Patients with schizophrenia are profoundly impaired in perceptual closure as indicated by both impaired performance and impaired N(cl) generation. The selective impairment in dorsal stream P1 is consistent with prior reports of impaired magnocellular processing in schizophrenia. By contrast, intact ventral N1 generation suggests that the initial stages of ventral stream processing are relatively preserved and that impaired magnocellular dorsal stream functioning in schizophrenia may lead to secondary dysregulation of ventral stream object recognition processing.

BackgroundThe NeuroTrax Mindstreams computerized cognitive assessment system was designed for widespread clinical and research use in detecting mild cognitive impairment (MCI). However, the capability of Mindstreams tests to discriminate elderly with MCI from those who are cognitively healthy has yet to be evaluated. Moreover, the comparability between these tests and traditional neuropsychological tests in detecting MCI has not been examined.MethodsA 2-center study was designed to assess discriminant validity of tests in the Mindstreams Mild Impairment Battery. Participants were 30 individuals diagnosed with MCI, 29 with mild Alzheimer's disease (AD), and 39 healthy elderly. Testing was with the Mindstreams battery and traditional neuropsychological tests. Receiver operating characteristic (ROC) analysis was used to examine the ability of Mindstreams and traditional measures to discriminate those with MCI from cognitively healthy elderly. Between-group comparisons were made (Mann-Whitney U test) between MCI and healthy elderly and between MCI and mild AD groups.ResultsMindstreams outcome parameters across multiple cognitive domains significantly discriminated among MCI and healthy elderly with considerable effect sizes (p < 0.05). Measures of memory, executive function, visual spatial skills, and verbal fluency discriminated best, and discriminability was at least comparable to that of traditional neuropsychological tests in these domains.ConclusionsMindstreams tests are effective in detecting MCI, providing a comprehensive profile of cognitive function. Further, the enhanced precision and ease of use of these computerized tests make the NeuroTrax system a valuable clinical tool in the identification of elderly at high risk for dementia.

Object recognition is achieved even in circumstances when only partial information is available to the observer. Perceptual closure processes are essential in enabling such recognitions to occur. We presented successively less fragmented images while recording high-density event-related potentials (ERPs), which permitted us to monitor brain activity during the perceptual closure processes leading up to object recognition. We reveal a bilateral ERP component (N(cl)) that tracks these processes (onsets approximately 230 msec, maximal at approximately 290 msec). Scalp-current density mapping of the N(cl) revealed bilateral occipito-temporal scalp foci, which are consistent with generators in the human ventral visual stream, and specifically the lateral-occipital or LO complex as defined by hemodynamic studies of object recognition.

Selective attention operates in visual cortex by facilitating processing of selected stimuli and by filtering out unwanted information from nearby distracters over circumscribed regions of visual space. The neural representation of unattended stimuli outside this focus of attention is less well understood. We studied the neural fate of unattended stimuli using functional magnetic resonance imaging by dissociating the activity evoked by attended (target) stimuli presented to the periphery of a visual hemifield and unattended (distracter) stimuli presented simultaneously to a corresponding location of the contralateral hemifield. Subjects covertly directed attention to a series of target stimuli and performed either a low or a high attentional-load search task on a stream of otherwise identical stimuli. With this task, target-search-related activity increased with increasing attentional load, whereas distracter-related activity decreased with increasing load in areas V4 and TEO but not in early areas V1 and V2. This finding presents evidence for a load-dependent push-pull mechanism of selective attention that operates over large portions of the visual field at intermediate processing stages. This mechanism appeared to be controlled by a distributed frontoparietal network of brain areas that reflected processes related to target selection during spatially directed attention.

Integrity of early visual sensory processing in schizophrenia was assessed using the well characterized P1 and N1 components of the visual evoked potential (VEP) as our dependent measures. VEPs were recorded in response to successively less fragmented line drawings of common objects. P1 amplitudes were significantly reduced across all stimulus conditions for patients versus controls. Further, this decrement was relatively greater at parieto-occipital than occipito-temporal electrode sites. No differences in N1 amplitude were found. The finding of P1 deficits in patients, particularly over dorsal scalp, supports the view that schizophrenia is associated with impairment of early dorsal visual stream processing. On the other hand, the finding of normal N1 amplitudes in patients suggests that early stages of ventral stream processing may be relatively more intact. These results imply that the cognitive impairment seen in schizophrenia is not just due to deficits in higher order aspects of cognition but also encompasses significant deficits in early sensory processing.