Inhibitory neural mechanisms in spatial orienting

Canadian Psychology, Jan 1994 by Rafal, Robert

This talk reviews studies in normal and braininjured humans which have revealed separate neural structures which control covert attention and others which regulate eye movements. These studies have also identified subcortical brain regions critical for reflexive orienting, and cortical regions important for endogenously controlled orienting.

Converging evidence for a special role of the midbrain pathway in reflexive orienting in humans comes from three sources: 1) Patients with lesions of the midbrain have been studied to define what visually guided behaviours are impaired; 2) Patients with hemianopia due to lesions of the visual cortex are examined to determine what visually guided behaviours are preserved when only the midbrain visual pathway is available; 3) Extrageniculate vision is studied in normal subjects by comparing the orienting of attention into the temporal and nasal hemifields (an anatomic asymmetry which is present in the phylogenetically older midbrain visual pathways.)

This strand of research has shown that patients with midbrain lesions are impaired in both eye movements and reflexive covert visual orienting. Both hemianopic patients and newborns (whose visual function is almost exclusively collicular) do demonstrate activation of the oculomotor system by signals in the temporal hemifield. Moreover, signals in the temporal visual field are more efficient in summoning attention even in normal adults. Studies of patients with lesions of the frontal eye fields (FEF) have shown that the FEF is necessary for generation of voluntary eye movements; and that it normally inhibits collicular mechanisms for reflexive eye movements. There is thus evidence both for separate neural systems for covert orienting and eye movements, and for reflexive and voluntary orienting within each of these systems. Moreover, the voluntary mechanisms function, in part, by inhibiting reflexive orienting.

The midbrain reflexively activates an inhibitory tagging mechanism. This inhibition of return is also impaired by midbrain lesions, and is more effectively activated by signals in the temporal hemifield of normal adults. Recent studies have suggested that inhibition of return may have separate components which influence perceptual and oculomotor processes.

The separation of attentional processes into those which influence perceptual processing and others involved in spatial orienting is further supported by studies which have demonstrated both space and objectbased attention. Normal subjects cued to part of an object (or an element within a gestalt group) are more quickly able to reorient attention to a signal that occurs in the same object, than to a signal that occurs equidistant but within another object or grouping. Right parietal lesions seem to affect the space based attention representation; where - as left parietal lesions produce a deficit in moving attention between objects.

Observations in patients with visual neglect and with Balint's syndrome have shown that unattended information outside of the patient's ken are processed to a high level. Flanker interference and semantic priming are produced by information in the patients' neglected field. Moreover, extinction (the failure to report an object in the neglected field when another is present in the intact field) is more likely to occur if the two objects are the same, than if they are different. This advantage for becoming aware of a different object occurs only if the two objects are different in the attribute to be reported in a given task. This phenomenon is similar to "repetition blindness" which can be demonstrated in normal individuals. Extinction must therefore reflect an inhibitory processes acting at a late level of selection for action and awareness.