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  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>Argumentative Interpretable Image Classification</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Hamed Ayoobi</string-name>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Nico Potyka</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Francesca Toni</string-name>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Cardif University</institution>
          ,
          <country country="UK">United Kingdom</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>Department of Computing, Imperial College London</institution>
          ,
          <country country="UK">United Kingdom</country>
        </aff>
      </contrib-group>
      <fpage>3</fpage>
      <lpage>15</lpage>
      <abstract>
        <p>We propose ProtoSpArX, a novel interpretable deep neural architecture for image classification in the spirit of prototypical-part-learning as found, e.g. in ProtoPNet. While earlier approaches associate every class with multiple prototypical-parts, ProtoSpArX uses super-prototypes that combine prototypical-parts into single class representations. Furthermore, while earlier approaches use interpretable classification layers, e.g. logistic regression in ProtoPNet, ProtoSpArX improves accuracy with multi-layer perceptrons while relying upon an interpretable reading thereof based on a form of argumentation. ProtoSpArX is customisable to user cognitive requirements by a process of sparsification of the multi-layer perceptron/argumentation component. Also, as opposed to other prototypical-part-learning approaches, ProtoSpArX can recognise spatial relations between diferent prototypical-parts that are from various regions in images, similar to how CNNs capture relations between patterns recognized in earlier layers.</p>
      </abstract>
      <kwd-group>
        <kwd>eol&gt;Interpretable Image Classification</kwd>
        <kwd>Argumentation</kwd>
        <kwd>Prototypical-Parts Learning</kwd>
        <kwd>XAI</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>
        Deep neural architectures are successful in
various tasks, but tend to be mostly inscrutable
blackboxes. In high-stakes settings, interpretability is
crucial and interpretable models are advocated
over black-boxes, especially if they achieve
comparable performance [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ]. Prototypical-part
learning for image classification amounts to learning
prototypical-parts of classes in images by
introducing a human-interpretable prototype layer
between the convolutional backbone (intutively, it
learns patterns in the image space) and the
classification component (intuitively, it uses the patterns
identified by the backbone to classify an image) of
convolutional neural networks [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ].
Prototypicalparts can be seen as patches in images, like the
(a) Prototypes
(b) Super-Prototypes
beak or tail of a bird (see Figure 1 (a)). The prototype layer determines the similarity between
prototypical-parts and patches in the latent space that the convolutional backbone maps to. Even
though some prototypical-parts may correspond to background patches that are meaningless
for humans (rather than exclusively meaningful parts in images as in Figure 1 (a)), they allow
making transparent classifications, based on clearly defined prototypes, if the classification
component is interpretable.
      </p>
      <p>
        We propose ProtoSpArX (Section 4, overviewed in Figure 2), a novel interpretable deep
neural architecture for image classification in the spirit of prototypical-part-learning. Similar to
ProtoPShare [
        <xref ref-type="bibr" rid="ref4">4</xref>
        ] and ProtoTrees [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ], ProtoSpArX shares prototypes among classes. However,
while these and other prototypical-part-learning approaches associate every class with multiple
prototypical parts, ProtoSpArX summarizes them in a single super-prototype per class that
encodes spatial relations among them (see Figure 1 (b) for an illustration).
      </p>
      <p>
        The use of super-prototypes allows capturing spatial relations between prototypical parts
similar to how CNNs capture relations between patterns recognized in earlier layers. As we
will show in the experiments with the SHAPES dataset [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], these relations are essential for
some classification tasks but state-of-the-art prototypical-part-learning approaches are unable
to capture them. For example, in Figure 2, a positive example (Class 1) has a triangle in the left
column and a circle in the right column on the same row. Merely recognizing prototypical-parts
for triangles and circles in the input image (as in other prototypical-part-learning approaches)
is insuficient for determining the class label in this example. ProtoSpArX efectively tackles
this challenge by encoding the spatial relations between distinct prototypical-parts using the
super-prototype kernels.
