Large-scale Cortical Displacement of a Human Retinotopic Map

Neuroreport. Jan, 2002  |  Pubmed ID: 11924891

The aim of the current study was to determine the retinotopic organization of a patient with congenital cortical dysgenesis and normal visual function. Using functional magnetic resonance imaging (fMRI), detailed retinotopic maps corresponding to the four visual field quadrants were projected onto cortical surfaces. Similar to control subjects, the upper right visual field mapped onto ventral left hemisphere and was retinotopically organized. The lower right visual field's cortical representation was also retinotopically organized, yet was displaced many centimeters anteriomedially. Moreover, the entire left visual field was represented in non-retinotopically organized islands in both hemispheres. These results indicate retinotopic maps can shift in both location and topography illustrating cortical reorganization presumably due to either cortical dysgenesis or functional displacement. NeuroReport

Interactions Between Thalamic and Cortical Rhythms During Semantic Memory Recall in Human

Proceedings of the National Academy of Sciences of the United States of America. Apr, 2002  |  Pubmed ID: 11972063

Human scalp electroencephalographic rhythms, indicative of cortical population synchrony, have long been posited to reflect cognitive processing. Although numerous studies employing simultaneous thalamic and cortical electrode recording in nonhuman animals have explored the role of the thalamus in the modulation of cortical rhythms, direct evidence for thalamocortical modulation in human has not, to our knowledge, been obtained. We simultaneously recorded from thalamic and scalp electrodes in one human during performance of a cognitive task and found a spatially widespread, phase-locked, low-frequency rhythm (7-8 Hz) power decrease at thalamus and scalp during semantic memory recall. This low-frequency rhythm power decrease was followed by a spatially specific, phase-locked, fast-rhythm (21-34 Hz) power increase at thalamus and occipital scalp. Such a pattern of thalamocortical activity reflects a plausible neural mechanism underlying semantic memory recall that may underlie other cognitive processes as well.

A Prospective Study of Motor Recovery Following Multiple Subpial Transections

Neuroreport. Apr, 2002  |  Pubmed ID: 11973467

A prospective study of motor recovery was undertaken in a patient scheduled to undergo multiple subpial transections (MST) of right sensorimotor cortex. Pre-transection, functional MRI (fMRI) and cortical stimulation mapping confirmed left hand motor control within right primary motor cortex. Immediately post-transection, behavioral testing demonstrated preserved strength bilaterally but decreased dexterity in the left hand. Seven weeks post-transection, dexterity returned to normal and left hand finger tapping corresponded with multiple bilateral foci of fMRI activation. At 16 weeks, fMRI activation returned to pre-transection levels. These data indicate that cortical injury due to MST resulted in the temporary recruitment of distant cortical sites which presumably subserved normal motor function during recovery.

Darkness Beyond the Light: Attentional Inhibition Surrounding the Classic Spotlight

Neuroreport. May, 2002  |  Pubmed ID: 11997685

The aim of the present investigation was to determine the nature and spatial distribution of selective visual attention. Using cortical source localization of ERP data corresponding to 60 task-irrelevant stimuli across the visual field, we assessed attention effects on visual processing. Consistent with previous findings, visual processing was enhanced at the attended spatial location. In addition, this facilitation of processing extended from the attended location to the point of fixation resulting in a region of facilitation. Furthermore, a large region of inhibition was found surrounding this region of facilitation. The latter result is inconsistent with a simple facilitative spotlight model of attention and indicates that attention effects can be both facilitatory and inhibitory.

Efficient Acquisition of Human Retinotopic Maps

Human Brain Mapping. Jan, 2003  |  Pubmed ID: 12454909

A bifield stimulation method for rapidly obtaining retinotopic maps in human occipital cortex using functional MRI was compared to conventional unifield stimulation. While maintaining central fixation, each participant viewed the conventional display, consisting of a single rotating checkerboard wedge and, in a separate run, the bifield display, consisting of two symmetrically placed rotating checkerboard wedges (a "propeller" configuration). Both stimulus configurations used wedges with 30 degree polar angle width, 6.8 degrees visual angle extension from fixation, and 8.3 Hz contrast polarity reversal rate. Retinotopic maps in each condition were projected onto a distortion corrected computationally flattened cortical surface representation obtained from a high-resolution structural MRI. An automated procedure to localize borders between early visual areas revealed, as expected, that map precision increased with duration of data acquisition for both conditions. Bifield stimulation required 40% less time to yield maps with similar precision to those obtained using conventional unifield stimulation.

