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.

    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 diagram to the 
left. 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 
following diagram.