The programming of sequences of saccades

Contextual saccade adaptation induced by sequential saccades

Saccade adaptation is the gradual adjustment of saccade end point to maintain spatial accuracy. Contextual adaptation refers to a situation in which the adaptation-related change in saccade end point is contingent on the behavioral context in which the saccade is made. For example, in some situations, the same saccade to the same retinotopic location can be simultaneously adapted in opposite directions depending on the context in which it is made. Saccade adaptation has traditionally been studied in isolated movements, but in everyday life, saccades are often planned and executed in sequences. The oculomotor system may therefore have adaptive mechanisms specific to sequential saccades. Here, in five experiments, we investigated contextual saccade adaptation in sequences of saccades. In the first experiment, we demonstrate that saccades to a given retinotopic location can be simultaneously adapted in opposite directions depending on whether they occur in isolation or in a sequence. In the other experiments, we measured the extent to which properties of the previous and following saccades in a sequence can induce contextual saccade adaptation. Overall, we find that the existence, direction, and amplitude of previous and subsequent saccades, as well as the order of the current saccade within a movement sequence, can all induce contextual adaptation. These novel findings demonstrate the surprising flexibility of the system in maintaining end point accuracy, and support the idea that saccades made in a movement sequence are planned concurrently rather than independently.NEW & NOTEWORTHY This study reveals a new type of contextual saccade adaptation: sequential saccades are able to induce contextual saccade adaptation when direction, amplitude, or the existence of preceding and following saccades are used as contexts. These novel findings are also consistent with the idea that saccades made in a sequence are planned concurrently rather than independently.

The effect of subliminal incentives on goal-directed eye movements

Over the last decades, several studies have demonstrated that conscious and unconscious reward incentives both affect performance in physical and cognitive tasks, suggesting that goal-pursuit can arise from an unconscious will. Whether the planning of goal-directed saccadic eye movements during an effortful task can also be affected by subliminal reward cues has not been systematically investigated. We employed a novel task where participants made several eye movements back and forth between a fixation point and a number of peripheral targets. The total number of targets visited by the eyes in a fixed amount of time determined participants' monetary gain. The magnitude of the reward at stake was briefly shown at the beginning of each trial and masked by pattern images superimposed in time so that at shorter display durations participants perceived reward incentives subliminally. We found a main effect of reward across all display durations as higher reward enhanced participants' oculomotor effort measured as the frequency and peak velocity of saccades. This effect was strongest for consciously perceived rewards but also occurred when rewards were subliminally perceived. Although we did not find a statistically significant dissociation between the reward-related modulation of different saccadic parameters, across two experiments the most robust effect of subliminal rewards was observed for the modulation of the saccadic frequency but not the peak velocity. These results suggest that multiple indices of oculomotor effort can be incentivized by subliminal rewards and that saccadic frequency may provide the most sensitive indicator of subliminal incentivization of eye movements.

Searching Through Alternating Sequences: Working Memory and Inhibitory Tagging Mechanisms Revealed Using the MILO Task

We used the Multi-Item Localisation (MILO) task to examine search through two sequences. In Sequential blocks of trials, six letters and six digits were touched in order. In Mixed blocks, participants alternated between letters and digits. These conditions mimic the A and B variants of the Trail Making Test (TMT). In both block types, targets either vanished or remained visible after being touched. There were two key findings. First, in Mixed blocks, reaction times exhibited a saw-tooth pattern, suggesting search for successive pairs of targets. Second, reaction time patterns for vanish and remain conditions were identical in Sequential blocks-indicating that participants could ignore past targets-but diverged in Mixed blocks. This suggests a breakdown of inhibitory tagging. These findings may help explain the elevated completion times observed in TMT-B, relative to TMT-A.

Preparation of saccade sequences and eye programming affect endogenous covert attention

Endogenous attention can be allocated in parallel to at least two saccade target locations of a planned sequence, but attentional resources are larger in the location of the first than the second saccade. The meridian effect that is observed in endogenous attention can be explained by eye programming, but it is not known how eye-movement preparation and eye programming can together affect endogenous attention during sequential saccades. We used a double-task paradigm to investigate this issue. In two experiments, we confirmed the relation between the preparation of sequential saccades and attentional selection and also showed that eye programming could eliminate deterioration of attentional resources in the second saccade location. The finding of the meridian effect in both the saccadic task and the target discrimination task additionally indicates the important role of eye programming in endogenous attention. The results were discussed in terms of the Premotor Theory of Attention.

Reading Increases the Compositionality of Visual Word Representations

Reading causes widespread changes in the brain, but its effect on visual word representations is unknown. Learning to read may facilitate visual processing by forming specialized detectors for longer strings or by making word responses more predictable from single letters—that is, by increasing compositionality. We provided evidence for the latter hypothesis using experiments that compared nonoverlapping groups of readers of two Indian languages (Telugu and Malayalam). Readers showed increased single-letter discrimination and decreased letter interactions for bigrams during visual search. Importantly, these interactions predicted subjects’ overall reading fluency. In a separate brain-imaging experiment, we observed increased compositionality in readers, whereby responses to bigrams were more predictable from single letters. This effect was specific to the anterior lateral occipital region, where activations best matched behavior. Thus, learning to read facilitates visual processing by increasing the compositionality of visual word representations.

The role of fixation disengagement in the parallel programming of sequences of saccades

One of the core mechanisms involved in the control of saccade responses to selected target stimuli is the disengagement from the current fixation location, so that the next saccade can be executed. To carry out everyday visual tasks, we make multiple eye movements that can be programmed in parallel. However, the role of disengagement in the parallel programming of saccades has not been examined. It is well established that the need for disengagement slows down saccadic response time. This may be important in allowing the system to program accurate eye movements and have a role to play in the control of multiple eye movements but as yet this remains untested. Here, we report two experiments that seek to examine whether fixation disengagement reduces saccade latencies when the task completion demands multiple saccade responses. A saccade contingent paradigm was employed and participants were asked to execute saccadic eye movements to a series of seven targets while manipulating when these targets were shown. This both promotes fixation disengagement and controls the extent that parallel programming can occur. We found that trial duration decreased as more targets were made available prior to fixation: this was a result both of a reduction in the number of saccades being executed and in their saccade latencies. This supports the view that even when fixation disengagement is not required, parallel programming of multiple sequential saccadic eye movements is still present. By comparison with previous published data, we demonstrate a substantial speeded of response times in these condition ("a gap effect") and that parallel programming is attenuated in these conditions.