Binocular Coordination, Fixation Disparity, and Ocular Dominance

In a series of experiments we have quantified the spatial and temporal dynamics of binocular coordination. Tasks studied ranged from simple scanning and letter detection to complex visual processing in text reading. In all of these paradigms we found similar eye movement characteristics: in 70% to 90% of the observations, the saccade of the abducting eye is larger, relative differences being in the order of 5% to 15% of the amplitude. During the subsequent fixation the disparity is typically reduced by a convergence movement (about 1 deg s−1), which sometimes exceeds the initial saccade amplitude asymmetry. Interestingly, the relative vergence contributions of the eyes depend on saccade length. For progressive 2-letter reading saccades, the left (adducting) eye accounts for only 20% of the total movement as compared to about 70% for 14-letter saccades.

Up to now our analysis was limited to relative rather than absolute estimates of fixation disparity. To overcome this restriction, we measured disparity using the psychophysical method of dichoptically presented nonius lines as well as direct infrared pupil-reflection registration of binocular vs monocular fixation. Both measures were independent of target eccentricity (within a range typical for reading) and produced similar subject rank orders (Spearman's \rho=0.75).

When we studied vergence movements in a letter detection task using autostereograms with different levels of virtual depth, it became clear that spatiotemporal vergence parameters can be quite asymmetric for both eyes. This led to the question of whether unequal contributions to vergence may be related to ocular dominance. This hypothesis is currently being investigated with a new procedure that provides a reliable estimate of subjective visual direction (the ‘cyclopean eye’) under static viewing conditions.

Long-term effects of cannabis on oculomotor function in humans

Cannabis is known to affect human cognitive and visuomotor skills directly after consumption. Some studies even point to rather long-lasting effects, especially after chronic tetrahydrocannabinol (THC) abuse. However, it is still unknown whether long-term effects on basic visual and oculomotor processing may exist. In the present study, the performance of 20 healthy long-term cannabis users without acute THC intoxication and 20 control subjects were examined in four basic visuomotor paradigms to search for specific long-term impairments. Subjects were asked to perform: 1) reflexive saccades to visual targets (prosaccades), including gap and overlap conditions, 2) voluntary antisaccades, 3) memory-guided saccades and 4) double-step saccades. Spatial and temporal parameters of the saccades were subsequently analysed. THC subjects exhibited a significant increase of latency in the prosaccade and antisaccade tasks, as well as prolonged saccade amplitudes in the antisaccade and memory-guided task, compared with the control subjects. The results point to substantial and specific long-term deficits in basic temporal processing of saccades and impaired visuo-spatial working memory. We suggest that these impairments are a major contributor to degraded performance of chronic users in a vital everyday task like visual search, and they might potentially also affect spatial navigation and reading.

Oculomotor inhibition in children with and without attention-deficit hyperactivity disorder (ADHD)

The aim of the present study was to distinguish between a general deficit in oculomotor control and a deficit restricted to inhibitory functions in children with attention deficit hyperactivity disorder (ADHD). In addition, we were interested in differentiating between a general inhibition deficit and deficient subfunctions of inhibition. We used a prosaccade task to measure general oculomotor abilities in 22 children with ADHD and in age- and gender-matched healthy controls. A fixation, an antisaccade and a countermanding saccade task were used to measure specific aspects of oculomotor inhibition. Two major results were obtained: First, our prosaccade task suggests similar saccadic response preparation and saccadic accuracy in the ADHD compared to the control children. Secondly, the fixation and the countermanding saccade task indicate deficits on measures of oculomotor inhibition in the ADHD group. While patients were specifically impaired in stopping an already initiated response or in suppressing exploratory saccades in a novel situation, inhibition of a prepotent response was not deficient. Our data thus indicate an underlying impairment in cognitive inhibition in ADHD that has been associated with prefrontal lobe functions. More specifically, as the anterior cingulate gyrus has been associated with the countermanding saccade task and group differences were most pronounced in this paradigm our data are in line with imaging data stressing the importance of this cortical structure in the pathophysiology of ADHD.