docs/acquisition_protocol.md

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