 2

Read data from eeg-data.csv

We use the Pandas library to read the eeg-data.csv file and display the first 5 rows using the .head() command

eeg = pd.read_csv("eeg-data.csv")

We remove unlabeled samples from our dataset as they do not contribute to the model. We also perform a .drop() operation on the columns that are not required for training data preparation

unlabeled_eeg = eeg[eeg["label"] == "unlabeled"]
eeg = eeg.loc[eeg["label"] != "unlabeled"]
eeg = eeg.loc[eeg["label"] != "everyone paired"]

eeg.drop(
    [
        "indra_time",
        "Unnamed: 0",
        "browser_latency",
        "reading_time",
        "attention_esense",
        "meditation_esense",
        "updatedAt",
        "createdAt",
    ],
    axis=1,
    inplace=True,
)

eeg.reset_index(drop=True, inplace=True)
eeg.head()

In the data, the samples recorded are given a score from 0 to 128 based on how well-calibrated the sensor was (0 being best, 200 being worst). We filter the values based on an arbitrary cutoff limit of 128.

def convert_string_data_to_values(value_string):
    str_list = json.loads(value_string)
    return str_list


eeg["raw_values"] = eeg["raw_values"].apply(convert_string_data_to_values)

eeg = eeg.loc[eeg["signal_quality"] < QUALITY_THRESHOLD]
eeg.head()

Visualize one random sample from the data

We visualize one sample from the data to understand how the stimulus-induced signal looks like

def view_eeg_plot(idx):
    data = eeg.loc[idx, "raw_values"]
    plt.plot(data)
    plt.title(f"Sample random plot")
    plt.show()


view_eeg_plot(7)

Pre-process and collate data

There are a total of 67 different labels present in the data, where there are numbered sub-labels. We collate them under a single label as per their numbering and replace them in the data itself. Following this process, we perform simple Label encoding to get them in an integer format.

print("Before replacing labels")
print(eeg["label"].unique(), "\n")
print(len(eeg["label"].unique()), "\n")


eeg.replace(
    {
        "label": {
            "blink1": "blink",
            "blink2": "blink",
            "blink3": "blink",
            "blink4": "blink",
            "blink5": "blink",
            "math1": "math",
            "math2": "math",
            "math3": "math",
            "math4": "math",
            "math5": "math",
            "math6": "math",
            "math7": "math",
            "math8": "math",
            "math9": "math",
            "math10": "math",
            "math11": "math",
            "math12": "math",
            "thinkOfItems-ver1": "thinkOfItems",
            "thinkOfItems-ver2": "thinkOfItems",
            "video-ver1": "video",
            "video-ver2": "video",
            "thinkOfItemsInstruction-ver1": "thinkOfItemsInstruction",
            "thinkOfItemsInstruction-ver2": "thinkOfItemsInstruction",
            "colorRound1-1": "colorRound1",
            "colorRound1-2": "colorRound1",
            "colorRound1-3": "colorRound1",
            "colorRound1-4": "colorRound1",
            "colorRound1-5": "colorRound1",
            "colorRound1-6": "colorRound1",
            "colorRound2-1": "colorRound2",
            "colorRound2-2": "colorRound2",
            "colorRound2-3": "colorRound2",
            "colorRound2-4": "colorRound2",
            "colorRound2-5": "colorRound2",
            "colorRound2-6": "colorRound2",
            "colorRound3-1": "colorRound3",
            "colorRound3-2": "colorRound3",
            "colorRound3-3": "colorRound3",
            "colorRound3-4": "colorRound3",
            "colorRound3-5": "colorRound3",
            "colorRound3-6": "colorRound3",
            "colorRound4-1": "colorRound4",
            "colorRound4-2": "colorRound4",
            "colorRound4-3": "colorRound4",
            "colorRound4-4": "colorRound4",
            "colorRound4-5": "colorRound4",
            "colorRound4-6": "colorRound4",
            "colorRound5-1": "colorRound5",
            "colorRound5-2": "colorRound5",
            "colorRound5-3": "colorRound5",
            "colorRound5-4": "colorRound5",
            "colorRound5-5": "colorRound5",
            "colorRound5-6": "colorRound5",
            "colorInstruction1": "colorInstruction",
            "colorInstruction2": "colorInstruction",
            "readyRound1": "readyRound",
            "readyRound2": "readyRound",
            "readyRound3": "readyRound",
            "readyRound4": "readyRound",
            "readyRound5": "readyRound",
            "colorRound1": "colorRound",
            "colorRound2": "colorRound",
            "colorRound3": "colorRound",
            "colorRound4": "colorRound",
            "colorRound5": "colorRound",
        }
    },
    inplace=True,
)

print("After replacing labels")
print(eeg["label"].unique())
print(len(eeg["label"].unique()))

le = preprocessing.LabelEncoder()  # Generates a look-up table
le.fit(eeg["label"])
eeg["label"] = le.transform(eeg["label"])

