Chapter 4

Glossary

Agnosia The failure to recognise or interpret stimuli despite adequate sensory function. It is usually classified by sensory modality, so visual agnosia is the failure to recognise objects that are seen.

Congenital prosopagnosia This is thought to be present from birth and is thought to occur without any apparent brain injury.

Developmental prosopagnosia This is thought to be a result of early neurological trauma that might be caused by accident or injury.

Electroencephalography (EEG) Recording the brain’s electrical activity via electrodes placed against the scalp. Can be used to continuously record rhythmic patterns in brain function or particular responses to events (event-related potentials).

Event-related potentials (ERP) Systematic changes in the brain’s electrical responses linked to the presentation of a stimulus. Typically the stimulus is presented numerous times with the electroencephalographic (EEG) signals time-locked to its occurrence then being averaged to separate the signal from noise.

Form agnosia This is now the generally accepted term for patients who are unable to discriminate between objects and are unable to copy line drawings of objects (this was previously termed apperceptive agnosia).

Functional magnetic resonance imaging (fMRI) A medical imaging technology that uses very strong magnetic fields to measure changes in the oxygenation of the blood in the brain and thus map levels of activity in the brain. It produces anatomical images of extremely high resolution.

Fusiform face area (FFA) The fusiform area has been shown to be a key structure in face and object processing; numerous studies have shown that the fusiform gyrus contains an area dedicated to face processing – the fusiform face area (FFA).

Integrative agnosia This is the generally accepted term for associative agnosia. It refers to patients who can perceive the individual shapes and elements of objects but are unable to integrate these into a representation of the whole object.

Modular system A system in which different types of processing are carried out by separate and relatively independent sub-systems.

Prosopagnosia An inability to recognise faces despite adequate visual acuity.

Scotoma A blind area within the visual field, resulting from damage to the visual system (plural = scotomata).

Synaesthesia A condition in which individuals presented with sensory input of one modality consistently and automatically experience a sensory event in a different modality (for example seeing colour on hearing musical notes).

Synaesthete A person who has the condition synaesthesia.

Transcranial magnetic stimulation (TMS) This technique uses an electrical coil placed near the surface of the head to induce a rapid change in the magnetic field, which, in turn, produces a weak electrical current in underlying brain tissue. This can cause depolarisation or hyperpolarisation. The technique can use single bursts or repetitive stimulation. It can be used to support inferences about the role of that brain region in a particular task (e.g. by showing that repetitive stimulation slows responses in task a but not task b, that the region is involved in task a).

Unilateral spatial neglect A difficulty in noticing or acting on information from one side of space typically caused by a brain lesion to the opposite hemisphere (e.g. right-hemisphere damage producing lack of awareness for information on the left). Also called hemispatial neglect or hemispatial inattention

Summary

Damage to different brain regions can result in a variety of perceptual and attention disorders that can display a surprising degree of specificity.
The pattern of disorders observed suggests a highly modular system in which a series of independent processes each contribute towards the goal of perception.
Synaesthesia tells us that information in one modality, such as the sound of a letter, can reliably induce activity in another modality (such as colour vision) in some people.
Blindsight provides an example of a dissociation between conscious experience (which may be absent) and the ability to respond quite accurately to the location and some other features of visual objects.
Prosopagnosia also provides evidence of this dissociation, as does unilateral neglect, where some patients show evidence of knowledge about familar faces, or of neglected objects, of which they are not conscious.
There is some evidence that the nature of the representation formed might be dependent on the task to be performed and, in particular, an important distinction between the perceptual processes that support action and those that result in object recognition.
Two broad patterns of impairment in visual agnosia have been identified and these were originally termed apperceptive agnosia (now generally known as form agnosia) and associative agnosia (now known as integrative agnosia).
In form agnosia patients are unable to discriminate between objects and cannot copy line drawings. Integrative agnosia is characterised by an ability to perceive the individual shapes and elements of objects with an inability to integrate these elements into a representation of the whole object.
Prosopagnosia involves an impairment of face processing. It can be acquired or it can emerge early in development. In all forms of prosopagnosia there is considerable variation in the severity of impairment, associated deficits and types of face processing skills.
In acquired prosopagnosia there is variability in the location and type of lesion although impairment in areas such as the Fusiform Face Area seems to be particularly important.