Dorsal simultanagnosia: An impairment of visual processing or visual awareness?

When presented with two objects patients with simultanagnosia show a marked impairment at naming both items. This has led many authors to conclude that the second item is not being processed (e.g., Robinson, 2003). However, this deficit may instead reflect a deficit with explicit, or conscious report. We investigated this issue using a semantic priming paradigm that allowed us to assess implicit processing of the second "unseen" item. We presented a patient, with bilateral parietal damage, with pairs of pictures that were either from the same or a different semantic category. The patient was asked to either classify one of the pictures or to name both pictures. When the items were from different categories the patient's classification performance was significantly poorer than when they were from the same category, even though he could rarely explicitly report both items. These findings are consistent with the notion that the meaning of the "unseen" item influenced the reporting of the "seen" item. Consequently, the deficit seen in this patient does not seem to reflect an inability to process more than one item simultaneously but rather a deficit in explicitly identifying multiple items.     

Brain potentials in a memory-scanning task. III. Potentials to the items being memorized

Cerebral potentials evoked by items presented for memorization in a memory-scanning task were recorded from subjects ranging in age from 18 to 86 years old. Subjects were divided into younger (average age = 29 years) and older groups (average age = 66 years). Both verbal (digits) and non-verbal (musical notes) stimuli were used. Digits were presented in the auditory as well as the visual modality, and notes were presented acoustically. Potentials are described in terms of their scalp distribution, latency, and amplitude and are compared between the young and old subjects. Potentials evoked by the memorized items consisted of a positive (P50-90), negative (N100-150), positive (P185-225) sequence in the first 250 msec following stimulus onset. A sustained potential shift then followed whose amplitude differed with the items being memorized. The shift was positive in the parietal region being largest (5 microV) with verbal items presented visually and slightly smaller (3 microV) with non-verbal auditory stimuli (the notes); in contrast, verbal auditory digits were not associated with a detectable sustained parietal potential shift. In the frontal recordings there was a sustained potential shift accompanying all stimulus types, which was more negative in the young subjects. The amplitude of these sustained potential shifts differed as a function of the position of the item in the memorized set. These results provide electrophysiological evidence of brain activity during memorization that varies with the items being processed as well as differing between young and old subjects.     

Changes in neuronal activity related to the repetition and relative familiarity of visual stimuli in rhinal and adjacent cortex of the anaesthetised rat

Employing the same techniques as have been used with conscious rats, this study describes neuronal responses signalling information concerning the prior occurrence of visual stimuli in unconscious rats. Recordings of the activity of 387 neurones were made while anaesthetised rats were shown objects. Changes in neuronal responses related to stimulus repetition and the relative familiarity of visual stimuli were sought. The areas sampled were lateral occipital cortex, area TE of temporal cortex, perirhinal cortex and the hippocampal formation. The response to the first presentations of unfamiliar objects was significantly different from that to their second presentations for 30 (35%) of 86 visually responsive neurones; for 23 of the neurones was smaller when the stimulus was repeated, whereas for 7 it was larger. For all of these neurones the response change was maintained across intervening trials on which other stimuli were shown. For 4 (25%) of 16 neurones so tested, the response decrement persisted across at least 10 intervening trials. The activity of 63 neurones was recorded while rats were shown highly familiar as well as unfamiliar objects. The response to unfamiliar objects was significantly different from that to highly familiar objects for 3 (23%) of 13 visually responsive neurones. The types of neuronal response and their incidence expressed as a proportion of the number of visually responsive neurones were similar to those found in unanaesthetised rats (though the proportion of visually responsive neurones encountered in the anaesthetised rat was lower). The results indicate that information concerning the prior occurrence of stimuli is processed even under anaesthesia.     

