THE IMAGE-FORMING MECHANISM OF THE EYE 



663 



awareness of voluntary effort to readjust the triad 

 mechanism is an awareness of switching attention 

 from an object percei\eci to be located at one distance 

 to an object perceived to be located at another dis- 

 tance (60). As a matter of fact a readjustment of the 

 triad mechanism can be evoked when the subject is 

 in total darkness by ha\ing him switch his attention 

 from an imaginary far point to an imaginary near 

 point (47). The problem of focusing the eye, however, 

 is not quite this simple. If the subject starts by paying 

 attention to a given object, then covers one eye and 

 places a minus or plus lens in front of the other to 

 throw the image out of focus, and then concentrates 

 on the object or attempts to clear up the blur, the 

 object eventually comes into focus without the subject 

 perceiving any change in distance. We have yet to 

 learn whether this response is a result of the voluntary 

 effort to clear up the blur or a reflex response to the 

 blur which is akin to the reaction of an automatic 

 focusing device. Regardless of whether this accommo- 

 dative response to blur is voluntary or reflex, it ap- 

 pears to involve the same tie-up with convergence and 

 pupillary constriction as the accommodative response 

 to a change in the distance of attention. Considerable 

 attention is being devoted today to the problem of 

 whether the eye can detect ahead of time from some 

 aspect of a blurred image whether to increase or de- 

 crease accommodation to clear up the blur. Fincham 

 (28, 29) has investigated the response to blur and has 

 found evidence that the colored fringes on the target 

 resulting froin chromatic aberration determine the 

 direction of the response. Allen (6) has also investi- 

 gated what determines the direction of the response 

 when the subject is confronted with a blurred stimulus 

 with all cues of distance eliminated. In 19 of the 20 

 trials in which the response was recorded, the subject's 

 first response was in the right direction and in only 

 one trial did he make an initial response in the wrong 

 direction which had to be corrected by a second 

 adjustment. Astigmatism as well as chromatic aberra- 

 tion could provide the subject with a cue as to the 

 right direction. 



It is possible that the cortical center which controls 

 the triad response transmits impulses simultaneously 

 to the centers in the midbrain controlling convergence, 

 accommodation and pupil constriction. On the other 

 hand it is entirely possible that the triad innervation 

 from the cortex is first transmitted to the center con- 

 trolling accommodation and relayed from there to 

 the centers controlling convergence and pupil con- 

 striction. This is possible because the brain-stem 

 center controlling accommodation never responds 



without the simultaneous occurrence of a convergence 

 and a pupillary response. However, the accommoda- 

 tive response could not be mediated either through 

 the brain-stem center controlling convergence or 

 through the center controlling pupillary constriction 

 becau.se these same centers mediate other types of 

 pupillary and convergence responses which are not 

 associated with accommodation. 



The same center in the midbrain which mediates 

 the pupillary part of the triad response also mediates 

 the pupillary response to light. It is assumed to be 

 located in the Edinger-Westphal nucleus. 



Furthermore the brain-stem center which mediates 

 the convergence part of the triad response also medi- 

 ates fusional convergence. Both types of convergence 

 induce an excycloductive movement of the two eyes, 

 i.e. the two eyes rotate around their lines of sight with 

 the tops of the eyes turning outward. This indicates 

 that both types of convergence are mediated by the 

 same mechanism in the brain stem. There is no cyclo- 

 rotational movement associated with simple conjugate 

 movements of the eye to the right or left (5). 



On the other hand there is ample evidence that the 

 brain-stem mechanism for accommodative and fu- 

 sional convergence receives innervation from two 

 different cortical centers in mediating these two types 

 of responses. Fusional convergence is a reflex response 

 to stimulation of disparate points of the two retinas. 

 This is probably a feed-back type of response in which 

 the eyes constantly tend to drift to the phoria position, 

 i.e. that which they would assume if one eye were 

 placed under a cover, but are brought back to the 

 fusion position by the retinal disparity resulting from 

 the drift. 



Knoll (46) and Marg & Morgan (61, 62) have 

 demonstrated that a marked pupil constriction is 

 associated with a change in accommodation and ac- 

 commodative convergence, but the pupil response 

 associated with fusional convergence is almost negligi- 

 ble. The very fact that fusional convergence can be 

 manipulated without affecting accommodative con- 

 vergence is in itself evidence that it involves a separate 

 cortical mechanism. 



Reese & Hofstetter (7;^) have reported a case 

 in which accommodation and accommodative con- 

 vergence were absent, but positive fusional conver- 

 gence was still operative. An ordinary concomitant 

 squinter may have a normal amount of accommoda- 

 tive convergence but a total absence of fusional con- 

 vergence. 



The relationship of accommodative and fusional 

 convergence (8, 31, 33) at various levels of accommo- 