Distinct Prefrontal Cortex Activity Associated with Item Memory and Source Memory for Visual Shapes

Brain Research. Cognitive Brain Research. Jun, 2003  |  Pubmed ID: 12763194

In contrast to item memory, which refers to recognition or recall of previously presented information, source memory refers to memory for the context of previously presented information. The relatively few functional MRI (fMRI) source memory studies conducted to date have provided evidence that item memory and source memory are associated with differential activity in right and left prefrontal cortex, respectively. To both confirm this distinction in prefrontal cortex and to determine whether other differences in the neural substrates associated with these cognitive functions exist, an event-related fMRI study was conducted. In this study, item memory and source memory encoding phases were identical; participants viewed a series of abstract visual shapes presented on the left or right side of the screen and were instructed to remember each shape and its spatial location. During the item memory retrieval phase, shapes from the encoding phase were intermixed with new shapes and participants made an old-new judgment. During the source memory retrieval phase, all shapes were from the encoding phase and participants made a left-right judgment. An event-related analysis of item memory and source memory revealed a right and left prefrontal cortex distinction. Moreover, only item memory was associated with activity in the medial temporal lobes. These results confirm and extend previous findings that item memory and source memory are associated with distinct neural substrates.

Retinotopic Mapping Reveals Extrastriate Cortical Basis of Homonymous Quadrantanopia

Neuroreport. Jul, 2003  |  Pubmed ID: 12824761

It has been conventionally assumed that cortically based quadrantic visual field deficits (homonymous quadrantanopias) are caused by lesions in striate cortex (V1), extending precisely to the horizontal meridian representation. A more recent model, supported by anatomic MRI evidence, consists of an exclusively extrastriate cortical basis (e.g. V2, V3, VP, V4v). Employing fMRI, we sought to distinguish between these models through retinotopic mapping of a patient with an upper right homonymous quadrantanopia. As expected, maps of the lower right quadrant and left hemifield were normal. The map corresponding to the impaired upper right quadrant was normal in V1 and V2, with little or no activity in VP and V4v. These results provide functional evidence that extrastriate cortical lesions can elicit homonymous quadrantanopias.

Model Fitting in (n+1) Dimensions

Behavior Research Methods, Instruments, & Computers : a Journal of the Psychonomic Society, Inc. May, 2003  |  Pubmed ID: 12834091

Conventionally, fitting a mathematical model to empirically derived data is achieved by varying model parameters to minimize the deviations between expected and observed values in the dependent dimension. However, when functions to be fit are multivalued (e.g., an ellipse), conventional model fitting procedures fail. A novel (n+1)-dimensional [(n+1)-D] model fitting procedure is presented which can solve such problems by transforming the n-D model and data into (n+1)-D space and then minimizing deviations in the constructed dimension. While the (n+1)-D procedure provides model fits identical to those obtained with conventional methods for single-valued functions, it also extends parameter estimation to multivalued functions.

Attentional Inhibition of Visual Processing in Human Striate and Extrastriate Cortex

NeuroImage. Aug, 2003  |  Pubmed ID: 12948715

Allocating attention to a spatial location in the visual field is associated with an increase in the cortical response evoked by a stimulus at that location, compared to when the same stimulus is unattended. We used event-related functional magnetic resonance imaging to investigate attentional modulation of the cortical response to a stimulus probe at an attended location and to multiple probes at unattended locations. A localizer task and retinotopic mapping were used to precisely identify the cortical representations of each probe within striate (V1) and extrastriate cortex (V2, VP, V3, V4v, and V3A). The magnitude and polarity of attentional modulation were assessed through analysis of event-related activity time-locked to shifts in spatial attention. Attentional facilitation at the attended location was observed in striate and extrastriate cortex, corroborating earlier findings. Attentional inhibition of visual stimuli near the attended location was observed in striate cortex, and attentional inhibition of more distant stimuli occurred in both striate and extrastriate cortex. These findings indicate that visual attention operates both through facilitation of visual processing at the attended location and through inhibition of unattended stimulus representations in striate and extrastriate cortex.

Cortical Mechanisms of Feature-based Attentional Control

Cerebral Cortex (New York, N.Y. : 1991). Dec, 2003  |  Pubmed ID: 14615298

A network of fronto-parietal cortical areas is known to be involved in the control of visual attention, but the representational scope and specific function of these areas remains unclear. Recent neuroimaging evidence has revealed the existence of both transient (attention-shift) and sustained (attention-maintenance) mechanisms of space-based and object-based attentional control. Here we investigate the neural mechanisms of feature-based attentional control in human cortex using rapid event-related functional magnetic resonance imaging (fMRI). Subjects viewed an aperture containing moving dots in which dot color and direction of motion changed once per second. At any given moment, observers attended to either motion or color. Two of six motion directions and two of six colors embedded in the stimulus stream cued subjects either to shift attention from the currently attended to the unattended feature or to maintain attention on the currently attended feature. Attentional modulation of the blood oxygenation level dependent (BOLD) fMRI signal was observed in early visual areas that are selective for motion and color. More importantly, both transient and sustained BOLD activity patterns were observed in different fronto-parietal cortical areas during shifts of attention. We suggest these differing temporal profiles reflect complementary roles in the control of attention to perceptual features.

