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Optomotor System - an overview

Study the Brain

Human brain functions can be studied indirectly by having subjects doing specified perceptual and/or motor tasks. Their performance can be measured by counting correct versus incorrect responses and/or by recording the reaction times, correction times and correction rate.

The idea behind the latter approach is that any brain function has a certain time consume. This time will be included in the reaction time depending on whether this function was needed or not to perform the task on that particular trial.

Our Goals

We study the human optomotor (= eye movement) system with two goals:

1. The optomotor system is an example of a sensorimotor coordination. We want to understand its functional principles: how does the brain solve the problem of vision with moving eyes?

2. The anatomy and physiology of the primate visual and the oculomotor system is well studied. We are using eye movements to identify and study functional subprocesses of visual attention and cognition, learning, reading and the aspects of their development.

Eye Movements

The main brain structures involved in the generation of saccades are the superior colliculus (tectum), the primary visual cortex, the prestriate cortex, the parietal cortex, and the frontal cortex, see this schematic drawing.

Accordingly, the time between the presentation of a target and the start of a prosaccade, the saccadic reaction time, can have different values. This can readily be seen in the distribution of saccadic reaction times (see below) exhibiting three separate peaks.

The shortest are the express saccades, which can be mediated by the superior colliculus without the frontal eye field. The second peak consists of fast regular saccades which need the frontal eye fields. The last peak consists of slow regular saccades, which occur in cases where all presaccadic processes are started only at the occurance of the target stimulus.

Antisaccades, can not be of the express type and also need an intact frontal cortex. Dysfunctions anywhere in the system may lead to difficulties in reading and dyslexia.

Definitions:

Prosaccades: saccades to a visual target
Antisaccades: saccades in the direction opposite to the stimulus
Reflexive saccades: prosaccades of extremely short reaction time (express saccades).
Remembered saccades: are made upon fixation point offset to locations where a stimulus had been briefly presented before
Delayed saccades: are made upon fixation point offset to targets that remain visible.

Typical values:

Saccadic Reaction time in human (monkey):
anticipations      < 80 ms (   < 60 ms)
express saccade  90-130 ms ( 65-100 ms)
fast regular    140-170 ms (120-150 ms)
slow regular    190-230 ms (170-190 ms)
afferent and efferent delays: about 25 (20) ms.

These modes of saccadic reaction times can occur in a single session resulting in multimodal distributions (characteristic example in the figure above). The modes have different weights depending on the task. The gap task favours the express mode, the overlap task favours the regular modes. The difference in reaction time between these to tasks is called the gap-effect.

Engagement/disengagement of active fixation and of visual attention both influence saccade generation and contribute to the gap-effect.

Author:Dr.B.Fischer
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