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Biomedical subjects

M R Lamb

Publications and source records attributed to M R Lamb.

At least 19 recordsLinked to original sources
PubMed2001

Characterization of the EYE2 gene required for eyespot assembly in Chlamydomonas reinhardtii.

The unicellular biflagellate green alga Chlamydomonas reinhardtii can perceive light and respond by altering its swimming behavior. The eyespot is a specialized structure for sensing light, which is assembled de novo at every cell division from components located in two different cellular compartments. Photoreceptors and associated signal transduction components are localized in a discrete patch of the plasma membrane. This patch is tightly packed against an underlying sandwich of chloroplast membranes and carotenoid-filled lipid granules, which aids the cell in distinguishing light direction. In a prior screen for mutant strains with eyespot defects, the EYE2 locus was defined by the single eye2-1 allele. The mutant strain has no eyespot by light microscopy and has no organized carotenoid granule layers as judged by electron microscopy. Here we demonstrate that the eye2-1 mutant is capable of responding to light, although the strain is far less sensitive than wild type to low light intensities and orients imprecisely. Therefore, pigment granule layer assembly in the chloroplast is not required for photoreceptor localization in the plasma membrane. A plasmid-insertion mutagenesis screen yielded the eye2-2 allele, which allowed the isolation and characterization of the EYE2 gene. The EYE2 protein is a member of the thioredoxin superfamily. Site-directed mutagenesis of the active site cysteines demonstrated that EYE2 function in eyespot assembly is redox independent, similar to the auxiliary functions of other thioredoxin family members in protein folding and complex assembly.

Amino Acid Motifs↗
PubMed2000

Contributions of automatic and controlled processes to the analysis of hierarchical structure.

Three experiments provide evidence that 2 mechanisms, 1 automatic and 1 controlled, produce variations in the efficiency with which local and global forms are processed. Targets are identified faster if they appear at the same level (global or local) as the target on the previous trial. M. R. Lamb, B. London, H. M. Pond, and K. A. Whitt (1998) provided evidence that the beneficial effect of level repetition is due to an automatic process that is outside voluntary control. In the present experiments, pretrial cues informed participants as to the level of the upcoming target. Valid cues benefited performance, whereas invalid cues harmed performance relative to noninformative neutral cues. This was so even when the relation between the cue and the level it signaled was arbitrary, indicating that the cues initiated voluntary shifts of attention. The benefit associated with level repetition, however, was unaffected by the cues. These data suggest that the benefit of level repetition results from a process that is not subject to voluntary control.

Adolescent↗
PubMed2000

The role of spatial frequency in cued shifts of attention between global and local forms.

It has been suggested that shifts of attention between global and local forms might be based on selection between, or differential activation of, low- and high-spatial-frequency channels. In the present study, pretrial cues indicated which level (global or local) was likely to contain the target on each trial. There was a response time (RT) advantage for validly cued trials and an RT cost for invalidly cued trials relative to a neutral cue baseline. This cuing effect was the same for broadband stimuli and for contrast-balanced stimuli in which low spatial frequencies were eliminated. Thus, cued attentional shifts between global and local forms occur even when selection cannot be based on spatial frequency.

Adolescent↗
PubMed1999

Is attentional selection to different levels of hierarchical structure based on spatial frequency?

Target identification is faster when the target level (global or local) is the same as that on the previous trial, presumably because attention is directed to the appropriate level. L. C. Robertson (1996) found that eliminating low spatial frequencies by contrast balancing eliminated this level repetition effect and concluded that attentional selection between different levels of structure is based on spatial frequency. In contrast, M. R. Lamb and E. W. Yund (1996a) found no effect of contrast balancing on the level repetition effect and thus concluded that attentional selection is not based on spatial frequency. In this study, the authors identified the procedural difference between the 2 studies responsible for this difference in results and replicated both findings. The data show that spatial frequency is not a necessary basis for attentional selection between global and local forms. Although it remains possible that spatial frequency is the basis of attentional selection under some circumstances, the data supporting this proposition are not yet compelling.

