AMOR, T. M. A. Characterizing and modeling visual persistence, search strategies and fixation times. 2017. 114 f. Tese (Doutorado em Física) – Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2017. / Submitted by Pós-Graduação em Física (posgrad@fisica.ufc.br) on 2017-04-05T18:55:10Z
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Previous issue date: 2017 / To gather information from the world around us, we move our eyes constantly. In different
occasions we find ourselves performing visual searches, such as trying to find someone in a
crowd or a book in a shelf. While searching, our eyes “jump” from one location to another
giving rise to a wide repertoire of patterns, exhibiting distinctive persistent behaviors.
Initially, by focusing on saccadic directions and intersaccadic angles, we disclose that the
probability distributions of these measures show a clear preference of participants towards a
reading-like mechanism (geometrical persistence), whose features and potential advantages
for searching/foraging are discussed.We then perform a Multifractal Detrended Fluctuation
Analysis (MF-DFA) over the time series of jump magnitudes in the eye trajectory and find
that it exhibits a typical multifractal behavior arising from the sequential combination
of saccades and fixations. By inspecting the time series composed of only fixational
movements, our results reveal instead a monofractal behavior with a Hurst exponent
H ∼ 0.7, which indicates the presence of long-range power-law positive correlations
(statistical persistence). Motivated by the experimental findings from the study of the
distribution of the intersaccadic angles, we developed a simple visual search model that
quantifies the wide variety of possible search strategies. From our experiments we know
that when searching a target within an image our brain can adopt different strategies. The
question then is which one does it choose? We present a simple two-parameter visual search
model (VSM) based on a persistent random walk and the experimental inter-saccadic
angle distribution. The model captures the basic observed visual search strategies that
range from systematic or reading-like to completely random. We compare the results
of the model to the experimental data by measuring the space-filling efficiency of the
searches. Within the parameter space of the model, we are able to quantify the strategies
used by different individuals for three searching tasks and show how the average search
strategy changes along these three groups. Even though participants tend to explore a vast
range of parameters, when all the items are placed on a regular lattice, participants are
more likely to perform a systematic search, whereas in a more complex field, the search
trajectories resemble a random walk. In this way we can discern with high sensitivity
the relation between the visual landscape and the average strategy, disclosing how small
variations in the image induce strategy changes. Finally, we move beyond visual search
and study the fixation time distributions across different visual tasks. Fixation times are
commonly associated to some cognitive process, as it is in this instances where most of the
visual information is gathered. However, the distribution for the fixation durations exhibits
certain similarities across a wide range of visual tasks and foveated species. We studied
how similar these distributions are, and found that, even though they share some common
properties, such as similar mean values, most of them are statistically different. Because
fixations durations can be controlled by two different mechanisms: cognitive or ocular, we
focus our research into finding a model for the fixation times distribution flexible enough
to capture the observed behaviors in experiments that tested these concepts. At the same
time, the candidate function to model the distribution needs to be the response of some
very robust inner mechanism found in all the aforementioned scenarios. Hence, we discuss
the idea of a model based on the microsacaddic inter event time statistics, resulting in the
sum of Gamma distributions, each of these related to the presence of a distinctive number
of microsaccades in a fixation. / To gather information from the world around us, we move our eyes constantly. In different
occasions we find ourselves performing visual searches, such as trying to find someone in a
crowd or a book in a shelf. While searching, our eyes “jump” from one location to another
giving rise to a wide repertoire of patterns, exhibiting distinctive persistent behaviors.
Initially, by focusing on saccadic directions and intersaccadic angles, we disclose that the
probability distributions of these measures show a clear preference of participants towards a
reading-like mechanism (geometrical persistence), whose features and potential advantages
for searching/foraging are discussed.We then perform a Multifractal Detrended Fluctuation
Analysis (MF-DFA) over the time series of jump magnitudes in the eye trajectory and find
that it exhibits a typical multifractal behavior arising from the sequential combination
of saccades and fixations. By inspecting the time series composed of only fixational
movements, our results reveal instead a monofractal behavior with a Hurst exponent
H ∼ 0.7, which indicates the presence of long-range power-law positive correlations
(statistical persistence). Motivated by the experimental findings from the study of the
distribution of the intersaccadic angles, we developed a simple visual search model that
quantifies the wide variety of possible search strategies. From our experiments we know
that when searching a target within an image our brain can adopt different strategies. The
question then is which one does it choose? We present a simple two-parameter visual search
model (VSM) based on a persistent random walk and the experimental inter-saccadic
angle distribution. The model captures the basic observed visual search strategies that
range from systematic or reading-like to completely random. We compare the results
of the model to the experimental data by measuring the space-filling efficiency of the
searches. Within the parameter space of the model, we are able to quantify the strategies
used by different individuals for three searching tasks and show how the average search
strategy changes along these three groups. Even though participants tend to explore a vast
range of parameters, when all the items are placed on a regular lattice, participants are
more likely to perform a systematic search, whereas in a more complex field, the search
trajectories resemble a random walk. In this way we can discern with high sensitivity
the relation between the visual landscape and the average strategy, disclosing how small
variations in the image induce strategy changes. Finally, we move beyond visual search
and study the fixation time distributions across different visual tasks. Fixation times are
commonly associated to some cognitive process, as it is in this instances where most of the
visual information is gathered. However, the distribution for the fixation durations exhibits
certain similarities across a wide range of visual tasks and foveated species. We studied
how similar these distributions are, and found that, even though they share some common
properties, such as similar mean values, most of them are statistically different. Because
fixations durations can be controlled by two different mechanisms: cognitive or ocular, we
focus our research into finding a model for the fixation times distribution flexible enough
to capture the observed behaviors in experiments that tested these concepts. At the same
time, the candidate function to model the distribution needs to be the response of some
very robust inner mechanism found in all the aforementioned scenarios. Hence, we discuss
the idea of a model based on the microsacaddic inter event time statistics, resulting in the
sum of Gamma distributions, each of these related to the presence of a distinctive number
of microsaccades in a fixation.
| Identifer | oai:union.ndltd.org:IBICT/oai:www.repositorio.ufc.br:riufc/22495 |
| Date | January 2017 |
| Creators | Amor, Tatiana María Alonso |
| Contributors | Herrman, Hans Jurgen, Andrade Júnior, José Soares de |
| Source Sets | IBICT Brazilian ETDs |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/doctoralThesis |
| Source | reponame:Repositório Institucional da UFC, instname:Universidade Federal do Ceará, instacron:UFC |
| Rights | info:eu-repo/semantics/openAccess |
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