Size matters, but not for everyone: individual differences for contrast discrimination

Area of Science:

• Visual perception
• Psychophysics
• Computational neuroscience

Background:

• Contrast detection thresholds improve with stimulus size.
• This benefit is thought to be lost for contrast discriminations above threshold.
• Previous models explain size effects at detection but not discrimination.

Purpose of the Study:

• To investigate the effect of stimulus size on contrast detection and discrimination.
• To challenge the established view on size effects in suprathreshold contrast discrimination.
• To model individual differences in contrast discrimination using a gain control framework.

Main Methods:

• Performed contrast detection and discrimination tasks using circular sine wave grating patches.
• Varied stimulus size systematically across multiple observers.
• Applied an extended gain control model to analyze area summation effects.

Main Results:

• Confirmed sensitivity improves with the fourth-root of stimulus area at detection threshold (log-log slope of -0.25).
• Observed significant individual differences in suprathreshold contrast discrimination performance.
• Some observers showed performance unaffected by area, while others showed improvement then a plateau (log-log slope up to -0.5).

Conclusions:

• The effect of stimulus size on contrast discrimination varies among individuals.
• Individual differences in discrimination are linked to variations in the relationship between suppression and stimulus area.
• Area summation in contrast discrimination arises from suppression that grows slower than excitation with increasing stimulus size.