Adult↗
PubMed1999

Eyespot-assembly mutants in Chlamydomonas reinhardtii.

Chlamydomonas reinhardtii is a single-celled green alga that phototaxes toward light by means of a light-sensitive organelle, the eyespot. The eyespot is composed of photoreceptor and Ca(++)-channel signal transduction components in the plasma membrane of the cell and reflective carotenoid pigment layers in an underlying region of the large chloroplast. To identify components important for the positioning and assembly of a functional eyespot, a large collection of nonphototactic mutants was screened for those with aberrant pigment spots. Four loci were identified. eye2 and eye3 mutants have no pigmented eyespots. min1 mutants have smaller than wild-type eyespots. mlt1(ptx4) mutants have multiple eyespots. The MIN1, MLT1(PTX4), and EYE2 loci are closely linked to each other; EYE3 is unlinked to the other three loci. The eye2 and eye3 mutants are epistatic to min1 and mlt1 mutations; all double mutants are eyeless. min1 mlt1 double mutants have a synthetic phenotype; they are eyeless or have very small, misplaced eyespots. Ultrastructural studies revealed that the min1 mutants are defective in the physical connection between the plasma membrane and the chloroplast envelope membranes in the region of the pigment granules. Characterization of these four loci will provide a beginning for the understanding of eyespot assembly and localization in the cell.

Animals↗
PubMed1996

Spatial frequency and attention: effects of level-, target-, and location-repetition on the processing of global and local forms.

Is attentional selection between local and global forms based on spatial frequency? This question was examined by having subjects identify local or global forms of stimuli that had been "contrast balanced," a technique that eliminates low spatial frequencies. Response times (RTs) to global (but not local) forms were slowed for contrast-balanced stimuli, suggesting that low spatial frequencies mediate the global RT advantage typically reported. In contrast, the beneficial effect of having targets appear at the same, as opposed to a different, level as that on the immediately preceding trial was unaffected by contrast balancing. This suggests that attentional selection between different levels of structure is not based on spatial frequency. The data favor an explanation in terms of "priming," rather than in terms of adjustments in the diameter of an attentional "spotlight."

Adolescent↗
PubMed1993

Spatial attention and cuing to global and local levels of hierarchical structure.

Five experiments investigated the role of attention in identifying global and local targets in hierarchically structured patterns. Hierarchical patterns were presented at a variable stimulus onset asynchrony (SOA) either after a cue (4 arrows or shaded boxes) or after no cue. Targets occurred at the cued level 80% of the time and at the uncued level 20%. On uncued trials, target level probability was .5. Global cues produced benefits for both global and local targets over SOA on cued trials. Local cues produced benefits only for local targets. For uncued trials, responses favored local targets overall when interspersed with locally cued trials but favored global targets when interspersed with globally cued trials. The role of an attentional window and discrete distribution of attention over global and local levels of hierarchical patterns are discussed.

Adult↗
PubMed1993

The role of spatial frequency in the processing of hierarchically organized stimuli.

Can spatial frequency differences between local and global forms account for differences in the way different levels of structure are analyzed? We examined this question by having subjects identify local or global forms of hierarchical stimuli that had been contrast balanced. Contrast balancing eliminates low spatial frequencies, so that both local and global forms must be identified on the basis of high spatial frequency information. Response times (RTs) to global (but not local) forms were slowed for contrast-balanced stimuli, suggesting that low spatial frequencies mediate the global RT advantage typically found. In contrast, interference between local and global forms was little affected by contrast balancing or by shifts of attention between local and global forms, suggesting that it does not result from inhibitory interactions between spatial frequency channels or from temporal precedence of low versus high spatial frequency information. Finally, shifts of attention between local and global forms were also little affected by contrast balancing, suggesting that they were not based on spatial frequency.

