Effect of target shape on movement time in a Fitts task

Area of Science:

• Human-Computer Interaction
• Cognitive Psychology
• Motor Control

Background:

• Fitts's Law is a predictive model of human movement.
• Previous studies primarily used rectangular targets.
• The impact of non-rectangular target shapes on movement time is less understood.

Purpose of the Study:

• To investigate how target shape influences movement time in Fitts's Law tasks.
• To determine if different geometric shapes (square, circle, diamond, triangle) affect the movement time versus index of difficulty relationship.
• To develop a model that explains the effect of target shape on task difficulty.

Main Methods:

• Conducted two experiments varying target shapes (square, circular, diamond, triangular) in a Fitts movement task.
• Measured movement time and calculated the index of difficulty based on hit distribution.
• Analyzed the ratio of standard deviations of hits in vertical and horizontal directions.
• Applied a 'cookie-cutter' model to quantify shape effects.

Main Results:

• Movement time correlated with the index of difficulty across all tested shapes.
• The ratio of hit standard deviations was constant across shapes, indicating consistent directional constraint.
• All data fit a single line when movement time was plotted against the index of difficulty based on directional constraint.
• A 'cookie-cutter' model best explained the observed data, incorporating a 'shape factor'.

Conclusions:

• Target shape itself does not independently alter movement time in Fitts's Law tasks.
• The distribution of hits within a target's boundaries is the primary determinant of task difficulty, regardless of shape.
• The 'cookie-cutter' model effectively quantifies the influence of target shape on task difficulty relative to rectangular targets.