Invariants in the ipsilateral retinotectal visual projection of frogs

We determined whether the sensitivity of the ipsilateral type II units of Rana esculenta to prey (W/H-oriented bars) and non-prey (A/V-oriented bars)-like targets remains invariant under various experimental conditions. We show that the shape of the 'discrimination' curve is largely unaffected by the level of general illumination and by the background texture. An increase in the stimulus velocity and in the width of the bars moderately affects the salient points (negative peak and preference reversal) of the curve, but does not alter the overall configurational preference of these units. As for retinal ganglion cells: (i) this curve expresses better a 'contrast' between 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 the spatial and temporal properties of the neuronal elements forming the antagonistic center-surround arrangement of the receptive field.