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Eye movements and the maximization of value
Only the central region of the retina, the fovea, can provide us with high-acuity details of our visual environment. In the periphery however, resolution fades away with increasing eccentricity. As a consequence, humans and other animals with a foveated visual system move their eyes to redirect t...
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| Format: | Dissertation |
| Sprache: | Englisch |
| Veröffentlicht: |
Philipps-Universität Marburg
2017
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| Online Zugang: | PDF-Volltext |
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| Zusammenfassung: | Only the central region of the retina, the fovea, can provide us with high-acuity details of
our visual environment. In the periphery however, resolution fades away with increasing
eccentricity. As a consequence, humans and other animals with a foveated visual system move
their eyes to redirect their gaze towards objects of interest. And with each saccadic eye movement,
we choose a different region of the visual field for high-acuity processing. In the recent decades,
the eye movement system has thus evolved as a role model to study decision making (Glimcher,
2003), which is also because the oculomotor system is sensitive to valuation processes. Moreover,
our eye movements are tightly linked to visual perception, because where we look determines what
we see and every eye movement poses a major challenge to the visual system as it shifts the whole
visual image on the retina. In three studies, this dissertation project examined whether the eye
movement system can adjust saccade latencies to maximize informational and motivational value
and whether the visual system can maximize all the information available despite making eye
movements.
The first study investigated whether the eye movement system is sensitive to the information
that can be gained by executing an eye movement. Participants saccaded to a peripherally appearing
target and perform a perceptual task. By exchanging the target while the saccade was in flight, we
could independently manipulate the pre-saccadic peripheral and the post-saccadic foveal visibility
and thus create conditions where participants either lost or gained information by making an eye
movement. In the loss condition, the probability of correctly identifying the target increased with
saccade latency because participant could benefit longer from high resolution peripheral vision.
The opposite pattern was observed in the gain condition. However, eye movement latencies did not
differ no matter whether participants could gain or lose information and thus could not maximize
the all the information available. Instead, latencies decreased with the probability that visual
information at the saccade target was task-relevant, suggesting that saccade eye movements are
influenced by the motivation to foveate task-relevant information, but not by the information that
can be gained by saccade execution.
In Study II, we tested whether the visual system is able to integrate pre-saccadic peripheral
and post-saccadic foveal information and whether it weighs the incoming visual information
according to its reliability, that is, according to how well something can be seen. This optimal integration would minimize the perceptual uncertainty and thus maximize all the information
available to the visual system. For every individual, we separately measured discrimination
performance in the fovea and the periphery. Using maximum-likelihood integration (Ernst &
Bülthoff, 2004), we predicted the optimal weight given to peripheral information as well as the
optimal uncertainty associated with the trans-saccadic percept. Both, in terms of weighting and
uncertainty, trans-saccadic performance was not distinguishable from optimality. We thus could
show that the visual system is able to integrate information across saccades and that it is close to
optimal in doing so. This highlights that the visual system is able to maximize all the visual
information available despite making eye movements.
Study III investigated whether the influence of expected motivational value on saccades
(Milstein & Dorris, 2007, 2011) can only be found in contexts where participants additionally have
to choose between multiple rewarded targets. We recorded saccade latencies to rewarded targets
differing in reward and manipulated the proportion of interleaved choices within one block. In
choice-trials, two targets were displayed and participants could choose between the two to obtain
the corresponding reward. Without choices present, we found no evidence for single target
saccades to be affected by reward. When choices were interleaved, latencies to less rewarded
targets were delayed and the magnitude of this delay increased with the proportion of choices. This
delay was elicited by the expectation of an upcoming choice-trial as well as inter-trial priming:
After a choice, saccadic reactions to the non-chosen target were delayed. We thus could show that
there is no direct relationship between expected motivational value on the one hand and saccade
latencies on the other hand. Rather, this relationship only persists in contexts where humans can
maximize their reward outcome by preferring one target over the other.
In sum, the present dissertation shows that there is no direct relationship between saccade
latencies on the one hand and motivational value (Study III) or informational value (Study I) on
the other hand. Instead, saccade latencies are sensitive to the probability that information acquired
at the saccade target becomes task-relevant (Study I) and the preference of one target over the other
(Study III). For perception we could show that the visual system can optimally integrate
information about saccades and thus that vision does not correspond to disconnected snapshots, but
rather to an integrated stream of continuous information (Study II). |
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| Umfang: | 135 Seiten |
| DOI: | 10.17192/z2018.0080 |