Adolescent↗
PubMed1991

Neuropsychological contributions to theories of part/whole organization.

Recent evidence from neuropsychology has proven useful in defining neural and cognitive subsystems involved in certain cognitive processes such as spatial attention (Posner, Walker, Friedrich, & Rafal, 1984) and imagery (Farah, 1988; Kosslyn, 1986). In this article we discuss neuropsychological evidence that suggests a modular system is responsible for the organization of parts and wholes by the visual system. We review both normal and neuropsychological evidence that suggest that hierarchical organization of objects within objects involves the coordination of at least four separate subsystems, one that emphasizes the global properties of a figure (association with right posterior superior temporal-parietal regions), one that emphasizes the local properties of a figure (associated with left posterior superior temporal-parietal regions), one that controls the distribution of attentional resources to these subsystems (associated with right or left lateral parietal lobe), and one that interconnects global and local properties and relies on the integrity of posterior temporal-temporal pathways.

Attention↗
PubMed1991

Normal global-local analysis in patients with dorsolateral frontal lobe lesions.

The role of prefrontal cortex in the analysis of global and local levels of a visual stimulus was assessed by measuring reaction time to identify a target at one level or the other. Unlike patients with temporal-parietal lesions (STG), there were no global-local performance deficits in right or left prefrontal groups (RFL or LFL). Reallocation of attention to global and local levels was measured by examining changes in performance when the probability of a target appearing at one level or the other varied. While patients with lateral parietal lesions (IPL) have been shown to have deficits in these conditions, both RFL and LFL showed normal changes in performance. In sum, the performance of prefrontal groups differed from both IPL and STG groups but not from normal controls. These results strengthen previous arguments that posterior association cortex is crucial in responding to global and local levels of a pattern.

Adult↗
PubMed1991

Attention in humans and animals: is there a capacity limitation at the time of encoding?

Investigators have suggested that pigeons have a limited capacity input channel and that as a result performance suffers when information load is great. This information overload hypothesis is based on converging findings from experiments using the matching-to-sample paradigm. Fundamental among these findings are that (a) performance improves as sample duration increases, (b) performance is better when the sample contains one relevant feature (elements) than when it contains two (compounds), and (c) this element superiority effect can be reduced by making the relevant feature of compound samples predictable. Experiments 1 and 2 show that these effects occur for humans as well. However, Experiments 3, 4, and 5 show that in humans at least some of these effects do not result from information overload at the time of encoding. Thus, the assertion that these effects do reflect such a limitation in pigeons must be reevaluated.

Adult↗
PubMed1990

Component mechanisms underlying the processing of hierarchically organized patterns: inferences from patients with unilateral cortical lesions.

Subjects identified target letters that occurred randomly at the local or global level in a divided attention task. The visual angle of the stimuli was varied. Neurologically intact controls showed a reaction time advantage for local targets which increased as visual angle increased. Patients with lesions centered in the posterior superior temporal gyrus (STG) showed a larger local advantage than controls if the lesion was on the right and a global advantage if the lesion was on the left. STG patients were no more influenced by visual angle than were controls. Control subjects also showed the usual interference of global distractors on responding to local targets. STG patients showed little evidence of interference. Control patients with lesions centered in the rostral inferior parietal lobe performed normally. The findings suggest that several component mechanisms are involved in the processing of hierarchical levels of structure, each linked to specific anatomical regions.

Attention↗
PubMed1990

The effect of visual angle on global and local reaction times depends on the set of visual angles presented.

