Apraxia: Definition, clinical contexts, neurobiological profiles and clinical treatments

Apraxia (from the Greek a-praxía, inability to do) is a neuropsychological disturbance of the voluntary movement, defi ned as the inability to perform coordinated gestures directed towards a certain goal, although the will of the subject and his ability are maintained unaltered physical education; as in many other neurological diseases, people with this disorder generally don’t know they are. To exclude that it is hyposthenia, however, as a differential diagnosis, we observe the various muscles engaged as a whole, for the chosen action, as well as a family history to better defi ne the exact diagnosis (as certain forms can be also family members). Computed tomography and magnetic resonance imaging can ultimately prove useful in that they precisely defi ne the nature of the injury-causing the disorder [1-4].

surprisingly, a defi cit of the fi rst level involves an apperceptive agnosia, while a defi cit of the second level leads to associative agnosia. Therefore, starting from the sensory data, as a product of stimulation and receptor reactivity, through the fi ve senses (sight, hearing, smell, touch, and taste), we arrive at perception, such as the elaboration of the elementary sensation that reaches the sense organ [2].
Thanks to the studies of Weber (on the diff erential threshold or threshold of relevance of the perceived stimulus) and Fechner (on the sensation, directly proportional to the logarithm of the intensity of the stimulus), we came to say that [2]: 1) The thesis that supported "sensation = perception" (naive realism) is incorrect, while the hypothesis that foresees the analytical difference (critical realism) is correct; 2) Only the stimuli to which the sense organ is sensitive are perceived; 3) The stimulus, to be registered by the sense organ, must be suffi ciently intense, in an absolute sense. In the context of "critical realism" several theoretical approaches are distinguished [2]. 1) An associationist or atomistic approach by Helmholtz (1878), which provided for a perceptive system made up of 2 systems that are relatively separate but communicating with each other: elementary sensations (I) and cognitive apperceptive layer (II); 2) Gestalt approach, which preferred to pay attention to form and representation. Therefore, several authors contributed: a) For Metzger, the level of visual perception is not fl at but three-dimensional; b) For Rubin, the visual fi eld is differentiated in the background and fi gure; c) For Wertheimer, the perceptual fi eld was organized by a series of rules: good shape (the perceived structure is always the simplest); proximity (the elements are grouped according to distances); similarity (tendency to group similar elements based on similarities); proximity (tendency to group similar elements based on close distance); closure (tendency to group similar elements based on known closed forms); continuity (all elements are perceived as belonging to a coherent whole); common destiny (if the elements are moving, those with a coherent movement are grouped); fi gure-background (all parts can be interpreted both as an object and as a background); pregnancy (in case the stimuli are ambiguous, the perception will be good based on the information taken from the retina).
3) The cognitive approach, which provided for the integration of the models: a) Top-down, i.e. processing from top to bottom (guided by experience that infl uences perception); b) Bottom-up, i.e. the processing from the bottom up (guided by the sensory data processed than in the cortical way). A typical example is Gibson's ecological approach, which provides for very specifi c assumptions: the stimulus is described not in terms of retinal projection but in terms of "optical structure" (ie image that reaches the retina); the stimulus (except pathological defi cits) is perfect as it is and then it all depends on the subsequent interpretation; the set of information given by the context and movement of the observer is called "environmental optical set-up".

