Trends in Neurosciences
ReviewChanges in visual perception at the time of saccades
Section snippets
Saccadic suppression
Another idea to emerge early in the last century was that visual sensitivity is actively reduced during saccades. Holt 9 concluded that saccades ‘condition a momentary visual central anaesthesia’, that is, a complete loss of sensitivity. However, evidence for suppression by saccades is contradictory. Many researchers 10., 11., 12. have reported weak threshold elevation for detecting spots of light flashed briefly during saccades (two to threefold) and Krauskopf et al. 13 found no threshold
Physiological studies
Whereas psychophysical studies indicate an early site for the action of saccadic suppression, perhaps as early as LGN, direct physiological evidence is less clear. In humans, visual activity during saccades has been studied using evoked potentials (VEPs) and, more recently, with fMRI and positron emission tomography (PET) imaging. Early studies using VEP (44., 45.) showed a strong (>80%) attenuation of response amplitude to stimuli that were presented at about the same time as the start of a
Perceived position
Retinal motion is not the only problem introduced by saccades. A related (but not identical) problem is how to perceive a stable external world from extremely unstable retinal images. Helmholtz 1 believed that the constancy of perceived position was maintained during and after saccades, because both extra-retinal (the ‘effort of will’) and retinal (sensed but not perceived image motion) information were used to recalibrate the direction of gaze.
Leonard Matin and colleagues 68., 69., 70., 71.
Physiological mechanisms
Compression cannot be explained by a slow extra-retinal position signal, but instead it requires changes in the properties of receptive fields or position codes associated with them. Duhamel et al. 95 showed that there are early changes in the receptive field properties of some neurones in the lateral intraparietal area (LIP) of monkeys making saccades that anticipate their consequences (Fig. 5). LIP neurones begin to respond up to 80 ms before the onset of a saccade to stimuli that will fall
Concluding remarks
Saccades result in several physiological events in anticipation of their threat to visual stability. They suppress visual motion, an effect that is not entirely the result of ‘visual masking’ (although the motion of patterned stimuli might play an important role). Suppression is selective and particularly strong for rapid, low frequency luminance modulation, suggesting that only magnocellular function is suppressed, whereas parvocellular function is spared or even slightly enhanced. Suppression
Acknowledgements
This work was supported by grants from the Human Frontiers Science Program, the Italian Ministry of Education (MURST 40%) and the Australian Research Council.
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