We extract the number of unique classes present in the data

num_classes = len(eeg["label"].unique())
print(num_classes)

We now visualize the number of samples present in each class using a Bar plot.

plt.bar(range(num_classes), eeg["label"].value_counts())
plt.title("Number of samples per class")
plt.show()

Scale and split data

We perform a simple Min-Max scaling to bring the value-range between 0 and 1. We do not use Standard Scaling as the data does not follow a Gaussian distribution.

scaler = preprocessing.MinMaxScaler()
series_list = [
    scaler.fit_transform(np.asarray(i).reshape(-1, 1)) for i in eeg["raw_values"]
]

labels_list = [i for i in eeg["label"]]

We now create a Train-test split with a 15% holdout set. Following this, we reshape the data to create a sequence of length 512. We also convert the labels from their current label-encoded form to a one-hot encoding to enable use of several different keras.metrics functions.

x_train, x_test, y_train, y_test = model_selection.train_test_split(
    series_list, labels_list, test_size=0.15, random_state=42, shuffle=True
)

print(
    f"Length of x_train : {len(x_train)}\nLength of x_test : {len(x_test)}\nLength of y_train : {len(y_train)}\nLength of y_test : {len(y_test)}"
)

x_train = np.asarray(x_train).astype(np.float32).reshape(-1, 512, 1)
y_train = np.asarray(y_train).astype(np.float32).reshape(-1, 1)
y_train = keras.utils.to_categorical(y_train)

x_test = np.asarray(x_test).astype(np.float32).reshape(-1, 512, 1)
y_test = np.asarray(y_test).astype(np.float32).reshape(-1, 1)
y_test = keras.utils.to_categorical(y_test)

Prepare tf.data.Dataset

We now create a tf.data.Dataset from this data to prepare it for training. We also shuffle and batch the data for use later.

train_dataset = tf.data.Dataset.from_tensor_slices((x_train, y_train))
test_dataset = tf.data.Dataset.from_tensor_slices((x_test, y_test))

train_dataset = train_dataset.shuffle(SHUFFLE_BUFFER_SIZE).batch(BATCH_SIZE)
test_dataset = test_dataset.batch(BATCH_SIZE)

Make Class Weights using Naive method

As we can see from the plot of number of samples per class, the dataset is imbalanced. Hence, we calculate weights for each class to make sure that the model is trained in a fair manner without preference to any specific class due to greater number of samples.

We use a naive method to calculate these weights, finding an inverse proportion of each class and using that as the weight.

vals_dict = {}
for i in eeg["label"]:
    if i in vals_dict.keys():
        vals_dict[i] += 1
    else:
        vals_dict[i] = 1
total = sum(vals_dict.values())

# Formula used - Naive method where
# weight = 1 - (no. of samples present / total no. of samples)
# So more the samples, lower the weight

weight_dict = {k: (1 - (v / total)) for k, v in vals_dict.items()}
print(weight_dict)

Define simple function to plot all the metrics present in a keras.callbacks.History

object

def plot_history_metrics(history: keras.callbacks.History):
    total_plots = len(history.history)
    cols = total_plots // 2

    rows = total_plots // cols

    if total_plots % cols != 0:
        rows += 1

    pos = range(1, total_plots + 1)
    plt.figure(figsize=(15, 10))
    for i, (key, value) in enumerate(history.history.items()):
        plt.subplot(rows, cols, pos[i])
        plt.plot(range(len(value)), value)
        plt.title(str(key))
    plt.show()

Define function to generate Convolutional model

def create_model():
    input_layer = keras.Input(shape=(512, 1))

    x = layers.Conv1D(
        filters=32, kernel_size=3, strides=2, activation="relu", padding="same"
    )(input_layer)
    x = layers.BatchNormalization()(x)

    x = layers.Conv1D(
        filters=64, kernel_size=3, strides=2, activation="relu", padding="same"
    )(x)
    x = layers.BatchNormalization()(x)

    x = layers.Conv1D(
        filters=128, kernel_size=5, strides=2, activation="relu", padding="same"
    )(x)
    x = layers.BatchNormalization()(x)

    x = layers.Conv1D(
        filters=256, kernel_size=5, strides=2, activation="relu", padding="same"
    )(x)
    x = layers.BatchNormalization()(x)

    x = layers.Conv1D(
        filters=512, kernel_size=7, strides=2, activation="relu", padding="same"
    )(x)
    x = layers.BatchNormalization()(x)

