## Scatter Plot Grid: Fraction of Variance Explained by Principal Components (PCs)
### Overview
The image displays a 2x3 grid of six scatter plots. The overall title is "Fraction of variance in centered and averaged activations explained by PCs." Each subplot shows the explained variance (y-axis) for the first 10 principal components (x-axis) for different combinations of linguistic conditions. The data points are blue circles. The plots collectively analyze how variance in activation data is distributed across principal components under varying experimental conditions.
### Components/Axes
* **Overall Title:** "Fraction of variance in centered and averaged activations explained by PCs"
* **Y-axis Label (Common to all plots):** "Explained variance"
* **X-axis Label (Common to all plots):** "PC index"
* **X-axis Scale:** Linear scale from 1 to 10, with major ticks at 2, 4, 6, 8, and 10.
* **Y-axis Scale:** Linear scale from 0.0 to approximately 0.5, with major ticks at 0.0, 0.1, 0.2, 0.3, and 0.4. The exact upper limit varies slightly per subplot.
* **Subplot Titles (Positioned above each plot):**
1. Top-left: "affirmative"
2. Top-center: "affirmative, negated"
3. Top-right: "affirmative, negated, conjunctions"
4. Bottom-left: "affirmative, affirmative German"
5. Bottom-center: "affirmative, affirmative German, negated, negated German"
6. Bottom-right: "affirmative, negated, conjunctions, disjunctions"
* **Data Series:** Each plot contains a single data series represented by blue dots. There is no legend, as the title of each subplot defines the data series.
### Detailed Analysis
The following table reconstructs the approximate data points for each subplot. Values are estimated from the grid lines.
| PC Index | "affirmative" | "affirmative, negated" | "affirmative, negated, conjunctions" | "affirmative, affirmative German" | "affirmative, affirmative German, negated, negated German" | "affirmative, negated, conjunctions, disjunctions" |
| :--- | :--- | :--- | :--- | :--- | :--- | :--- |
| **1** | ~0.48 | ~0.33 | ~0.33 | ~0.48 | ~0.30 | ~0.31 |
| **2** | ~0.29 | ~0.29 | ~0.26 | ~0.30 | ~0.29 | ~0.25 |
| **3** | ~0.10 | ~0.15 | ~0.14 | ~0.09 | ~0.14 | ~0.13 |
| **4** | ~0.04 | ~0.06 | ~0.06 | ~0.04 | ~0.06 | ~0.06 |
| **5** | ~0.02 | ~0.05 | ~0.04 | ~0.02 | ~0.05 | ~0.05 |
| **6** | ~0.02 | ~0.03 | ~0.03 | ~0.02 | ~0.03 | ~0.04 |
| **7** | ~0.00 | ~0.02 | ~0.02 | ~0.01 | ~0.03 | ~0.03 |
| **8** | ~0.00 | ~0.02 | ~0.02 | ~0.01 | ~0.02 | ~0.03 |
| **9** | ~0.00 | ~0.01 | ~0.02 | ~0.00 | ~0.01 | ~0.03 |
| **10** | ~0.00 | ~0.01 | ~0.01 | ~0.00 | ~0.01 | ~0.03 |
**Trend Verification:** In all six plots, the data series follows the same fundamental trend: a steep, monotonic decrease in explained variance from PC1 to PC3 or PC4, followed by a long tail where the explained variance approaches zero for higher-index PCs. This is the classic "scree plot" pattern expected from PCA.
### Key Observations
1. **Dominance of First PCs:** Across all conditions, the first 2-3 principal components capture the vast majority of the variance (often over 70% combined). The explained variance drops sharply after PC2.
2. **Condition-Dependent Variance Distribution:**
* The "affirmative" only condition (top-left) shows the highest variance for PC1 (~0.48) and a very steep drop, suggesting a simpler, more dominant primary pattern.
* Adding more conditions (negated, conjunctions, German translations) generally reduces the variance explained by PC1 (to ~0.30-0.33) and slightly flattens the curve, indicating a more complex variance structure spread across more components.
* The plot with the most conditions (bottom-right: "affirmative, negated, conjunctions, disjunctions") shows a slightly more gradual decline, with PC10 still explaining a non-negligible fraction (~0.03).
3. **Similarity Between Related Conditions:** The two plots involving German translations (bottom-left and bottom-center) show very similar variance profiles to their English-only counterparts (top-left and top-center, respectively), suggesting the principal components capture language-invariant patterns.
### Interpretation
This set of plots performs a **Peircean investigative** analysis of the dimensional structure of neural or computational activation data. The "explained variance" is a sign of how much information (or structure) each principal component captures.
* **What the data suggests:** The data demonstrates that the core variance in "centered and averaged activations" for these linguistic tasks is low-dimensional. A few principal components (likely representing fundamental features like sentence polarity, presence of negation, or logical connective type) account for most of the systematic variation.
* **How elements relate:** The progression from simple ("affirmative") to complex ("...disjunctions") conditions acts as a controlled experiment. As the linguistic complexity of the input data increases, the variance becomes less concentrated in the very first component and more distributed, though still dominated by the first few. This implies the underlying representational space expands to accommodate the new distinctions.
* **Notable anomalies/trends:** The near-zero variance for PCs 7-10 in the simple "affirmative" case is notable; it suggests that beyond a few key features, the remaining dimensions capture noise or irrelevant variation. In contrast, the more complex conditions maintain small but measurable variance in these higher components, indicating they encode meaningful, albeit subtle, information necessary to distinguish the broader set of conditions.
* **Why it matters:** This analysis is crucial for dimensionality reduction and understanding model representations. It tells a researcher that they can likely project their high-dimensional activation data into a 3- to 5-dimensional space (using the top PCs) while retaining most of the meaningful signal for these tasks. The differences between plots guide how many components are needed for different experimental scopes.