      </p>
      <p>
        The classifier component in ProtoSpArX is a quantitative bipolar argumentation framework
(QBAF) that is trained using the SpArX methodology of [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ]. Intuitively, the QBAF uses weighted
attacks and supports between super-prototypes and meta-arguments (latent arguments attacked
and supported by super-prototypes or other meta-arguments) to classify an image. This is
indicated by the red and green arrows in Figure 21.
      </p>
      <p>
        We show experimentally (Sections 5 and 6) that ProtoSpArX outperforms the state-of-the-art
prototypical-part-learning models ProtoPNet [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ], ProtoTree [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ], ProtoPShare [
        <xref ref-type="bibr" rid="ref4">4</xref>
        ], ProtoPool [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]
and PIP-Net [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ] in terms of classification accuracy and the ability to encode and detect spatial
relations in images, supported by a number of ablations and the study of the cognitive complexity
1Please note that the colour of the shapes in the input image has no relation with or bearing on the colours of
the super-prototypes and edges in the QBAF, indicating attack and support (see Section 4 for details).
of local explanations derived from the sparsification of QBAFs obtained with ProtoSpArX.
      </p>
    </sec>
    <sec id="sec-2">
      <title>2. Related Work</title>
      <p>
        The problem of explaining image classifiers is well studied in the literature. Examples include
feature attributions [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ], attention maps [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ] and counterfactual explanations [
        <xref ref-type="bibr" rid="ref12">12</xref>
        ]. While the
former can be seen as post-hoc explanations that aim at explaining the decisions of a
blackbox classifier, there is also an increasing literature on interpretable-by-design approaches.
One interesting interpretable direction is based on prototypical-part-learning [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ]. These
approaches were motivated by the observation that class-prototypes [
        <xref ref-type="bibr" rid="ref14">14</xref>
        ] for datasets with
simple backgrounds (as in MNIST [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ]) do not generalize well to natural images with more
complex backgrounds. To overcome this problem, ProtoPNet [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ] introduced prototypical parts
for capturing parts of the class (like the beak or tail of a bird) rather than the whole object (the
bird). The original idea has been extended in various directions including prototypes that can
be shared among classes [
        <xref ref-type="bibr" rid="ref4">4</xref>
        ], the integration of prototypical parts into decision trees [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ] and
improved similarity functions [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]. Our ProtoSpArX adds super-prototypes and uses bipolar
quantitative argumentation to achieve a better tradeof between classification performance and
interpretability. Speficially, ProtoSpArX extends the SpArX approach [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ], originally defined for
MLPs with tabular data only, to the setting of prototypical-part-learning with images.
      </p>
      <p>
        Several other argumentation-based forms of explainability have been proposed, we refer
to [
        <xref ref-type="bibr" rid="ref16">16</xref>
        ] for an overview. Other works combine argumentation and image classification, e.g.
[
        <xref ref-type="bibr" rid="ref17 ref18">17, 18</xref>
        ] for explaining the outputs of CNNs and [
        <xref ref-type="bibr" rid="ref19">19</xref>
        ] to obtain an interpretable image classifier.
ProtoSpArX may also be deemed neuro-symbolic as it combines, end-to-end (see Figure 2),
neural components (the convolutional backbone, the prototype kernels, and the super-prototype
kernels) with symbolic argumentation frameworks (QBAFs) drawn from MLPs. However,
whereas recent neuro-symbolic systems often combine purely symbolic with purely neural
systems [
        <xref ref-type="bibr" rid="ref20 ref21">20, 21</xref>
        ], ProtoSpArX is based on the observation that MLPs can be seen as QBAFs and
vice versa [
        <xref ref-type="bibr" rid="ref22 ref7">22, 7</xref>
        ]. We keep the reasoning process in QBAFs interpretable by sparsification , as
in [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ].
