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3. VISUAL PERCEPTION
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3.1 Visual masking and conscious awareness (Forster, Marcel, Sunderland, Wilkins)
Experimental and theoretical work by Marcel (131; 132; 133) has related visual masking to consciousness; and work by Forster has related it to spatial frequency characteristics of visual stimuli. Several extensions are now being conducted of Marcel's original demonstrations that when words are masked so that they are undetectable, their visual and semantic characteristics nevertheless affect subsequent processes.
Two extensions of Marcel's theoretical work on conscious and unconscious visual perception (133) are being conducted by Marcel and Wilkins. In one, patients with lesions of the visual cortex are being investigated. Some patients, while having no phenomenal vision in one half of the visual field, show significant ability to point to and grasp objects of which they are not aware (so-called "blinUsight"). In addition, after-images in the blind field are apparently facilitated by stimulation of the sighted field. This is in contrast to patients with "visual neglect", studied by Sunderland, where awareness of stimuli in one field is impeded by stimulation in the contralateral field.
Another extension is the creation of situations whereby normal subjects experience illusory conjunctions of separate visual features (shape, colour) under conditions of attentional demand. Such experiments mimic the experiences of certain visual agnosic patients and address the issue of how we normally experience a cohesive visual world.
3.2 Object perception (Evett, Hinton, Marcel, Morton)
Morton and Evett are attempting to use experiments on picture perception to develop an analytic model applicable to clinical failures of object recognition. Their experiments have shown chat when we recognise and name the drawing of an object (e.g. a bicycle), another drawing with the same name (a different bicycle) is more easily recognised up to at least 45 minutes later. Reading the name of the object out loud, on the other hand, has no subsequent effect on object recognition (223 U). The result has been applied to problems of object recognition in brain damaged patients, notably giving rise to a functional distinction between optic aphasia and visual agnosia.
Both Hinton and Marcel are using preparatory hand adjustments in grasping to explore the internal representation of perceived objects and their spatial location. An important aspect of this work is the notion of structural descriptions, which Hinton has developed theoretically and experimentally (105) in relation to visual imagery. The importance of this is to emphasise, at a certain level of representation, the spatial coordinates and axes with reference to which an object or pattern is described. Related to this aspect of imagery is Baddeley's (21) work suggesting a separation of spatial and visual components of working memory.
3.3 Visual attention (Duncan)
Duncan's work on visual attention is related theoretically to Hinton's and Marcel's concern in its use of levels of visual processing, and to Baddeley's techniques in focussing on attentional demands. Duncan's work suggests that while all aspects of a visual stimulus may be completely analysed prior to attention (79; 80 U), it is nonetheless easier to direct attention on the basis of one rather than a combination of features. In addition while people have little difficulty attending to different aspects of a stimulus (size, form), they do have difficulties attending to spatially separate stimuli.
A project which has now been completed is that by Poulton and Edwards (204; 205) on the use of colour in sonar displays used for tracking submarines. They showed that for a constant ratio of signal to noise, tracks are easier to detect when the gain of the display is turned up, even if this means displaying a lot of background noise. The use of colour does not substantially improve performance.
3.4 Colour coding in sonar displays (Edwards, Poulton)
The experimental evidence on which the above inferences are based has appeared in several papers (40; 41 U; 246; 247; 249 U) and been reviewed in more detail (248).
The theory that seizures occur when physiological excitation exceeds a critical level within a region of "equipotencial" hyper-excitable cortex has proved to be useful in interpreting the "reflex" aechanism not only of the photosensitive seizures but also of seizures induced by thought (253 U).
3.5 Photosensitive epilepsy (Wilkins in collaboration with colleagues at Runwell Hospital and the Instituut voor Epilepsie-bestrijding, Holland)
Certain patients with epilepsy are photosensitive and suffer attacks that are precipitated by visual stimuli such as flashing lights or patterns of stripes. The nature of the stimuli that provoke this EEC response is of interest both from the point of view of treatment and from the inferences that can be drawn about the chain of neural events that culminates in a seizure.
3.5.1 Theoretical inferences
Our study of the characteristics of epileptogenic visual stimuli and of the paroxysmal EEC responses they evoke had earlier suggested
1. that the epileptiform activity is triggered when normal physiological excitation in the striate cortex exceeds a critical threshold level, and
2. that the threshold varies from one patient to another. More recently our studies have shown
3. that the critical level of excitation is similar throughout different areas of the striate cortex of each hemisphere
4. that excitation of the striate cortex alone can be sufficient to induce epileptiform EEG activity
5. that the hyperexcitability of the two cerebral hemispheres is not uniform but can differ considerably, even in patients with primary generalised epilepsy in whom there is no evidence of cerebral trauma.
3.5.2 Practical consequences
Television viewing is responsible for many of the seizures suffered by photosensitive patients. We have shown that the epileptogenic properties of the television derive mainly from the pattern of stripes which "reverses" at 25 Hz. Televisions with small screens are less likely to cause an attack partly because they stimulate a relatively small area of retina but mainly because the line pattern is difficult to resolve (251). Generally speaking the television appears to be more likely to cause a seizure under conditions of high ambient illumination (39). The consequences of the above work for the design and use of visual display units have been reviewed (242).
A substantial proportion of photosensitive patients have been found to induce their attacks by rolling their eyes upwards under closed eyelids (38). This action may result in a rapid tremor of the lids and thereby cause an intermittent stimulation of the retina (75).
There are several arguments that favour the introduction of tests for pattern sensitivity as part of the routine EEC examination. Simple techniques and the rationale for their selection have been described in detail (76).