Thursday, 2 October 2008

Saccade hypothesis

Thinking about how the visual cortex works I came to the conclusion that the 'movies' must be produced by the cortex itself. This is to make sure that the changes of the picture are not the changes in the world. One example for this can be saccades. I started looking for more information about this subject and surprisingly found that the main reason for saccades is difference in retina resolution. I do not agree with this.

According to the current belief the main reason for saccades of the human eye is that the central part of the retina, the fovea, plays a critical role in resolving objects. By moving the eye so that small parts of a scene can be sensed with greater resolution, body resources can be used more efficiently.

In my opinion this is not the case. The main reason for saccades is building abstract invariant of the picture. In visual cortex the information propagates up the higher level by building more abstract notation of the picture. The voluntary changing of the picture is the way for the cortex to deduce the common information. The crucial role here is that the brain (cortex) knows that the real cause (objects being seen) does not change. So the cortex must be working differently when the picture coming from the retina changes due to external factors or changes due to internally motivated saccades.

The memory prediction mechanism allows comparing the changed picture with the predicted according to a particular distance and direction of the saccade. This is done in partly learned memory. But the same mechanism works as finding the nvariants in the changing pictures produced by slightly different view.

The analogy of this process is the theory and the experiment. The theory is proven by different experiments giving the different results but fitting in the theory prediction. The assumption is that the essence does not change with the experiments.

"Saccades are a widespread phenomenon across animals with image-forming visual systems. They have been observed in animals across three phyla, including animals that do not have a fovea (most vertebrates do not) and animals that cannot move their eyes independently of their head (such as insects)." [Land, MF. "Motion and vision: why animals move their eyes". J Comp Physiol A. 1999 185:341–352.] Although in this paper the author argues that the reason for saccades "is the need to avoid the blur that results from the long response time of the photoreceptors".

Assuming that building image invariant is the main reason for saccades, one would come to a conclusion that the other voluntary picture changes should happen in the eye. Indeed, some animals allow their eye rotate [above paper]. Why human eyes do not rotate I do not know. Maybe our vision system is complex enough (3D) to allow less emphasis on obtaining picture.

Sensory cortex and saccade motor cortex must be closely coupled. Because the sensory cortex governs the motor cortex and motor cortex has to feed back its lower level information to sensory cortex. There should be many effects which can easily be tested by experiments to support this hypothesis.