According to wiki, “In physics, the principle of relativity is the requirement that the equations, describing the laws of physics, have the same form in all admissible frames of reference.” The importance of reference frame has been less appreciated in other sciences and it is the purpose of this blog to understand its relevance to visual perception.
Why do we need the frame of reference? A common example related to motion-related illusion – sometimes one feels an still train or airplane is moving because the observe his/her own motion. The underlying reason for such illusion appears to be the lack of coordination between vestibular system and vision system. It also shows the relativity of motion perception – in the above illusion example, ambiguity is often easily resolved if the observer looks at somewhere else (change of the reference frame).
The story does not end here. Pioneering studies by Nobel Laureates Hubel and Wiesel in 1950s have shown the abundance of movement-sensitive cells in visual cortex of a cat. It is easy to understand their role in dorsal pathway from the perspective of motion detection for the survival but how about their role in ventral pathway – how did movement-sensitive cells analyze a stationary landscape? It involves both saccade and microsaccade which really echoes J. Gibson’s saying “We move because we see, we see because we move”. More subtle implication lies in the scale of movement on visual perception – if saccade (global motion) is for the purpose of accumulating local information into a holistic understanding; the role played by microsaccade (local motion) is more relevant to the functioning of ventral pathway involving object recognition.
In other words, out eyes seldom process “absolutely stationary” images (the cells simply won’t fire); the perception of even stationary scene is the consequence of moving eyes around (both globally and locally). Therefore, it is really a lame approach to understand the biological counterpart of image processing because there is none. A biologically inspired approach toward image processing is to cast images as the subspace of video and understand color and texture along with motion and disparity. The principle of relativity implies that both saccade and microsaccade are important to the functioning of movement-sensitive cells because they sense “relative” changes all the time.
