# PolyCAT Acquisition Protocol

## Equipment

| Component | Specification |
|-----------|--------------|
| Eye tracker | EyeLink 1000+ (SR Research) |
| Mounting | Head-mounted (desktop mount) |
| Recording mode | Binocular |
| Sampling rate | 500 Hz per eye (1000 Hz hardware, split binocular) |
| Display | 27" 4K LCD monitor |
| Resolution | 3840 x 2160 pixels |
| Refresh rate | 144 Hz |
| Viewing distance | 70 cm |
| Background luminance | Gray (0.3 normalized) |

## Display Geometry

The pixels-per-degree (PPD) value is computed from the physical display properties:

```
Screen diagonal = 27" = 68.58 cm
Aspect ratio = 16:9
Screen width  = 68.58 * (16/sqrt(16^2 + 9^2)) = 59.77 cm
Screen height = 68.58 * (9/sqrt(16^2 + 9^2))  = 33.62 cm
Pixel pitch = 59.77 / 3840 = 0.01557 cm/px

PPD = viewing_distance * tan(1 deg) / pixel_pitch
    = 70 * tan(1 deg) / 0.01557
    = 70 * 0.017455 / 0.01557
    ≈ 78.5 px/deg
```

Note: This gives the conversion for uniform pixel density. Since pixels are square on modern displays, horizontal and vertical PPD are equal.

## Calibration

- **Calibration type:** HV13 (13-point horizontal-vertical)
- **Validation:** Performed before each block
- **Drift check:** Before each trial (fixation on cue position)
- **Drift retry limit:** 1 attempt, max 10 seconds
- **Re-calibration:** Triggered if drift check fails

## Saccade Detection Parameters

The EyeLink built-in saccade detection was configured with:

| Parameter | Value |
|-----------|-------|
| Velocity threshold | 30 deg/s |
| Acceleration threshold | 8000 deg/s^2 |

Fixations are defined as periods between saccades (EyeLink cognitive configuration).

## Experiment Design

### Structure

Each participant completed 2 parts (A and B), typically on separate days.

Each part consists of:
- 9 mini-blocks of 39 trials each = 351 trials
- 36 image trials + 3 empty trials per block
- 1 memory probe after each block

### Trial Sequence

1. **Drift check** — Participant fixates on a cue at one of 9 grid positions (3x3 grid at 960/1920/2880 x 540/1080/1620 px). Max 10 s, 1 retry.
2. **Stimulus presentation** — Image displayed through polygon aperture for 4.0 seconds.
3. **Inter-trial interval** — Blank screen for 0.5 seconds.

### Stimuli

- **Images:** 600 images from the CAT2000 dataset across 6 categories:
  - Fractal, Object, OutdoorNatural, Random, Satelite, Sketch
  - 100 images per category (102 available, 100 used per part assignment)
- **Polygon apertures:** 27 shapes with systematically varied geometric properties
  - See `data/manifests/polygon_geometry.csv` for center coordinates
  - See `data/stimuli/polygons/*.json` for vertex definitions
- **Aperture scale factor:** 1987 (applied uniformly)

### Memory Task

After each block, a memory probe image was shown for 3 seconds:
- Participants judged whether the image was "old" (seen in that block) or "new"
- Response time and accuracy recorded
- Purpose: ensure attentive viewing

### Fixation Cue Positions

A 3x3 grid of possible fixation cue locations:

| Grid ID | X (px) | Y (px) |
|---------|--------|--------|
| grid_11 | 960    | 540    |
| grid_12 | 1920   | 540    |
| grid_13 | 2880   | 540    |
| grid_21 | 960    | 1080   |
| grid_22 | 1920   | 1080   |
| grid_23 | 2880   | 1080   |
| grid_31 | 960    | 1620   |
| grid_32 | 1920   | 1620   |
| grid_33 | 2880   | 1620   |

## Data Collection Timeline

Data was collected between January 15, 2026 and February 25, 2026. Most participants completed Part A and Part B on separate days.

## Split Sessions

Some participants required multiple session folders to complete a single part (due to experiment restarts or technical issues). In these cases:
- Trials are concatenated across sessions using `trial_uid` as the unique key
- For overlapping trials (recorded in multiple sessions), the later session's data takes precedence
- The `sessions.csv` metadata file labels these as `split_primary`