City Research Online

Abstract

A novel visual psychophysical technique has been developed and validated. The technique allowed the development of the ‘EMAIL’ (Eye Movement and Integrated Latency) test which has been optimised and used extensively in this thesis. This technique allows the measurement of eye movement response latencies and visual processing times, without the use of eye-tracking. The computed value produced by the test - labelled as ISL (Integrated Saccade Latency) - captures the overall time the subject requires to compute and generate an appropriate eye movement response and to process a specific stimulus attribute at the end of each saccade. In addition, the EMAIL test also measures the subject’s decision response time (DRT). The latter is largely a measure of the motor response and indicates the time the subject needs to press the appropriate response button. To account for all the parameters that make up the ISL and to validate the EMAIL test as an effective visual psychophysical technique, three studies have been designed and carried out to answer the following questions:

1. Can a psychophysical test designed to measure the overall time subjects need to execute the eye movement and to perform a visual task at the end of each saccade be used as a viable testing procedure to assess saccadic response latencies and visual processing times?

2. Can the EMAIL test capture the known asymmetries that exist between the horizontal and vertical saccadic eye movements through ISLs?

3. Can the EMAIL test capture reliably the effects of aging associated with saccadic eye movements?

In the first study, the eye tracking equipment in conjugation with the EMAIL test, was used to determine the value of each component that makes up the ISL time. The latter also includes the post saccadic visual processing time (VPT). This set up makes it possible to test the hypothesis that VPT, when saccades are involved, are longer than those measured with identical stimuli in the absence of eye movements. The experiments carried out confirm this to be the case. I therefore proposed a second hypothesis to account for the extended VPTs. I hypothesise that the extended VPTs measured in this study can be attributed to instabilities of the eyeball or / and fixation errors. The experimental results confirm my hypothesis by demonstrating clearly that instabilities of the eyeball cause increased VPTs for small saccades as well as fixation errors which also contribute to the increased VPTs, particularly for large saccade amplitudes.

The second study aimed to test the hypothesis that the known vertical versus horizontal directional asymmetries in eye movement responses affect the measured ISL responses. By presenting the visual target along different meridians at specific locations in the visual field, the directional asymmetries in eye movement responses were investigated to test this hypothesis. The results demonstrate clearly that ISL responses reveal the expected directional asymmetries with saccades along the horizontal meridian being faster than those measured along the vertical meridian.

The last study investigates the effects of age on oculomotor and decision response times. Since brain structures change significantly during both developmental and degenerative stages and such changes have been shown to translate into behavioural responses, it is reasonable to test the hypothesis that both ISL and in particular DRT, are also affected by age. The experimental findings show that the EMAIL test captures reliably the expected changes in both oculomotor and decision response times. The results of the last study show that both ISL and DRT times increase non-linearly with age, a finding consistent with earlier reports from eye-movement and reaction time studies.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QM Human anatomy
Departments:	School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses School of Health & Psychological Sciences School of Science & Technology

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