Visual acuity and moving objects

Crawford, W. A (1960) Visual acuity and moving objects. PhD thesis, University of Glasgow.

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Abstract

This study was undertaken to describe the ability of man to see detail in objects which have an angular velocity relative to the observer. In flight all objects viewed beyond the physical parameters of the aircraft have an angular velocity relative to the pilot or passengers. A loss of visual acuity was anticipated and has been shown to occur, and an attempt has been made to examine the functions of the oculomotor system to discern the aetiology of this loss. An appreciation of the practical significance of the loss of visual acuity to aviation was made by trials conducted as the pilot of fast low flying jet aircraft in service in the Royal Air Force. The ability to perceive detail in moving objects can be related to the size of the object viewed, the angular velocity of the object relative to the observer, the method of observation, and the time available for observing the object. The larger the object the greater the ability, the greater the angular velocity the greater the loss in ability, co-ordinated movements of the head and eyes are more effective than eye movements alone and increasing the viewing time improves ability irrespective of the method of viewing. Studies of eye movements during the visual pursuit revealed that the smooth following eye movements were interrupted by jerky movements known as saccades and that as the relative angular velocity of the object viewed increased so also did the occurrence of these saccades. These second saccades are considered to be evidence of the failure of the eye movement to achieve fixation and are corrective in function. The failure may be ascribed to insufficient or inaccurate retinal information, inaccurate prediction of the eye movement required, inaccurate response of the oculomotor system to the demand. The interval between the end of the initial rapid eye movement and the beginning of the second saccade may be considered as one of reassessment of the movement required of the eyes to achieve fixation. During this interval which may be as short as 50-70 msec, a new eye movement must be made based on the error of fixation present, a new temporal pattern of muscular activity developed, and the signs of this activity shown by a change in the recorded eye movement patttern. The estimated time for afferent volleys from the retina to reach the visual cortex is 30-70 msec. It is not unreasonable to assume that the more complex motor system from cortex to eye muscles will take at least as long. If these currently accepted estimations of the time for the impulses to pass along the nervous pathways are accurate, little, if any, time remains for integration of information from the retina into an eye movement pattern. A re-assessment of the mechanism and the temporal characteristics of the oculomotor system appears necessary. The relationship of the free movements of the head to eye movements was investigated. The latency of head movement is 250-350 msec, which is 80-i00 msec, greater than eye movement latency. The head rapidly attains a velocity reasonably, comparable to that of the moving object after an overtaking phase and then trails behind the object by an arc varying from less than 5

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: R C Garry
Keywords: Neurosciences
Date of Award: 1960
Depositing User: Enlighten Team
Unique ID: glathesis:1960-73545
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56
URI: http://theses.gla.ac.uk/id/eprint/73545

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