8 ployed as a means of studying vision,it allows more direct access to the higher vels of information processing without contamination from lower levels.There is considerable evidence that some of the same anatomical mechanisms are imagery as are involved in some aspects of pattern recognition (Farab,1988; osslyn,1988). Patients with right parietal lesions,who show deficits in visual ienting of the type we have described above,also fail to report the ntrales of visual images (Bisiach,Luzzatti Perani,1981).Whea asked a a nc, they make elaborate reports of the right side but not the :glecte paris of the are reported when facing in one direction are This suggests that the deficit arises at the time scanning the image rmal sub) imag ine the selves ing on fami route, blood activa up parieta (Roland, 85) ug study.m of ghts the pe 10 gery. 93 previously,the o pa terna ati pcoselytela involved attend ing to an external location when subjects scan a visual image NTERIOR ATTENTION SYSTEM ber paper on the topog of」 cogni a-Rakic (1988) de lob cribes medial and tex alled appe olu -ur1a1973 ave called the posterior attention system,to focs g by conscious attention Cognitive ave ofter shown tha detec 78. It haa oduces little curs (Duncan 10g ral aler ate.and one in which d the h ttle trouble in he ultiple pro modalities of interfer The of he widespread erference the de ys hat there unified 4 system8 iployed as a means of studying vision, it allows more direct access to the higher vels of information processing without contamination from lower levels. There is now considerable evidence that some of the same anatomical mechanisms are ed in imagery as are involved in some aspects of pattern recognition (Farah, 1988; 3sslyn, 1988). Patients with right parietal lesions, who show deficits in visual ienting of the type that we have described above, also fail to report the intralesional side of visual images (Bisiach, Luzzatti & Perani, 1981). When asked imagine a familiar scene, they make elaborate reports of the right side but not the ft. The parts of the image that are reported when facing in one direction are -glected when facing in the other. This suggests that the deficit arises at the time scanning the image. When normal subjects imagine themselves walking on a familiar route, blood ow studies show activation of the superior parietal lobe on both sides (Roland, ?85). Although many other areas of the brain are also active in this study, most of tern are common to other verbal and arithmetical thoughts, but activation of the iperior parietal lobe seems more unique to imagery. As discussed previously, the arietal lobe seems to be central to spatial attention to external locations. Thus, it ppears likely that the neural systems involved in attending to an external location re closely related to those used when subjects scan a visual image. ,NTERIOR ATTENTION SYSTEM In her paper on the topography of cognition, Goldman-Rakic (1988) describes le strong connectionb between the posterior parietal lobe and areas of the lateral ad medial frontal cortex. This anatomical organization is appealing as a basis for elating what has been called involuntary orienting by Luria (1973), and what we ave called the posterior attention system, to focal or conscious attention. Cognitive studies of attention have often shown that detecting a target roduces widespread interference with most other cognitive operations (Posner, 978). It has been shown that monitoring many spatial locations or modalities roduces little or no interference over monitoring a single modality, unless a target ccurs (Duncan, 1980). This finding supports the distinction between a general alert Late, and one in which attention is clearly oriented and engaged in processing ,formation. In the alert but disengaged state, any target of sufficient intensity has ttle trouble in summoning the mechanisms that produce detection. Thus monitoring iultiple modalities or locations produce only small amounts of interference. The nportance of engaging the focal attention system in the production of widespread iterference between signals supports the idea that there is a unified system