Independent contributions of the orienting of attention, fixation offset and bilateral stimulation on human saccadic latencies

Abstract

In a series of experiments we examined the effects of the endogenous orienting of visual attention on human saccade latency. Three separate manipulations were performed: the orienting of visual attention, the prior offset of fixation (gap paradigm) and the bilateral presentation of saccade targets. Each of these manipulations was shown to make an independent contribution to saccade latency. In experiments 1 and 2 subjects were instructed to orient their attention covertly to a location by a verbal pre-cue; targets could appear in the attended hemifield (valid) or in the non-attended hemifield (invalid) together with a no-instruction (neutral) condition. Saccades were made under fixation gap and overlap conditions, to either single target or two bilaterally presented targets which appeared at equal and opposite eccentricities in both hemifields. The results showed a large increase (cost) of saccade latency to invalid targets and a small non-significant decrease (benefit) of saccade latency to valid targets. The cost associated with invalid targets replicates the "meridan crossing effect" shown in manual reaction time experiments and is consistent with the hemifield inhibition and premotor models of attentional orienting. The use of a "gap" procedure produced a generalised facilitation of saccade latency, which was not modified by the prior orienting of visual attention. The magnitude of the gap effect was similar for saccades made to attended and non-attended stimulis. This suggests that the gap effect may be due to ocular motor disengagement, or a warning signal effect, rather than to the prior disengagement of visual attention. When two targets were presented simultaneously, one in each hemifield, saccade latency was slowed compared with the single target condition. The magnitude of this slowing was unaffected by the prior orienting of visual attention or by the fixation condition. The slowing was examined in more detail in experiment 3, by presenting targets with brief offset delays. The latency increase was maximal if the two targets were presented simultaneously and decreased if the distractor appeared at short intervals (20-80 ms) before or after the saccade target onset. If the non-attended stimulus was presented at greater intervals (160, 240 ms) before the saccade target, then a facilitation effect was observed. This demonstrates that the onset of a distractor in the non-attended hemifield can have both an inhibitory and a facilitatory effect on a saccade production.

Inhibition of return, gap effect and saccadic reaction time to a visual target

“…Similar effects of voluntary orienting of attention on saccadic eye movements have also been observed (13). Moreover, inhibition has been found for saccadic eye reaction time when the visual target is presented at the same position as the cue (8,14,15) or in another peripheral position in the same hemifield (12,14,15).…”

A non-spatial bias favouring fixated stimuli revealed in patients with spatial neglect

“…Moreover, our patients even showed shorter saccade latencies compared to the foveal distracter condition when the distracter appeared ipsilesionally and the target was in the contralesional hemifield, a situation that should have induced a disengagement deficit. Finally, it has been shown that whether attention is engaged at fixation or not does not change the extent of facilitation observed in the gap condition (Kingstone and Klein, 1993;Walker et al, 1995). These considerations suggest that attentional disengagement primarily affects processing of stimuli presented in the visual periphery, but is unlikely to explain the bilateral effect of a foveal distracter on saccadic latencies and the absence of an effect on manual RTs.…”

“…First, in comparison to an overlap condition the offset of a central fixation stimulus shortly prior to appearance of the target significantly reduces saccade latency (Saslow, 1967), a phenomenon known as the gap effect. Second, a distracter presented simultaneously with the target, but in the opposite hemifield, systematically increases saccade latency, which has been termed the remote distracter effect (Walker et al, 1995(Walker et al, , 1997. Both effects are specific to eye movements and have been attributed to inhibitory interactions within the oculomotor system, in particular the superior colliculus Wurtz, 1992, 1993b;Walker et al, 1997;Gandhi and Keller, 1999).…”

Eye scanpaths during visual imagery reenact those of perception of the same visual scene

“…Indeed, several investigators have suggested that covert shifts of attention may operate and have evolved as preparatory mechanisms for the control of overt shifts in eye fixations (cf. Rizzolatti et al, 1987;Umiltà et al, 1991;Walker, Kentridge, and Findlay, 1995). Shifts of attention precede shifts of the eyes to the same location (cf.…”