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Primary saccades are often followed by small secondary saccades, which are generally thought to reduce the distance between the saccade endpoint and target location. Accumulated evidence demonstrates that secondary saccades are subject to various influences, among which retinal feedback during postsaccadic fixation constitutes only one important signal. Recently, we reported that target eccentricity and an orientation bias influence the generation of secondary saccades. In the present study, we examine secondary saccades in the absence of postsaccadic visual feedback. Although extraretinal signals (e.g., efference copy) have received widespread attention in eye-movement studies, it is still unclear whether an extraretinal error signal contributes to the programming of secondary saccades. We have observed that secondary saccade latency and amplitude depend on primary saccade error despite the absence of postsaccadic visual feedback. Strong evidence for an extraretinal error signal influencing secondary saccade programming is given by the observation that secondary saccades are more likely to be oriented in a direction opposite to the primary saccade as primary saccade error shifts from target undershoot to overshoot. We further show how the functional relationship between primary saccade landing position and secondary saccade characteristics varies as a function of target eccentricity. We propose that initial target eccentricity and an extraretinal error signal codetermine the postsaccadic activity distribution in the saccadic motor map when no visual feedback is available.
Parameters of a formal working-memory model were estimated for verbal and spatial memory updating of children. The model proposes interference though feature overwriting and through confusion of whole elements as the primary cause of working-memory capacity limits. We tested 2 age groups each containing 1 group of normal intelligence and 1 deficit group. For young children the deficit was developmental dyslexia; for older children it was a general learning difficulty. The interference model predicts less interference through overwriting but more through confusion of whole elements for the dyslexic children than for their age-matched controls. Older children exhibited less interference through confusion of whole elements and a higher processing rate than young children, but general learning difficulty was associated with slower processing than in the age-matched control group. Furthermore, the difference between verbal and spatial updating mapped onto several meaningful dissociations of model parameters.
Neuronal activity in area LIP is correlated with the perceived direction of ambiguous apparent motion (Z. M. Williams, J. C. Elfar, E. N. Eskandar, L. J. Toth, & J. A. Assad, 2003). Here we show that a similar correlation exists for small eye movements made during fixation. A moving dot grid with superimposed fixation point was presented through an aperture. In a motion discrimination task, unambiguous motion was compared with ambiguous motion obtained by shifting the grid by half of the dot distance. In three experiments we show that (a) microsaccadic inhibition, i.e., a drop in microsaccade frequency precedes reports of perceptual flips, (b) microsaccadic inhibition does not accompany simple response changes, and (c) the direction of microsaccades occurring before motion onset biases the subsequent perception of ambiguous motion. We conclude that microsaccades provide a signal on which perceptual judgments rely in the absence of objective disambiguating stimulus information.
A mathematical model of working-memory capacity limits is proposed on the key assumption of mutual interference between items in working memory. Interference is assumed to arise from overwriting of features shared by these items. The model was fit to time-accuracy data of memory-updating tasks from four experiments using nonlinear mixed effect (NLME) models as a framework. The model gave a good account of the data from a numerical and a spatial task version. The performance pattern in a combination of numerical and spatial updating could be explained by variations in the interference parameter: assuming less feature overlap between contents from different domains than between contents from the same domain, the model can account for double dissociations of content domains in dual-task experiments. Experiment 3 extended this idea to similarity within the verbal domain. The decline of memory accuracy with increasing memory load was steeper with phonologically similar than with dissimilar material, although processing speed was faster for the similar material. The model captured the similarity effects with a higher estimated interference parameter for the similar than for the dissimilar condition. The results are difficult to explain with alternative models, in particular models incorporating time-based decay and models assuming limited resource pools.
Binocular eye movements of normal adult readers were examined as they read single sentences. Analyses of horizontal and vertical fixation disparities indicated that the most prevalent type of disparate fixation is crossed (i.e., the left eye is located further to the right than the right eye) while the left eye frequently fixates somewhat above the right eye. The Gaussian distribution of the binocular fixation point peaked 2.6 cm in front of the plane of text, reflecting the prevalence of horizontally crossed fixations. Fixation disparity accumulates during the course of successive saccades and fixations within a line of text, but only to an extent that does not compromise single binocular vision. In reading, the version and vergence system interact in a way that is qualitatively similar to what has been observed in simple nonreading tasks. Finally, results presented here render it unlikely that vergence movements in reading aim at realigning the eyes at a given saccade target word.