Figure 3. Example neurons tested with tilted horizon stimuli while the monkey remained in an upright orientation.

(a, b) Stimuli used to study two different neurons, demonstrating example object orientations in two conditions, with the ground surface, horizon, and sky gradient tilted –25° (clockwise, top row) or with ground surface, etc. tilted +25° (counterclockwise, second row). The monkey was in a normal upright orientation during these experiments, producing conflicting vestibular/somatosensory cues. The retinal orientation discovered in the genetic algorithm experiments is arbitrarily labeled 0°. (c, d) For one of the example IT neurons, tested with the stimuli in (a), object orientation tuning with respect to the visually cued direction of gravity was consistent across the two ground tilts. (e, f) Correspondingly, the neuron gave very different responses to retinal object orientation values between the two ground tilts. (g, h) This different example IT neuron, tested with the stimuli in (b), did not exhibit consistent object orientation tuning in visually-cued gravitational space. (i, j) Instead, this neuron maintained consistent tuning for retinal-screen orientation despite changes in ground tilt.