4)
Constructivist approach, which has its foundation in the following assumption: "perception is an inferential process (ie based on hypotheses), given that the sensory information coming from the external environment is ambiguous and incomplete, to reach perception it is necessary to the intervention of top-down processes ". Therefore, perception: it is an inferential and active process; it is the fi nal process of the interaction between stimulus (from below) and experience (from above); it is a process that is infl uenced by various external factors (example, visual disturbances, psychiatric disorders, wrong perception, illusion, hallucinations, delusions, suggestion, impossibility of perception due to the sense organ). Among the major exponents we mention: a) Gregory: Starting from Gibson, he says that the starting point is the external stimulus but then we activate the best possible interpretation to explain the complex object); b) Necker: Between two possible interpretations, one will always prevail; c) Allport: Proposed the "perceptual set", that is the idea that the fi eld is infl uenced by subjective motivations, emotions, experiences and expectations, reorganized according to two cognitive operations that we will see later: "generalization" and "categorization". 5) Synthetic approach, or the schools of thought adhering to neuroscience, which attempt to overcome the top-down/ bottom-up relationship, effectively integrating two models. Among the major exponents we mention: a) Bruner: Perception depends on one's needs, expectations, moods, subjective values, emotional meaning, and personality characteristics (New Look School). b) Neisser: The brain is a computer and perception is nothing but an analysis by synthesis, the result of a three-stage sequence: I) the selection of the stimulus through an automatic pre-attentional (bottom-up) process; II) the voluntary shift of attention on the stimulus (top-down); III) the fi nal mental representation. c) Marr: Perception can be investigated on at least 3 levels (computational theory): I) computational level (the goal); II) algorithmic level (the means used); III) procedural level (the how). Marr's approach is strongly bottom-up (as it focuses on sensory processing) but includes the intervention of "topdown (ie information previously learned from the world)" factors: << the chair must have four legs to be able to stand. So our knowledge of the world (top-down) acts on sensory input (bottom-up), reaching a perceptual synthesis >>. The analysis of the visual sensory input proceeds through four specifi c stages: a) description of the gray levels; b) geometric primitive primary sketch; c) two-dimensional sketch and ½ (example, depth); d) 3D representation (three dimensions). However, this model is strongly criticized because it does not take into account an adequate explanation of how the factors at playwork and there are no functional tests outside the laboratory context.
Regarding the human ability to recognize faces and objects: the ability to recognize faces is a typical basic characteristic that has a high degree of social relevance and is vital for newborn babies who at thirty minutes of life seem to already show a preference for faces. The action of recognizing faces is something we can do quickly and automatically, it is a function that is acquired early during the development phase and that does not need to be taught. Face processing takes into account a large number of perceptual and cognitive processes. Two scholars, Bruce and Young, in 1986, proposed a theoretical model for face recognition. This model involves a sequential and hierarchical organization of different degrees of processing. According to the two authors, face recognition is based on an abstract unit that contains different types of structural information of each face that is present in memory. able, however, to recognize familiar objects. This could happen because the differentiation between one face and another must be more precise and detailed than, for example, that there can be between classes of objects or because there are specifi c mechanisms used only for the recognition of faces and which do not concern the recognition of objects. If the processing of faces concerned specifi c mechanisms, the existence of distinct brain regions associated with the recognition of faces and objects should be considered realistic. However, many studies conducted with PET and fMRI have revealed confl icting results with such a hypothesis [4].
The recognition of the objects, therefore, implies the comparison between the information that derives from the visual stimulus and that which is recorded in the memory.
Both proponents of theories of shapes and those of theories of characteristics agree on this. The theory of shapes is based on the fact that a miniature copy exists, called a template that is stored in long-term memory and which corresponds to each visual confi guration that we know. The simpler the more similar to the stimulus presented is the easier the confi guration is recognized. This theory, however, is not very realistic because there can be a huge number of visual stimuli that can combine with the same shape. to grasping the differences within the same category [3,4].
Appreciative agnosia: We speak of apperceptive agnosia when the subject, in the absence of sensory defi cit, is unable to compose the stimulus data and integrate them into a structured perceptual unit. A patient with apperceptive agnosia in the visual modality is unable to perform a drawing on a copy, to accurately describe it in its details and to distinguish it from visually similar objects. Consequently, the comparison with the mental representations of known stimuli also fails, and therefore the recognition of the stimulus does not take place.
Humphreys and Riddoch, about Marr's model of perceptual processes, classify three main forms of perceptive agnosia: a) Agnosia for shape: The patient correctly analyzes the individual sensory characteristics of the stimulus, but is unable to derive the external confi guration of the object. If tested, it is unable to match identical geometric shapes or distinguish different shapes, and is unable to copy simple shapes.

