The maps of visual and auditory space within the superior colliculus are in approximate register both with each other and with the underlying motor maps associated with orienting responses. The fact that eyes and ears can move independently poses a problem for the sensorimotor organization of these two modalities. By monitoring eye and pinna positions in alert, head-fixed cats, we showed that the accuracy of saccadic eye movements to auditory targets was little affected by eye eccentricity (range +/- 15 deg) at the onset of the sound. A possible neural basis for this behavioral compensation was suggested by recordings from superior colliculus neurons. The preferred sound directions of some neurons in the deep layers of this midbrain nucleus exhibited a shift with the direction of gaze, while in others the response throughout the auditory receptive field was either increased or decreased, suggesting that changes in eye position alter the gain of the auditory response.

investigation in Alzheimer's disease (AD) has indicated that the concentration of neurofibrillary tangles in inferotemporal cortex (IT) is greater than that found in posterior parietal cortex (PPC). Researchers hypothesized that the relative degree of impairment of visual function subserved by each of these cortical areas should reflect the disproportionate distribution of neuropathological changes. Eleven AD patients and 16 elderly controls received 8 tests of visual function, 4 of which have been shown previously to be selectively affected by IT lesions and 4 that are selective for PPC lesions. AD patients were significantly impaired on all 8 tests, but multivariate analysis indicated a relatively greater impairment on tests of IT function. The greater impairment of visual function mediated by IT relative to function mediated by PPC is consistent with differential degradation of the respective cortical areas. Several types of visual impairments are reported to occur in the early and intermediate stages of Alzheimer's disease (AD; for review, Katz & Rimmer, 1989; Mendez, Tomsak, & Remler, 1990). Unlike memory impairment, which is an obligatory correlate of AD, visual impairments vary in prevalence and severity (Mendola, Cronin-Golomb, Corkin, & Growdon, 1995). Cases range from individuals who are free from any observable dysfunction to those who manifest Balint's syndrome (Hof, Bouras, Constantinidis, & Morrison, 1989), visual agnosia (Katz & Rimmer, 1989), or simultanagnosia (Trobe & Bauer, 1986). The heterogeneity of visual function likely reflects individual differences in the regional distribution of neuropathological changes. This idea is supported indirectly by positron emission tomography (PET) glucose metabolism studies (Grady et al., 1993) and by direct histological examination of AD brains

The pattern of cerebral degeneration in Alzheimer's disease (AD) patients suggests that basic auditory capacities should be normal in AD, whereas progressively higher levels of auditory function should be increasingly impaired. To test this hypothesis, we administered tests of auditory capacities associated with primary auditory cortex (sound localization and perception of complex tones) and auditory association cortex (phoneme discrimination, timbre discrimination, and tonal memory) to 19 mildly to moderately demented AD patients, 21 elderly control subjects (ECS), and 14 young control subjects (YCS). The results showed significant differences between YCS and ECS on phoneme discrimination with synthetic speech and on tonal memory. The AD group differed from the ECS group on sound localization, one measure of synthetic speech discrimination, and timbre discrimination. Performance did not correlate with age, dementia severity, or duration of illness on any test condition. These findings indicate that although AD is accompanied by specific auditory deficits, the increase in neuropathologic change between primary auditory and auditory association cortices is not reflected in an increased impairment of functions that are mediated by these areas. Degraded aural language comprehension, which is characteristic of AD, likely reflects disruption of language processes, rather than dysfunction specific to auditory circuits.

An investigation was made of the time course of perceptual grouping that is based on two qualitatively different spatial relationships: proximity and alignment. An index of grouping capacity was used to assess the processing time required before a backward pattern mask interfered with grouping. Stimuli consisted of bistable arrays of disjunct dots that were followed by a mask. Grouping cues, either proximity or alignment, were randomly assigned to either the horizontal or vertical orientation, and subjects indicated whether the dots appeared grouped as a series of horizontal or vertical lines. Spatial metrics of the cues were systematically altered until they no longer served as a cue for grouping, thereby determining the grouping threshold. The stimulus onset asynchrony (SOA)of the mask, relative to the test stimulus, ranged from 33.3 to 150 msec. The SOAat which grouping thresholds first became elevated identified the point at which the mask first interfered with the grouping process, thereby identifying the processing time required for grouping by the specified cue. The processing time for grouping by proximity and alignment differed significantly, requiring means of 87.6and 118.8msec, respectively, for processing to be completed. These measurements serve to identify the processing time necessary for spatially integrating stimulus elements into unified forms, thereby delineating temporal constraints at this stage of visual processing.In order to organize complex stimuli in preparation for object recognition, the visual system quickly and automatically associates and segregates elements of the visual scene. To accommodate novel stimuli with speed and accuracy, the process ofgrouping likely follows established algorithms that mediate grouping strategies. These algorithms operate by extracting relevant information from the stimuli and then applying a set ofcriteria to that information which specify the grouping arrangement best suited for form identification. Although many of the characteristics associated with perceptual grouping have been described, the principles by which grouping functions, and the time course of its operation, are not well understood.Grouping strategies are based on several characteristics of stimuli, including feature similarities, such as color or element shape, and spatial relationships, such as element proximity or regularity. The present study examined grouping that is based on two spatial relationships among stimulus elements: element proximity and element alignment. In each case, a single spatial parameter was systematically altered while observers made judgments about the manner in which elements appeared to be grouped. In this way, the association between a specific spatial metric and the process of grouping was isolated.Grouping by element proximity and element alignment differ qualitatively in the nature of the spatial cue from which grouping is derived. Grouping by proximity (Rush, 1937) is based on the tendency for more proximal images to produce unified configurations, and for...