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G L Shulman

Publications and source records attributed to G L Shulman.

34 records · Page 2Linked to original sources
PubMed1991

Selective attention in vibrotactile tasks: detecting the presence and absence of amplitude change.

Selective spatial attention has a greater effect on detection of the absence of an amplitude change than it has on detection of the presence of such a change. Attention to one of four fingertips was manipulated by an 80% valid tactile cue in two-interval forced-choice tasks. In one task, the target was a vibrotactile amplitude change appearing among constant-amplitude distractors; in the other task, targets of constant amplitude had to be detected amid amplitude changes at the other fingertips. Cuing had a greater effect on the latter task than it did on the former. This asymmetry is consistent with the presence-absence asymmetry found in visual search and does not depend on the difficulty of the two tasks. A statistical model shows that a pooled activity mechanism could account for these experimental results.

Adult↗
PubMed1991

Selective and divided attention during visual discriminations of shape, color, and speed: functional anatomy by positron emission tomography.

Positron emission tomography (PET) was used to identify the neural systems involved in discriminating the shape, color, and speed of a visual stimulus under conditions of selective and divided attention. Psychophysical evidence indicated that the sensitivity for discriminating subtle stimulus changes in a same-different matching task was higher when subjects selectively attended to one attribute than when they divided attention among the attributes. PET measurements of brain activity indicated that modulations of extrastriate visual activity were primarily produced by task conditions of selective attention. Attention to speed activated a region in the left inferior parietal lobule. Attention to color activated a region in the collateral sulcus and dorsolateral occipital cortex, while attention to shape activated collateral sulcus (similarly to color), fusiform and parahippocampal gyri, and temporal cortex along the superior temporal sulcus. Outside the visual system, selective and divided attention activated nonoverlapping sets of brain regions. Selective conditions activated globus pallidus, caudate nucleus, lateral orbitofrontal cortex, posterior thalamus/colliculus, and insular-premotor regions, while the divided condition activated the anterior cingulate and dorsolateral prefrontal cortex. The results in the visual system demonstrate that selective attention to different features modulates activity in distinct regions of extrastriate cortex that appear to be specialized for processing the selected feature. The disjoint pattern of activations in extravisual brain regions during selective- and divided-attention conditions also suggests that preceptual judgements involve different neural systems, depending on attentional strategies.

Adult↗
PubMed1990

Attentional modulation of neural processing of shape, color, and velocity in humans.

Positron emission tomography (PET) was used to measure changes in regional cerebral blood flow of normal subjects, while they were discriminating different attributes (shape, color, and velocity) of the same set of visual stimuli. Psychophysical evidence indicated that the sensitivity for discriminating subtle stimulus changes was higher when subjects focused attention on one attribute than when they divided attention among several attributes. Correspondingly, attention enhanced the activity of different regions of extrastriate visual cortex that appear to be specialized for processing information related to the selected attribute.

Adult↗
PubMed1989

Visual performance on CRT screens and hard-copy displays.

Three experiments examined the effect of hard-copy print and CRT screens of different resolution/addressability ratios (RAR) on accommodation and visual search performance. Three different display modes were generated with an IBM personal computer and a Princeton Graphics Terminal: capital letters in high-RAR mode, capital letters in low-RAR mode, and capital letters in high-RAR mode programmed to simulate the low-RAR mode. Same-sized letters were also presented on hard-copy print. Experiment 1 demonstrated that accommodation to hardcopy print and high-RAR screens was more accurate than to low-RAR screens. In Experiment 2 the spatial frequency channels activated by each display were evaluated by measuring the effect of display adaptation on the contrast-sensitivity function. The results suggested that high-RAR screens and hard-copy print activated higher-frequency channels than did low-RAR screens. In Experiment 3 significantly better visual search performance was obtained for high-RAR screens and for hard-copy print than for low-RAR screens. It was concluded that screen RAR is an important variable to consider in the design of CRTs.

Accommodation, Ocular↗
PubMed1987

Spatial frequency and selective attention to local and global information.

