(A),(D) Heading tuning curves measured in the horizontal plane for the congruent cell (Red: vestibular; Green: visual). 0° heading denotes straight forward translation, whereas positive/negative numbers indicate rightward/leftward directions, respectively. (B),(E) Responses of the same neurons during the heading discrimination task, tested with a narrow range of heading angles placed symmetrically around straight ahead (0°). (C), (F) Neurometric functions computed by ROC analysis. Smooth curves show best-fitting cumulative Gaussian functions. Neuronal thresholds were: Congruent cell (A–C): 8.6°, 10.6° and 4.7°; Opposite cell: 5.2°, 4.6° and 19.7°, for vestibular, visual and combined data, respectively.