Human MT+ Mediates Perceptual Filling-in During Apparent Motion

NeuroImage. Apr, 2004  |  Pubmed ID: 15050597

During apparent motion, spatially distinct items presented in alternation cause the perception of a visual stimulus smoothly traversing the intervening space where no physical stimulus exists. We used fMRI to determine whether the perceptual 'filling-in' that underlies this phenomenon has an early or late cortical locus. Subjects viewed a display comprised of concentric rings that elicited apparent motion (two concentric rings presented in alternation), flicker (the same rings presented simultaneously), or real motion. We independently localized the cortical regions corresponding to the path of apparent motion in early visual areas (V1, V2, VP, V3, V4v, V3A), as well as the human motion processing complex (MT+). Cortical activity in the path of apparent motion in early visual areas was similar in amplitude during both apparent motion and flicker. In contrast, cortical activity in MT+ was higher in amplitude during apparent motion than during flicker, but was lower in amplitude than during real motion. In addition, we observed overlap in the cortical loci of MT+ and the lateral occipital complex (LOC), a region involved in shape and object processing. This overlap suggests that these regions could directly interact and thereby support perceived object continuity during apparent motion.

A Sensory Signature That Distinguishes True from False Memories

Nature Neuroscience. Jun, 2004  |  Pubmed ID: 15156146

Human behavioral studies show that there is greater sensory/perceptual detail associated with true memories than false memories. We therefore hypothesized that true recognition of abstract shapes would elicit greater visual cortical activation than would false recognition. During functional magnetic resonance imaging (fMRI), participants studied exemplar shapes and later made recognition memory decisions ("old" or "new") concerning studied exemplars (old shapes), nonstudied lures (related shapes) and new shapes. Within visual processing regions, direct contrasts between true recognition ("old" response to an old shape; old-hit) and false recognition ("old" response to a related shape; related-false alarm) revealed preferential true recognition-related activity in early visual processing regions (Brodmann area (BA)17, BA18). By comparison, both true and false recognition were associated with activity in early and late (BA19, BA37) visual processing regions, the late regions potentially supporting "old" responses, independent of accuracy. Further analyses suggested that the differential early visual processing activity reflected repetition priming, a type of implicit memory. Thus, the sensory signature that distinguishes true from false recognition may not be accessible to conscious awareness.

The Cognitive Neuroscience of Memory Distortion

Neuron. Sep, 2004  |  Pubmed ID: 15450167

Memory distortion occurs in the laboratory and in everyday life. This article focuses on false recognition, a common type of memory distortion in which individuals incorrectly claim to have encountered a novel object or event. By considering evidence from neuropsychology, neuroimaging, and electrophysiology, we address three questions. (1) Are there patterns of neural activity that can distinguish between true and false recognition? (2) Which brain regions contribute to false recognition? (3) Which brain regions play a role in monitoring or reducing false recognition? Neuroimaging and electrophysiological studies suggest that sensory activity is greater for true recognition compared to false recognition. Neuropsychological and neuroimaging results indicate that the hippocampus and several cortical regions contribute to false recognition. Evidence from neuropsychology, neuroimaging, and electrophysiology implicates the prefrontal cortex in retrieval monitoring that can limit the rate of false recognition.

Visual Memory and Visual Perception Recruit Common Neural Substrates

Behavioral and Cognitive Neuroscience Reviews. Dec, 2004  |  Pubmed ID: 15812107

This human neuroimaging review aims to determine the degree to which visual memory evokes activity in neural regions that have been associated with visual perception. A visual perception framework is proposed to identify cortical regions associated with modality-specific processing (i.e., visual, auditory, motor, or olfactory), visual domain-specific processing (i.e., "what" versus "where," or face versus visual context), and visual feature-specific processing (i.e., color, motion, or spatial location). Independent assessments of visual item memory studies and visual working memory studies revealed activity in the appropriate cortical regions associated with each of the three levels of visual perception processing. These results provide compelling evidence that visual memory and visual perception are associated with common neural substrates. Furthermore, as with visual perception, they support the view that visual memory is a constructive process, in which features or components from disparate cortical regions bind together to form a coherent whole.

Support for a Continuous (single-process) Model of Recognition Memory and Source Memory

Memory & Cognition. Jan, 2005  |  Pubmed ID: 15915801

Does memory retrieval occur in a continuous or an all-or-none manner? The shape of the receiver operating characteristic (ROC) has been used to answer this question, with curvilinear and linear memory ROCs indicating continuous and all-or-none retrieval processes, respectively. Signal detection models (e.g., the unequal variance model) correspond to a continuous retrieval process, whereas threshold models (including the multinomial model and the recollection component of the dual-process model) correspond to an all-or-none process. In studies of source memory, Slotnick et al. (2000) and others have observed curvilinear ROCs (supporting the unequal variance model), whereas Yonelinas (1999) observed linear ROCs (supporting the dual-process model). We resolve these seemingly inconsistent results, showing that source memory ROCs are naturally curvilinear but can appear linear when nondiagnostic source information is included in the analysis. Furthermore, the unequal variance model accounted for both recognition memory and source memory ROCs, supporting a continuous process of memory retrieval.