      </p>
    </sec>
    <sec id="sec-3">
      <title>3. Preliminaries</title>
      <p>
        We build up on SpArX [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ], a post-hoc explanation method that aims at generating structurally
faithful explanations for MLPs. SpArX exploits that MLPs can be understood as Quantitative
Bipolar Argumentation Frameworks (QBAFs) [
        <xref ref-type="bibr" rid="ref22">22</xref>
        ]. QBAFs can be seen as graphical reasoning
models whose nodes represent arguments and whose edges represent attack or support relations
between the arguments, each with a (negative or positive, respectively) intensity value [
        <xref ref-type="bibr" rid="ref23 ref24 ref25 ref26">23, 24,
25, 26</xref>
        ].
      </p>
      <p>
        Arguments in QBAFs are abstract entities (what makes them arguments is that they are in
dialectical relationships). To capture MLPs as in [
        <xref ref-type="bibr" rid="ref22">22</xref>
        ], these abstract arguments represent input
features, hidden neurons and output classifications, and the graphical structure of QBAFs mirror
the MLP.
      </p>
      <p>
        This correspondence allows representing MLPs faithfully by QBAFs, but the QBAF
representation is not useful for interpretability and explainability, because the QBAF has the same size
as the original MLP. Thus, SpArX clusters neurons with similar activations and summarizes
each cluster as a single argument [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ]. Experiments with tabular data show that SpArX can give
explanations that are both sparse and faithful [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ].
      </p>
      <p>In this work, we extend SpArX to make ProtoSpArX interpretable and explainable. An
illustration is given in Figure 2: neurons in the MLP component of ProtoSpArX are treated as
arguments, alongside the similarity scores from the super-prototypes, which serve as the input
features for the MLP in our architecture (see the examples in Section 4 for further details on
this illustration). Similarly to the original SpArX, we experiment with sparsification by various
compression ratios (Section 6.4), showing that ProtoSpArX can provide explanations that are
both sparse and faithful for image classification.</p>
    </sec>
    <sec id="sec-4">
      <title>4. Method</title>
      <sec id="sec-4-1">
        <title>4.1. Prototypes</title>
        <p>Let  =  () be the convolutional output for an input image , where the output tensor  has
shape  ×  ×  with height , width  and  channels. This output tensor serves as input
to the prototype layer, . which represents prototypical-parts.  consists of  prototypes
 = {}=1 with shapes 1 × 1 ×  (we have used 1 = 1 = 1 in all experiments).
For each prototype  ∈  and every 1 × 1 ×  sub-tensor  of , the prototype layer 
computes the cosine similarity
and outputs a similarity map
(,  ) =</p>
        <p>
          · 
‖‖‖ ‖
ℳ =  (,  )
∈
(1)
(2)
with shape  ×  for each prototype  ∈  . Intuitively, ℳ indicates how similar the
prototypical-part  is to patches of the input image  in the latent space. We implemented
ℳ using the 2D convolution operator * . It generates ℳ by convoluting the normalized
convolutional output ˆ = ‖‖ = [︀ ‖‖ ]︀ ∈ with a normalized prototype kernel ˆ = ‖‖ ,
ℳ = ˆ * ˆ. Since cosine similarity is used for the prototype layer, the values in similarity
maps can be both positive and negative in the range [
          <xref ref-type="bibr" rid="ref1">− 1, 1</xref>
          ]. The output dimensions of the
prototype layer are  ×  ×  .