Event-related potential correlates of the retrieval of emotional and nonemotional context

In two experiments, we examined event-related potentials (ERPs) elicited in an old/new recognition memory test by emotionally neutral visual objects that, at encoding, had been associated with neutrally, negatively, or positively valenced background contexts. In Experiment 2, subjects also judged the context in which the item had been studied. In Experiment 1, "left parietal" old/new ERP effects were elicited by correctly recognized items. Items encoded in emotional contexts, but not those studied in neutral contexts, elicited additional effects early in the recording epoch over lateral temporal scalp and, later, over left temporo-frontal scalp. In Experiment 2, "left parietal" and "right frontal" ERP effects were elicited by recognized items that attracted correct source judgments. Additional effects, an early lateral temporal positivity and a late-onset, left-sided positivity, were elicited by items studied in emotionally valenced contexts and attracting correct source judgments. Together, the findings indicate that retrieval processing is influenced by the emotional valence of the context in which an item is encoded, regardless of whether contextual information is task relevant.     

Appearing and disappearing stimuli trigger a reflexive modulation of visual cortical activity

From the vast array of stimuli continually inundating our senses, only a very small portion is selected for higher-order processing. This selection is influenced by voluntary and reflexive mechanisms that may act at multiple stages of analysis. Extensive research has revealed that top-down voluntary mechanisms modulate information processing at both "early" (e.g., perceptual) and "late" (e.g., semantic) stages. Bottom-up sensory-driven mechanisms, however, are less well understood. Previous investigations of bottom-up mechanisms may have been influenced by top-down mechanisms because the stimuli were task-relevant and required overt responses. Here, we directly measured bottom-up influences on visual information processing by recording event-related brain potentials (ERP) to sequences of task-irrelevant visual stimuli. We found that abrupt visual events triggered an automatic enhancement of extrastriate visual activity (the P1 ERP component) to subsequent visual stimuli occurring at the same location. In contrast to theories suggesting that the abrupt appearance of a new object is unique in being able to trigger bottom-up effects, we found that disappearing objects triggered the same enhancement of subsequent stimulus processing as did appearing objects. The present data, however, also provide new electrophysiological evidence for a level of analysis in the brain that may be specific to the appearance of new objects. These data thus provide evidence that abruptly appearing objects may evoke specialized processing at certain stages of analysis in the brain but that, despite this difference, appearing and disappearing objects both trigger reflexive mechanisms that bias neural activity in human extrastriate visual cortex.     

Simultanagnosia: when a rose is not red

Abstract Information regarding object identity ("what") and spatial location ("where/how to") is largely segregated in visual processing. Under most circumstances, however, object identity and location are linked. We report data from a simultanagnosic patient (K.E.) with bilateral posterior parietal infarcts who was unable to "see" more than one object in an array despite relatively preserved object processing and normal preattentive processing. K.E. also demonstrated a finding that has not, to our knowledge, been reported: He was unable to report more than one attribute of a single object. For example, he was unable to name the color of the ink in which words were written despite naming the word correctly. Several experiments demonstrated, however, that perceptual attributes that he was unable to report influenced his performance. We suggest that binding of object identity and location is a limited-capacity operation that is essential for conscious awareness for which the posterior parietal lobe is crucial.     

Coding space within but not between objects: evidence from Balint's syndrome

The ability to make spatial judgements was examined in a patient demonstrating poor perception of multiple objects following bilateral parietal lesions, under conditions in which the presence of the stimuli to which judgements were made could be detected. The tasks required judgements of spatial length or the position of coloured parts of stimuli. We manipulated the degree to which two uprights in a display could be encoded into a single perceptual object using either stored knowledge or bottom-up cues based on 2D or 3D image relations. Performance was dependent on the presence of both bottom-up grouping and familiarity. However, connectedness in the image was not sufficient to benefit performance, when stimuli were separate objects in 3D space. This deficit in spatial judgements, arising following detection of the relevant stimulus elements, is attributed to an impairment in coding the spatial relations between separate perceptual objects. This deficit could be overcome if stimuli could be grouped in 3D, using bottom-up cues and top-down knowledge.     