Common Neural Substrates for the Control and Effects of Visual Attention and Perceptual Bistability

Brain Research. Cognitive Brain Research. Jun, 2005  |  Pubmed ID: 15922162

Behavioral studies have suggested that bistable figure perception is mediated by spatial attention. We tested this hypothesis using event-related functional MRI. During central fixation, two tilted squares containing coherently moving dots were presented in the left and right hemifields. In the attention condition, participants were occasionally cued to shift attention between the squares. In the perception condition, corresponding corners of the squares were connected by horizontal lines producing a perceptually bistable Necker cube figure. Observers reported which of the two faces appeared 'forward' in depth; cues elicited voluntary perceptual reversals. Attending to either square during the attention condition or perceiving either square as forward during the perception condition yielded increased activity in contralateral visual areas. Furthermore, voluntary shifts of attention and voluntary shifts in perceptual configuration were associated with common activity in the posterior parietal cortex, part of the frontoparietal attentional control network. These results support the hypothesis that voluntary shifts in perceptual bistability are mediated by spatial attention.

Visual Mental Imagery Induces Retinotopically Organized Activation of Early Visual Areas

Cerebral Cortex (New York, N.Y. : 1991). Oct, 2005  |  Pubmed ID: 15689519

There is a long-standing debate as to whether visual mental imagery relies entirely on symbolic (language-like) representations or also relies on depictive (picture-like) representations. We sought to discover whether visual mental imagery could evoke cortical activity with precise visual field topography (retinotopy). Participants received three conditions: the perception condition consisted of a standard retinotopic mapping procedure, where two flickering checkerboard wedges rotated around a central fixation point. The imagery and attention conditions consisted of the same stimulus, but only the outer arcs of the wedges were visible. During imagery, participants mentally reproduced the stimulus wedges, using the stimulus arcs as a guide. The attention condition required either distributed attention or focused attention to where the stimulus wedges would have been. Event-related analysis revealed that the imagery (greater than either form of attention) retinotopic maps were similar to the perception maps. Moreover, blocked analysis revealed similar perception and imagery effects in human motion processing region MT+. These results support the depictive view of visual mental imagery.

The Neural Origins of Specific and General Memory: the Role of the Fusiform Cortex

Neuropsychologia. 2005  |  Pubmed ID: 15716157

Recognition of an object can be based on memory for specific details of a prior encounter with the object, or on a more general memory for the type of object previously encountered. Responding on the basis of general information alone can sometimes produce memory errors involving both distortion and forgetting, but little is known about the neural origins of general versus specific recognition. We extended the standard subsequent memory paradigm to examine whether neural activity at encoding predicts whether an object will subsequently elicit specific as compared to general memory. During event-related functional magnetic resonance imaging (fMRI), participants viewed objects and made size judgments about them. Later, they viewed same, similar, and new objects, labeling each as "same," "similar," or "new." Specific recognition was indicated by a "same" response to a same object. By contrast, general, non-specific recognition was indicated by either a "same" response to a similar object (false memory) or a "similar" response to a same object (partial memory). As predicted, specific recognition, as compared to non-specific recognition, was associated with encoding-related activity in the right fusiform cortex, while non-specific recognition, as compared to forgetting, was associated with encoding-related activity in the left fusiform cortex. Furthermore, all successful recognition (specific and general), as compared to forgetting, was associated with encoding-related activity in bilateral fusiform cortex. These results suggest that the right fusiform cortex is associated with specific feature encoding, while the left fusiform cortex is involved in more general object encoding.

Spatial Working Memory Specific Activity in Dorsal Prefrontal Cortex? Disparate Answers from FMRI Beta-weight and Timecourse Analysis

Cognitive Neuropsychology. Oct, 2005  |  Pubmed ID: 21038282

Visual spatial processing and object processing rely on dorsal and ventral cortical pathways, respectively. Whether this functional segregation exists in the prefrontal cortex is currently a source of debate. Using functional MRI (fMRI), there has been some evidence that the superior frontal sulcus (within dorsal prefrontal cortex) is specialised for spatial working memory, while ventral prefrontal cortex is associated with object working memory. Employing beta-weight analysis, Postle, Berger, Taich, and D'Esposito (2000) challenged these results, finding no differential activity associated with spatial working memory versus two-dimensional saccades in the superior frontal sulcus. In the present reanalysis of Postle et al.'s data, both beta-weight analysis and event-related timecourse analysis were utilised. Beta-weight analysis results replicated Postle et al.; however, timecourse analysis revealed greater activity associated with spatial working memory versus two-dimensional saccades in the superior frontal sulcus. Thus, identical fMRI data analysed via distinct methods yielded results with different theoretical conclusions.

Valid FMRI Timecourse Analysis with Tasks Containing Temporal Dependencies

Cognitive Neuropsychology. Oct, 2005  |  Pubmed ID: 21038284

Not All False Memories Are Created Equal: the Neural Basis of False Recognition

Cerebral Cortex (New York, N.Y. : 1991). Nov, 2006  |  Pubmed ID: 16400158

False recognition, a type of memory distortion where one claims to remember something that never happened, can occur in response to items that are similar but not identical to previously seen items (i.e., related false recognition) or in response to novel items (i.e., unrelated false recognition). It is unknown whether these 2 types of memory errors arise from the same or distinct neural substrates. Using functional magnetic resonance imaging, we compared the neural activity associated with true recognition, related false recognition, and unrelated false recognition for abstract shapes. True recognition and related false recognition were associated with similar patterns of neural activity, including activity in the prefrontal cortex, the parietal cortex, and the medial temporal lobe. By contrast, unrelated false recognition was associated with activity in language-processing regions. These results indicate that false recognition is not a unitary phenomenon, but rather can reflect the operation of 2 distinct cognitive and neural processes.

