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Fixation disengagement and eye-movement latency

W J Tam1, H Ono

  • 1York University, Ontario, Canada.

Perception & Psychophysics
|September 1, 1994
PubMed
Summary

The gap effect, where eye movements are faster when a fixation stimulus disappears before a target appears, was observed for vergence and saccadic eye movements during fixation, pursuit, and extrafoveal viewing.

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Area of Science:

  • Oculomotor control
  • Human sensorimotor systems
  • Visual neuroscience

Background:

  • The gap effect is a well-documented phenomenon in oculomotor research.
  • It describes shorter saccadic latencies when a fixation stimulus is removed before target onset (gap condition) compared to when it overlaps with target onset (overlap condition).
  • Previous research primarily focused on saccadic eye movements during foveal fixation.

Purpose of the Study:

  • To investigate the presence and extent of the gap effect beyond standard saccadic eye movements during foveal fixation.
  • To determine if the gap effect applies to vergence eye movements.
  • To examine if the gap effect persists during dynamic visual conditions like smooth pursuit and during extrafoveal fixation.

Main Methods:

  • Experiments involved presenting visual stimuli under gap and overlap conditions.
  • Eye movements (saccades and vergence) were recorded during fixation, smooth pursuit of a moving stimulus, and extrafoveal fixation.
  • Target appearance was manipulated relative to the offset of the fixation stimulus.

Main Results:

  • The gap effect was confirmed for saccadic eye movements during foveal fixation.
  • A significant gap effect was demonstrated for vergence eye movements during visual fixation.
  • The gap effect was also observed for saccadic eye movements during smooth visual pursuit and extrafoveal fixation.

Conclusions:

  • The findings extend the known parameters of the gap effect to vergence movements and dynamic/peripheral visual tasks.
  • The results suggest that the underlying mechanisms of the gap effect are robust across different oculomotor response types and viewing conditions.
  • The study highlights the interplay between fixation-maintenance and target-directed eye movements, particularly the conflict between fixation-locking and fixation-shifting responses.

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