Consistent with previous reports (Duysens et al., 1982a; Rapaport et al., 1982; Pettigrew and Dreher, 1987), most area 19 cells exhibited only a weak direction selectivity in control condition (low DSI—Figures 2A, 3Ai, 4A). Inactivation of PTV resulted in substantial changes in their DSIs (Figures 4A,B,Di). Thus, when DSI was calculated for velocities preferred before cooling (see Preferred velocities), a large proportion of cells exhibited substantial (≥20%) increases (21.5%; 6/28 cells) or decreases (6/28 cells) in their DSIs (Figure 4Di). Indeed, a couple of cells started with large (>70%) DSIs became absolutely direction selective (DSI = 100%) during inactivation (Figure 4Di). However, three cells that were weakly direction selective (DSIs < 40%) became completely non-direction selective (DSI = 0%) during inactivation (Figure 4Di). In addition, in a proportion (18%; 5/28) of cells, inactivation resulted in a 180° directional shift, that is, a complete reversal of their preferred direction (Figures 4A,Di**). Note that four of these cells also exhibited DSI changes of 20% or more. Overall however, low mean DSI, characteristic of area 19 neurons before cooling (mean: 48.81 ± 25.68%; range: 11.32%–100%, n = 28) was almost identical (P = 0.57, Wilcoxon test) with that (46.34 ± 34.09%; range: 0%–100%) during inactivation. Similarly, for the complex cells, the DSIs before PTV cooling (47.12 ± 26.69%, range: 11.32%–100%; n = 18) were not significantly (P = 0.62, Wilcoxon test) different from those (50.68 ± 27.96%, range: 0%–100%) during PTV cooling. However, for the simple cells, the DSIs in the control condition (49.44 ± 25.09%, range: 29.50%–100%; n = 9) were significantly (P = 0.04, Wilcoxon test) higher than PTV cooling (31.71 ± 40.22%, range: 0%–100%). Note that after rewarming of PTV, DSIs of most cells were much closer to these before PTV cooling (Figure 4Dii).