Visual event-related potentials index focused attention within bilateral stimulus arrays. II. Functional dissociation of P1 and N1 components.

Event-related potentials (ERPs) were recorded from 12 subjects as they attended to the left or right hemifield of a visual display while fixating a central point. Stimuli were presented to the left or right visual fields on separate trials (unilateral stimuli) or to both fields simultaneously (bilateral stimuli). In different conditions, the stimulus sequences contained only bilateral stimuli, only unilateral stimuli, or a mixture of unilateral and bilateral stimuli. Bilateral stimuli elicited an enhanced positivity lasting from about 75 to 250 msec that was largest at posterior electrode sites contralateral to the attended hemifield. The early phase of this attention-related positivity appeared to be an enhancement of the exogenous P1 component. In contrast, both the posterior P1 and N1 components were enhanced in response to attended unilateral stimuli. Moreover, the N1 attention effect was reduced when the preceding stimulus contained elements in the attended field. It was concluded that modulations of the N1 and P1 components in these experiments represent different aspects of visual spatial attention: N1 may represent the orienting of attention to a task-relevant stimulus, whereas P1 may represent a facilitation of early sensory processing for items presented to a location where attention is already focused.     

Neural correlates of memory for items and for associations: an event-related functional magnetic resonance imaging study

Although results from cognitive psychology, neuropsychology, and behavioral neuroscience clearly suggest that item and associative information in memory rely on partly different brain regions, little is known concerning the differences and similarities that exist between these two types of information as a function of memory stage (i.e., encoding and retrieval). We used event-related functional magnetic resonance imaging to assess neural correlates of item and associative encoding and retrieval of simple images in 18 healthy subjects. During encoding, subjects memorized items and pairs. During retrieval, subjects made item recognition judgments (old vs. new) and associative recognition judgments (intact vs. rearranged). Relative to baseline, item and associative trials activated bilateral medial temporal and prefrontal regions during both encoding and retrieval. Direct contrasts were then performed between item and associative trials for each memory stage. During en- coding, greater prefrontal, hippocampal, and parietal activation was observed for associations, but no significant activation was observed for items at the selected threshold. During recognition, greater activation was observed for associative trials in the left dorsolateral prefrontal cortex and superior parietal lobules bilaterally, whereas item recognition trials showed greater activation of bilateral frontal regions, bilateral anterior medial temporal areas, and the right temporo-parietal junction. Post hoc analyses suggested that the anterior medial temporal activation observed during item recognition was driven mainly by new items, confirming a role for this structure in novelty detection. These results suggest that although some structures such as the medial temporal and prefrontal cortex play a general role in memory, the pattern of activation in these regions can be modulated by the type of information (items or associations) interacting with memory stages.     

Event-related potential correlates of the serial position effect in short-term memory.

Event-related potential (ERP) correlates of the serial position effect in short-term memory were investigated using a memory scanning task. Nine normal young adults (18-39 years) indicated whether a probe item was a member of a previously presented 5-item memory set by pressing 1 of 2 reaction-time buttons. Three types of stimuli were used: verbal digits presented both auditorily and visually, and musical notes presented auditorily. The ERPs to the probes were separately averaged according to the serial position of the probe (1, 2, 3, 4 or 5) in the memory set. The ERPs to the memory set items in positions 1, 3 and 5 also were separately averaged. Both baseline-to-peak and average amplitudes of a late positive parietal potential to the probes were larger to probe items presented in the last position in the memory set than to probes presented in the middle positions (2, 3 and 4), showing a significant recency effect, but only for auditory digits. Reaction time reflected significant recency effects for both auditory digits and notes, but not for visual digits. Response accuracy (percent correct) showed a significant recency effect only for notes. For each stimulus type, both the baseline-to-peak and average amplitudes of a late frontal component to the memory set items became more negative (in the case of the visual digits, less positive) in the third and last serial position of the memory set compared to the first. These findings provide electrophysiological evidence of serial position effects in short-term memory, which, during memory scanning, are dependent on stimulus modality (auditory, visual) and type (verbal, non-verbal).     

