S cone increments and decrements: Nearly-linear perceptual scales and variable noise

Area of Science:

• Vision science
• Psychophysics
• Photoreceptor physiology

Background:

• The human visual system processes light stimuli through cone photoreceptors, with short-wavelength (S) cones playing a role in color vision.
• Understanding the processing of increases (S+) and decreases (S-) in S cone stimulation is crucial for visual perception models.

Purpose of the Study:

• To investigate perceptual asymmetries between increases and decreases in S cone photoreceptor stimulation.
• To compare suprathreshold perceptual scaling and discrimination thresholds for S+ and S- stimuli.
• To develop a model reconciling perceptual scales and thresholds for S cone contrast.

Main Methods:

• Two psychophysical experiments were conducted using incremental (S+) and decremental (S-) contrasts.
• Experiment 1 utilized the Maximum Likelihood Difference Scaling (MLDS) procedure for suprathreshold perception.
• Experiment 2 employed a two-alternative forced-choice (2AFC) method to measure pedestal discrimination thresholds.

Main Results:

• Substantial asymmetry was observed in pedestal discrimination thresholds between S+ and S- stimuli.
• Perceptual scales for S cone contrast were nearly linear and showed no significant asymmetry for S+ and S-.
• A proposed model, assuming noise proportional to the square root of stimulus contrast, successfully explained both perceptual scales and discrimination thresholds.

Conclusions:

• S+ and S- cone signals are processed asymmetrically, despite similar perceived contrast scaling.
• The asymmetry in discrimination thresholds is likely attributable to physiological differences between S ON and S OFF pathways.
• The noise model provides a unified explanation for both scaling and discrimination data in S cone pathways.