3.3.3 Conv4, Conv5 and Pool5: High-level
Feature Representation
With regards to the discussion above, this is an inter-
esting case. We observe that the classification accura-
cies with learned features from conv4 and conv5 lay-
ers are very high. As for the low-level features, in this
case too, the t-SNE plot (Figures. 4(d) and 4(e)) show
a clear linear discrimination between the two classes.
Typically, these feature representation is known
to incorporate more semantic information for object
/ scene images. However, for the cell image classifi-
cation case, the images do not seem to have such high
level semantic information, and the activation maps in
Figure 5 seem to support this argument. Thus, while
the activation maps are again not as clearly represen-
tative as those for low-level features, one hypothesis
for the high performance is that the activation maps
are also highly sparse. This could indicate that filter
responses of irrelevant filters (in conv3) might be sup-
pressed in high-level features and only few relevant
filters might activated with discriminative responses.
Again, as for conv3 responses, this is also worth ex-
ploring further to better analyze and interpret the per-
formance of high-level features in this case.
3.4 Comparison with the Baseline CNN
Classifier
Finally, we compare with the baseline CNN classifier
wherein fully connected (FC) layers in the Alexnet
are retrained for classification (Table 2). Note that
both the baseline classifier and one case with the
SVM classifier, operate on the pool5 features. In-
terestingly, in the undersampled case, the baseline
classifier shows a much lower performance than the
SVM classifier, especially with low-amount of train-
ing data. We believe that this is due to over-fitting,
as the amount of data is low. As indicated earlier, the
SVM classifier could be more robust here, as it effec-
tively models the classifier using less number of sam-
ples. Typically, in many applications, the CNN clas-
sifier is trained with oversampled data. In the over-
sampled case, as the data size increases, the baseline
CNN learns better and the accuracy increases. How-
ever, the note that the SVM operating on undersam-
pled data performs equally well.
To the best of our knowledge, there are very few
approaches proposed for this task, and indeed this
is the first work on this dataset. Hence, we do not
provide any comparisons with any other approaches.
However, even in an absolute sense the best results
achieved in this work are quite high and demonstrate
that the approach is very effective to discriminate the
mitotic and non-mitotic class samples.
4 CONCLUSION
In the proposed work, a mitotic cell detection frame-
work for HEp-2 cell images is proposed via learned
feature representation with a pre-trained CNN. We
achieve high quality performance with low-level and
high-level layered features of the architecture. Fur-
thermore, we discuss some useful observations with
respect to the features at various levels, and compari-
son with a baseline CNN. In future, we mean to build
our own classification CNN architecture or re-train
selected layers (transfer learning), which may also
help in achieving better insights.
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