Both separation and eccentricity can limit precise position judgements: a reply to Morgan and Watt

Area of Science:

• Visual perception
• Psychophysics
• Computational neuroscience

Background:

• Weber's law describes how position thresholds scale with reference feature separation in position acuity tasks.
• Two sensory processes, target separation and eccentricity, may limit position acuity and contribute to Weber's law for position.

Purpose of the Study:

• To investigate whether target eccentricity or separation limits position acuity.
• To address criticisms regarding the role of eccentricity in spatial interval discrimination tasks.
• To reconcile findings with previous studies and a two-mechanism model of position acuity.

Main Methods:

• Measuring spatial interval discrimination thresholds on an iso-eccentric arc to pit separation against eccentricity.
• Analyzing arc length discrimination tasks and comparing them to chord length judgments.
• Evaluating the impact of cognitive demands and task difficulty on performance.

Main Results:

• Position discrimination thresholds were independent of separation at large separations on an iso-eccentric arc.
• Eccentricity, not separation, limits precise position judgments at larger distances.
• Chord length judgments showed thresholds proportional to eccentricity at large separations, unlike arc length tasks.

Conclusions:

• Eccentricity is a critical factor limiting position acuity, particularly at larger target separations.
• The spatial interval discrimination task (chord length) is a more direct measure of sensory limits than arc length discrimination.
• Findings support a two-mechanism model where eccentricity plays a significant role in position acuity limits.