Tilt representation beyond the retinotopic level

We perceive a stable visual world, which enables successful interaction with our environment, despite movements of the eyes, head and body. How are such perceptions formed? One possibility is that retino-centric visual input is transformed into representations at higher levels, such as head-, body-, or world-centered representations. We investigated this hypothesis using the tilt aftereffect in a balanced adaptation paradigm designed to isolate head, body and world-centered aftereffects. Observers adapted to two oppositely tilted stimuli, each contingent on one of two different gaze, head or body directions. We found aftereffects contingent on gaze direction, but not head or body direction. This demonstrates that adaptable tilt representations exist in a head-centric frame but not in higher reference frames. These aftereffects may be attributed to adaptation of retinotopic tilt sensitive neurons whose responses are modulated by gaze direction (gain fields). Such neurons could support functionally head-centric tilt representations and are found as early as V1. On the basis of our results we would not expect activity in tilt sensitive neurons to be modulated by head or body direction. The balanced adaptation paradigm is a useful tool for examining properties of the process responsible for gaze-modulation of activity in visual neurons

An Investigation of the Gaze Contingent Tilt Aftereffect

In order to act on the world around us, the brain needs to encode the location of visible objects. This begins with representing the location of visual features on the retina. However, a retinal representation alone does not provide information about the direction of visual features with respect to the head or body whenever the eyes, head and body move. Therefore, successful action requires more than a retinal representation of location. One possibility is that the locations of visible objects may be encoded in retino-centric, head-centric and body-centric frames of reference. To investigate this, we used a well-known visual phenomenon: the tilt aftereffect (TAE). We investigated whether visual feature tilt is represented beyond a retino-centric representation, using a gaze contingent adaptation paradigm. The results of seven experiments yielded four key findings: 1) The TAE was contingent on gaze, suggesting that the TAE is not just retino-centric. We found evidence of a head-centric representation of tilt, but no evidence of a body-centric or world-centric representation of tilt. 2) The gaze contingent TAE showed different characteristics compared to the conventional TAE. While the conventional TAE was sensitive to test stimulus duration, the gaze contingent TAE was not. The gaze contingent TAE was also significantly smaller in aftereffect magnitude. Both TAEs shared similar characteristics with respect to stimulus contrast. 3) We found no evidence that either TAE was modulated by attention. 4) Our findings support the conclusion that the conventional TAE is the result of adaptation of two mechanisms: a tilt sensitive mechanism and a gaze direction encoding mechanism. This result fits with neurophysiological findings of neurons jointly sensitive to tilt and gaze direction (Trotter & Celebrini, 1999). Concluding, our results in this thesis provide psychophysical evidence that our impression of the world is based on head-centric visual representations