Invariants in the ipsilateral retinotectal visual projection of frogs

We determined whether the sensitivity of the ipsilateral type II units of R ana esculenta to prey (W/H-oriented bars) and non-prey (A/V-oriented bars)- like targets remains invariant under various experimental conditions. We sh ow that the shape of the 'discrimination' curve is largely unaffected by th e level of general illumination and by the background texture. An increase in the stimulus velocity and in the width of the bars moderately affects th e salient points (negative peak and preference reversal) of the curve, but does not alter the overall configurational preference of these units. As fo r retinal ganglion cells: (i) this curve expresses better a 'contrast' betw een two vertically oriented edges of different dimensions than a 'contrast' between two edges of equal dimension but of different orientation; and (ii ) the experimentally induced variations can be explained on the basis of th e spatial and temporal properties of the neuronal elements forming the anta gonistic center-surround arrangement of the receptive field. (C) 1999 Elsev ier Science Inc. All rights reserved.