Probe methods were used to investigate whether the distribution of attention to the local or the global structure of a stimulus affects the detectability of different spatial frequencies. Four experiments are reported in which the detectability of threshold probe gratings of different spatial frequencies was measured while subjects analyzed either the local or the global information from a display. A relative shift in the detectability of low and high frequencies was observed. Low frequencies were facilitated during global processing and/or high frequencies were facilitated during local processing.

Adult↗
PubMed1987

Spatial frequency and selective attention to spatial location.

The effect of spatial attention on the detectability of gratings of different spatial frequency was measured using a probe technique. Three experiments are reported in which the detectability of full-field probe gratings was measured while subjects analyzed stimuli presented in either the central or the peripheral visual field. Selective attention to peripheral stimuli produced a facilitation at low frequencies and a decrement at high frequencies. These effects disappeared under forced-choice presentation.

Attention↗
PubMed1986

Reaction times to different spatial frequencies as a function of detectability.

Simple reaction time to sine-wave gratings of 1, 4 and 10 c/deg was measured as a function of the detectability of the gratings. Reaction time increased with spatial frequency over a range of detectabilities. Assuming that equally detectable gratings produce equivalent levels of response in the visual system, this increase in reaction time reflects an increase in perceptual latency as spatial frequency is increased.

Form Perception↗
PubMed1986

The role of spatial-frequency channels in the perception of local and global structure.

Adaptation and reaction-time techniques were used to examine the role of different spatial-frequency channels in the perception of local and global structure. Subjects were shown figures consisting of a large C composed of smaller Cs and asked to identify the orientation of either the global C or its local elements. Prior to performing the task subjects were adapted to different spatial frequencies and the effect on subsequent performance was assessed. Two main results were found. First, the adapting frequency that most affected the global task was often lower than that most affecting the local task, suggesting that high and low frequencies independently code the structure of an image. Second, reaction time to global figures was often faster than to local figures at all levels of detectability, again suggesting a role of low-frequency channels in global processing.

Adaptation, Ocular↗
PubMed1986

Directionally selective channels mediate the discrimination of moving aperiodic stimuli.

Adaptation procedures were used to demonstrate that directionally selective channels mediate the discrimination of aperiodic stimuli moving at a common speed. Subjects were adapted to a grating that drifted to either the left or the right. They then determined the orientation of the gap in a 'C' that was also moving to either the left or the right. Performance in identifying the gap orientation was most impaired when the C moved in the same direction as the adapting grating.

Adaptation, Ocular↗
PubMed1979

Moving attention through visual space.

Several experiments have shown that attention as measured by simple reaction time to luminance increments can be shifted in the visual field while the eyes are kept in a fixed position. The shift of attention appears to take place within 50 msec following a cue indicating the most likely position of the target. The present study reports that these shifts of attention can be time locked to a central cue. Moreover, they show that a probe event located between the cue and the target receives maximal facilitation from attention at a time prior to maximal facilitation at the target. These results provide support for an analogue movement of attention across the visual field that does not involve the suppression found during saccades.

Attention↗
PubMedDate not supplied

Top-down modulation of early sensory cortex.

Data from nine previous studies of human visual information processing using positron emission tomography were reanalyzed to contrast blood flow responses during passive viewing and active discriminations of the same stimulus array. The analysis examined whether active visual processing (i) increases blood flow in medial visual regions early in the visual hierarchy and (ii) decreases blood flow in auditory and somatosensory cortex. Significant modulation of medial visual regions was observed in six of nine studies, indicating that top-down processes can affect early visual cortex. Modulations showed several task dependencies, suggesting that in some cases the underlying mechanism was selective (e.g. analysis-or feature-specific) rather than non-selective. Replicable decreases at or near auditory Brodmann area (BA) left 41/42 were observed in two of five studies, but in different locations. Analyses that combined data across studies yielded modest but significant decreases. Replicable decreases were not found in primary somatosensory cortex but were observed in an insular region that may be a somatosensory association area. Decreases were also noted in the parietal operculum (perhaps SII) and BA 40. These results are inconsistent with a model in which the precortical input to task-irrelevant sensory cortical areas is broadly suppressed.

Auditory Cortex↗