Different classifiers in decoding the Haxby dataset

Here we compare different classifiers on a visual object recognition decoding task.

# We start by loading data using nilearn dataset fetcher
from nilearn import datasets
from nilearn.image import get_data
from nilearn.plotting import plot_stat_map, show

Loading the data

# by default 2nd subject data will be fetched
haxby_dataset = datasets.fetch_haxby()

# print basic information on the dataset
print(
    "First subject anatomical nifti image (3D) located is "
    f"at: {haxby_dataset.anat[0]}"
)
print(
    "First subject functional nifti image (4D) is located "
    f"at: {haxby_dataset.func[0]}"
)

# load labels
import numpy as np
import pandas as pd

labels = pd.read_csv(haxby_dataset.session_target[0], sep=" ")
stimuli = labels["labels"]

# identify resting state (baseline) labels in order to be able to remove them
resting_state = stimuli == "rest"

# extract the indices of the images corresponding to some condition or task
task_mask = np.logical_not(resting_state)

# find names of remaining active labels
categories = stimuli[task_mask].unique()

# extract tags indicating to which acquisition run a tag belongs
run_labels = labels["chunks"][task_mask]


# Load the fMRI data
# For decoding, standardizing is often very important
mask_filename = haxby_dataset.mask_vt[0]
func_filename = haxby_dataset.func[0]


# Because the data is in one single large 4D image, we need to use
# index_img to do the split easily.
from nilearn.image import index_img

fmri_niimgs = index_img(func_filename, task_mask)
classification_target = stimuli[task_mask]

First subject anatomical nifti image (3D) located is at: /home/himanshu/nilearn_data/haxby2001/subj2/anat.nii.gz
First subject functional nifti image (4D) is located at: /home/himanshu/nilearn_data/haxby2001/subj2/bold.nii.gz

Training the decoder

# Then we define the various classifiers that we use
classifiers = [
    "svc_l2",
    "svc_l1",
    "logistic_l1",
    "logistic_l2",
    "ridge_classifier",
]

# Here we compute prediction scores and run time for all these
# classifiers
import time

from sklearn.model_selection import LeaveOneGroupOut

from nilearn.decoding import Decoder

cv = LeaveOneGroupOut()
classifiers_data = {}

for classifier_name in sorted(classifiers):
    print(70 * "_")

    # The decoder has as default score the `roc_auc`
    decoder = Decoder(
        estimator=classifier_name,
        mask=mask_filename,
        standardize="zscore_sample",
        cv=cv,
    )
    t0 = time.time()
    decoder.fit(fmri_niimgs, classification_target, groups=run_labels)

    classifiers_data[classifier_name] = {"score": decoder.cv_scores_}
    print(f"{classifier_name:10}: {time.time() - t0:.2f}s")
    for category in categories:
        mean = np.mean(classifiers_data[classifier_name]["score"][category])
        std = np.std(classifiers_data[classifier_name]["score"][category])
        print(f"    {category:14} vs all -- AUC: {mean:1.2f} +- {std:1.2f}")

    # Adding the average performance per estimator
    scores = classifiers_data[classifier_name]["score"]
    scores["AVERAGE"] = np.mean(list(scores.values()), axis=0)
    classifiers_data[classifier_name]["score"] = scores

______________________________________________________________________
/home/himanshu/Desktop/nilearn_work/nilearn/nilearn/image/resampling.py:492: UserWarning: The provided image has no sform in its header. Please check the provided file. Results may not be as expected.
  warnings.warn(
logistic_l1: 150.01s
    scissors       vs all -- AUC: 0.00 +- 0.00
    face           vs all -- AUC: 0.17 +- 0.37
    cat            vs all -- AUC: 0.45 +- 0.46
    shoe           vs all -- AUC: 0.00 +- 0.00
    house          vs all -- AUC: 1.00 +- 0.00
    scrambledpix   vs all -- AUC: 0.98 +- 0.02
    bottle         vs all -- AUC: 0.16 +- 0.35
    chair          vs all -- AUC: 0.00 +- 0.00
______________________________________________________________________
/home/himanshu/Desktop/nilearn_work/nilearn/nilearn/image/resampling.py:492: UserWarning: The provided image has no sform in its header. Please check the provided file. Results may not be as expected.
  warnings.warn(
logistic_l2: 404.51s
    scissors       vs all -- AUC: 0.90 +- 0.08
    face           vs all -- AUC: 0.97 +- 0.04
    cat            vs all -- AUC: 0.97 +- 0.04
    shoe           vs all -- AUC: 0.91 +- 0.08
    house          vs all -- AUC: 1.00 +- 0.00
    scrambledpix   vs all -- AUC: 0.97 +- 0.08
    bottle         vs all -- AUC: 0.82 +- 0.17
    chair          vs all -- AUC: 0.87 +- 0.17
______________________________________________________________________
/home/himanshu/Desktop/nilearn_work/nilearn/nilearn/image/resampling.py:492: UserWarning: The provided image has no sform in its header. Please check the provided file. Results may not be as expected.
  warnings.warn(
ridge_classifier: 37.09s
    scissors       vs all -- AUC: 0.94 +- 0.05
    face           vs all -- AUC: 0.98 +- 0.02
    cat            vs all -- AUC: 0.95 +- 0.04
    shoe           vs all -- AUC: 0.94 +- 0.07
    house          vs all -- AUC: 1.00 +- 0.00
    scrambledpix   vs all -- AUC: 1.00 +- 0.00
    bottle         vs all -- AUC: 0.89 +- 0.09
    chair          vs all -- AUC: 0.93 +- 0.07
______________________________________________________________________
/home/himanshu/Desktop/nilearn_work/nilearn/nilearn/image/resampling.py:492: UserWarning: The provided image has no sform in its header. Please check the provided file. Results may not be as expected.
  warnings.warn(
svc_l1    : 43.99s
    scissors       vs all -- AUC: 0.92 +- 0.05
    face           vs all -- AUC: 0.98 +- 0.03
    cat            vs all -- AUC: 0.96 +- 0.04
    shoe           vs all -- AUC: 0.92 +- 0.07
    house          vs all -- AUC: 1.00 +- 0.00
    scrambledpix   vs all -- AUC: 0.99 +- 0.01
    bottle         vs all -- AUC: 0.89 +- 0.08
    chair          vs all -- AUC: 0.93 +- 0.04
______________________________________________________________________
/home/himanshu/Desktop/nilearn_work/nilearn/nilearn/image/resampling.py:492: UserWarning: The provided image has no sform in its header. Please check the provided file. Results may not be as expected.
  warnings.warn(
svc_l2    : 99.14s
    scissors       vs all -- AUC: 0.90 +- 0.09
    face           vs all -- AUC: 0.96 +- 0.05
    cat            vs all -- AUC: 0.96 +- 0.04
    shoe           vs all -- AUC: 0.91 +- 0.08
    house          vs all -- AUC: 1.00 +- 0.00
    scrambledpix   vs all -- AUC: 0.96 +- 0.10
    bottle         vs all -- AUC: 0.82 +- 0.17
    chair          vs all -- AUC: 0.87 +- 0.16