    x = layers.Conv1D(
        filters=1024,
        kernel_size=7,
        strides=2,
        activation="relu",
        padding="same",
    )(x)
    x = layers.BatchNormalization()(x)

    x = layers.Dropout(0.2)(x)

    x = layers.Flatten()(x)

    x = layers.Dense(4096, activation="relu")(x)
    x = layers.Dropout(0.2)(x)

    x = layers.Dense(
        2048, activation="relu", kernel_regularizer=keras.regularizers.L2()
    )(x)
    x = layers.Dropout(0.2)(x)

    x = layers.Dense(
        1024, activation="relu", kernel_regularizer=keras.regularizers.L2()
    )(x)
    x = layers.Dropout(0.2)(x)
    x = layers.Dense(
        128, activation="relu", kernel_regularizer=keras.regularizers.L2()
    )(x)
    output_layer = layers.Dense(num_classes, activation="softmax")(x)

    return keras.Model(inputs=input_layer, outputs=output_layer)

Get Model summary

conv_model = create_model()
conv_model.summary()

Define callbacks, optimizer, loss and metrics

We set the number of epochs at 30 after performing extensive experimentation. It was seen that this was the optimal number, after performing Early-Stopping analysis as well. We define a Model Checkpoint callback to make sure that we only get the best model weights. We also define a ReduceLROnPlateau as there were several cases found during experimentation where the loss stagnated after a certain point. On the other hand, a direct LRScheduler was found to be too aggressive in its decay.

epochs = 30

callbacks = [
    keras.callbacks.ModelCheckpoint(
        "best_model.keras", save_best_only=True, monitor="loss"
    ),
    keras.callbacks.ReduceLROnPlateau(
        monitor="val_top_k_categorical_accuracy",
        factor=0.2,
        patience=2,
        min_lr=0.000001,
    ),
]

optimizer = keras.optimizers.Adam(amsgrad=True, learning_rate=0.001)
loss = keras.losses.CategoricalCrossentropy()

Compile model and call model.fit()

We use the Adam optimizer since it is commonly considered the best choice for preliminary training, and was found to be the best optimizer. We use CategoricalCrossentropy as the loss as our labels are in a one-hot-encoded form.

We define the TopKCategoricalAccuracy(k=3), AUC, Precision and Recall metrics to further aid in understanding the model better.

conv_model.compile(
    optimizer=optimizer,
    loss=loss,
    metrics=[
        keras.metrics.TopKCategoricalAccuracy(k=3),
        keras.metrics.AUC(),
        keras.metrics.Precision(),
        keras.metrics.Recall(),
    ],
)

conv_model_history = conv_model.fit(
    train_dataset,
    epochs=epochs,
    callbacks=callbacks,
    validation_data=test_dataset,
    class_weight=weight_dict,
)

Visualize model metrics during training

We use the function defined above to see model metrics during training.

plot_history_metrics(conv_model_history)

Evaluate model on test data

loss, accuracy, auc, precision, recall = conv_model.evaluate(test_dataset)
print(f"Loss : {loss}")
print(f"Top 3 Categorical Accuracy : {accuracy}")
print(f"Area under the Curve (ROC) : {auc}")
print(f"Precision : {precision}")
print(f"Recall : {recall}")


def view_evaluated_eeg_plots(model):
    start_index = random.randint(10, len(eeg))
    end_index = start_index + 11
    data = eeg.loc[start_index:end_index, "raw_values"]
    data_array = [scaler.fit_transform(np.asarray(i).reshape(-1, 1)) for i in data]
    data_array = [np.asarray(data_array).astype(np.float32).reshape(-1, 512, 1)]
    original_labels = eeg.loc[start_index:end_index, "label"]
    predicted_labels = np.argmax(model.predict(data_array, verbose=0), axis=1)
    original_labels = [
        le.inverse_transform(np.array(label).reshape(-1))[0]
        for label in original_labels
    ]
    predicted_labels = [
        le.inverse_transform(np.array(label).reshape(-1))[0]
        for label in predicted_labels
    ]
    total_plots = 12
    cols = total_plots // 3
    rows = total_plots // cols
    if total_plots % cols != 0:
        rows += 1
    pos = range(1, total_plots + 1)
    fig = plt.figure(figsize=(20, 10))
    for i, (plot_data, og_label, pred_label) in enumerate(
        zip(data, original_labels, predicted_labels)
    ):
        plt.subplot(rows, cols, pos[i])
        plt.plot(plot_data)
        plt.title(f"Actual Label : {og_label}\nPredicted Label : {pred_label}")
        fig.subplots_adjust(hspace=0.5)
    plt.show()


view_evaluated_eeg_plots(conv_model)