There are at least three reasons in the chapter on consciousness where this issue is broached again. The first concerns the way our eyes are constantly moving back and forth. Some patients say there is a smear or smudge around the area of blindness. The second relates to the issue of where in consciousness perception is registered. Are we conscious of the exact visual image that we process at V1 or is it a more process-less spatially exact image at a later stage, say V3 or 4. A later level of processing may be more influenced by top-down processing or what we are expecting to see. The image at this level has been found to be less reflective of the definition available at V1. A third possibility is that the damage to neurons at V1 is compensated. A damaged V1 neuron also has lateral connections to other V1 neurons. The neighbouring healthy neurons therefore have the potential to take over or extrapolate from their damaged neighbours. One or more of these possibilities may be true, but as yet there is no confirmed mechanism.

There are two processes. The first is to imagine the two radar scanners or curved dishes as a representation of the curved eye lens as you look down above the eyes. The patient is unable to see part of the left visual field and therefore the right sides of the two eyes are not processing information. The right side of each eye projects to the right hemisphere so it is known that the lesion is in the right hemisphere. Now imagine that you are viewing the retina from the side. The patient is unable to see above and therefore the bottom of the retina is not processing visual information. The lower area of the retina ultimately projects to the lower part of the visual cortex via the Myer loop that swoops around within the temporal lobe on its travels towards the visual cortex. This lower optic radiation terminates below the Calcarine sulcus, which is the sulcus that divides the primary visual cortex. In other words, the lesion is placed most likely somewhere along the lower right optic radiations that project from the thalamus to the lower aspect of the visual cortex within the right hemisphere. This could, for example, involve the posterior cerebral artery within the visual cortex. The neuropsychologist might be interested in checking out other functions associated with a right lesion in such an area to firm up this hypothesis.

Clearly a description of the hierarchical system as described by Hubel and Wiesel (pronounced like weasel) is required here. Extended reading allows for the description of specialised modules e.g. colour and motion; also, how the hierarchy extends to advanced visual processing within the temporal cortex which is to be described further. Finally, a more updated view is that there is additionally a two-way communication between V1 and brain areas as far distant as the prefrontal cortex and that the perceptual process during the milliseconds after presentation includes a back-and-forth reverberation during which time the final percept is stabilised.

Broadly here there is a discussion of the size and speed of cells and the quality of the transferred information: is it detailed and in colour or is it monochrome and less detailed?

Perhaps the most challenging aspect of this is a system that allows for top-down processing. Even if the robot were programmed to analyse information at very high speed from the bottom up, to have meaning the information needs to be compared to past association and contextual memory. The machine must therefore be a complex learning machine that must process the vast amount of information that a human is presented with in their first years of life and then keeps on learning.

Patients with either tactile agnosia or astereoagnosia may have difficulty with work that requires associating meaning with touch such as wool classing. Anyone who works in the dark may also have difficulty. The patient can still feel accurately but is unable to know what they are feeling. Akinotopsia patients would have difficulty with any sport that requires the judgement of motion.

Tactile agnosia and asteroagnosia are both associated with difficulties of associating the haptic sense of touch and therefore can be seen as perceptual disorders. However, tactile agnosia is also associated with poor dexterity. It might be difficult to determine the cause of such poor dexterity without further research, but an additional sensory disorder cannot be ruled out. Akinotopsia is seen as a perceptual disorder of judging movement and a difficulty in judging motion. In one sense this is also a sensory difficulty since the patient would have difficulty in matching two types of motion. However, the patient may have additional difficulty in recognising the motion of a man versus that of a woman. If the motion were displayed by lights attached to joints with the figure in darkness, this interpretive impairment is clearly perceptual.

There is the assumption that many movements are fast, automatic and often have little conscious deliberation. The need for a quick response in defence or attack has obvious advantages for survival. The collicular pathway contains magno cells that allow for a fast reflexive response which is part of the flight or fight orienting system based on the superior colliculus. Blindsight patients appear to be more aware of motion compared to other stimuli, although the studies showing this have been criticised according to the method used. The advantage of the collicular pathway hypothesis is that such a pathway is also capable of bypassing the primary visual cortex (V1). However, the lenticular pathway also contains a magno pathway and a branch of the pathway that goes directly to V5.

Apperceptive visual agnosia is associated with difficulties in perceiving the form of an object and is often associated with right parietal damage while associative visual agnosia is associated with difficulties in recognising the meaning of an object and a left posterior lesion. An improved answer would refer to the associated impairments linked with associative agnosia such as alexia or letter-by-letter reading. Also, the findings of face perception sometimes are associated with apperceptive agnosia. Often a patient with apperceptive agnosia may know the meaning of an object but have difficulty in matching objects of a similar form. Severe apperceptive agnosia can be described as a syndrome in which the perception of form is so impaired that the meaning of the object is also lost.

Patient DF is one of the patients who provides evidence that the "what" pathway is distinct from "where action" pathway. She had great difficulty in perceiving the form of an object but could reach out accurately towards an object showing spatial processing when acting towards the objects that she could not perceive accurately.

Because associative agnosia is due to either a disconnection with the area that analyses meaning or is due to atrophy of this area e.g. Alzheimer's disease, other aspects of perception will be deprived of meaning e.g. reading, colour.