Prefrontal Cortex Hemispheric Specialization for Categorical and Coordinate Visual Spatial Memory

Neuropsychologia. 2006  |  Pubmed ID: 16516248

During visual spatial perception of multiple items, the left hemisphere has been shown to preferentially process categorical spatial relationships while the right hemisphere has been shown to preferentially process coordinate spatial relationships. We hypothesized that this hemispheric processing distinction would be reflected in the prefrontal cortex during categorical and coordinate visual spatial memory, and tested this hypothesis using functional magnetic resonance imaging (fMRI). During encoding, abstract shapes were presented in the left or right hemifield in addition to a dot at a variable distance from the shape (with some dots on the shape); participants were instructed to remember the position of each dot relative to the shape. During categorical memory retrieval, each shape was presented centrally and participants responded whether the previously corresponding dot was 'on' or 'off' of the shape. During coordinate memory retrieval, each shape was presented centrally and participants responded whether the previously corresponding dot was 'near' or 'far' from the shape (relative to a reference distance). Consistent with our hypothesis, a region in the left prefrontal cortex (BA10) was preferentially associated with categorical visual spatial memory and a region in the right prefrontal cortex (BA9/10) was preferentially associated with coordinate visual spatial memory. These results have direct implications for interpreting previous findings that the left prefrontal cortex is associated with source memory, as this cognitive process is categorical in nature, and the right prefrontal cortex is associated with item memory, as this process depends on the precise spatial relations among item features or components.

Temporally Unfolding Neural Representation of Pictorial Occlusion

Psychological Science. Apr, 2006  |  Pubmed ID: 16623695

The human visual system possesses a remarkable ability to reconstruct the shape of an object that is partly occluded by an interposed surface. Behavioral results suggest that, under some circumstances, this perceptual process (termed amodal completion) progresses from an initial representation of local image features to a completed representation of a shape that may include features that are not explicitly present in the retinal image. Recent functional magnetic resonance imaging (fMRI) studies have shown that the completed surface is represented in early visual cortical areas. We used fMRI adaptation, combined with brief, masked exposures, to track the amodal completion process as it unfolds in early visual cortical regions. We report evidence for an evolution of the neural representation from the image-based feature representation to the completed representation. Our method offers the possibility of measuring changes in cortical activity using fMRI over a time scale of a few hundred milliseconds.

The Nature of Memory Related Activity in Early Visual Areas

Neuropsychologia. 2006  |  Pubmed ID: 16901520

Memory for visual items can evoke activity in visual processing regions, which is typically assumed to reflect conscious remembering. However, based on previous findings, we hypothesized that such activity in early visual areas (BA17, BA18) may reflect priming, a form of nonconscious memory. We tested this hypothesis in two fMRI experiments with similar stimulus protocols, but explicit or implicit task instructions. During initial runs, abstract shapes were presented to either side of fixation, filled with parallel lines of random orientation and color. In subsequent runs, old and new shapes (plus related shapes in Experiment 2) were presented at fixation. In Experiment 1, participants were instructed to remember each shape and its spatial location during initial runs; during subsequent runs they classified each shape as old and on the "left", old and on the "right", or "new". A right fusiform gyrus region (BA18) and a left lingual gyrus region (BA18) were preferentially associated with shapes previously presented on the left and right, respectively. In support of our hypothesis, this early visual area activity was independent of response accuracy for spatial location. In Experiment 2, for each shape, participants identified parallel line orientation relative to horizontal. Consistent with our hypothesis, specific neural activity was observed in early visual regions (BA17, BA18, extending into BA19), with old activity greater than related and new activity (likely reflecting priming). The results of these experiments provide convergent evidence that memory related early visual area activity (BA17, BA18) can reflect nonconscious processing.

Aging, Source Memory, and Misrecollections

Journal of Experimental Psychology. Learning, Memory, and Cognition. Jan, 2007  |  Pubmed ID: 17201560

The authors propose an illusory recollection account of why cognitive aging is associated with episodic memory deficits. After listening to statements presented by either a female or a male speaker, older adults were prone to misrecollecting past events. The authors' illusory recollection account is instantiated in a new illusory recollection signal detection model that provides a better fit of older adults' data than does the standard signal detection model. They observed that age-related differences in source memory (as measured by source d' scores) virtually disappear after accounting for the occurrence of illusory recollections. These data suggest that age-related source memory impairments are not due to older adults' remembering less diagnostic source information and having to guess more. Instead, older adults appear to misremember past events more often than younger adults.

