Eye, Brain, and Vision
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further suppose that the inhibitory path involves a delay, perhaps produced by still another intermediate cell. Then, if the stimulus moves in one direction, say, right to left, as in the illustration of Barlow and Levick's model, the intermediate cell is excited by one of its inputs just as the inhibition arrives from the other, whose field has just been crossed. The two effects cancel, and the cell does not fire. For left-to-right movement, the inhibition arrives too late to prevent firing. If many such intermediate cells converge on a third cell, that cell will have the properties of a directionally selective complex cell.
We have little direct evidence for any schemes that try to explain the behavior of cells in terms of a hierarchy of complexity, in which cells at each successive level are constructed of building blocks from the previous level. Nevertheless, we have strong reasons for believing that the nervous system is organized in a hierarchical series. The strongest evidence is anatomical: for example, in the cat, simple cells are aggregated in the fourth layer of the striate cortex, the same layer that receives geniculate input, whereas the complex cells are located in the layers above and below, one or two synapses further along.
Thus although we may not know the exact circuit diagram at each stage, we have good reasons to suppose the existence of some circuit.
The main reason for thinking that complex cells may be built up from center-surround cells, with a step in between, is the seeming necessity of doing the job in two logical steps. I should emphasize the word logical because the whole transformation presumably could be accomplished in one physical step by having center-surround inputs sum on separate dendritic branches of complex cells, with each branch doing the job of a simple cell, signaling electrotonically (by passive electrical spread) to the cell body, and hence to the axon, whenever a line falls in some particular part of the receptive field. The cell itself would then be complex. But the very existence of simple cells suggests that we do not have to imagine anything as complicated as this.



                                  THE SIGNIFICANCE                       OF MOVEMENT-SENSITIVE CELLS,                          INCLUDING SOME COMMENTS                                        ON HOW WE SEE
Why are movement-sensitive cells so common? An obvious first guess is that they tell us if the visual landscape contains a moving object. To animals, ourselves included, changes in the outside world are far more important than static conditions, for the survival of predator and prey alike. It is therefore no wonder that most cortical cells respond better to a moving object than to a stationary one. Having carried the logic this far, you may now begin to wonder how we analyze a stationary landscape at all if, in the interests of having high movement sensitivity, so many oriented cells are insensitive to stationary contours. The answer requires a short digression, which takes us to some basic, seemingly counterintuitive facts about how we see.
First, you might expect that in exploring our visual surroundings, we let our eyes freely rove around in smooth, continuous movement. What our two eyes in fact do is fixate on an object: we first adjust the positions of our eyes so that the images of the object fall on the two foveas; then we hold that position for a brief period, say, half a second; then our eyes suddenly jump to a new position by fixating on a new target whose presence somewhere out in the visual field has asserted itself, either by moving slightly, by contrasting with the background, or by presenting an interesting shape. During the jump, or saccade, which is French for "jolt", or "jerk" (the verb), the eyes move so rapidly that our visual system does not even respond to the resulting movement of the scene across the retina; we are altogether unaware of the violent change. (Vision may also in some sense be turned off during saccades by a complex circuit linking eye-movement centers with the visual path.) Exploring a visual scene, in reading or just looking around, is thus a process of having our eyes jump in rapid succession from one place to another.
Detailed monitoring of eye movements shows vividly how unaware we are of any of this. To monitor eye movements we first attach a tiny mirror to a contact lens, at the side, where it does not block vision; we then reflect a spot of light off the mirror onto a screen. Or, using a more modern version invented by David Robinson at the Wilmer Institute at Johns Hopkins, we can mount a tiny coil of wire around the rim of a contact lens, with the subject seated between two orthogonal pairs of bicycle-wheel size hoops containing coils of wire; currents in these coils induce currents in the contact-lens coil, which can be calibrated to give precise monitoring of eye movements. Neither method is what you would call a picnic for the poor subject.
In 1957, Russian psychophysicist A. L. Yarbus recorded eye movements of subjects as they explored various images, such as a woods or female faces (see the illustrations below), by showing the stopping places of a subject's gaze as dots joined by lines indicating the eyes' trajectory during the jumps. A glance at these amazing pictures gives us a world of information about our vision—even about the objects and details that interest us in our environment.
So the first counterintuitive fact is that in visual exploration our eyes jump around from one point of interest to another: we cannot explore a stationary scene by swinging our eyes past it in continuous movements. The visual system seems intent instead on keeping the image of a scene anchored on our retinas, on preventing it from sliding around. If the whole scene moves by, as

 

 

 

 

 

 

 

 

 

 

 

occurs when we look out a train window, we follow it by fixating on an object and maintaining fixation by moving our eyes until the object gets out of range, whereupon we make a saccade to a new object. This whole sequence—
following with smooth pursuit, say, to the right, then making a saccade to the left—is called nystagmus. You can observe the sequence—perhaps next time you are in a moving train or streetcar—by looking at your neighbor's eyes as he or she looks out a window at the passing scene—taking care not to have


   
 
 
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Horace Barlow and William Levick proposed this circuit to explain directional selectivity. Synapses from purple to green are excitatory, and from green to white, inhibitory. We suppose the three white cells at the bottom converge on a single master cell.
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A picture is viewed by an observer while we monitor eye position and hance directinon of gaze. the eyes jump, come to rest momentarily (producing a small dot on the record), then jump to a new locus of interest.
It seems difficult to jump to a void - a
place lacking abrupt luminance changes.