OPTIC ATAXIA
Susan Radtke and Sarah Leonard
University of Waterloo

    Ataxia usually follows damage to the neocortex and is the inability to conduct meaningful movements
or movements on command in the absence of paralysis or other sensory impairments (Kolb & Whishaw, 1996).  Optic ataxia occurs when the patient has a deficit in reaching under visual guidance that cannot be explained by motor, somatosensory, visual field deficits or acuity deficits.  The involvement of the posterior parietal cortex in visuomotor behavior has been extensively illustrated.  The parietal lobe is divided into two functional regions. One involves sensation and perception, the other involves integrating sensory input, primarily with the visual system. It is this second region that is of concern for optic ataxia. The diagram below indicates the location of the parietal lobe.

SOME EVIDENCE:
    Monkeys loose the ability to correctly reach for an object when the inferior parietal lobe is lesioned.  Misreaching is most evident on the contralateral side to lesion.  The lesion also impairs the ability to accurately shape the hand according to the objects configuration, and therefore produces a severe deficit in grasping or manipulating tools.  In humans, a similar pattern is found following posterior parietal lesions, this is called optic ataxia.

SYMPTOMS:
    Patients suffering from this syndrome produce inaccurate reaching movements towards a target or object in space, this is especially true with their contralesional hand.  The kinematics of their reach is also altered, the reach takes longer with a lower velocity and a longer deceleration phase.  Grasping of objects is also impaired, correctly orientating the hand and inadequate pre-shaping with respect to the target occurs.  The patient's performance is even more severely deteriorated when vision of either the hand or the target is prevented (Kolb & Whishaw, 1996).  For more information on visuo-motor control go to Consciousness and the Visuomotor Transformation and New Views of Vision and Action.

VENTRAL AND DORSAL PATHWAYS:
    Within recent models of visual information processing, optic ataxia is considered a deficit of the dorsal
pathway that connects the pre-striate areas to the posterior part of the parietal lobe.  This pathway is
involved in the perception of spatial relations and probably most object attributes which are used to
control movement.
    The figure below shows the major roots of visual input into the dorsal and ventral streams.  Approximate roots of the cortico-cortical projections from the primary visual cortex to the posterior parietal and the inferotemperal cortex respectively.  (LGNd = Lateral Geniculate Nucleus, Pulv = Pulvinar, SC = Superior Colliculus).  Taken from Milner, AD. The Visual Brain in Action.

BALINT'S SYNDROME:
    Optic ataxia is a common symptom of what is known as Balint's Syndrome.  Patients with Balint's Syndrome will only perceive stimuli that is presented at 35 to 40 degrees to the right, they are able to move their eyes but cannot fixate on specific visual stimuli.  A second symptom of this syndrome involves the patients' field of attention which is limited to one object at a time.  This makes activities like reading difficult because each letter is perceived separately.  The third symptom is optic ataxia in which the patient has severe deficits in reaching under visual guidance.

SUBTYPES OF OPTIC ATAXIA:
    Some researchers have divided optic ataxia into subtypes. One subtype is "non-foveal" ataxia, caused by a problem mediating spatio-motor transformations for the eye and hand. These people can successfully reach to objects focussed on the fovea but can not reach to objects located in the peripheral field of vision (Buxbaum, Branch & Coslett, 1997).

REFERENCES
Buxbaum, L., Branch Coslett, H., (1997). Subtypes of Optic Ataxia: Reframing the Disconnection       Account. Neurocase, 3, pp.  159-166.

Kolb, B & Whishaw, I. Q., (2000). Human Neuropsychology, Fourth Edition. Lethbridge: Freeman Worth Publishers.


For a list of references on this and related topics click here.