Keeping an Eye on the Periphery
Authors:   , van Heusden Elle Martina 
Published: December 2023

After reading the work of this thesis I hope you are convinced that eccentricity
is a major factor that cannot be ignored when it comes to understanding
visual selection. More specifically, in Chapter 2 we showed that while the
proportion of selecting targets and salient items does not change with
eccentricity, the dynamics of saliency- and relevance-driven selection do
change. That is, the effect of saliency was protracted at a further eccentricity,
while the effect of relevance was delayed. This discrepancy between overall
selection and performance over time can be explained by the difference in
saccade latencies between conditions. That is, as the saccade latency
distribution shifted in time with increasing eccentricity, so did the effects of
saliency and relevance.

In Chapter 3, consistent with earlier work, we challenged existing models of
visual selection, showing that the time at which a saccade is initiated greatly
influences whether it will be saliency- or relevance-driven. That is, short
latency eye movements are more likely to be saliency driven while later eye
movements are more likely to be relevance driven. Importantly, we showed
for the first time that this separation in time leads to a brief period in between
saliency-driven and relevance-driven selection in which the eyes appear to be
in ‘limbo’. That is, selection appears to operate randomly, irrespective of
saliency and relevance. By fitting different models on the data, we showed
that the dynamics of saliency- and relevance-based selection are best
described as two independent processes that do not influence each other. We
propose an alternative view on the classic priority map model, in which
saliency effects are actually a byproduct of a difference in processing speed
between different items. That is, on the priority map, salient items are
available for selection earlier than non-salient items as they are processed
more quickly and elicit therefore more activation at an earlier point in time.
After a while, this difference in activation disappears because then non-salient
items are processed as well, resulting in a period of non-selectivity. After this,
the influence of behavioral relevance takes effect, and activity for the relevant
item increases.

In Chapter 4 we showed that subjects are more likely to select items that are
presented close to fixation than items presented further away. This central
selection bias was larger than would be expected based on low-level sensory
differences between eccentricities suggesting an important role for
attentional competition.

In Chapter 5 we were able to determine, for the first time, the time course
that the effect of eccentricity follows. Here we showed that eccentricity mainly
influences those saccades that are initiated early. That is, eccentricity operates
in a similar time window as saliency. As a consequence, the effects of saliency
were diminished as the eccentricity difference between the two items grew,
but those of relevance were unaffected.

In Chapter 6 we showed that attentional capture by salient distractors is
modulated by the bias that is described in Chapter 4. That is, even though we
saw no effect of eccentricity on attentional capture in overall manual RTs,
using eye movement data we showed that participants are more likely to
select an item closer to fixation than an item presented further away. Crucially,
on those trials in which an eye movement was made towards the distractor
reaction times increased with distractor eccentricity while the likelihood of
making an eye movement to the distractor in the first place decreases with
increasing distractor eccentricity. As these effects go in opposite directions,
overall RT showed no effect of distractor eccentricity when all trials were
combined together.