Involuntary Capture of Attention Produces Domain-specific Activation

Neuroreport. Jul, 2007  |  Pubmed ID: 17558280

In this study, we ask whether or not the involuntary capture of attention by a specific information type can produce domain-specific activation. Participants indicated the presence of a flicker in rapidly alternating letter-string masks presented in the periphery. Despite letters not being relevant to the task, we found, using functional MRI and a novel task that allowed us to contrast attended and unattended processing of the same visual information, a robust modulation by attentional capture in a localized letter-processing region. This finding suggests that the involuntary capture of attention is sufficient to produce domain-specific activation in early visual processing.

The Hippocampus is Preferentially Associated with Memory for Spatial Context

Journal of Cognitive Neuroscience. Mar, 2008  |  Pubmed ID: 18004952

Abstract The existence of a functional-anatomic dissociation for retrieving item versus contextual information within subregions of the medial temporal lobe (MTL) is currently under debate. We used a spatial source memory paradigm during event-related functional magnetic resonance imaging to investigate this issue. At study, abstract shapes were presented to the left or right of fixation. During test, old and new shapes were presented at fixation. Participants responded whether each shape had been previously presented on the "left," the "right," or was "new." Activity associated with contextual memory (i.e., source memory) was isolated by contrasting accurate versus inaccurate memory for spatial location. Item-memory-related activity was isolated by contrasting accurate item recognition without contextual memory with forgotten items. Source memory was associated with activity in the hippocampus and parahippocampal cortex. Although item memory was not associated with unique MTL activity at our original threshold, a region-of-interest (ROI) analysis revealed item-memory-related activity in the perirhinal cortex. Furthermore, a functional-anatomic dissociation within the parietal cortex for retrieving item and contextual information was not found in any of three ROIs. These results support the hypothesis that specific subregions in the MTL are associated with item memory and memory for context.

Imagery: Mental Pictures Disrupt Perceptual Rivalry

Current Biology : CB. Jul, 2008  |  Pubmed ID: 18644335

Are mental images like pictures? Yes, according to a new study showing that imagery can have a high degree of visual specificity and thereby disrupt perceptual rivalry.

Functional MT + Lesion Impairs Contralateral Motion Processing

Cognitive Neuropsychology. Jul, 2008  |  Pubmed ID: 18651258

Human motion processing region MT + is retinotopically organized with perception of and attention to motion in the right visual field preferentially associated with left MT + activity and vice versa. However, the degree to which MT + is crucial for motion processing is uncertain. We report an epilepsy patient with visual symptoms early in his seizure evolution and a left temporal-occipital seizure onset electrographically in whom we hypothesized a functional left MT + lesion. The patient was impaired in his right but not left visual field on a hemifield motion attention task and demonstrated worse performance on a hemifield picture identification task when pictures implying motion were presented in the right as opposed to the left visual field. Functional MRI (fMRI) during a full-field motion detection task activated right MT + but failed to activate left MT + despite activating both left and right MT + in each of 10 controls. Furthermore, fMRI during a hemifield motion attention task also showed a lack of left MT + attention effects in the patient. Together these results suggest that MT + is necessary for normal motion processing.

Rapid Retinotopic Reactivation During Spatial Memory

Brain Research. May, 2009  |  Pubmed ID: 19272364

Memories are thought to be constructed from features processed in different cortical regions. However, it is unknown how the retrieval process unfolds over time. The present investigation aimed to address this issue by combining evidence from event-related potentials (ERPs) and functional magnetic resonance imaging (fMRI). During study, abstract shapes were presented to the left or right of fixation and participants were instructed to remember each shape and its spatial location. At test, studied (old) and new shapes were presented at fixation and participants classified each shape as old and on the "left", old and on the "right", or "new". Accurate memory for items previously presented on the left or right produced fMRI activity in the right or left extrastriate cortex (BA18), respectively. ERP results revealed these retinotopic memory effects occurred within 100-250 ms after stimulus onset indicating memory construction can occur very rapidly.

Two Forms of Spatial Imagery: Neuroimaging Evidence

Psychological Science. Oct, 2009  |  Pubmed ID: 19765238

Spatial imagery may be useful in such tasks as interpreting graphs and solving geometry problems, and even in performing surgery. This study provides evidence that spatial imagery is not a single faculty; rather, visualizing spatial location and mentally transforming location rely on distinct neural networks. Using 3-T functional magnetic resonance imaging, we tested 16 participants (8 male, 8 female) in each of two spatial imagery tasks--one that required visualizing location and one that required mentally rotating stimuli. The same stimuli were used in the two tasks. The location-based task engendered more activation near the occipito-parietal sulcus, medial posterior cingulate, and precuneus, whereas the transformation task engendered more activation in superior portions of the parietal lobe and in the postcentral gyrus. These differences in activation provide evidence that there are at least two different types of spatial imagery.