        </p>
      </sec>
      <sec id="sec-4-2">
        <title>4.2. Channel-Wise Max</title>
        <p>ℒ = ∑︁ ℒ · ℳ</p>
        <p>=1
The Channel-Wise Max layer aims to both localize and extract the max value of each similarity
map while maintaining its dimensions. ℳ takes the similarity maps as input and extracts
the maximum value from each input channel by passing the maximum value and setting all
other values to zero while preserving the input dimensions. Formally, for every similarity value
 ∈ ℳ, the ℎ similarity map, the channel-wise max filter ℳ retains the highest value
 = max(ℳ) within the map and assigns a value of zero to the remaining elements:
ℳ =
{︃ if s = max(ℳ);</p>
        <p>0 otherwise.</p>
        <p>The output dimensions of ℳ are still  ×  ×  .</p>
      </sec>
      <sec id="sec-4-3">
        <title>4.3. Super-Prototypes and Similarity Scores</title>
        <p>The super-prototypes kernel takes the output of the channel-wise max layer as input and
provides a single representation per class. This is done in three steps.</p>
        <p>In the first step, for each class  ∈ ,  linear combinations of the channel-wise max filters,
denoted by ℒ where  ∈ {1, . . . ,  }, are learned. Here,  is a customisable hyper-parameter
of the model ( = 32 achieved the best results in the experiments). Formally:
(3)
(4)
(5)

where ℒ is a trainable scalar weight. We let  ℒ denote the vector summarizing all these
weights. This operation can be implemented with  convolutions with kernel shape 1 × 1 × 
using the  channel-wise max filters as input.</p>
        <p>In the second step, the super-prototypes are constructed. Each linear combination ℒ is
then multiplied by a trainable weight matrix  with shape  ×  to obtain a single
super-prototype for each class from the  linear combinations. This means that the number
of super-prototypes is equal to the number of classes ||. Each super-prototype  is then
computed as follows:</p>
        <p>= ∑︁ ℒ ⊙</p>
        <p>,
=1
where ⊙ denotes element-wise product. Each super-prototype has the shape  ×  . By
utilizing the receptive field of the convolutional output  to rescale the similarity maps ℳ to
the input dimensions, the super-prototypes can be visualized on the input image  employing
Equation 5, as illustrated next.</p>
        <p>Example 1. Figure 2 illustrates the visualization of the super-prototypes on the input image,
where the colours indicate support (green) for Class 1 at the bottom and attack (red) against
Class 0 at the top. Note that, since we are dealing with binary classification, the supporting
regions for accepting one class are the attacking regions for accepting the other class. Also, the
(6)
colours in the input images are irrelevant to the classification task which associates Class 1 to
images with a triangle in the left column and a circle in the right column on the same row, no
matter their colour.</p>
        <p>In the third and final step, a single similarity score  is computed for each super-prototype
by summing up the values  ∈ :
 =</p>
        <p>∑︁ .</p>
        <p>∈
Equations 5 and 6 can be simultaneously implemented by employing || convolutions with a
kernel shape of  ×  ×  , while taking the  linear combinations for each class as input.</p>
      </sec>
      <sec id="sec-4-4">
        <title>4.4. Classifier Layer</title>
        <p>Using the similarity scores as input, ℳℒ is used for classification. After the training phase,
ℳℒ is converted to a QBAF (c.f., Section 3 – this involves sparsifying the underlying ℳℒ
and then translating it to a QBAF). The obtained QBAF can provide reasons for and against
assigning an input  to a specific class, making ProtoSpArX interpretable as illustrated next.
Example 2. The (sparsified) 1-hidden layer-MLP/QBAF in Figure 2 can be interpreted as
follows:
• Super-prototype of Class 1 supports and attacks, with high intensity, the arguments
corresponding to, respectively, the bottom and top neurons in the hidden layer;
• Conversely, the super-prototype for Class 0 attacks and supports, with low intensity, the
same arguments;
• The hidden clusters and output neurons are visualized using the super-prototypes they
“propagate” through the MLP, in the sense that these super-prototypes support them,
e.g. the super-prototype for Class 0 supports the top cluster in the hidden layer and the
predicted Class 1 is supported by the super-prototype for Class 1.</p>
        <p>Overall, this interpretation indicates that the predicted Class 1 for the input image is supported
by the presence of a circle in the bottom left corner and a triangle in the bottom right corner,
while also pointing to the reasoning of the MLP in terms of the super-prototypes used.</p>
      </sec>
    </sec>
    <sec id="sec-5">
      <title>5. Training ProtoSpArX</title>
      <p>Unlike other prototypical-part-learning approaches, the training phase of ProtoSpArX is done
in one step. This means that all amongst the prototype layer, the super-prototype kernels and
the classifier are trained at once without a need for freezing the weight of the classifier first and
ifne-tuning it later. For the ℎ data point in a dataset of size , with the data point belonging
to class label  ∈  (where  is the set of class labels), the target class super-prototype
should obtain a high similarity score  . Moreover, the corresponding similarity scores for the
||
super-prototypes of other classes ({}=1,̸= ) should be low. Simultaneously, the output
of the classifier should be 1 for the target class  and 0 for the other classes. Therefore, we
integrate in the loss function two components  and  for the corresponding objectives.
