Blind saccades: an asynchrony between seeing and looking.
Saccades may not always wait for the completion of the perceptual analysis. By taking advantage of a motion-induced illusion of position and of the spontaneous scatter of saccade latency, we showed that in normal observers, regular saccades (latency, approximately 200 ms) were accurately directed to the target, whereas at higher latencies, saccades were increasingly biased by visual motion until they reflected the perceptual illusion. We reconstructed the time course of saccadic direction coding and identified an early phase in which saccades are mostly predictive (latencies less than approximately 100 ms), followed by a phase in which saccades are guided by the target position signal (latencies approximately 100-250 ms), and a later phase associated with the buildup of mislocalization (approximately 250-450 ms). This transient dissociation between action and perception indicates that seeing and looking are based on asynchronous processes, possibly because of independent thresholds for saccades and perceptual localization. The metrics of a saccade would then reflect the evolution of cortical visual signals from a predictive state to a perceptual state, passing through an intermediate visuomotor state. If saccades occur during the visuomotor state, they escape the tricks of perception.
de'Sperati
Claudio
C
Visuomotor Functions Laboratory, University Vita-Salute San Raffaele, 20132 Milan, Italy. desperati.claudio@hsr.it
Baud-Bovy
Gabriel
G
D000328:Adult; D000818:Animals; D001766:Blindness / Q000503:physiopathology; D005260:Female; D006801:Humans; D007088:Illusions / Q000502:physiology*; D008297:Male; D009039:Motion Perception / Q000502:physiology*; D010775:Photic Stimulation / Q000379:methods; D011698:Pursuit, Smooth / Q000502:physiology; D011930:Reaction Time / Q000502:physiology; D012438:Saccades / Q000502:physiology*; D014785:Vision, Ocular / Q000502:physiology*