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Measuring V1 receptive fields despite eye movements in awake monkeys
Jenny C A Read1, Bruce G Cumming
1Laboratory of Sensorimotor Research, National Eye Institute, National Institutes of Health, Bethesda, Maryland 20892-4435, USA. jcr@lsr.nei.nih.gov
Journal of Neurophysiology
|April 25, 2003
Summary
Measuring receptive fields in awake monkeys is challenging due to small eye movements. Scleral search coils introduce errors, but a new fitting technique accurately determines receptive field parameters despite these movements.
Area of Science:
- Neuroscience
- Ophthalmology
- Computational Biology
Background:
- Accurate measurement of neuronal receptive fields in awake primates is crucial for understanding visual processing.
- Small, involuntary eye movements during fixation (fixational eye movements) blur receptive fields, leading to underestimates of neuronal precision.
- Scleral search coils are commonly used to track eye movements, but their accuracy and impact on receptive field measurements are not fully understood.
Purpose of the Study:
- To assess the accuracy of scleral search coils in measuring eye movements during primate fixation.
- To develop and validate methods for correcting receptive field measurements affected by eye movements.
- To determine the true stability of vergence in awake monkeys.
Main Methods:
- Implanted two scleral search coils in a single monkey eye.
- Utilized the small receptive field size of V1 neurons to assess coil accuracy.
- Developed maximum-likelihood fitting techniques to extract receptive field parameters while accounting for eye movements.
- Investigated the time course of coil errors relative to fixational eye movements.
Main Results:
- Un-sutured scleral search coils exhibit an error (SD ≈ 0.1 degrees) comparable to natural eye movements.
- This coil error is sufficient to explain the variability in measured vergence, suggesting vergence is more stable than previously thought.
- Coil measurements are more accurate at reflecting differences in eye position between trials than absolute positions within a trial.
- Receptive field estimates are not improved by assuming coil data is veridical; a fitting technique accounting for eye movements is necessary.
Conclusions:
- Scleral search coils, when not sutured, introduce significant errors that limit their utility for precise receptive field mapping.
- A novel fitting technique allows for reliable determination of receptive field parameters despite inherent eye movement measurement inaccuracies.
- Monkeys, similar to humans, exhibit highly stable vergence, contrary to some previous interpretations of coil data.
- These findings offer a method for assessing the reliability of eye-position measurements in neurophysiological studies.