The Role of Parietal Cortex During Sustained Visual Spatial Attention

Brain Research. Dec, 2009  |  Pubmed ID: 19765554

The control of spatial attention-shifting attention between visual field locations or sustaining attention to one location-involves the prefrontal cortex and parietal cortex. Within the parietal cortex, shifting attention has been linked to the superior parietal lobule; however, the neural substrates associated with sustained attention are still unknown. In the present fMRI study, we aimed to identify generalized control regions associated with sustained attention using two different protocols. The motion protocol alternated between periods of moving or stationary dots, and the flicker protocol alternated between periods of flickering or stationary checkerboards (each period lasted 14 s). During moving and flickering periods, the behavioral task alternated between sustained attention and perception. A region-of-interest analysis confirmed that the motion but not flicker protocol produced attention effects-greater activity associated with sustained attention than perception-in motion processing region MT+. A whole brain conjunction analysis identified regions commonly associated with sustained attention for both protocols, which included the right intraparietal sulcus (BA 7/40), the right middle frontal gyrus (BA 9/46), the right superior temporal gyrus (BA 22), the right insula (BA 13), and the left cerebellum. Coupled with previous results, the present findings suggest a functional-anatomic organization of parietal cortex where shifts in attention are mediated by superior regions and sustained attention is mediated by more lateral regions.

Memory for Color Reactivates Color Processing Region

Neuroreport. Nov, 2009  |  Pubmed ID: 19858765

Memory is thought to be constructive in nature, where features processed in different cortical regions are synthesized during retrieval. In an effort to support this constructive memory framework, the present functional magnetic resonance imaging study assessed whether memory for color reactivated color processing regions. During encoding, participants were presented with colored and gray abstract shapes. During retrieval, old and new shapes were presented in gray and participants responded 'old-colored', 'old-gray', or 'new'. Within color perception regions, color memory related activity was observed in the left fusiform gyrus, adjacent to the collateral sulcus. A retinotopic mapping analysis indicated this activity occurred within color processing region V8. The present feature specific evidence provides compelling support for a constructive view of memory.

"Remember" Source Memory ROCs Indicate Recollection is a Continuous Process

Memory (Hove, England). Jan, 2010  |  Pubmed ID: 19937493

The dual process model assumes memory is based on recollection (retrieval with specific detail) or familiarity (retrieval without specific detail). A current debate is whether recollection is a threshold process or, like familiarity, is a continuous process. In the present study two continuous models and two threshold models of recollection were evaluated using receiver operating characteristic (ROC) analysis. These models included the continuous signal detection unequal variance model and the threshold dual process model. In the study phase of three experiments, objects were presented to the right or left of fixation. At test, participants made either remember-know responses or item confidence responses followed by source memory (spatial location) confidence ratings. Recollection-based ROCs were generated from source memory confidence ratings associated with "remember" responses (in Experiments 1-2) or the highest item confidence responses (in Experiment 3). Neither threshold model adequately fit any of the recollection-based ROCs. By contrast, one or both of the continuous models adequately fit all of the recollection-based ROCs. The present results indicate recollection and familiarity are both continuous processes.

Conscious and Nonconscious Memory Effects Are Temporally Dissociable

Cognitive Neuroscience. Mar, 2010  |  Pubmed ID: 20200601

Intentional (explicit) retrieval can reactivate sensory cortex, which is widely assumed to reflect conscious processing. In the present study, we used an explicit visual memory event-related potential paradigm to investigate whether such retrieval related sensory activity could be separated into conscious and nonconscious components. During study, abstract shapes were presented in the left or right visual field. During test, old and new shapes were presented centrally and participants classified each shape as "old-left", "old-right", or "new". Conscious activity was isolated by comparing accurate memory for shape and location (old-hits) with forgotten shapes (old-misses), and nonconscious activity was isolated by comparing old-left-misses with old-right-misses and vice versa. Conscious visual sensory activity had a late temporal onset (after 800 ms) while nonconscious visual sensory activity had an early temporal onset (before 800 ms). These results suggest explicit memory related sensory activity reflects both conscious and nonconscious processes that are temporally dissociable.

Attentional Inhibition Mediates Inattentional Blindness

Consciousness and Cognition. Jun, 2010  |  Pubmed ID: 20227894

Salient stimuli presented at unattended locations are not always perceived, a phenomenon termed inattentional blindness. We hypothesized that inattentional blindness may be mediated by attentional inhibition. It has been shown that attentional inhibition effects are maximal near an attended location. If our hypothesis is correct, inattentional blindness effects should similarly be maximal near an attended location. During central fixation, participants viewed rapidly presented colored digits at a peripheral location. An unexpected black circle (the critical stimulus) was concurrently presented. Participants were instructed to maintain central fixation and name each color/digit, requiring focused attention to that location. For each participant, the critical stimulus was presented either near to or far from the attended location (at the same eccentricity). In support of our hypothesis, inattentional blindness effects were maximal near the attended location, but only at intermediate task accuracy.