Levels of processing for visual stimuli in an "extinguished" field.

Volpe et al. (Nature 282, 722, 1979 [19]) described an experimental study of four patients with parietal tumours who were able to judge whether two simultaneous stimuli were identical or different, even when they were unable to name the stimulus contralateral to their brain injury. We report the case of another patient, E.M., in whom we have investigated this phenomenon further. E.M. had undergone a right temporal lobectomy to prevent recurrent seizures. She could correctly name photographs of objects presented in isolation to either the left or right visual field, at 150 msec exposure (although she was impaired for single objects on the left at 10 msec exposures). She was able to judge correctly whether two simultaneous objects on the left and right had the same or different names, even though she was often unable to name the object on the left. These judgements remained above chance when same-name pairs of stimuli showed the same object but seen from two different viewpoints, or even when they showed visually dissimilar exemplars of the same name category. This implies that the patient based her same-different judgements on categorical information about the pair of objects, even though she was often unable to name the contralateral object.     

Altitudinal neglect

Previous clinical observations on patients with hemispatial neglect from unilateral hemispheric lesions suggest the brain's attentional mechanisms are organized along the horizontal dimensions of extrapersonal space. We now report a patient with Balint's syndrome caused by bilateral parieto-occipital infarctions, who demonstrated altitudinal neglect. On visual and tactile bisection of vertical rods, the patient consistently placed her mark well above the true midpoint, and this performance remained unchanged when the stimuli were simultaneously explored visually and tactually. She also showed altitudinal inattention in the visual modality by extinguishing the stimulus presented in the lower quadrants during double simultaneous stimulation across the horizontal meridian. These findings suggest that bilateral damage to the parietal lobes can lead to multimodal attentional and exploratory deficits along the vertical dimensions of extrapersonal space.     

The effects of study task on prestimulus subsequent memory effects in the hippocampus

Functional magnetic resonance imaging (fMRI) was employed to examine the effects of a study task manipulation on pre‐stimulus activity in the hippocampus predictive of later successful recollection. Eighteen young participants were scanned while making either animacy or syllable judgments on visually presented study words. Cues presented before each word denoted which judgment should be made. Following the study phase, a surprise recognition memory test was administered in which each test item had to be endorsed as “Remembered,” “Known,” or “New.” As expected, “deep” animacy judgments led to better memory for study items than did “shallow” syllable judgments. In both study tasks, pre‐stimulus subsequent recollection effects were evident in the interval between the cue and the study item in bilateral anterior hippocampus. However, the direction of the effects differed according to the study task: whereas pre‐stimulus hippocampal activity on animacy trials was greater for later recollected items than items judged old on the basis of familiarity (replicating prior findings), these effects reversed for syllable trials. We propose that the direction of pre‐stimulus hippocampal subsequent memory effects depends on whether an optimal pre‐stimulus task set facilitates study processing that is conducive or unconducive to the formation of contextually rich episodic memories. © 2015 Wiley Periodicals, Inc.     

The human medial temporal lobe processes online representations of complex objects

There has been considerable debate as to whether structures in the medial temporal lobe (MTL) support both memory and perception, in particular whether the perirhinal cortex may be involved in the perceptual discrimination of complex objects with a large number of overlapping features. Similar experiments testing the discrimination of blended images have obtained contradictory findings, and it remains possible that reported deficits in object perception are due to subtle learning in controls, but not patients. To address this issue, a series of trial-unique object "oddity" tasks, in which subjects selected the odd stimulus from a visual array, were administered to amnesic patients with either selective bilateral damage to the hippocampus or more extensive damage to MTL regions, including the perirhinal cortex. Whereas patients with damage limited to the hippocampus performed similarly to controls on all conditions, patients with perirhinal damage were significantly impaired when the task required discrimination between objects with a large number of features in common. By contrast, when the same stimuli could be discriminated using simple visual features, patients with perirhinal damage performed normally. These results are consistent with a theoretical view which holds that rostral inferotemporal cortical regions, including perirhinal cortex, represent the complex conjunctions of stimulus features necessary for both perception and memory of objects.     

