Disentangling visual and motor contributions to continuous psychophysics

Continuous psychophysics is a method of measuring visual function by having observers track a randomly moving target. This approach provides more information than traditional discrete judgments and can be used to estimate visual sensitivity in a relatively short amount of time. When analyzing tracking performance, researchers often summarize the data by examining the cross-correlation between target and cursor velocities.

The resulting cross-correlogram reveals how strongly, quickly, and sharply a person responds to the target, with key metrics including the peak, lag, and width.

However, tracking involves both seeing the target and moving the cursor towards it, so poor tracking performance may be due to impaired vision, impaired motor control, or a combination of both factors. In this study, researchers aimed to determine whether visual and motor contributions to tracking performance can be separated. Participants tracked a target using a computer mouse while wearing glasses that simulated two levels of visual impairment. They used either their dominant or non-dominant hand for the task.

The researchers found that both visual impairment and the use of the non-dominant hand affected the cross-correlogram metrics, but in different ways. The non-dominant hand mainly lowered the peak correlation, indicating a motor control issue. On the other hand, visual impairment mainly delayed and broadened the response, with the extent of the effect directly proportional to the participant's degree of visual acuity loss.

Two distinct signatures of motor control emerged as key to differentiating visual and motor contributions to tracking performance. When using the non-dominant hand, participants paused more frequently and for longer durations, and their movements were noisier between pauses. In contrast, visual impairment did not affect these behaviors.

By analyzing response lag, the time participants paused, and movement noise in the tracking data, the researchers were able to determine whether changes in performance were primarily due to visual impairment or motor control issues in almost every participant.

These findings demonstrate that the same tracking data used to measure visual function can also provide valuable insights into the role of motor control in tracking performance. With a baseline measurement of performance, researchers can use the same tracking data to identify whether changes in tracking are primarily visual or motor in nature.