Electrophysiological correlates of recollecting faces of known and unknown individuals.

We recorded brain potentials from healthy human subjects during a recognition test in order to monitor neural processing associated with face recollection. Subjects first attempted to memorize 40 faces; half were accompanied by a voice simulating that person speaking (e.g., "I'm Jimmy and I was a roadie for the Grateful Dead") and half were presented in silence. In the test phase, subjects attempted to discriminate both types of old faces (i.e., "named" and "unnamed" faces) from new faces. Recognition averaged 87% correct for named faces, 74% correct for unnamed faces, and 91% correct for new faces. Potentials to old faces were more positive than those to new faces from 300 to 600 ms after face onset. For named faces, the old-new ERP difference was observed at anterior and posterior scalp locations. For unnamed faces, the old-new ERP difference was observed only at posterior scalp locations. Results from a prior experiment suggest that these effects do not reflect perceptual priming of faces. The posterior portion of the old-new ERP difference was thus interpreted as a neural correlate of retrieval of visual face information and the anterior portion as an indication of retrieval of person-specific semantic information.

Brain waves following remembered faces index conscious recollection.

At a glance, one can often determine whether a face belongs to a known individual. To investigate brain mechanisms underlying this memory feat, we recorded EEG signals time-locked to face presentations. In the study phase, 40 unknown faces were presented, 20 of which were accompanied by a voice simulating that person speaking. Instructions were to remember the faces with spoken biographical information (R-faces) and to forget the others (F-faces). In the test phase, famous and non-famous faces were presented in a visually degraded manner. Subjects made two-choice fame judgments and priming was observed in the form of faster and more accurate responses for old than for new non-famous faces. Priming did not differ between R-faces and F-faces. In a second experiment, faces were not degraded at test and behavioral responses were made only when faces were presented twice in immediate succession. Brain potentials elicited 300 to 900 ms after stimulus onset from frontal and parieto-occipital scalp regions were larger for R-faces than for F-faces. Recognition tested later was more accurate for R-faces than for F-faces. Because the study-phase manipulation influenced recognition but not priming, we conclude that this procedure succeeded in isolating neural correlates of recollective processing from more automatic uses of face memory as indexed by priming.

The Interaction of Spatial and Object Pathways: Evidence from Balint's Syndrome.

An earlier report described a patient (RM) with bilateral parietal damage who showed severe binding problems between shape and color and shape and size (Friedman-Hill, Robertson, & Treisman, 1995). When shown two different-colored letters, RM reported a large number of illusory conjunctions (ICs) combining the shape of one letter with the color of the other, even when he was looking directly at one of them and had as long as 10 sec to respond. The lesions also produced severe deficits in locating and reaching for objects, and difficulty in seeing more than one object at a time, resulting in a neuropsychological diagnosis of Balint's syndrome or dorsal simultanagnosia. The pattern of deficits supported predictions of Treisman's Feature Integration Theory (FIT) that the loss of spatial information would lead to binding errors. They further suggested that the spatial information used in binding depends on intact parietal function. In the present paper we extend these findings and examine other deficits in RM that would be predicted by FIT. We show that: (1) Object individuation is impaired, making it impossible for him correctly to count more than one or two objects, even when he is aware that more are present. (2) Visual search for a target defined by a conjunction of features (requiring binding) is impaired, while the detection of a target defined by a unique feature is not. Search for the absence of a feature (0 among Qs) is also severely impaired, while search for the presence (Q among 0s) is not. Feature absence can only be detected when all the present features are bound to the nontarget items. (3) RM's deficits cannot be attributed to a general binding problem: binding errors were far more likely with simultaneous presentation where spatial information was required than with sequential presentation where time could be used as the medium for binding. (4) Selection for attention was severely impaired, whether it was based on the position of a marker or on some other feature (color). (5) Spatial information seems to exist that RM cannot access, suggesting that feature binding relies on a relatively late stage where implicit spatial information is made explicitly accessible. The data converge to support our conclusions that explicit spatial knowledge is necessary for the perception of accurately bound features, for accurate attentional selection, and for accurate and rapid search for a conjunction of features in a multiitem display. It is obviously necessary for directing attention to spatial locations, but the consequences of impairments in this ability seem also to affect object selection, object individuation, and feature integration. Thus, the functional effects of parietal damage are not limited to the spatial and attentional problems that have long been described in patients with Balint's syndrome. Damage to parietal areas also affects object perception through damage to spatial representations that are fundamental for spatial awareness.

Role of human prefrontal cortex in attention control.

Without a functioning dorsolateral prefrontal cortex, humans are stimulus bound and have little confidence in their ability to interact with the environment. Deficits in inhibitory control of external and internal processes coupled with impaired temporal coding of stimuli and detection capacity for novel events leave the patient functioning in a noisy internal environment without critical spatiotemporal cues. Some of these proposals are similar to those of Nauta (104). Based on connectivity of the prefrontal cortex, Nauta suggested that this region was ideally suited to generate and evaluate internal models of action. It is proposed that, in addition to this generation function, the prefrontal cortex is crucial for detecting changes in the external environment and for discriminating internally and externally derived models of the world. This chapter has described a cascade of deficits that result from damage to the dorsolateral prefrontal cortex. Awareness of the sensory world, and of the apparent stream of internal and external events, is impaired by deficits in novelty detection. Changes in the world, internal or external, may not be noticed in a noisy internal milieu. These deficits contribute to impaired reality monitoring and to a subsequent lack of confidence in behavior. An inability to bridge temporal gaps and temporally sequence internal events, together with deficits in inhibitory control systems, contribute to an impairment in the ability to generate coherent representations of alternate or counterfactual realities.

Preattentive processes guide visual search: evidence from patients with unilateral visual neglect.

Abstract Preattentive processes such as perceptual grouping are thought to be important in the initial guidance of visual attention and may also operate in unilateral neglect by contributing to the definition of a task-appropriate reference frame. We explored this question with a visual search task in which patients with unilateral visual neglect (5 with right-, 2 with left-hemisphere damage) searched a diamond-shaped matrix for a conjunction target that shared one feature with each of two distractor elements. Additional grouping stimuli appeared as flanks either on the left, right, or both sides of the central matrix, and significantly changed performance in the search task. As expected, when flanks appeared only on the ipsilesional side a decrement in search performance was observed, but the further addition of contralesional flanks actually reduced the decrement and returned performance to near baseline levels. These data suggest that flanking stimuli on the neglected contralesional side of visual space can influence the reference frame by grouping with task-relevant stimuli, and that this preattentive influence can be preserved in patients with unilateral visual neglect.