Idiosyncratic Search: Biases in the deployment of covert attention.

Eye tracking of visual search tasks shows that the probability that the eyes will move from the current fixation to a nearby target on the next saccade is only ~50%. How can observers fail to find clearly identifiable targets close to fixation (even if they find it later)? One possibility is that processing within the Functional Visual Field (FVF) around fixation is not homogenous. If so, is that inhomogeneity random or systematic? To answer this question, we asked observers to move their eyes to a cue. 300 msec after cue onset, a ring of 7 black Ls and one black T was briefly flashed. Observers made 4AFC decisions about the orientation of Ts. After response, a new fixation location appeared, and this process repeated for two blocks of 360 trials. We found reliably idiosyncratic patterns of accuracy as a function of radial angle (10 of 16 observers were significantly different from normalized group average accuracy, assessed by Chi-sq, p<0.001. Four more p<0.05). Is idiosyncratic accuracy a function of idiosyncratic deployment of attention or retinotopic variation in basic visual processing? To test this, we made the T red, allowing it to summon attention without need for search. Duration was staircased to produce ~25% errors. This eliminated systematic idiosyncrasies in accuracy (Only 1 of 20 observers with p<0.05). Did the original idiosyncrasies depend on making successive saccades to new fixation points? We repeated the experiment with fixation held at a single location. Idiosyncratic patterns were seen, though they seem weaker than with a moving fixation (7 of 20 observers with p<0.001. Three more with p<0.05). We do not yet know if the idiosyncratic patterns for one observer would be the same with saccades and with steady fixation. These results suggest that attentional deployment is systematically inhomogeneous in the immediate vicinity of fixation.