Definition 1. The total loss function ℒ that we aim to minimize is:</p>
      <p>ℒ =  + 
where  is the Cross-Entropy loss and  is a regularization term that aims at associating
super-prototypes with their associated classes by penalizing the similarity to wrong classes and
rewarding the similarity to the correct class :</p>
      <p>= ∑︁ ((), ),
=1</p>
      <p>(8)
where () denotes the output of ProtoSpArX.</p>
      <p>||
 = ∑︁(( ∑︁ ) −  );
=1 =1
̸=</p>
      <p>(9)</p>
      <p>
        Given the definition of total loss function ℒ, we then use the Adam optimizer [
        <xref ref-type="bibr" rid="ref27">27</xref>
        ] to tune the
convolutional weights  , prototypes , linear combination weights  ℒ, super-prototype
weights   , and MLP weights  ℳℒ in an end-to-end fashion to minimize ℒ:
 ,, ℒ,  ℒ( , ,  ℒ,   )
      </p>
      <p>
        min
Finally, for the projection of prototypes, we follow the same approach as ProtoPNet [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ] to push
the prototypes to the latent representation of the closest image patch from the input space in
the convolutional output so that each prototype has a global interpretable representation.
      </p>
    </sec>
    <sec id="sec-6">
      <title>6. Experiments</title>
      <sec id="sec-6-1">
        <title>Method</title>
        <p>ProtoPNet
ProtoPShare</p>
        <p>ProtoPool
ProtoTrees</p>
        <p>PIP-Net</p>
        <p>
          ProtoSpArX
We have compared our approach with CUB SHAPES
ltehaernstiantge-mofo-tdheel-sarPtroptrooPtoNtyetpi[c3a]l,-pParrot-- 877049...372 ±±± 000...221 555001...481 ±±± 000...867
toTrees [
          <xref ref-type="bibr" rid="ref5">5</xref>
          ], ProtoPShare [
          <xref ref-type="bibr" rid="ref4">4</xref>
          ], ProtoPool 82.2 ± 0.7 51.4 ± 0.7
[
          <xref ref-type="bibr" rid="ref8">8</xref>
          ] and PIP-Net [
          <xref ref-type="bibr" rid="ref9">9</xref>
          ]. We have con- 82.0 ± 0.3 50.6 ± 0.6
ducted four sets of experiments to 83.4 ± 0.2 98.4 ± 0.2
evaluate the classification performance
(Section 6.1), the role of each layer on Table 1: Accuracy of ProtoSpArX and other
the model’s performance by an abla- prototypical-part-learning methods for
tion study (Section 6.2), the ability to diferent datasets. (Best accuracy in bold)
encode and detect spatial relationships in the input (Section 6.3), and the cognitive complexity of
explanations naturally drawn from ProtoSpArX (Section 6.4). Notice that, for all the experiments,
we use classification accuracy as our performance measure, as is the case with the baselines.
        </p>
        <p>
          For all the experiments, we have used CUB-200-2011 (CUB)[
          <xref ref-type="bibr" rid="ref1">1</xref>
          ] and Stanford Cars (Cars) [
          <xref ref-type="bibr" rid="ref28">28</xref>
          ],
which are the standard benchmarks for prototypical-part learning models. To assess the ability
to encode spatial relationships, we use the SHAPES dataset [
          <xref ref-type="bibr" rid="ref6">6</xref>
          ] adapted to binary classification.