Visualization

# Then we make a rudimentary diagram
import matplotlib.pyplot as plt

plt.figure(figsize=(8, 6))

all_categories = np.sort(np.hstack([categories, "AVERAGE"]))
tick_position = np.arange(len(all_categories))
plt.yticks(tick_position + 0.25, all_categories)
height = 0.1

for color, classifier_name in zip(["b", "m", "k", "r", "g"], classifiers):
    score_means = [
        np.mean(classifiers_data[classifier_name]["score"][category])
        for category in all_categories
    ]

    plt.barh(
        tick_position,
        score_means,
        label=classifier_name.replace("_", " "),
        height=height,
        color=color,
    )
    tick_position = tick_position + height

plt.xlabel("Classification accuracy (AUC score)")
plt.ylabel("Visual stimuli category")
plt.xlim(xmin=0.5)
plt.legend(ncol=1, bbox_to_anchor=(1.3, 0.2))
plt.title(
    "Category-specific classification accuracy for different classifiers"
)
plt.tight_layout()

We can see that for a fixed penalty the results are similar between the svc and the logistic regression. The main difference relies on the penalty ($ell_1$ and $ell_2$). The sparse penalty works better because we are in an intra-subject setting.

Visualizing the face vs house map

Restrict the decoding to face vs house

condition_mask = np.logical_or(stimuli == "face", stimuli == "house")
stimuli = stimuli[condition_mask]
assert len(stimuli) == 216
fmri_niimgs_condition = index_img(func_filename, condition_mask)
run_labels = labels["chunks"][condition_mask]
categories = stimuli.unique()
assert len(categories) == 2

for classifier_name in sorted(classifiers):
    decoder = Decoder(
        estimator=classifier_name,
        mask=mask_filename,
        standardize="zscore_sample",
        cv=cv,
    )
    decoder.fit(fmri_niimgs_condition, stimuli, groups=run_labels)
    classifiers_data[classifier_name] = {}
    classifiers_data[classifier_name]["score"] = decoder.cv_scores_
    classifiers_data[classifier_name]["map"] = decoder.coef_img_["face"]

/home/himanshu/Desktop/nilearn_work/nilearn/nilearn/image/resampling.py:492: UserWarning: The provided image has no sform in its header. Please check the provided file. Results may not be as expected.
  warnings.warn(
/home/himanshu/Desktop/nilearn_work/nilearn/nilearn/image/resampling.py:492: UserWarning: The provided image has no sform in its header. Please check the provided file. Results may not be as expected.
  warnings.warn(
/home/himanshu/Desktop/nilearn_work/nilearn/nilearn/image/resampling.py:492: UserWarning: The provided image has no sform in its header. Please check the provided file. Results may not be as expected.
  warnings.warn(
/home/himanshu/Desktop/nilearn_work/nilearn/nilearn/image/resampling.py:492: UserWarning: The provided image has no sform in its header. Please check the provided file. Results may not be as expected.
  warnings.warn(
/home/himanshu/Desktop/nilearn_work/nilearn/nilearn/image/resampling.py:492: UserWarning: The provided image has no sform in its header. Please check the provided file. Results may not be as expected.
  warnings.warn(

Finally, we plot the face vs house map for the different classifiers Use the average EPI as a background

from nilearn.image import mean_img

mean_epi_img = mean_img(func_filename)

for classifier_name in sorted(classifiers):
    coef_img = classifiers_data[classifier_name]["map"]
    threshold = np.max(np.abs(get_data(coef_img))) * 1e-3
    plot_stat_map(
        coef_img,
        bg_img=mean_epi_img,
        display_mode="z",
        cut_coords=[-15],
        threshold=threshold,
        title=f"{classifier_name.replace('_', ' ')}: face vs house",
        figure=plt.figure(figsize=(3, 4)),
    )

show()

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Estimated memory usage: 1368 MB