Fig. 7. Effect of the surround on the field potential equivalent to the sensory response (FPSR). (A) An artificial stimulus was given via a dipole electrode simultaneously with the electric organ discharge (EOD) at the receptive field center (RFC; red dot; 0 on the horizontal axis of the graph) and at different points along the rostro-caudal axis (dotted line), illustrated relative to the fish's head. Potentiation of the field potential reafferent sensory response (relative to the basal response, =0) was maximal when the simultaneous artificial stimulation was applied close to the receptive field center but was also observed when the artificial stimulation was rostral to the receptive field center or up to 20 mm caudal to this point. Surround inhibition was not seen when the artificial and natural stimuli occurred simultaneously. (B) Comparison of the basal FPSR with that obtained when an artificial excitatory stimulus was applied synchronously with the EOD, in the center of the receptive field. The effect of this stimulus is represented by difference between the two FPSRs, shown by the blue and red areas. (C) The basal FPSR (blue trace; control) and FPSRs obtained at the same recording point when a metal cylinder (green trace) or a plastic cylinder (red trace) were placed facing the non-electroreceptive area of the flank labeled `object' in A. Electrical stimulation at the same point had no visible effect on the reafferent sensory response.