Regularization Revisited: Advanced Techniques

While standard regularization methods like L2 weight decay and basic Dropout are fundamental tools for combating overfitting, training the very deep and complex CNNs discussed in this course often benefits from more refined approaches. As models grow deeper and wider, they gain expressive capacity but also become more susceptible to overfitting the training data and sensitive to the specifics of the training process. This section examines advanced regularization techniques designed to improve the generalization capabilities of modern CNNs.

Dropout Variants for Convolutional Layers

Standard Dropout randomly sets a fraction of neuron activations to zero during training. While effective in fully connected layers, its application in convolutional layers, where adjacent pixels or feature map activations often share correlated information, can be suboptimal. Simply dropping individual activations might not introduce sufficient noise or prevent co-adaptation effectively due to the strong spatial structure. Advanced Dropout variants address this.

Spatial Dropout

Instead of dropping individual activation values, Spatial Dropout (sometimes called 2D Dropout) randomly drops entire feature maps (channels) during training. If a specific channel is selected for dropout, all activations within that feature map are set to zero.

Consider a feature map tensor with shape (batch_size, height, width, channels). Standard Dropout would operate independently on each element within the height * width * channels dimensions. Spatial Dropout, however, applies the same dropout mask across the height and width dimensions for a given channel.

Comparison showing standard dropout acting pixel-wise versus spatial dropout acting channel-wise. Gray squares indicate dropped units or channels.

This approach encourages the network to learn redundant representations across different feature maps, making it more resilient to the absence of entire channels and better suited for convolutional layers processing spatially correlated data.

DropConnect

DropConnect is another variation where, instead of zeroing out activations (outputs of neurons), it randomly sets weights within the network to zero during the forward pass. Each connection between layers has a probability of being dropped.

While Dropout affects the activations ($a$) in a layer ($y = a(Wx + b)$), DropConnect applies a mask ($M$) directly to the weights ($W$) and biases ($b$):

$$y = a((M_W \odot W)x + (M_b \odot b))$$

Here, $\odot$ represents element-wise multiplication, and $M_W, M_b$ are binary masks sampled for each training example. DropConnect can be seen as a more general form of regularization compared to Dropout, potentially introducing more noise and requiring careful tuning. It's computationally more intensive than standard Dropout as it requires sampling different masks for weights for each example, rather than just for activations.

Label Smoothing Regularization (LSR)

Classification models are typically trained using one-hot encoded labels and a cross-entropy loss function. This encourages the model to produce output probabilities that are extremely close to 1 for the correct class and 0 for all incorrect classes. For example, for a 3-class problem with the true label being class 1, the target probability vector is $[1, 0, 0]$. The model is penalized heavily if it assigns even a small probability to the incorrect classes.

While this seems intuitive, it can lead to issues:

1. Overconfidence: The model learns to become excessively confident in its predictions, which might not reflect the true underlying uncertainty. This can hinder generalization, especially if the training data contains noise or ambiguity.
2. Sensitivity to Noise: Hard targets make the model highly sensitive to potential mislabeled examples in the training set.
3. Reduced Adaptability: Extremely large logit differences between the correct and incorrect classes can make the model less adaptable during fine-tuning or transfer learning.

Label Smoothing addresses this by replacing the hard 0 and 1 targets with "softer" probabilities. Instead of demanding a probability of 1.0 for the correct class, we assign it a target probability slightly less than 1, like $1 - \alpha$. The remaining probability mass $\alpha$ is distributed evenly among the incorrect classes.

For a classification problem with $K$ classes, if the original one-hot target for an example is $y_k = 1$ for the true class $k_{true}$ and $y_k = 0$ for $k \neq k_{true}$, the smoothed label $y'_{k}$ becomes:

$$y'_{k} = y_{k} (1 - \alpha) + \frac{\alpha}{K}$$

Let's illustrate with an example. Suppose we have $K=5$ classes, the true class is index 2, and we use a smoothing factor $\alpha = 0.1$.

• Original one-hot target: $[0, 0, 1, 0, 0]$
• Smoothed target:
  • For the true class (k=2): $y'_{2} = 1 \times (1 - 0.1) + \frac{0.1}{5} = 0.9 + 0.02 = 0.92$
  • For any incorrect class (e.g., k=0): $y'_{0} = 0 \times (1 - 0.1) + \frac{0.1}{5} = 0 + 0.02 = 0.02$
• Resulting smoothed target vector: $[0.02, 0.02, 0.92, 0.02, 0.02]$

When training with cross-entropy loss using these smoothed targets, the model is discouraged from producing extremely large logit values for the correct class relative to the others. It encourages finite differences between the logits, leading to a model that is better calibrated (its confidence scores are more indicative of actual likelihood) and often generalizes slightly better.

A typical value for the smoothing factor $\alpha$ is 0.1, but this can be tuned as a hyperparameter. LSR is widely used in training state-of-the-art image classification models.

Interaction with Other Techniques

It's important to note that these advanced regularization techniques interact with other components of the training process. For instance:

• Batch Normalization: BN itself has a slight regularizing effect due to the noise introduced by mini-batch statistics. The combination of BN and Dropout requires careful consideration, sometimes leading to adjustments in dropout placement or rate.
• Data Augmentation: Techniques like Cutout, Mixup, or RandAugment, which modify training images significantly, act as powerful regularizers. The strength of explicit regularization (like Dropout or LSR) might need adjustment when strong data augmentation is used.

Choosing the right combination and strength of regularization techniques often involves experimentation. Monitoring validation loss and accuracy is essential to find the configuration that prevents overfitting without excessively hindering the model's ability to learn from the training data. These advanced methods provide valuable additions to the deep learning practitioner's toolkit for building more effective and reliable CNN models.