---
title: 3.12 PLR Signal Analysis Enhancements
description: "Version 6.1+ substantially extends the PLR video mode (§2.6) with a quantitative signal-analysis layer. The existing pupil-diameter time series is unchanged; the new features all operate on that series after capture. --- ### 3.12.1 Constriction Kinetics — Velocity, Amplitude, Latency…"
url: "https://cnri.edu/docs/3-12-plr-signal-analysis-enhancements/"
updated: "2026-04-22"
category: Reading the Analysis Results
---

# 3.12 PLR Signal Analysis Enhancements

Version 6.1+ substantially extends the PLR video mode (§2.6) with a quantitative signal-analysis layer. The existing pupil-diameter time series is unchanged; the new features all operate on that series after capture.

---

### 3.12.1 Constriction Kinetics — Velocity, Amplitude, Latency

The constriction kinetics panel extracts the four canonical PLR parameters from the diameter trace:

| Parameter | Symbol | Definition | Normal adult range |
|-----------|--------|-----------|-------------------|
| **Latency** | T<sub>L</sub> | Time from stimulus onset to the first detectable diameter reduction | 200 – 280 ms |
| **Maximum constriction amplitude** | ΔD<sub>max</sub> | Difference between baseline diameter and minimum diameter during the response | 1.0 – 2.5 mm |
| **Maximum constriction velocity** | V<sub>c</sub> | Peak of the first derivative of the diameter trace | 3 – 7 mm/s |
| **Constriction duration** | T<sub>c</sub> | Time from onset to minimum diameter | 700 – 1,000 ms |

Each value is shown with its normal adult reference range and flagged if outside the range. The ranges are adjustable per age group using the age-norm table already implemented in §3.2.

---

### 3.12.2 Redilation Time (T75)

T75 is the time required for the pupil to re-dilate to 75% of its pre-stimulus baseline diameter after the constriction minimum. It is a widely used marker of parasympathetic–sympathetic balance in pupillometry research.

| T75 value | Interpretation |
|-----------|----------------|
| < 1.2 s | Rapid redilation — sympathetic dominance |
| 1.2 – 2.0 s | Normal range |
| > 2.0 s | Slowed redilation — parasympathetic dominance, fatigue, or pharmacological influence |

---

### 3.12.3 Hippus — Spontaneous Oscillation Detection

**Hippus** is the normal physiological low-frequency oscillation of pupil diameter at rest (typically 0.1 – 1 Hz). Increased hippus amplitude is a research-observational marker of autonomic instability.

After the constriction trace settles into its steady-state baseline (starting ~3 seconds after the stimulus), PupilMetrics computes:

| Metric | Meaning |
|--------|---------|
| **Hippus amplitude** | Peak-to-peak baseline oscillation amplitude, normalised to mean diameter |
| **Hippus frequency** | Dominant frequency in the 0.05 – 1.5 Hz band |
| **Hippus irregularity** | Coefficient of variation of successive peak intervals |

---

### 3.12.4 Spectral (Fourier) Analysis of the PLR Trace

A **Spectrum** tab on the PLR results screen shows the power spectral density of the baseline diameter trace (Welch's method, 4-second windows, 50% overlap). The spectrum is drawn with shaded bands for the three physiological frequency ranges:

| Band | Range | Physiological association |
|------|-------|--------------------------|
| Very low frequency | 0.04 – 0.15 Hz | Thermoregulation, humoral activity |
| Low frequency | 0.15 – 0.4 Hz | Sympathetic modulation (pupillary analogue of heart-rate LF) |
| High frequency | 0.4 – 1.5 Hz | Parasympathetic modulation (pupillary analogue of heart-rate HF) |

The LF/HF ratio is displayed as a single research-observational autonomic-balance proxy.

> **Research-only caveat.** All PLR signal-analysis metrics are experimental. They are influenced by ambient light, fixation stability, blinks, screen flash consistency, and camera frame rate. Published normative ranges assume laboratory-grade pupillometers; mobile-camera values may show systematic offsets. Use only for longitudinal comparison within the same patient and same device.