Influence of foveal distractors on saccadic eye movements: A dead zone for the global effect

Abstract

Three experiments investigated the global effect with foveal distractors displayed in the same hemifield as more eccentric saccade targets. Distractors were x-letter strings of variable length and targets corresponded to the central letter of letter strings (e.g., 'xxxkxxx'). Results showed that only foveal distractors longer than four letters (about 1 degree) deviated the eyes in a center-of-gravity manner thus suggesting a dead zone for the global effect. Short distractors influenced the likelihood of small-amplitude saccades (less than about 1 degree) and the latency of longer saccades. The findings were interpreted based on the dissociation between fixation and saccadic neurons. Implications for eye movements in reading were discussed.

On the effect of remote and proximal distractors on saccadic behavior: A challenge to neural-field models

“…Rejected trials represented 19.7% of the total number of trials across participants. This was in accordance with rejection rates obtained with a similar task and setup (see Vitu, Lancelin, Jean, & Farioli, 2006) and was mainly due to button press occurring before stimulus presentation or stimuli being displayed during a saccade and not during a fixation (8.6%) as well signal artifacts and blinks (5.8%). Other rejected trials were trials where no saccade was being made (1.4%), the saccade was anticipatory (3.7%), the saccade was in wrong direction (2%), or the initial fixation was not within the fixation window around the bars (0.8%).…”

“…We previously reported that a foveal distractor stimulus fails to deviate the eyes from a more eccentric ipsilateral target when it extends within less than 1.258 from the center of the fovea, thus suggesting a foveal deadzone for the global effect (Vitu, 2008;Vitu et al, 2006). Here, we found a global effect with ipsilateral distractors in the central region, but this was greatly reduced as compared to the effect of more eccentric distractors.…”

“…In turn, saccade onset would be delayed, and gradually more as the ratio of distractor to target eccentricity decreases (see also Griffiths et al, 2006;Walker et al, 1997). However, as shown here, the latency increase would be greatest when both distractor and target stimuli are displayed in the central 18 region (for related findings see Vitu, 2008;Vitu et al, 2006), possibly as a result of the slight overlap of the neurons' receptive fields across the vertical midline (see Goldberg & Wurtz, 1972) and/or rostro-rostral excitatory connections (e.g., Munoz & Istvan, 1998). These would reinforce neuronal activity in the rostral pole of the SC, and in turn the brainstem omnipause region, but more greatly than when only the distractor falls in the central region.…”

“…One particular study, reported by White et al (2005), was interpreted most favorably in terms of visual saliency, but could be another manifestation of the fixationmove balance. This revealed that the effect on saccade latency of a foveally-centered distractor patch decreased as the patch became greater in size (but see Vitu et al, 2006). It may well be that as the extent of the distractor increased, the omnipause fixation system was being activated to gradually smaller extents in comparison with the move system, which in turn progressively reduced its potentially inhibitory influence on saccade initiation.…”

On the reduced influence of contour on saccade metrics and its competition with stimulus size

“…Trials were rejected when the stimuli were displayed during a saccade and not a fixation (4.8%), the averaged eye position before saccade onset (as measured offline) deviated from the fixation cross by more than ±0.5°and ±1°in the horizontal and the vertical directions respectively (10.4%), a blink occurred before or after the saccade (6.94%), the saccade was anticipatory (latency less than 80 ms; 9.66%) or it landed no further than 1°t o the right of fixation (10.39%), resulting in a total of 24.6% rejected trials on average across participants. Note that rejection percentages in the range of 20% are commonly observed in this type of task and setup (e.g., Vitu et al, 2006). After rejection, there remained 527 trials on average (range: 504-544) in each In all analyses, means were calculated for each participant, and these were then averaged across participants.…”