Synchronous Retinotopic Frontal-temporal Activity During Long-term Memory for Spatial Location

Brain Research. May, 2010  |  Pubmed ID: 20307512

Early visual areas in occipital cortex are known to be retinotopic. Recently, retinotopic maps have been reported in frontal and parietal cortex during spatial attention and working memory. The present event-related potential (ERP) and functional magnetic resonance imaging (fMRI) study determined whether spatial long-term memory was associated with retinotopic activity in frontal and parietal regions, and assessed whether retinotopic activity in these higher level control regions was synchronous with retinotopic activity in lower level visual sensory regions. During encoding, abstract shapes were presented to the left or right of fixation. During retrieval, old and new shapes were presented at fixation and participants classified each shape as old and previously on the "left", old and previously on the "right", or "new". Retinotopic effects were manifested by accurate memory for items previously presented on the left producing activity in the right hemisphere and accurate memory for items previously presented on the right producing activity in the left hemisphere. Retinotopic ERP activity was observed in frontal regions and visual sensory (occipital and temporal) regions. In frontal cortex, retinotopic fMRI activity was localized to the frontal eye fields. There were no significant ERP or fMRI retinotopic memory effects in parietal regions. The present long-term memory retinotopic effects complement previous spatial attention and working memory findings (and suggest retinotopic activity in parietal cortex may require an external peripheral stimulus). Furthermore, ERP cross-correlogram analysis revealed that retinotopic activations in frontal and temporal regions were synchronous, indicating that these regions interact during retrieval of spatial information.

Does the Hippocampus Mediate Objective Binding or Subjective Remembering?

NeuroImage. Jan, 2010  |  Pubmed ID: 19786107

Human functional magnetic resonance imaging (fMRI) evidence suggests the hippocampus is associated with context memory to a greater degree than item memory (where only context memory requires item-in-context binding). A separate line of fMRI research suggests the hippocampus is associated with "remember" responses to a greater degree than "know" or familiarity based responses (where only remembering reflects the subjective experience of specific detail). Previous studies, however, have confounded context memory with remembering and item memory with knowing. The present fMRI study independently tested the binding hypothesis and remembering hypothesis of hippocampal function by evaluating activity within hippocampal regions-of-interest (ROIs). At encoding, participants were presented with colored and gray abstract shapes and instructed to remember each shape and whether it was colored or gray. At retrieval, old and new shapes were presented in gray and participants classified each shape as "old and previously colored", "old and previously gray", or "new", followed by a "remember" or "know" response. In 3 of 11 hippocampal ROIs, activity was significantly greater for context memory than item memory, the context memory-item memory by remember-know interaction was significant, and activity was significantly greater for context memory-knowing than item memory-remembering. This pattern of activity only supports the binding hypothesis. The analogous pattern of activity that would have supported the remembering hypothesis was never observed in the hippocampus. However, a targeted analysis revealed remembering specific activity in the left inferior parietal cortex. The present results suggest parietal cortex may be associated with subjective remembering while the hippocampus mediates binding.

Memory for Motion and Spatial Location is Mediated by Contralateral and Ipsilateral Motion Processing Cortex

NeuroImage. Mar, 2011  |  Pubmed ID: 21134469

Memory and perception have been associated with common sensory cortical activity. However, previous studies have only investigated memory and perception effects associated with a single feature (i.e., spatial location or color). The aim of the present functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) study was to assess whether memory for multiple (two) features would produce sensory cortical activity that mirrored perceptual processing of the same features. During encoding, moving or stationary abstract shapes were presented to the right or left of fixation. During retrieval, shapes were presented at fixation and participants classified each item as previously in motion or stationary within the right or left visual field. Memory for items in motion, regardless of spatial location, produced fMRI activity in perceptual motion processing region MT+. Memory for motion and spatial location produced contralateral and ipsilateral fMRI activity in perceptual motion processing sub-region MT. Following TMS to MT, memory for motion was impaired, but performance did not differ between the contralateral and ipsilateral visual fields. The present results are consistent with previous findings in that memory for motion produced fMRI activity in MT+ and was impaired following TMS to MT. However, memory for motion and spatial location produced contralateral and ipsilateral fMRI and TMS effects, deviating from the primarily contralateral perceptual processing organization of MT. The present evidence suggests that during memory for motion and spatial location only motion information is coded in motion processing cortex, while previous findings suggest spatial location information is coded in earlier extrastriate cortex.

Disruption of MT Impairs Motion Processing

Neuroscience Letters. Mar, 2011  |  Pubmed ID: 21195742

Functional magnetic resonance imaging (fMRI) studies have associated motion processing with cortical region MT+, which includes sub-region MT that preferentially processes motion in the contralateral visual field. Transcranial magnetic stimulation (TMS) has been used to temporarily disrupt MT+ which impaired motion perception, suggesting this region is necessary for motion processing. In the present study, we used fMRI guided TMS to disrupt MT and determine whether this sub-region is necessary for motion processing. On an individual participant basis, MT was localized in each hemisphere using motion related fMRI activity on the posterior bank of the ascending limb of the inferior temporal sulcus. In the first experiment, 1 Hz TMS of left MT preferentially impaired motion detection in the contralateral versus ipsilateral visual field. In the second experiment, single-pulse TMS of MT impaired motion processing to a greater degree than color processing. These results provide convergent evidence that sub-region MT is necessary for motion processing.