Factors Associated with Saccade Latency

Abstract

Part of the aim of this thesis was to explore a model for producing very fast saccade latencies in the 80 to 120ms range. Its primary motivation was to explore a possible interaction by uniquely combining three independent saccade factors: the gap effect, target-feature-discrimination, and saccadic inhibition of return (IOR). Its secondary motivation was to replicate (in a more conservative and tightly controlled design) the surprising findings of Trottier and Pratt (2005), who found that requiring a high resolution task at the saccade target location speeded saccades, apparently by disinhibition. Trottier and Pratt’s finding was so surprising it raised the question: Could the oculomotor braking effect of saccadic IOR to previously viewed locations be reduced or removed by requiring a high resolution task at the target location? Twenty naïve untrained undergraduate students participated in exchange for course credit. Multiple randomised temporal and spatial target parameters were introduced in order to increase probability of exogenous responses. The primary measured variable was saccade latency in milliseconds, with the expectation of higher probability of very fast saccades (i.e. 80-120ms). Previous research suggested that these very fast saccades could be elicited in special testing circumstances with naïve participants, such as during the gap task, or in highly trained observers in non-gap tasks (Fischer & Weber, 1993). Trottier and Pratt (2005) found that adding a task demand that required naïve untrained participants to obtain a feature of the target stimulus (and to then make a discriminatory decision) also produced a higher probability of very fast saccade latencies. They stated that these saccades were not the same as saccade latencies previously referred to as express saccades produced in the gap paradigm, and proposed that such very fast saccades were normal. Carpenter (2001) found that in trained participants the probability of finding very fast saccades during the gap task increased when the horizontal direction of the current saccade continued in the same direction as the previous saccade (as opposed to reversing direction) – giving a distinct bimodality in the distribution of latencies in five out of seven participants, and likened his findings to the well known IOR effect. The IOR effect has previously been found in both manual key-press RT and saccadic latency paradigms. Hunt and Kingstone (2003) stated that there were both cortical top-down and oculomotor hard-wired aspects to IOR. An experiment was designed that included obtain-target-feature and oculomotor-prior-direction, crossed with two gap level offsets (0ms & 200ms-gap). Target-feature discrimination accuracy was high (97%). Under-additive main effects were found for each factor, with a three-way interaction effect for gap by obtain-feature by oculomotor-prior-direction. Another new three-way interaction was also found for anticipatory saccade type. Anticipatory saccades became significantly more likely under obtain-target-feature for the continuing oculomotor direction. This appears to be a similar effect to the increased anticipatory direction-error rate in the antisaccade task. These findings add to the saccadic latency knowledge base and in agreement with both Carpenter and Trottier and Pratt, laboratory testing paradigms can affect saccadic latency distributions. That is, salient (meaningful) targets that follow more natural oculomotor trajectories produce higher probability of very fast latencies in the 80-120ms range. In agreement with Hunt and Kingstone, there appears to be an oculomotor component to IOR. Specifically, saccadic target-prior-location interacts differently for obtain-target-feature under 200-ms gap than under 0ms-gap, and is most likely due predominantly to a predictive disinhibitory oculomotor momentum effect, rather than being due to the attentional inhibitory effect proposed for key-press IOR. A new interpretation for the paradigm previously referred to as IOR is offered that includes a link to the smooth pursuit system. Additional studies are planned to explore saccadic interactions in more detail.