If the pupil measurements in the youngest 10 subjects (mean age 24, range 20-26) are compared to those in the oldest 10 subjects (mean age 65, range 53–75), no significant difference is found in their normalized velocity estimates (mean 3.51 mm/s in the younger subjects, 3.42 m/s in the older subjects, t = 0.488, P = 0.632). Figure 4B shows a scatter plot of normalized velocity estimates against a wide range of ages (20–75 years), but the linear regression coefficient is not significant ( R = 0.193, P = 0.215). A similar approach also reveals no apparent influence from age. No clear relationship emerges across a wide range of pupil size (3.87–7.84 mm), and linear regression analysis gives a coefficient R = 0.242 ( P = 0.118). A scatter plot of these normalized velocity estimates (“NV”) against corresponding measurements of the resting pupil diameter is shown in Figure 4A. The mean value for these estimates of normalized velocity was 3.56 mm/s (SD 0.40, range 2.79–4.41). Only one randomly chosen eye was selected from each subject to prevent duplication of the independent variable in the subsequent analysis. This was achieved in each subject by using the regression plot to extrapolate the constriction velocity that would be expected if the stimulus intensity had been adjusted to produce exactly a 1.0 mm response amplitude ( Fig. In the second protocol, performed a few minutes later and also after 10 seconds of darkness, each subject was presented with a pseudo-random sequence of 10 light stimuli of varying intensity (from 0.0 to −4.0 log units attenuation of this “standard” brightness) projected into both eyes with an inter-stimulus interval of 8 seconds (test duration 90 seconds).īecause of this interdependence between amplitude and velocity of constriction, it was necessary first to normalize the velocity measurements with respect to response amplitude before investigating the possible influence of pupil size and age on peak constriction velocity. In the first, after 10 seconds of darkness the subject was presented with a single pulse of “standard intensity” light (3.96 log trolands) projected into both eyes: this light intensity was enough to produce approximately 30% constriction of the pupil (i.e., not bright enough to saturate the pupillary reflex or produce an afterimage). In an emmetropic eye this stimulus illuminates the central 7 0 of retina, centered on the fovea. In each eye the stimulus beam was converged to a width no more than 1.0 mm in the plane of the pupil, ensuring identical light flux in all subjects irrespective of starting pupil size (“open-loop” or Maxwellian optics). The light stimulus consisted of a square-wave pulse of white light (duration 1.0 second) projected through both pupils. Clinicians and researchers must keep this interdependence in mind when drawing inferences from the observed (or measured) speed of the pupillary constriction to light. This relationship is unaffected by the stimulus intensity, size of the pupil, or age of the subject.
There is a strong linear correlation between amplitude and peak velocity of constriction for the pupil light reflex in normal subjects. In each subject the regression plot was used to normalize the velocity estimates for A = 1.0 mm these normalized velocity estimates showed no significant correlation with the starting size of the pupil or the age of the subject.
Over a 4.0 log unit range of stimulus intensities measurements of A and V were seen to co-vary with the data being best fit by the equation V = 0.86 + 2.65 A (linear regression coefficient, R = 0.919, P < 0.001). In response to the “standard-intensity” light stimulus, mean measurements of A and V were 1.92 mm (SD 0.39) and 5.65 mm/s (SD 1.17), respectively. The pupil response to a variable intensity 1.0 second light stimulus presented under open-loop conditions (Maxwellian optics) was measured using infrared video techniques in 43 healthy subjects aged 20 to 75 years old. To investigate the correlation between measurements of amplitude (A) and peak velocity (V) of constriction in the pupil light reflex of normal subjects, and to determine the effects of stimulus intensity, pupil size, and age on this relationship.
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