Another prominent group, the cells of the vertical system, called VS cells, in general respond most strongly to vertical image motion; downward is their preferred direction and upward is their null

direction. However, precise mapping of the cells’ local preferred directions revealed a spatially nonuniform receptive field; the different preferred directions in different parts of the fly’s visual field resemble an optic flow pattern as might be elicited by the fly during certain flight maneuvers (Krapp and Hengstenberg, 1996 and Krapp et al., 1998). These large and elaborate receptive fields could be shown to result from a combination of direct feed-forward input the tangential cells receive from columnar motion-sensitive elements and lateral synaptic interactions between the various tangential cells within the lobula plate (Borst and Weber, 2011; for review, see Borst et al., 2010). As for

the nature of their retinotopic selleck inhibitor input elements, the lobula plate tangential cells have been subjected to numerous tests investigating whether they conform to the Reichardt model in blow flies, 3-MA mw hover flies, and fruit flies. In these experiments, tangential cells were stimulated by periodic gratings moving at a constant velocity (Haag et al., 2004, Joesch et al., 2008 and Schnell et al., 2010) or with a dynamic velocity profile (Egelhaaf and Reichardt, 1987, Egelhaaf and Borst, 1989, Borst et al., 2003, Reisenman et al., 2003, Borst et al., 2005 and Spavieri et al., 2010). Some

studies investigated the local motion response by restricting the field of view to a small window through which the pattern was shown to the fly (Egelhaaf et al., 1989) or by using intracellular calcium concentration changes as a readout for local activity in the dendrite (Single and Borst, 1998 and Haag et al., 2004). Tangential cells were also stimulated by natural images (Dror et al., 2001) or by apparent motion stimuli consisting of spatially displaced sequences of discrete brightness steps (Egelhaaf and Borst, 1992). All these studies concluded that the Reichardt detector accurately describes no the behavior of these input elements. As an example, the responses of Drosophila HS cells have been measured as a function of pattern contrast ( Figure 2D): Although the response does not rise quadratically as predicted by a perfect multiplication, it clearly increases with increasing pattern contrast, thus ruling out a division of temporal by spatial gradient as specified in the gradient detector. When stimulated by a periodic grating drifting at different velocities, the response of HS cells displays a velocity optimum, as predicted by the Reichardt detector ( Figure 2E, black trace). Furthermore, when the test is repeated with a grating of twice the spatial wavelength, the optimum velocity is doubled ( Figure 2E, gray trace).