A disconnection disallowing communication within the left hemisphere but also a disconnection from the right hemisphere is associated with the left posterior cerebral artery. However, an additional stroke involving the left middle cerebral artery may also cause an aphasic condition or semantic dementia also associated with a loss of meaning of objects.

This question refers to the way that perception may be a process of problem solving. When one process of perception fails other processes may compensate. Intuitively, when we fail to recognise a person we may use other methods of recognition such as being reminded of the last time you have seen that person. Some research has shown extra activation in areas of the visual cortex which has been interpreted as undamaged areas compensating for damaged brain function. This kind of reliance and interaction between perceptual processes may cause a person to exhibit difficulties in face recognition when there is actually a difficulty with other means of identification e.g. voice recognition.

There is another sense in which the perceptual system is interactive which concerns the top-down and bottom-up interaction. Initially we see the broad brush strokes (reverse hierarchies); feedback and interactive activation allow a back-and-forth exchange of information in which expectancies are seen to be reflected by the patterns of activation within the primary visual cortex. This interaction eventually stabilises after a matter of milliseconds.

For patients with form agnosia detail within a drawing adds clues to its identification. The integrative agnostic patient is unable to differentiate detail into its component parts. These patients who have difficulty in discerning the parts of overlapping figures appear to be less confused by less complicated silhouettes.

At least two factors seem to be influential. There is evidence that while both hemispheres are involved to some extent, the right hemisphere is more likely to be found to be influential in recognising faces. In contrast names are more obviously stored within the left hemisphere. This dislocation is not helped by the usual lack of a semantic relationship between a person's name and their face. Name retrieval, one of the most common memory complaints, may be improved by finding some kind of link between a person's name and their face. Would knowing information about the person improve recall of the name according to the model?

It is argued by some models that the emotional connotations of the face may help face recognition. While all the different means of recognising a face contribute to a cognitive system the serial nature of the model allows more interaction between some units compared to others.

As a social species it is important for us to detect emotions that broadly depict positive versus negative relationships. If natural selection has determined brain organisation then it is important to associate facial features with threatening actions and it is a further survival advantage to make associations such as the identification of the threatening person and all this information must be processed at high speed. Studies have shown that access to such information is locally and closely processed to allow the immediate collation of information. Recently, there have been interesting distinctions made between male and female face perception that also encourage an evolutionary perspective.

To a large degree yes. But Figure 2.30 shows a memory process that is specifically used in identification. Also, there is further face processing that analyses for example the direction of gaze that seems to be relatively independent of identification. In other words some parts of the model seemed to have their own recognition units without access to a more general "cognitive system". Access to identification seems to refer to a more lengthy process of memory retrieval.

Any answer to this question is speculative, but at this time there seems to be a convergence of evidence towards the view that well-known faces are stored in the right hemisphere e.g. right temporal lobe. This right hemisphere is consequently important for signalling a feeling of knowing. A feeling of knowing might occur in isolation when we see a person or place that seems to be familiar but is as yet unplaced. This process of feeling of familiarity may be triggered erroneously by a stranger or place similar to known images but may then be further checked for detailed facial familiarity which is more in keeping with a left hemisphere function. However, such a back-up may be less useful and so when a damaged right hemisphere sends a false familiarity a stranger might be falsely recognised as in the case of hyper-familiarity. This view is supported by the finding that patients with this difficulty tend to have right hemisphere damage. The same argument may be made for feelings of déjà vu. Although clinical wisdom supports that this is a sign of right temporal lobe epilepsy, reviews do not necessary give support for this. In contrast individual EEG studies tend to lend support that this type of aura occurs with abnormal EEG activity on the right side. Perhaps there should be a distinction between the electrical stimulation associated with paroxysmal activity that might stimulate erroneous feelings of familiarity and brain damage that might remove such feelings as in the case of prosopagnosia.

For a long time the line orientation task was seen as a right hemisphere test par excellence and it still may be. However there has been a suggestion that patients with right parietal lesions do more poorly on this task because they fail to attend to information on the left rather than judging the orientation. Similarly a test of rotated orientation as a test of matching two figures after they have been rotated in the "minds eye" may be judged by a different strategy of flipping the image rather than rotating when the two figures are orientated at 180 degrees.

There are large numbers of types of depth perception which suggests that different disorders of depth perception might exist, for example, size constancy in which there is a judgement of relative size is interpreted in terms depth with a smaller than expected size illustrating an object that is further away. Stereopsis in contrast is judgement of distance according to ocular muscles that allows a focus on nearer or further according to the extent the eyes have to focus inwards towards the viewed object.

This is a confusing area largely because different apparently overlapping terms have been used. One way of broadly dividing up the different types of topographical disorientation might be to distinguish between those disorders that show a spatial disorientation and those that have a difficulty in recognising scenes (see chapter on Memory) which involve the where action pathway and those disorders that show poor imagery and poor route learning due to semantic confusions e.g. failing to acknowledge the meaning of buildings. These could be further divided into sub-categories. Testing based on neuropsychological knowledge in this way would allow compensation for the underlying impairment that might be affecting the different types of topographical disorientation to a greater or lesser degree. This is just one speculative way of approaching this enigmatic area of research.