b) Integrative agnosia:
The patient fails to integrate the individual characteristics into a unifi ed global structure. For example, a fi gure with many details will not be recognized, while a fi gure with few details will not be recognized. The patient can perceive the parts of a dog (legs, ears, tail) but does not integrate them to represent the shape of the dog, or he does it with great diffi culty. Sensory agnosia: They are all typologies that concern a particular sense where its function is impaired, failing to recognize what it sees, feels, tastes, or touches, the most particular of these forms is that which concerns touch. In this case, it is caused by damage to the parietal-temporal cortex, varying in severity, where the worst form is given by left cerebral infarcts. Among the forms of "visual agnosis" can be mentioned: "prosopagnosia", agnosia for faces, "akinetopsia", agnosia for movement, and "achromatopsia", agnosia for colors. Among the "auditory agnoses": agnosia for environmental sounds, agnosia for the human voice, and agnosia for musical arias. Bilateral lesion of the temporal lobes is found in all three cases in particular. Among the "tactile agnosias" or "astereognosias": the "amorphognosy", which concerns the shape and size of the objects, the "ailognosia", which concerns the weight and the material and thermal characteristics of the objects, the "asymbolism" or " tactile agnosia ", which concerns the meaning of objects and" tactile agnosia ", which consists in the inability to recognize one's hands.
From an etiopathological point of view, agnosia is caused by damage to the parietal, temporal, or occipital lobe of the brain. These areas store memories of the use and importance of familiar objects, sight and sounds, and integrate memory with perception and identifi cation. Often, agnosia suddenly appears after a head injury or stroke. Other causes of agnosia include brain tumors, brain abscesses (pockets of pus), and disorders that cause progressive areas of the brain to degenerate, such as Alzheimer's disease [6].
The symptoms of agnosia therefore vary according to the damaged lobe [5]. a) "Parietal lobe": people have diffi culty identifying a familiar object (such as keys or a safety pin) positioned in the hand on the side of the body opposite to the damage (somatosensory agnosia). However, when they look at this object they immediately recognize it and can identify it. Some subjects with damage to a parietal lobe insist that everything is fi ne or ignore the problem, even if one side of the body is paralyzed (anosognosia); b) "Occipital lobe": people are unable to recognize familiar objects, such as a spoon or pencil, even if they can see them. This pathology is called visual agnosia. They may not recognize familiar faces (prosopagnosia); c) "Temporal lobe": people may not be able to recognize sounds, even if they can hear them. This pathology is called auditory agnosia; d) "Occipitotemporal lobe": people may not recognize familiar places (environmental agnosia) and suffer from color blindness.
The patient suffering from agnosia is diagnosed after an anamnestic evaluation and the use of diagnostic brain imaging equipment. The physical examination is performed to highlight primary defi cits in individual sensory modalities or communication skills that may interfere with neurobiological testing. For example, if there is damage to superfi cial tactile sensitivity, patients may not perceive an object, even when cortical function is intact; in addition, the presence of possible aphasia may interfere with patient expression. Neuropsychological examination may also help to identify more nuanced agnosias. Neurological examinations (CT or MRI with or without hagiographic sequences) are necessary to characterize a lesion of the central nervous system (e.g. heart attack, bleeding, mass) and to assess the presence of atrophy leading to degenerative disease [3].
To complete the clinical picture, the patient undergoes specifi c neurocognitive tests [4], which aim to assess individual abilities, and specifi c personality tests, which will provide detailed information on his mental health status; for example, the diagnosis of agnosia may exacerbate any anxiety [10,11] and mood disorders suffered by the patient as a result of the traumatic event [12,13], up to panic disorder [14], major depressive disorder [15], obsessive disorder [16,17], or even encroaching on psychotic symptoms where the plane of reality is wholly or partly compromised [18]. If the aetiology of agnosia