Eye, Brain, and Vision
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One scheme for explaining the behavior of a complex end-stopped cell. Three ordinary complex cells converge on the end-stopped cell: one, whose receptive field is congruent with the end-stopped cell's activating region (a), makes excitatory contacts; the other two, having fields in the outlying regions (b and c), make inhibitory contacts.
your attentions misunderstood! The process of making visual saccades to items of interest, in order to get their images on the fovea, is carried out largely by the superior colliculus, as Peter Schiller at MIT showed in an impressive series of papers in the 1970s.
The second set of facts about how we see is even more counterintuitive.
When we look at a stationary scene by fixating on some point of interest, our eyes lock onto that point, as just described, but the locking is not absolute.
Despite any efforts we may make, the eyes do not hold perfectly still but make constant tiny movements called microsaccades; these occur several times per second and are more or less random in direction and about1 to 2 minutes of arc in amplitude. In 1952 Lorrin Riggs and Floyd Ratliff, at Brown University, and R. W. Ditchburn and B. L. Ginsborg, at Reading University, simultaneously and independently found that if an image is optically artificially stabilized on the retina, eliminating any movement relative to the retina, vision fades away after about a second and the scene becomes quite blank! (The simplest way of stabilizing is to attach a tiny spotlight to a contact lens; as the eye moves, the spot moves too, and quickly fades.) Artificially moving the image on the retina, even by a tiny amount, causes the spot to reappear at once. Evidently, microsaccades are necessary for us to continue to see stationary objects. It is as if the visual system, after going to the trouble to make movement a powerful stimulus—wiring up cells so as to be insensitive to stationary objects—had then to invent microsaccades to make stationary objects visible.
We can guess that cortical complex cells, with their very high sensitivity to movement, are involved in this process. Directional selectivity is probably not involved, because microsaccadic movements are apparently random in direction. On the other hand, directional selectivity would seem very useful for detecting movements ot objects against a stationary background, by telling us that a movement is taking place and in what direction. To follow a moving object against a stationary background, we have to lock onto the object and track it with our eyes; the rest of the scene then slips across the retina, an event that otherwise occurs only rarely. Such slippage, with every contour in the scene moving across the retina, must produce a tremendous storm of activity in our cortex.



                                           END STOPPING
One additional kind of specificity occurs prominently in the striate cortex. An ordinary simple or complex cell usually shows length summation:
the longer the stimulus line, the better is the response, until the line is as long as the receptive field; making the line still longer has no effect. For an end

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

stopped cell, lengthening the line improves the response up to some limit, but exceeding that limit in one or both directions results in a weaker response, as shown in the bottom diagram on the facing page. Some cells, which we call completely end stopped, do not respond at all to a long line. We call the region from which responses are evoked the activating region and speak of the regions at one or both ends as inhibitory. The total receptive field is consequently made up of the activating region and the inhibitory region or regions at the ends. The stimulus orientation that best evokes excitation in the activating region evokes maximal inhibition in the outlying area(s). This can be shown by repeatedly stimulating the activating region with an optimally oriented line of optimal length while testing the outlying region with lines of varying orientation, as shown in the top diagram on this page.
We originally thought that such cells represented a stage one step beyond complex cells in the hierarchy. In the simplest scheme for elaborating such a cell, the cell would be excited by one or a few ordinary complex cells with fields in the activating region and would be inhibited by complex cells with similarly oriented fields situated in the outlying regions. I have illustrated this scheme in the bottom diagram on the preceding page. A second possibility is that the cell receives excitatory input from cells with small fields, marked (a) in the diagram on this page, and inhibition from cells with large fields, marked (b); we assume that the cells supplying inhibition are maximally excited by long slits but poorly excited by short ones. This second possibility (analogous to the model for center-surround cells) is one of the few circuits for which we have some evidence. Charles Gilbert, at Rockefeller University in New York, has shown that complex cells in layer 6 of the monkey striate cortex have just the right properties for supplying this inhibition and, furthermore, that disabling these cells by local injections causes endstopped cells in the upper layers to lose the end inhibition.
















 

After these models were originally proposed, Geoffery Henry, in Canberra, Australia, discovered end-stopped simple cells, presumably with the receptive-field arrangement shown in the lower margin of this page. For such a cell, the wiring would be analogous to our first diagram, except that the input would be from simple rather than from complex cells. Complex end-stopped cells could thus arise by excitatory input from one set of complex cells and


   
 
 
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This end-stopped simple cell is assumed to result from convergent input from three ordinary simple cells. (One cell, with the middle on-center field, could excite the cell in question; the two others could be off center and also excite or be on center and inhibit.) Alternatively, the input to this cell could come directly from center-surround cells, by some more elaborate version of the process illustrated on page 18.
 
 
 
 
 
In an alternative scheme, one cell does the inhibiting, a cell whose receptive field covers the entire area (b). For this to work, we have to assume that the inhibiting cell responds only weakly to a short slit when (a)
is stimulated, but responds strongly to a long slit.

For this end-stopped cell, stimulating the middle activating region alone with an optimally oriented slit produces a strong response. Including one of the inhibitory regions almost nullifies the response, but if the inhibitory region is stimulated with a different orientation, the response is no longer blocked. Thus the activating region and the inhibitory regions both have the same optimal orientations.
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Top: An ordinary complex cell responds to various lengths of a slit of light. The duration of each record is 2 seconds. As indicated by the graph of response versus slit length, for this cell the response increases with length up to about 2 degrees, after which there is no change. Bottom: For this end-stopped cell, responses improve up to 2 degrees but then decline, so that a line 6 degrees or longer gives no response.