Biomedical subjects

Scott D Slotnick

Publications and source records attributed to Scott D Slotnick.

15 recordsLinked to original sources

PubMed2005

Visual mental imagery induces retinotopically organized activation of early visual areas.

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.

Adult↗

PubMed2004

The neural origins of specific and general memory: the role of the fusiform cortex.

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.

Adult↗

PubMed2004

The cognitive neuroscience of memory distortion.

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.

Animals↗

PubMed2004

A sensory signature that distinguishes true from false memories.

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.

Adult↗

PubMed2004

Human MT+ mediates perceptual filling-in during apparent motion.

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.

Adult↗

PubMed2003

Retinotopic mapping reveals extrastriate cortical basis of homonymous quadrantanopia.

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.

Brain Mapping↗

PubMed2003

Efficient acquisition of human retinotopic maps.

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.

Brain Mapping↗

PubMed2003

Distinct prefrontal cortex activity associated with item memory and source memory for visual shapes.

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.

Adult↗

PubMed2003

Attentional inhibition of visual processing in human striate and extrastriate cortex.

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.

Adult↗

PubMed2003

Cortical mechanisms of feature-based attentional control.

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.

Action Potentials↗

PubMed2003

Model fitting in (n+1) dimensions.

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.

Algorithms↗

PubMed2002

Darkness beyond the light: attentional inhibition surrounding the classic spotlight.

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.

Adult↗

PubMed2002

Interactions between thalamic and cortical rhythms during semantic memory recall in human.

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.

Brain Mapping↗

PubMed2002

A prospective study of motor recovery following multiple subpial transections.

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.

Adult↗

PubMed2002

Large-scale cortical displacement of a human retinotopic map.

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

Adult↗