Difference in visual processing assessed by eye vergence movements

Abstract

Orienting visual attention is closely linked to the oculomotor system. For example, a shift of attention is usually followed by a saccadic eye movement and can be revealed by micro saccades. Recently we reported a novel role of another type of eye movement, namely eye vergence, in orienting visual attention. Shifts in visuospatial attention are characterized by the response modulation to a selected target. However, unlike (micro-) saccades, eye vergence movements do not carry spatial information (except for depth) and are thus not specific to a particular visual location. To further understand the role of eye vergence in visual attention, we tested subjects with different perceptual styles. Perceptual style refers to the characteristic way individuals perceive environmental stimuli, and is characterized by a spatial difference (local vs. global) in perceptual processing. We tested field independent (local; FI) and field dependent (global; FD) observers in a cue/no-cue task and a matching task. We found that FI observers responded faster and had stronger modulation in eye vergence in both tasks than FD subjects. The results may suggest that eye vergence modulation may relate to the trade-off between the size of spatial region covered by attention and the processing efficiency of sensory information. Alternatively, vergence modulation may have a role in the switch in cortical state to prepare the visual system for new incoming sensory information. In conclusion, vergence eye movements may be added to the growing list of functions of fixational eye movements in visual perception. However, further studies are needed to elucidate its role.

Figure 1. Schematic explanation of the angle of eye vergence.

The eyes focus on a single point in space. The angle of eye vergence relates to the distance of the focus point to the eyes. For a near point the vergence angle (α1) is larger than for a far point (α2). α represents the angle of eye vergence.

Figure 2. Illustration of the tasks.

A: The cue/no-cue task. B The matching task. Time is from fixation onset.

Figure 3. Behavioral performance.

Average reaction times from the cue/no-cue task (A) and from the matching task (B). Error bars are SEM.

Figure 4. Modulation in eye vergence while performing the cue/no-cue task.

A: Average modulation in the angle of eye vergence from all subjects in the cue (green) and no-cue (red) conditions for FI (continuous lines) and FD (dotted lines) subjects. Higher values of vergence angle represent convergence. Time is from cue/no-cue onset. B: Mean modulation in eye vergence for FI and FD subjects. Asterisks denote significant (p<0.01) differences. Error bars are SEM.

Figure 5. Modulation in eye vergence while performing the matching task.

A: Average modulation across all subjects in the angle of eye vergence during the task (black) and control task (blue) for FI (continuous lines) and FD (dotted lines) subjects. Higher values of vergence angle represent convergence. Time is from the onset of the peripheral change B: Mean modulation in eye vergence for FI and FD subjects. Asterisks denote significant (p<0.01) differences. Error bars are SEM.