Invalid Targets Research Articles

Localization of spatial attention processes with the aid of a probe technique

A sudden visual onset is thought to `attract attention to its location' within less than 100 ms. We attempted to measure the effect of this attentional process on the event-related potential (ERP) to a probe presented about 140 ms after the onset, and to delineate the spatio-temporal characteristics of such an effect, if any. ERPs were recorded from 30 channels from 6 subjects while they performed a target detection task. Both targets and probes could be located in each of the 4 quadrants (eccentricities 6.1° and 7°, respectively). For a given single target, the subsequent probe was either presented near the location of the target (`valid target') or at the diagonal opposite (`invalid target'). Appropriate `neutral' conditions (probes preceded by no target, or by simultaneous targets in all quadrants) were applied, and ERPs to probes were corrected for the contribution of the ERPs to targets. The earliest effect of (in)validity was found at about 120 ms after probe onset for lower field probes. This effect consisted of enhanced posterior positivity for valid relative to neutral relative to invalid conditions. This positivity was superposed on a contralateral, extrastriate negative ongoing wave peaking at about 150 ms (`N150'). Source localization suggested that the (in)validity effects originate from deep medial parietal areas. The source corresponding to the N150 activity was not influenced by (in)validity. An earlier deflection to the probe at 80 ms (`NP80') depended on location, but not on (in)validity, and seemed to be of striate origin. Results are discussed in terms of a model postulating an attention-independent `input module' from which activation is fed to a `location module' embodying the actual attention mechanism.

Spatial- and object-based attentional deficits in Alzheimer's disease. Relationship to HMPAO-SPECT measures of parietal perfusion.

The purpose of this study was to investigate the ability of patients with Alzheimer's disease to shift attention between spatial locations and between objects, and to examine the brain regions involved in these cognitive operations using single photon emission computed tomography (SPECT) imaging. A recent study of patients with focal lesions provided evidence that the right and left parietal lobes are differentially involved in shifting selective attention from invalidly cued spatial locations and objects, respectively (Egly et al. J Exp Psychol Gen 1994; 123: 161-77). Accordingly, in Alzheimer's disease patients, we hypothesized that right parietal hypoperfusion on SPECT would be associated with deficits on the spatial-based component of a cued reaction time task, and left parietal hypoperfusion would be associated with the deficits on the object-based component. Attentional performance of Alzheimer's disease patients (n = 29) was compared with aged-matched normal controls (n = 17) using a cued reaction time task based on Egly et. al. (1994). Regions of interest were defined semi-automatically on SPECT, and were anatomically localized with the aid of co-registered MRI. As hypothesized, in Alzheimer's disease patients, reaction time costs of invalid targets eliciting shifts of attention between spatial locations were selectively correlated with SPECT hypoperfusion in the right superior parietal lobe; while reaction time costs of between-object shifts of attention were correlated with hypoperfusion in the left inferior parietal lobe. These results provide evidence for the specialized roles of the right and left parietal regions in the spatial and object components of attentional shifting respectively, and suggest that the cognitive profile associated with Alzheimer's disease includes both spatial- and object-based attentional impairments.

Covert visual attention in schizophrenia spectrum personality disordered subjects: Visuospatial cuing and alerting effects

A recent study observed lateralized deficit in the disengagement of covert visual attention in schizophrenic patients. Subsequent attempts to replicate this finding have had mixed results. Differences in the neuroleptic treatment or other secondary factors associated with schizophrenia are some of the possible reasons for these inconsistent findings. In this study, we examined the ability to shift covert visual attention in neuroleptic-naive, schizophrenia spectrum personality disordered (SSPD; n = 35) subjects and normal controls ( n = 34) under a variety of spatial cuing and alerting conditions. We hypothesized that SSPD subjects would have difficulty with disengagement of covert visual attention from an invalidly cued left visual field when the target appeared in the right visual field in comparison to the normal subjects. As predicted, schizophrenia spectrum personality disordered subjects had significantly longer latencies for the right visual field invalid targets than normals ( p = .014). Under the remaining cue conditions, spectrum subjects performed normally. Consequently, the cost of left visual field invalid cueing for the right visual field target was significantly higher in spectrum personality subjects than in normals. The cost for the invalid right visual field cue and the benefits of valid cue in both fields were very similar in the two groups. The findings of an asymmetrical deficit in the disengagement and shift of covert visual attention in schizophrenia spectrum subjects are similar to the one's observed in patients with unilateral left hemisphere lesions.

Electrophysiologic correlates of age effects on visuospatial attention shift

The effects of age on the neural processes underlying shifts in visuospatial attention were studied by non-invasive recording of brain electrical activity. Event-related evoked potentials (ERPs) were recorded in young (age 28–36 years old) and elderly (59–74 years old) subjects during a target detection task. Targets were preceded by central or peripheral informative cues, which are hypothesized to provoke voluntary and reflexive shifts of spatial attention, respectively. There were no age-related differences in the effects of cue validity on reaction times. Elderly subjects had longer reaction times to both the valid and invalid targets. The ERP recordings demonstrated that attention shift-related negativities (ARNs) were generated over the hemisphere contralateral to the central cue direction with the same amplitude in young and elderly subjects. However, the onset latency was delayed for the elderly at anterior scalp sites. Peripheral cues enhanced the N1 component (140–200 ms post-cue) over the contralateral hemisphere comparably in the two age groups. The ARNs following N1 enhancement demonstrated delayed onset and restricted scalp distributions in the elderly group compared with the young group. These results suggest that resistance of cue validity effects to aging is consistent with existence of the N1 enhancement and ARNs, even in elderly subjects. Different age effects on ERPs generated by central and peripheral cues support the notion that voluntary and reflexive mechanisms of attention shift involve distinct neural systems with different vulnerability to aging.

The feasibility of Doppler sonar fish counting

In the course of monitoring and evaluating fish population in the seas, sonar systems have proved a practical and efficient measurement approach. The results obtained, however, from the use of sonar for monitoring fish migration in rivers have been somewhat disappointing-the most troublesome problems being the inability to recognize invalid targets. It has been proposed [1] that a high-resolution Doppler sonar which recognizes a valid target on the basis of its Doppler signature would be a solution to this problem. This paper examines the feasibility of such a target identification scheme. In particular, an examination is made of the nature of returns to be expected from a fish, and of interference sources-principally surface reverberation. From this it is concluded that the Doppler approach is indeed feasible, but that the use of a high-resolution pulsed system capable of separating multiple targets is only possible in a channel width of a few meters.