Separate representations of target and timing cue locations in the supplementary eye fields

When different stimuli indicate where and when to make an eye movement, the brain areas involved in oculomotor control must selectively plan an eye movement to the stimulus that encodes the target position and also encode the information available from the timing cue. This could pose a challenge to the oculomotor system since the representation of the timing stimulus location in one brain area might be interpreted by downstream neurons as a competing motor plan. Evidence from diverse sources has suggested that the supplementary eye fields (SEF) play an important role in behavioral timing, so we recorded single-unit activity from SEF to characterize how target and timing cues are encoded in this region. Two monkeys performed a variant of the memory-guided saccade task, in which a timing stimulus was presented at a randomly chosen eccentric location. Many spatially tuned SEF neurons encoded only the location of the target and not the timing stimulus, whereas several other SEF neurons encoded the location of the timing stimulus and not the target. The SEF population therefore encoded the location of each stimulus with largely distinct neuronal subpopulations. For comparison, we recorded a small population of lateral intraparietal (LIP) neurons in the same task. We found that most LIP neurons that encoded the location of the target also encoded the location of the timing stimulus after its presentation, but selectively encoded the intended eye movement plan in advance of saccade initiation. These results suggest that SEF, by conditionally encoding the location of instructional stimuli depending on their meaning, can help identify which movement plan represented in other oculomotor structures, such as LIP, should be selected for the next eye movement

Contribution of the Primate Frontal Cortex to Eye Movements and Neuronal Activity in the Superior Colliculus

Humans and non-human primates must precisely align the eyes on an object to view it with high visual acuity. An important role of the oculomotor system is to generate accurate eye movements, such as saccades, toward a target. Given that each eye has only six muscles that rotate the eye in three degrees of freedom, this relatively simple volitional movement has allowed researchers to well-characterize the brain areas involved in their generation. In particular, the midbrain Superior Colliculus (SC), is recognized as having a primary role in the generation of visually-guided saccades via the integration of sensory and cognitive information. One important source of sensory and cognitive information to the SC is the Frontal Eye Fields (FEF). The role of the FEF and SC in visually-guided saccades has been well-studied using anatomical and functional techniques, but only a handful of studies have investigated how these areas work together to produce saccades. While it is assumed that the FEF exerts its influence on saccade generation though the SC, it remains unknown what happens in the SC when the FEF is suddenly inactivated. To test this prediction, I use the combined approach of FEF cryogenic inactivation and SC neuronal recordings, although it also provides a valuable opportunity to understand how FEF inputs to the SC govern saccade preparation. Nonetheless, it was first necessary to characterize the eye movement deficits following FEF inactivation, as it was unknown how a large and reversible FEF inactivation would influence saccade behaviour, or whether cortical areas influence fixational eye movements (e.g. microsaccades). Four major results emerged from this thesis. First, FEF inactivation delayed saccade reaction times (SRT) in both directions. Second, FEF inactivation impaired microsaccade generation and also selectively reduced microsaccades following peripheral cues. Third, FEF inactivation decreased visual, cognitive, and saccade-related activity in the ipsilesional SC. Fourth, the delayed onset of saccade-related SC activity best explained SRT increases during FEF inactivation, implicating one mechanism for how FEF inputs govern saccade preparation. Together, these results provide new insights into the FEF\u27s role in saccade and microsaccade behaviour, and how the oculomotor system commits to a saccade

Biases in the perceived timing of perisaccadic perceptual and motor events

Subjects typically experience the temporal interval immediately following a saccade as longer than a comparable control interval. One explanation of this effect is that the brain antedates the perceptual onset of a saccade target to around the time of saccade initiation. This could explain the apparent continuity of visual perception across eye movements. Thisantedating account was tested in three experiments in which subjects made saccades of differing extents and then judged either the duration or the temporal order of key events. Postsaccadic stimuli underwent subjective temporal lengthening and had early perceived onsets. A temporally advanced awareness of saccade completion was also found, independently of antedating effects. These results provide convergent evidence supporting antedating and differentiating it from other temporal biases

Eye guidance in extrafoveal choice reaction.

Although it has been amply demonstrated that eye movements are influenced by the presence of complex information in visual displays , the nature of the control of this behavior remains unclear. Because of the brevity of the eye fixations, a major theoretical dispute has arisen over whether the eyes are guided to new fixation positions immediately, on the basis of information glimpsed during the immediately preceding fixation, or on a delayed basis , because new information is not available early enough in a fixation to influence the direction of the next eye movemen

Human Posterior Parietal Cortex Plans Where to Reach and What to Avoid

In this time-resolved functional magnetic resonance imaging (fMRI) study, we aimed to trace the neuronal correlates of covert planning processes that precede visually guided motor behavior. Specifically, we asked whether human posterior parietal cortex has prospective planning activity that can be distinguished from activity related to retrospective visual memory and attention. Although various electrophysiological studies in monkeys have demonstrated such motor planning at the level of parietal neurons, comparatively little support is provided by recent human imaging experiments. Rather, a majority of experiments highlights a role of human posterior parietal cortex in visual working memory and attention. We thus sought to establish a clear separation of visual memory and attention from processes related to the planning of goal-directed motor behaviors. To this end, we compared delayed-response tasks with identical mnemonic and attentional demands but varying degrees of motor planning. Subjects memorized multiple target locations, and in a random subset of trials targets additionally instructed (1) desired goals or (2) undesired goals for upcoming finger reaches. Compared with the memory/attention-only conditions, both latter situations led to a specific increase of preparatory fMRI activity in posterior parietal and dorsal premotor cortex. Thus, posterior parietal cortex has prospective plans for upcoming behaviors while considering both types of targets relevant for action: those to be acquired and those to be avoided

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

The Examination of Endogenous Attention: Stimulus-Cue Learning in 4- and 9-Month-Olds

The present study used a stimulus-to-response task to test the presence of endogenous attention in 4- and 9-month-old infants. Infants were presented with a cue stimulus that predicted the location of a peripheral target. If infants had the ability to endogenously guide eye movements, then the learned association between the cue and peripheral target location could be used to facilitate eye movements to the upcoming target location. Although a decrease in anticipation latency was observed, this decrease did not appear to be based systematically on the contingent relationship. This study also examined the effect of enhancing stimulus salience, and found that cue salience can affect the outcome of the stimulus-to-response task for younger infants. Cue salience, however, did not improve infants' performance on the task. This result coupled with an overall lack of contingent based facilitation indicates that infants at neither age showed evidence of endogenous attention