Drawing As the study of<unk>  impairments narrows, research is concentrating on analyzing drawing abilities. Drawing abilities may be decomposed into three steps: visual perception, visual imagery, and graphic production. According to the two-streams hypothesis, as information exits the occipital lobe it follows two pathways. The dorsal stream ("where pathway") ends in the parietal lobe while the ventral stream ("what pathway") terminates in the temporal lobe. Damage to the parietal lobe is highly correlated with<unk>  since it is involved in drawing and copying. The parietal lobe is also critical for remapping spatial position across saccades. There is an attentional subsystem responsible for moving the eyes, head, and body to focus on different images. Damage at various levels of this system could lead to trouble localizing a stimulus or hemispatial neglect which manifests as perseverative errors on the drawing. There are a couple theories used to describe the neurological mechanisms behind drawing. Kosslyn and Koeing model Kosslyn proposes that there is an early split of information in the dorsal stream. The first pathway captures coordinate relations by defining distances between points in space. These points become a continuum which can transform into other points through intermediate points. This coding of points would allow space to be perceived qualitatively, which would also help with movement. The other pathway encodes "categorical" information, which synthesizes information about the shape and spatial arrangement of an objects parts. It decomposes objects into their most basic form, by looking for boundaries, lines, or patches. These categorical relations in turn lead to abstract spatial relations that allow one to perceive objects as being "on top," "inside," "between," "beside," etc. Van Sommers model The Van Sommers model describes two hierarchical systems for drawing: one for visual perception, another for graphic production. The visual perception model utilizes David Marr's three stage system to describe visual perception in copying. In the first stage, an image us represented in 2D based on changes in intensity. Foreground and background are not distinguished. In the second stage, a 2.5D representation is formed which encodes the object in a viewer-centered coordinate system. Finally a 3D object-centered representation is established making it possible to appreciate volume. Visual representations of familiar drawings are stored in memory. This representation sends feedback to the other areas of the brain which encoded the spatial and physical properties of the object. Feedback from these areas allows the drawer to successfully encode the coordinate and categorical relations. In the graphic production model, the viewer begins by making a series of depiction decisions about the dimensions, amount of detail to include, etc. Depiction decisions are not used when copying a drawing because they're dictated by the situation. Next, the production strategy is formed. If the drawing is unfamiliar, then the drawer will divide and rank the different portions of the drawings. If the drawing is familiar (e.g. a sun), then the drawer will reproduce the item line by lie regardless of the pictures organization due to automatic execution. The third component, contingent planning, reflects the importance of planning in drawing. Contingent planning is a result of production strategy. If the drawing is unfamiliar and requires a segmented approach, then the most appropriate sequence is determined before the drawing. At this point, the drawing task becomes a problem solving task. The fourth and final component of the model refers to the articulatory and economic constraints placed on the drawer by using a pencil. Certain directions are favored due to the orientation of the hand and fingers, etc. However, some feel the Van Sommers model does not adequately account for all aspects of drawing. Other theories Drawing from memory in response to a verbal order requires the image to be recalled from associative memory and brought into the visual buffer. Once there, it can be successfully drawn and copied from memory. Familiar images (like the sun) may not require visual imagery to draw, as the production schemes and action programming stored in the associative memory and procedural memory may be sufficient to reproduce the drawing. Construction Construction problems are usually caused by visual perception deficits. They require normal vision and the ability to execute a series of motor activities. When looking at performance, it is important to consider perceptual and executive functioning. A patient with trouble visually recognizing patterns or spatial relations may have difficulty correctly building a model. In addition, problems planning, organizing, or carrying out action may impede the ability to solve a construction problem. Neuropsychological mechanisms Modern attempts to understand<unk>  have moved away from anatomical functions towards a cognitive neuropsychological approach. Both adults and children alike experience difficulty reproducing oblique lines. Some feel that these deficiencies may be attributed to planning since it is easier to plan horizontal and vertical lines than oblique lines. Research indicates that both adults and children are more able to draw squares than diamonds, although as children grow into adults they are more accurately able to depict diamonds. One study showed that<unk>  patients were significantly less accurate than the control patients in producing angles with vertical and horizontal orientations. In this study,<unk>  patients drew patterns usually found in children 8 and younger. Gregory argues that ontogenetically and phylogenetically earlier behavioral traits are present in the brain, but inhibited. When these inhibitory mechanisms become compromised, then the childlike behavior patterns re-emerge. Therefore, according to this theory, the inhibitory mechanisms in patients with<unk>  have failed, causing them to draw like young children who have difficulty drawing oblique lines.
Constructional apraxia
Ideomotor apraxia
Hemimotor neglect
Visuospatial dysgnosia
Neurapraxia
Pseudodyssynergia
Integrative agnosia
Dyssynergia