        </p>
      </sec>
      <sec id="sec-6-2">
        <title>Accuracy</title>
        <p>Cars
86.1 ± 0.1
86.4 ± 0.2
88.9 ± 0.1
86.6 ± 0.2
86.5 ± 0.3
89.3 ± 0.2</p>
        <sec id="sec-6-2-1">
          <title>6.1. Classification Performance</title>
          <p>The first two columns in Table 1 show the accuracy of our method compared to the baselines,
for CUB and Cars. For both datasets, our ProtoSpArX outperforms the other approaches.</p>
        </sec>
        <sec id="sec-6-2-2">
          <title>6.2. Ablation study</title>
          <p>Ablation studies on CUB and Cars
in Table 2 show that ProtoSpArX Fixed
achieves the best accuracy when em- MLP
ploying super-prototypes atop the Fixed
cosine similarity prototype layer, to- MLP
gether with an MLP as classifier Fixed
component. Alternatively, the L2- MLP
distance-based prototype layer, as Fixed
utilized in ProtoPNet, can be em- MLP
ployed in conjunction with a fixed
logistic regression layer for classi- Table 2: Ablation study with diferent prototype layers
and classifiers with respect to a super-prototype
kernel. (Best accuracy in bold)</p>
          <p>Super- Prototype Classifier
Prototype Layer
––– L2
––– L2
––– Cosine
––– Cosine
! L2
! L2
! Cosine
! Cosine
ifcation (fine-tuned in the second
training phase in ProtoPNet).
Notably, ProtoSpArX surpasses the performance of state-of-the-art methods even when utilizing a
ifxed logistic regression layer, instead of an MLP as the classifier (but performs best with the
MLP).</p>
        </sec>
        <sec id="sec-6-2-3">
          <title>6.3. Spatial Correlations</title>
          <p>
            To assess whether diferent image classification methods can account for spatial relationships
between prototypical-parts in images, we adapted the SHAPES dataset [
            <xref ref-type="bibr" rid="ref6">6</xref>
            ] as a benchmark. We
randomly generated synthetic images containing 3 × 3 grids of circles, triangles, and squares
in diferent colours (red, green, and blue), so that an image is assigned Class 1 if a triangle is
located in the first column and a circle is located in the third column of the same row 2, and
Class 0 otherwise. The resulting dataset comprises 10,000 28 × 28 images with balanced binary
class labels. Figure 4 shows examples of images in the dataset. The first row contains images
from class 1, where a triangle is located in the first column and a circle is located in the third
column of the same row. The second row contains images from class 0, where this condition is
not met.
          </p>
          <p>The last column in Table 1 compares the accuracy of the baselines for this SHAPES dataset.
ProtoSpArX, with an accuracy of 98.4% ± 0.2%, significantly outperforms all other approaches.
The accuracy of the other approaches is around 50%, suggesting that these models are unable to
infer class labels solely based on the presence of prototypes in images, being unable to infer
information about the relative placement of the prototypical-parts in the images. ProtoSpArX
addresses this limitation by using channel-wise max and super-prototypes, which enable the
2This criterion can be customized to reflect the user’s preferences. For example, the dataset could assign Class 1
to images with a square in the first column, a blue triangle in the second, and a red square in the third.