Preattentive surface and contour grouping in Kanizsa figures: Evidence from parietal extinction

Visual extinction commonly occurs after unilateral, parietal brain damage and manifests in a failure to identify contralesional stimuli when presented simultaneously with other, ipsilesional stimuli - but full awareness for single stimulus presentations. However, extinction can be substantially reduced when preattentive grouping operations link fragmentary items across hemifields into a coherent object. For instance, one study demonstrated preserved access to bilateral stimulus segments when these could be grouped to form a Kanizsa square [Mattingley, J. B., Davis, G., & Driver, J. (1997). Preattentive filling-in of visual surfaces in parietal extinction. Science, 275, 671-674]. Here, we investigated the relative contributions of distinct object attributes to the spared access in Kanizsa figure completion in extinction, by systematically varying the degree to which bilateral surface filling-in and contour interpolations group disparate items. We demonstrate that surface information can substantially reduce extinction, whereas contour completions showed comparably smaller influences. In summary, such graded influences of object attributes support recurrent models of grouping, first, linking fragmentary parts into coherent surfaces and, second, interpolating the precise boundaries.     

Hemisphere specialisation and inter-hemispheric cooperation during a phonological task: Effect of lexicality as assessed by the divided visual field approach

In the presented experiment we explored the effect of lexicality on hemisphere specialisation and cooperation during a phonological task. The divided visual field (DVF) method with bilateral presentation (BVF) of redundant (identical) stimuli is considered an appropriate approach to assess inter-hemispheric cooperation (IHC). IHC is supposed to increase the efficiency of cognitive processes. Specifically, it has been shown that, compared to unilateral hemifield presentation, word processing is significantly more efficient if stimuli were presented under bilateral redundancy conditions. The performance enhancement during bilateral vs. unilateral presentation is called bilateral redundant gain (BRG). In the present experiment a DVF was used and participants were required to perform a rhyme detection task in two blocks, one in words and another one in pseudowords. Each item was presented in two different modes, one unilateral (right or left hemi-visual field) and another one bilateral (simultaneous redundant presentation). Unilateral trials allow one to study hemispheric specialisation, while bilateral redundant trials allow one to study inter-hemispheric cooperation. We obtained left hemisphere specialisation for both types of items (word, pseudoword). Moreover, words were more efficiently processed than pseudowords. Additionally, words were processed more efficiently in BVF than in unilateral presentation, inducing BRG. No similar effect was obtained for pseudowords. These results are discussed in respect to findings reported by other studies suggesting that hemispheric specialisation depends on lexicality. Moreover, compared to lexical decision tasks used in previous studies, the phonological task used in the present study seems to modulate the inter-hemispheric cooperation less.     

Restricted ocular exploration does not seem to explain simultanagnosia

One major function of parietal cortex is to direct our attention towards salient stimuli. The present data suggest that it also plays an important role in visual gestalt perception. Patients with simultanagnosia following lesions in this area are not able to extract the meaning of a visual scene whereas being perfectly able to recognise individual objects of this scene. We tested two patients with simultanagnosia with hierarchical Navon figures combined with eye movements recordings. The patients' performance allowed us to compare directly the scan paths in trials in which the global letter shape was recognised with trials in which the global letter shape was not recognised. We did not find any obvious differences in the eye movement pattern related to the two perceptual situations. The two patients did not show a significant problem in shifting their eyes (and thus possibly also their attentional focus) to all aspects of the complex visual stimulus when attempting to bind together the different elements of spatially distributed information. The results demonstrate that restricted ocular exploration cannot be the reason for the patients' inability to recognise the global shape of stimuli. Our data rather suggest a role of parietal cortex in visual gestalt perception that is beyond its role of directing attention towards relevant objects.     

