Centrifugal control of the avian retina. IV. Effects of reversible cold block of the isthmo-optic tract on the receptive field properties of cells in the retina and isthmo-optic nucleus

Abstract

Single unit recording techniques were used in decerebrate, immobilised domestic chicks, to investigate the effects of reversible cold block of the isthmo-optic tract on the receptive field properties of cells in the isthmo-optic nucleus and contralateral retina. Tract cooling had no clear-cut effects on the visual responses of either retinal, or isthmo-optic, units to a variety of moving targets or to stationary spots of light of various sizes turned on and off in their receptive field centres. These negative findings are tentatively taken to suggest that the centrifugal system is effectively decoupled for many targets, though there may be other kinds of visual stimuli (excluded here for technical reasons) whose retinal responses normally do incur centrifugal modification; indeed, at least one functional suggestion is made in this regard. In 4 retinal units out of 25 successfully tested, the firing to large spots turned on in the receptive field was much augmented after the recent passage of a dark edge moving forwards through the visual field, an effect which was abolished by cooling the isthmo-optic tract. It is suggested that this effect is mediated through the centrifugal system and may have functional significance in aiding the animal's normal visual search in shadowed areas. A general functional hypothesis is advanced, based upon the findings in the present series of experiments and the literature relating to other systems, proposing that some centrifugal systems may have evolved specifically to deal with the sensory consequences of the animal's own movements. It is suggested that in some cases this might operate merely to minimise the disruption of sensory processing due to the animal's movements; in others, the centrifugal contribution might be much more elaborate, facilitating some positive aspect of sensory exploration, and hence incorporated as feedforward in the animal's normal search strategies.