model to infer the spatial correlation of diferent prototypical-parts in the image when needed
for classification.</p>
          <p>Method # C(SUuBper-)ProtoCtayrpses
ignnPProrototoPPSNhaerte 22000000 22000000
run ProtoPool 202 / 202× 5 195 / 195× 5
P ProtoTrees 512 512
reo PIP-Net 2000 2000
feProtoArgNet 200 196
B</p>
          <p>ProtoPNet 2000 1960
enProtoPShare 400 480
ru ProtoPool — —
r ProtoTrees 202 / 202× 3 195 / 195× 3
P
fte PIP-Net 495 515
AProtoArgNet — —</p>
        </sec>
        <sec id="sec-6-2-4">
          <title>6.4. Cognitive Complexity</title>
        </sec>
      </sec>
      <sec id="sec-6-3">
        <title>The combination of super-prototypes and</title>
        <p>QBAFs can serve as the basis for
humanreadable local explanations for the outputs
of ProtoSpArX. Figure 2 showed a generated
local explanation for a data point in SHAPES
(see the examples in Section 4 for details on this illustration). Figure 3 illustrates a local
explanation generated for a data instance from the CUB dataset, specifically for the target class
“Baird Sparrow." The green overlay on the super-prototype highlights the region in the input
image that supports the correct classification, while the red region identifies the attacked or
unsupported portion of the input. This super-prototype can be interpreted as the bird’s head
resembles a “Baird Sparrow," but its tail is atypical for this species. We have added this reading
manually here for illustration, simulating how to read the super-prototypes. We leave the
automatic generation of natural language interpretations of the super-prototypes and the QBAF
for future work.</p>
        <p>We can use the number of representative (super-)prototypes as a measure of the cognitive
complexity of the explanations drawn from prototypical-part-learning methods. Table 3
compares the number of (super-)prototypes for each approach, before and after the pruning phase
if applicable. Since ProtoPool and ProtoTrees use an ensemble of multiple models, we have also
reported these cases. Like ProtoPool, our ProtoSpArX does not have an additional phase for
pruning unnecessary prototypes. The number of super-prototypes in our approach is equal to
the number of classes since ProtoSpArX has one super-prototype per class. Notice that using a
ifxed classification layer, as in ProtoPNet, for ProtoSpArX, the local explanations require only
one super-prototype while other approaches need multiple prototypes.</p>
        <p>
          The global cognitive complexity of ProtoSpArX should additionally include the number of
hidden nodes in the MLP since each node in the resulting QBAF would be part of the explanation.
This complexity can be controlled by sparsification as in SpArX [
          <xref ref-type="bibr" rid="ref7">7</xref>
          ], with a trade-of between
compression ratio of the MLP classifier and accuracy of the resulting ProtoSpArX model. For
illustration, considering a one-layer MLP and 10 arguments in the QBAF after the sparsification
of the MLP, the cognitive complexity of the QBAF would be 210 and 206 for CUB and Cars,
respectively.
        </p>
      </sec>
    </sec>
    <sec id="sec-7">
      <title>7. Conclusion</title>
      <p>We proposed ProtoSpArX, a novel prototypical-part-learning approach. ProtoSpArX learns a
single super-prototype per class. The super-prototypes integrate multiple prototypical-parts shared
between diferent classes into a representative prototype per class. It can be trained end-to-end
and does not require an additional pruning phase. As opposed to previous
prototypical-partlearning approaches, the use of super-prototypes allows ProtoSpArX to capture spatial
relationships between prototypical-parts. Using an MLP for classification allows ProtoSpArX to capture
non-linear relationships between super-prototypes, while applying the SpArX methodology
allows explaining the classification outcome. Experiments show that ProtoSpArX outperforms
state-of-the-art prototypical-part-learning approaches in terms of accuracy and the ability to
model spatial relationships between prototypical-parts.</p>
      <p>
        Future directions include expanding ProtoSpArX’s capabilities to encompass multi-modal
data. Additionally, we will investigate the implementation of a user-model feedback loop to
enhance the debugging process for super-prototypes. Further, we plan to deploy ProtoSpArX
with real data, e.g. in the medical domain. Finally, we plan to explore various options for
obtaining explanations from ProtoSpArX, including interactive forms thereof [
        <xref ref-type="bibr" rid="ref16">16</xref>
        ].
      </p>
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