| dc.description.abstract | The functional roles of the Frontal Eye Field (FEF) are preparation for saccades, saccade triggering, and controlling eye movements. Saccade goals are based on a salience map. This salience map is a convergence of both top-down (i.e. the decision for selection) and bottom-up (i.e. target and distractor features) processes. The current study examines the role of the FEF in these processes through the effects of transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS). The distractor-related modulation of saccade latencies and trajectories are used as indicators. The saccade latencies are used to represent the saccade preparation time and the saccade trajectories represents the target selection and distractor suppression processes.
In the first series of experiments, the aim of the research was investigation of both top-down and bottom up processes and their relationship to the saliency map. The manipulation of target-distractor similarity and tDCS were used. In experiment 1-1, participants made reflexive saccades to a blue square target appearing with an accompanying distractor. The distractor was either a blue diamond or a green square, both of which share only one feature with the target. Behavioral results showed that the saccade latency was longer when the distractor was of the same color, implying that color has higher salience than shape.
In experiment 1-2, participants made voluntary saccades to peripheral locations specified by a central arrow-cue. The target was either a blue square or green diamond and was indicated on each trial. The distractors were the same as experiment 1-1, but the role of same color or same shape were defined by the target in each trial. The behavioral results showed no differences between these conditions. However, when tDCS was applied over participant’s rFEF, the saccades made curved more away from the distractor in the same color condition, and curved away less in same shape condition. This not only indicated the color has higher salience than shape, but also demonstrated a relationship between the saliency map hypothesis and FEF.
In experiment 2, the aim of the research was examining the distractor suppression role in FEF and comparing this with posterior parietal cortex (PPC). In this case, the appearance time of distractors was manipulated. The distractor and the TMS preceded the target by various lengths of time, i.e. short (70 ms), middle (150 ms), and long (300 ms). TMS was applied over either FEF or PPC, two critical areas for saccade generation. The results indicated that saccade latencies were significantly longer when distractors appeared slightly ahead of a target across all TMS conditions, but TMS over PPC increased saccade latency in all time conditions. Additionally, the initial slope of saccade curvatures decreased in the FEF TMS condition for all distractor time offsets. These results show that distractor suppression requires more than 70ms and also that the contingency between saccade latency and curvature was dissociated. Most importantly, evidence is provided for PPC and FEF involvement in saccade generation, but that this involvement is different, particularly in the case of distractor suppression.
The current study examined the degree to which distractor saliency modulates saccade curvature, and provides evidence that color has higher saliency than shape in this context. Increasing FEF activity by tDCS stimulation demonstrated its role in forming a saliency map for eye movement generation. Above all, differing roles of FEF and PPC are illustrated by the effects TMS in distractor suppression during the pre-saccadic period. FEF seems to have a role in top-down processes, and PPC a role in bottom-up processes. | en_US |