INHIBITORY INTERACTION IN THE RETINA 273 



enhancement of visual contrast at borders or steep intensity gradients in the 

 retinal image. Consider two contiguous regions of different retinal illumina- 

 tion with a sharp transition between them. A receptor that is within the 

 region of high illumination but close to the transition will receive less in- 

 hibition than a receptor that is well inside that region, where all of the closely 

 neighboring receptors are brightly lighted. Consequently, the receptors near 

 the transition will respond more vigorously than those well inside the region 

 and the briglit area will appear a little brighter near its border. Conversely, 

 the dimly lighted region will appear a httle dimmer near the border where it 

 adjoins the bright region. These bands of greater and lesser brightness, flank- 

 ing and accentuating a transition between two areas of unequal illumination, 

 are Mach's bands. They are specially noticeable if the transition is not per- 

 fectly sharp but is an extended though fairly steep gradation in intensity. The 

 penumbra of a shadow formed by a small extended source usually shows 

 Mach's bands clearly. A double shadow cast by an object illuminated by two 

 point sources of light usually shows striking variations in apparent brightness 

 across its double edge — variations that have no counterpart in the actual 

 distribution of light but are entirely the result of "border contrast" in the eye. 



We have demonstrated the physiological counterpart of Mach's bands in 

 the eye of Liniulus, recording from a single receptor unit as the eye was caused 

 to scan, slowly, a pattern consisting of two regions of different brightness 

 separated by a transitional gradient (Ratliff and Hartline, 1959). When the 

 eye was masked so that only the one receptor from wliich we were recording 

 viewed the pattern, then the frequency of optic nerve impulse discharge 

 mapped faithfully the distribution of physical brightness in the pattern. But 

 when the mask was removed, so that the entire eye viewed the pattern, then 

 as the pattern was scanned the receptor from which activity was being re- 

 corded showed maxima and minima of discharge rates correlated with those 

 regions of the pattern where a human observer saw Mach's bands (Fig. 22, 

 upper graph). Even more pronounced border contrast effects were recorded 

 when the intensity step was abrupt (Fig. 22, lower graph). In the Limulus eye, 

 this phenomenon is the inevitable consequence of mutual inhibitory inter- 

 action; for the human observer, a similar inhibitory interaction in the visual 

 system may be postulated to explain this and related subjective phenomena 

 of simultaneous "brightness contrast". Indeed, an inhibitory Wechsehvirkung 

 of adjacent retinal regions is precisely what Mach postulated to explain his 

 now well known bands. 



An exact theoretical treatment of "border contrast" cannot be given as yet 

 for the eye of Limulus or for any other visual system because the exact law 

 relating the magnitude of the inhibitory parameters to retinal distance be- 

 tween receptors is not known. We have, however, considered theoretically an 

 ideahzed system — a uniform, fine-grained mosaic of large extent with respect 

 to the range of inhibitory interaction. We postulated convenient plausible 



