Neglect 15 In macague monkeys,lesions to the frontal peri then be associated with greater contralesional arcuate areas and around the inferior parietal lobule neglect since the"what"and"where"of contrale- result in neglect.at least transiently (Duel.1987 sional obiects are no longer conioined.This Milner 1987)These monkeys are more likely to study highlights the difficulties in establishing the orient toward and act on stimuli in ipsilesiona appropriate homology between the monkey and space.Single-cell recordings of neurons arounc parietal cortex.While the intraparietal sulcus and prefrontal cortices neolect in humans is as (reviewed later)suggest that these regions are crit with lesions to the inferior posterior parietal co ical in the maintenance of spatial representations Brodmann's areas 39 and 40.it is not clear which. and preparation for actions directed at ecific loca if any regions.are the appropriate monkey analog tion 、From this one would expe ect that lesion to these areas these would produce neglec in Finally.Gaffan and Hornak (1997)emphasize in monke They find that beha obvio with eglect is esions which see at which human lesi tract les ad optic trac les owing way emote ing to be associa his 987 phere acquire glucos e commissure each en nuclei ithout access to ducing 3 did ne sed glucos abolism in the e pulvinar and opu lateral posterior thalamic nucle and in the tion about contral ona space is acquir deeper layers of the superior colliculus.It is inter the nonlesioned hemisphere becau with multiple est ing that recovery in these animals is also as ocular fixations.objects in contra space sociated with recovery of these remote some mes fall on the ipsilesional side of fixatio abnormalities.This idea that distributed abnormali The idea that short-term memories of contralesional ties are needed to produce neglect is reiterated in stimuli influence spatial behavior had not been con- a more recent study by Gaffan and Hornak (1997). sidered previously in animal models. They found in monkeys that transecting white matter tracts underlving the posterior parietal cortex Single-Cell Neurophvsiological Studies was important in producing more persistent neglect. Watson and colleagues (Watson.Valenstein Single-cell neurophysiological studies record the Day.Heilman.1994)reported that damage to activity of neurons in animals.often monkeys that monkeys'superior temporal sulcus produced more are engaged in various perceptual,motor,or cogni- profound neglect than damage to the inferior pari- tive tasks.These studies support the idea that etal lobule.They suggest that the superior temporal neurons in parietal and frontal association cortices sulcus in the monkey may serve as an importan mediate spatial attention and representations.These convergence zone for processing both the dorsal and neurons form a distributed network dedicated to a the ventral visual streams integrating the "where" variety of spatial behaviors,including attention and and"what"of obiects.Damage to this area might intention regarding spatial locations.memory ofIn macaque monkeys, lesions to the frontal peri￾arcuate areas and around the inferior parietal lobule result in neglect, at least transiently (Duel, 1987; Milner, 1987). These monkeys are more likely to orient toward and act on stimuli in ipsilesional space. Single-cell recordings of neurons around the intraparietal sulcus and prefrontal cortices (reviewed later) suggest that these regions are crit￾ical in the maintenance of spatial representations and preparation for actions directed at specific loca￾tions. From this, one would expect that lesions in these areas would produce profound neglect in animals. Yet such cortical lesions produce only mild and transient neglect (Milner, 1987). If anything, biased behavior seems more obvious with frontal lesions, which seems at odds with human lesion studies in which posterior lesions are associated more often with neglect. In monkeys, cortical lesions with remote meta￾bolic abnormalities are more likely to be associated with neglect (Duel, 1987). Frontal lesions produc￾ing neglect are associated with decreased glucose utilization in the caudate nucleus and the ventral anterior and dorsomedial thalamic nuclei. Parietal lesions producing neglect are associated with decreased glucose metabolism in the pulvinar and the lateral posterior thalamic nuclei and in the deeper layers of the superior colliculus. It is inter￾esting that recovery in these animals is also as￾sociated with recovery of these remote metabolic abnormalities. This idea that distributed abnormali￾ties are needed to produce neglect is reiterated in a more recent study by Gaffan and Hornak (1997). They found in monkeys that transecting white matter tracts underlying the posterior parietal cortex was important in producing more persistent neglect. Watson and colleagues (Watson, Valenstein, Day, & Heilman, 1994) reported that damage to monkeys’ superior temporal sulcus produced more profound neglect than damage to the inferior pari￾etal lobule. They suggest that the superior temporal sulcus in the monkey may serve as an important convergence zone for processing both the dorsal and the ventral visual streams integrating the “where” and “what” of objects. Damage to this area might then be associated with greater contralesional neglect since the “what” and “where” of contrale￾sional objects are no longer conjoined. This study highlights the difficulties in establishing the appropriate homology between the monkey and the human posterior temporoparietal cortex. While neglect in humans is associated most commonly with lesions to the inferior posterior parietal cortex, Brodmann’s areas 39 and 40, it is not clear which, if any regions, are the appropriate monkey analog to these areas. Finally, Gaffan and Hornak (1997) emphasize the importance of memory in monkeys’ behavioral manifestations of overt neglect. They find that neglect is associated with complete commissuro￾tomy and optic tract lesions, but not with isolated optic tract, parietal, or frontal cortex lesions. They interpret this finding in the following way: Section￾ing the optic tract makes one hemisphere blind to visual information. This hemisphere acquires visual information from the other hemisphere through interhemispheric commissures. If each hemisphere maintains a representation of contralateral space, then a monkey without access to information about contralesional space will act as if this space did not exist. With an isolated optic tract lesion, informa￾tion about contralesional space is acquired through the nonlesioned hemisphere because with multiple ocular fixations, objects in contralesional space sometimes fall on the ipsilesional side of fixation. The idea that short-term memories of contralesional stimuli influence spatial behavior had not been con￾sidered previously in animal models. Single-Cell Neurophysiological Studies Single-cell neurophysiological studies record the activity of neurons in animals, often monkeys that are engaged in various perceptual, motor, or cogni￾tive tasks. These studies support the idea that neurons in parietal and frontal association cortices mediate spatial attention and representations. These neurons form a distributed network dedicated to a variety of spatial behaviors, including attention and intention regarding spatial locations, memory of Neglect 15