DEL DIAGNÓSTICO SITUACIONAL DEL MANEJO POST COSECHA

Attempts have been made to selectively manipulate vertical gaze angle to gauge its role in the estimation of object distance. Ooi et al. (2001) manipulated the vertical gaze angle of a standing observer in an open field situation using base-up and base- down ophthalmic prisms. Wearing base-up prisms, which act to increase

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", was shown to decrease the perceived distance of an object on the ground plane and adaptation to these prisms increased perceived distance to the object. Ooi et al. interpreted the results in terms of a change in perceived eye height, in this interpretation perceived eye height is increased when wearing base-up prisms, which causes an increase in

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" and therefore a decrease in perceived distance. Similarly, after

prism adaptation, which causes a decrease in

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" perceived distance is increased (Ooi et al., 2001). It is also possible that the causality is reversed i.e. a change in " causes a change in perceived eye height.

Gardner and Mon-Williams (2001) have shown that height in the field influences the reaching and positioning movements of an observer seated at a table surface. Observers carried out a visuo-motor positioning task and a prehension task, whilst wearing either base-up prisms, which increase

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", base-down prisms, which decrease

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", or no prisms. The positioning task required observers to place a pin in the

underside of a table surface directly below a target viewed within arm’s reach. The prehension task required observers to reach out and grasp, but not pick up, an object. The prism manipulation had the predicted effect, visuo-motor positioning was reasonably accurate with no prisms, but overshot the target with base down prisms, and undershot the target with base up prisms. In the prehension task peak velocity, which has been shown to increase linearly with object distance (Jeannerod, 1988), was greater with base-down prisms and smaller with base-up prisms, when compared to reaches with no prisms. This indicates an increase and decrease in perceived distance respectively.

An issue related to using prisms to manipulate height in the field is that the prisms also act to alter the perceived slant of the support surface. Surfaces slant upwards with base-down prisms and downwards with base-up prisms (Gardner & Mon-Williams, 2001). If the ground plane is perceived to be slanting, height in the field will signal a different object distance to when the surface is perceived to be flat (as can be seen from Figure 4.1 and by comparing Equations 4.1 and 4.2). For the same gaze angle,

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", estimated distance,

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D , will increase when the supporting surface is perceived to be

sloping away from the observer (

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" <0), and decrease when the supporting surface is

perceived to be sloping toward the observer (

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" >0). However, the effect of a prism- induced change in slope on distance estimates is opposite to that demonstrated with the use of prisms for the manipulation of vertical gaze angle.

Studies using prisms to manipulate height in the field in the natural open field setting have not highlighted or reported a change in perceived slope of the supporting surface (Ooi et al., 2001). This suggests that the effect of manipulating vertical gaze angle on the perception of slope could be restricted to near distances (Gardner & Mon- Williams, 2001). A prism-altered estimate of slope from vertical gaze angle will conflict with other cues to slope such as the texture gradient, which will continue to

signal the true slope of the supporting surface. It is possible that the effect of vertical gaze angle on perceived slope might be reduced at far distances because whereas the reliability of vertical gaze angle is expected to decrease with distance (Gardner & Mon-Williams, 2001) the reliability of texture remains approximately constant (Hillis et al., 2004). This means that the weight given to vertical gaze angle will decrease relative to the weight given other cues as the viewing distance increases. However, manipulations of vertical gaze in a full cue open field setting have been shown to influence the perception of far distances, this suggests that the effect of vertical gaze angle on perceived slope might be less reliable or robust than its direct effect on perceived distance.

In addition to changing perceived slope, prisms can also introduce conflict regarding the height of the supporting surface, between visual and haptic information, and curvature distortion (Gardner & Mon-Williams, 2001). This highlights the fact that prisms used to manipulate vergence or vertical gaze angle (Gardner & Mon-Williams, 2001, Tresilian & Mon-Williams, 1999) affects more than just the angle of gaze, they also introduce large conflicts between cues for many perceptual attributes. This makes it difficult to interpret any effect they have unambiguously. In a situation with large cue conflicts complex interactions may occur between cues in the estimation properties such as distance, slant and shape. Under these situations the visual system may act as a robust estimator and veto the information provided by highly discrepant cues (Landy et al., 1995). It is also worth noting that the effects of prisms on perceived distance have also been demonstrated when viewing point light objects in the dark (Ooi et al., 2006). Here many, but not all, of the conflicts associated with the use of prisms will be eliminated.

Under many situations in the natural environment objects are not viewed on a single flat planar surface below eye level. For example, a person standing by an office desk viewing a mug of coffee positioned on a book resting on the desk surface. If an observer is to use height in the field to estimate the distance to the mug of coffee they must take into account the changing relationship between the angle of gaze and distance for each of the relevant surfaces (the office floor, the desk top and the book). These types of relationships have been termed ‘nested contact relations’ and evidence suggests that observers are able to take into account these geometrical relationships

when estimating object distance (Meng & Sedgwick, 2001). This suggests that height in the field could be a useful and accurate cue for estimating object distance in more diverse naturalistic circumstances. Somewhat contrary to this is evidence showing that distances are overestimated near dips/drops in the ground surface and underestimated over changes in ground plane texture (Sinai et al., 1998).

Overall, manipulations of height in the field, although complicated by the introduction of cue conflicts, indicate that it is a potentially useful source of information regarding absolute distance from the observer. However, there has been little direct assessment of its role in scaling binocular disparity. Given its potentially improved reliability compared to vergence, height in the field might be weighted more highly in the estimation of object distance during natural viewing (Jacobs, 2002, Landy et al., 1995). It could therefore be a valuable cue to distance for scaling disparity in estimation of 3-D shape. In a viewing situation where height in the field is removed, such as when viewing isolated disparity defined objects at eye height, the visual system would be forced to use potentially less reliable distance information from vergence. Systematic distortions in perceived 3-D shape have been demonstrated under these viewing situations (Johnston, 1991).