PubMed · 440428
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H B Barlow. 1979-05-17. Reconstructing the visual image in space and time.. https://doi.org/10.1038/279189a0
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Blindsight, the ability of some blind patients to describe attributes of stimuli they have no conscious awareness of seeing, has been attributed to a secondary (retinotectal) visual pathway. However, it has also been proposed that blindsight could be due to residual function within the primary (geniculostriate) visual pathway. Data have now been obtained that support the second alternative. With an image stabilizer ensuring the accurate retinal placement of stimuli, dense visual field mapping was carried out with a hemianopic patient. This perimetry revealed, embedded in the patient's scotoma, an isolated 1-degree island of residual vision that was not disclosed by conventional perimetric methods. Stimuli presented to this island could be detected and discriminated, although the subject reported he did not see them. The existence of this island of vision implies a corresponding island of functioning cortex within the patient's lesion. Other instances of blindsight may be mediated by similar islands of functioning cortex.
The small angle subtended by the human fovea places a premium on the ability to quickly and accurately direct the gaze to targets of interest. Thus the resultant saccadic eye fixations are a very instructive behaviour, revealing much about the underlying cognitive mechanisms that guide them. Of particular interest are the eye fixations used in hand-eye coordination. Such coordination has been extensively studied for single movements from a source location to a target location. In contrast, we have studied multiple fixations where the sources and targets are a function of a task and chosen dynamically by the subject according to task requirements. The task chosen is a copying task: subjects must copy a figure made up of contiguous coloured blocks as fast as possible. The main observation is that although eye fixations are used for the terminal phase of hand movements, they are used for other tasks before and after that phase. The analysis of the spatial and temporal details of these fixations suggests that the underlying decision process that moves the eyes leaves key decisions until just before they are required.
During movements of the head in pitch (yes-yes) or in yaw (no-no) the visual scene appears stable whereas rolling the head (ear down to shoulder) induces an apparent swinging of the world in the opposite direction. This visual instability is due to the inadequacy, in the roll plane, of the reflex eye movements which are effective in stabilising the eyes in space during pitch and yaw. We investigated whether head is stabilised in roll to protect against visual instability. Human subjects were fixed in a gimbal with their heads free and were exposed to unpredictable oscillatory movement in pitch and, for comparison roll, about axes aligned with the head. With vision, during roll motion, the head was displaced from upright by approximately half the amplitude of the gimbal motion. In comparison, with eyes closed relying on vestibular and proprioceptive cues and during pitch stimuli with or without vision, the magnitude of head displacement from upright was approximately equal to that to the gimbal. The superior head stability in roll, dependent on a visual frame of reference, compensates for poverty of eye movement in this plane.