Identical event-related potentials to target and frequent stimuli of visual oddball task recorded by intracerebral electrodes.

OBJECTIVE: The shape of visually elicited event-related potentials (ERP) of epileptic patients during their presurgical evaluation with intracerebral electrodes was investigated in the study. METHODS: Twenty intractable epileptic patients with depth electrodes at several intracranial locations in the frontal, temporal, parietal lobes, and in the amygdalo-hippocampal complex participated in the study. To evoke the ERP, a standard visual oddball task was used with target stimuli, and frequent non-habituated and habituated stimuli. The averaged responses of the 3 groups were superimposed and visually analyzed whether the shape appeared identical or non-identical. RESULTS: The EEG response to target and frequent stimuli was recorded in 660 intra-cerebral sites. In 88 sites (14 different patients) localized in the amygdala, parahippocampal gyrus, superior, middle, and inferior temporal gyri, fusiform and lingual gyri, sensorimotor cortex, prefrontal cortex, hippocampus, and cingulated gyrus, the identical ERPs to target and both groups of frequent stimuli were observed. In 442 sites located in the above listed structures, and in the basal ganglia and parietal cortex, the shape of the ERP differed from 0.3 to 0.47 s on after the stimulus. The remaining 130 sites did not yield the task-specific potential change. CONCLUSIONS: The existence of identical ERPs to target and frequent stimuli in the oddball task suggests that a part of mental operations underlying the brain engagement in this task is not dependent on the way of responding.     

Dissociable effects of top-down and bottom-up attention during episodic encoding

It is well established that the formation of memories for life's experiences-episodic memory-is influenced by how we attend to those experiences, yet the neural mechanisms by which attention shapes episodic encoding are still unclear. We investigated how top-down and bottom-up attention contribute to memory encoding of visual objects in humans by manipulating both types of attention during fMRI of episodic memory formation. We show that dorsal parietal cortex-specifically, intraparietal sulcus (IPS)-was engaged during top-down attention and was also recruited during the successful formation of episodic memories. By contrast, bottom-up attention engaged ventral parietal cortex-specifically, temporoparietal junction (TPJ)-and was also more active during encoding failure. Functional connectivity analyses revealed further dissociations in how top-down and bottom-up attention influenced encoding: while both IPS and TPJ influenced activity in perceptual cortices thought to represent the information being encoded (fusiform/lateral occipital cortex), they each exerted opposite effects on memory encoding. Specifically, during a preparatory period preceding stimulus presentation, a stronger drive from IPS was associated with a higher likelihood that the subsequently attended stimulus would be encoded. By contrast, during stimulus processing, stronger connectivity with TPJ was associated with a lower likelihood the stimulus would be successfully encoded. These findings suggest that during encoding of visual objects into episodic memory, top-down and bottom-up attention can have opposite influences on perceptual areas that subserve visual object representation, suggesting that one manner in which attention modulates memory is by altering the perceptual processing of to-be-encoded stimuli.     

Visual field asymmetries in schematic persistence

A modified partial-report technique was used as a tool to study hemispheric asymmetries in schematic persistence. Two types of stimuli were presented: (1) verbal stimuli consisting of function words and nonsense words, and (2) pictures of faces and nonfaces. The stimulus items were arranged bilaterally in a circular array around the fixation point and the participants' task was to decide whether the probed item was a correct or distorted version of each stimulus type. Delay time between stimulus set and marker ranged from 0 ms to 360 ms. Calculating a sensitivity measure of accuracy, distinctive visual field differences emerged which depended on the nature of the stimuli; that is, a left field advantage for facial stimuli and a right field advantage for lexical stimuli. In addition, performance decreased linearly as delay time between probe and marker increased. The present results indicate that on a level of schematic persistence perceptual hemispheric asymmetries are already strongly present.