Efficiently Measuring Magnocellular and Parvocellular Function in Human Clinical Studies

Abstract

Purpose: Pokorny and Smith (J Opt Soc Am A Opt Image Sci Vis. 1997;14:2477-2486) described a laboratory method to behaviorally measure magnocellular and parvocellular pathway sensitivity. We investigated whether their method may be more efficiently applied to clinical populations by reducing adaptation times.

Methods: We measured contrast detection thresholds to a 30-ms increment on a 30 cd/m² background every 2 seconds after a 1-minute preadaptation to either a bright (90 cd/m²) or dim (3 cd/m²) luminance, in four observers. We also measured increment thresholds atop a steady 60 cd/m² luminous pedestal (30 cd/m² above the background) that remained on for 80 seconds, and tracked thresholds for 60 seconds after pedestal offset. We also assessed the minimum number of stimulus presentations required to reliably estimate thresholds using our four alternative forced choice (4-AFC) zippy estimation by sequential testing (ZEST) procedure.

Results: Detection thresholds between the bright and dim preadaptation conditions were identical within seconds after the offset of the preadaptation luminance. Thresholds on the steady luminance pedestal reached stable values within approximately 10 seconds from pedestal onset, and recovered within 2 seconds of pedestal offset. Analysis of the 4-AFC ZEST procedure found little decrease in threshold variability after approximately 14 stimulus presentations.

Conclusions: Preadaptation and stimulus adaptation times may be reduced dramatically from those described by Pokorny and Smith, without altering thresholds.

Translational relevance: Experimental time with clinical populations often is limited. Increasing the efficiency of the method of Pokorny and Smith allows for either shorter test sessions, or for a more extensive range of experimental parameters to be explored in disease.

Keywords: ZEST; adaptation; magnocellular; parvocellular.

Figure 1

Schematic representation of a single run of each of the three experimental conditions. Conditions 1 and 2 commenced with 60 seconds preadaptation to a either a bright or dim blank screen, respectively (a), after which increment thresholds for a 30 ms square probe on a 30 cd/m² background were measured every 2 seconds for the following 140 seconds (b, c). Condition 3 commenced with 60 seconds preadaptation to a 30 cd/m² blank screen (a), after which 60 cd/m² pedestals appeared (i.e., 30 cd/m² above the background) and contrast discrimination thresholds for a 30 ms square probe presented on top of one of the pedestals measured every 2 seconds for the next 80 seconds (b). The pedestals then disappeared, and increment thresholds measured every 2 seconds until 140 seconds (c). Runs were repeated 20 times per observer, in an interleaved fashion, with thresholds estimated using a method of a thousands ZESTs. Time zero denotes the end of the preadaptation period.

Figure 2

Increment thresholds on a 30 cd/m² background, as a function of the time after a preadaptation luminance of 3 or 90 cd/m² (Conditions 1 and 2, respectively). Solid and dashed lines give the mean ±2 SD, calculated from the combined trials of Conditions 1 and 2 from 120 seconds onwards (grey shaded zone). Data are shown from participants A through D.

Figure 3

Steady pedestal thresholds (0–80 seconds), followed by increment thresholds with the pedestal turned off (from 80 seconds onwards, Condition 3). For each panel, the leftmost solid and dashed lines give the mean ± 2 SD, calculated for trials at 60 through 78 seconds. Rightmost solid and dashed lines give the mean ± 2 SD calculated for trials from 120 seconds onwards for Conditions 1 and 2, as presented in Figure 2.

Figure 4

Standard deviation of threshold estimates as a function of the number of presentations. Trials from 120 seconds onwards were pooled across Conditions 1 and 2. Solid lines give best fitting decaying exponentials of the form y = A.b^x + C, where A gives the magnitude of the decay, b gives the rate of the decay, and C gives the final asymptote. Parameters values were A = 0.68, 0.73, 0.64, and 0.72 (Participants A–D), b = 0.85, 0.78, 0.77, and 0.78, and C = 0.05, 0.10, 0.11, and 0.08.