The use of egocentric and exocentric location cues in saccadic programming

Abstract

Theoretically, the location of a visual target can be encoded with respect to the locations of other stimuli in the visual image (exocentric cues), or with respect to the observer (egocentric cues). Egocentric localization in the oculomotor system has been shown to rely on an internal representation of eye position that inaccurately encodes the time-course of saccadic eye movements, resulting in the mislocalization of visual targets presented near the time of a saccade. In the present investigation, subjects were instructed to localize perisaccadic stimuli in the presence or absence of a visual stimulus that could provide exocentric location information. Saccadic localization was more accurate in the presence of the exocentric cue, suggesting that localization is based on a combination of exocentric and egocentric cues. These findings indicate the need to reassess previously reported neurophysiological studies of spatial accuracy and current models of oculomotor control, which have focused almost exclusively on the egocentric localization abilities of the brain.

Colliding saccades evoked by frontal eye field stimulation: artifact or evidence for an oculomotor compensatory mechanism underlying double-step saccades?

“…For targets presented before the initial saccade onset, R"O et al 1992a). The remaining inaccuracy can be further reduced when allocentric cues become more salient (Dassonville et al 1995) -an important concern in our subsequent discussion.…”

“…To consider these questions in a larger context, we return to the original double-step observations of Hallet and Lightstone (1976), and the recent re-examination of this work by Dassonville et al (1995). In their re-examination, these authors determined that consecutive presentation of the two targets with no temporal gap separating them (NO-GAP condition) provides a form of exocentric (allocentric) spatial cueing that increases saccade accuracy considerably.…”

“…When a timing gap disrupts this temporal continuity by using brief target flashes while leaving the onset times the same (GAP condition), the accuracy is impaired. Dassonville et al (1995) suggested that when experimental conditions allow the integration of temporally adjacent target information [as in the 'NO-GAP' paradigms of Hallet and Lightstone (1976), Goldberg and Bruce (1990) and Gnadt and Andersen (1988)], brain mechanisms may take advantage of this information to directly calculate the required dimensions of a saccade from the first to second target. It is conceivable that under such conditions these cortical mechanisms would be responsible for the updated motor error code for the second target recorded in FEF, whereas the less accurate compensation observed in the more impoverished GAP condition would rely more exclusively on a subcortical mechanism like the one we have described here.…”

“…This discrepancy in characterizing the compensatory mechanism is at the heart of the debate that our research addresses. The problem of localizing this mechanism in the brain and understanding its underlying processes is compounded by the possibility of still other mechanisms that would rely on allocentric cues for correctly performing this transformation (Dassonville et al 1992a(Dassonville et al , 1995.…”

Visually guided adjustments of body posture in the roll plane

“…Similarly, for visual alignments (Cai et al 1997), estimated target location (Dassonville et al 1995) and spatial memory (Baker et al 2003) extra-retinal input was reported to be taken into consideration for tasks that theoretically may be solved based on retinal input solely. This, however, does not completely exclude a retinal origin of such a roll-angle dependent modulation (see below).…”