It has been shown that there is a transition from a global to a local advantage in reaction time as visual angle increases (Kinchla & Wolfe, 1979), and it has been assumed that this transition reflects lower level (e.g., retinal) processes. In three experiments, we examined whether higher level (e.g., attentional) processes play a role in this transition. In each experiment, subjects received a different stimulus set in each of two blocks of trials. In Experiment 1, stimuli subtending 1.5 degrees, 3 degrees, 4.5 degrees, or 6 degrees of visual angle vertically (small-stimuli set) were randomly presented in one block, while the other block consisted of random presentations of 3 degrees, 6 degrees, 9 degrees, or 12 degrees stimuli (large-stimuli set). The subjects' task was to identify targets that appeared randomly at either the local or the global level. It was found that the transition from a global to a local reaction-time advantage took place at a larger visual angle for the large-stimuli set than for the small-stimuli set. The same effects of stimulus set were found in Experiment 2, in which the small-stimuli set included 1.5 degrees, 3 degrees, or 6 degrees stimuli while the large-stimuli set included 3 degrees, 6 degrees, or 9 degrees stimuli. In Experiment 3, eye position was monitored to rule out the possibility that subjects adopted different fixation strategies depending on which stimulus set was being presented. The findings suggest that attention plays a major role in determining the relative speed of processing of local-and global-level information.

Aged↗
PubMed1989

Attention and interference in the processing of global and local information: effects of unilateral temporal-parietal junction lesions.

The processing of hierarchical stimuli was examined in patients with lesions in the temporal-parietal junction. In separate blocks of trials, subjects were instructed to identify the letter at the local or the global level of a hierarchical stimulus. Consistent with previous findings, reaction times for controls were faster in the globally than in the locally directed condition and reaction times to the local level were longer when the letters at the two levels were different (e.g. local "S"s forming a global "H") than when they were the same (e.g. local "S"s forming a global "S"). In other words, controls exhibited interference when locally directed. Patients with lesions centered in the rostral inferior parietal lobe (IPL) showed interference effects similar to controls. In contrast, patients with lesions centered in the posterior superior temporal gyrus and adjacent caudal inferior parietal lobe (STG) showed no interference. The data suggest that the posterior superior temporal plane and adjacent caudal inferior parietal lobe plays an important role in the integration of and/or attention to local and global level information.

Adult↗
PubMed1989

Judging the reflection of misoriented patterns in the right and left visual fields.

The reported study demonstrates that performance asymmetries between normal or reflected letters presented in the right and left visual field favors the right visual field when stimulus patterns are blocked and rotated 90 degrees clockwise and favors the left visual field when they are blocked and rotated 90 degrees counterclockwise. When stimuli are rotated, the orientations of the patterns define a coordinate system that is independent of upright. It is suggested that our results are due to subjects sampling the different areas in the visual field relative to this coordinate system. The right visual field was given priority when the patterns were oriented so their "tops" were pointing rightward and the left visual field was given priority when the patterns were oriented so their tops were pointing leftward. Neither differences in the stimulus image on the retina in the two rotated conditions, eye movements, nor direct access to one hemisphere or the other could account for these effects.

Adult↗
PubMed1989

Do response time advantage and interference reflect the order of processing of global- and local-level information?

Navon's (1977) global precedence hypothesis was based primarily on the joint occurrence of two effects: a response time (RT) advantage for identifying global targets, and interference by global distractors on responding to local targets. Although the hypothesis has been questioned on the basis of experiments in which it has been shown that a local RT advantage and local interference can occur, it is still frequently assumed that these two effects are a valid measure of the order in which local and global levels of structure are processed. In the present experiment, this assumption was examined. Subjects identified target letters that occurred randomly at the global or local level in a divided-attention task. The visual angle subtended by the stimulus pattern was varied, a manipulation known to affect the relative speed of response to local- or global-level information. Local targets were identified faster than global targets at the larger visual angles, but there was no difference in RT at the smallest visual angle. Despite this change in RT advantage, the interference effect did not change as a function of the visual angle of the stimulus pattern. Moreover, global distractors interfered with responding to local targets but local targets had no effect on responding to global targets, which is exactly the opposite of the finding one would expect if RT advantage and interference reflected order of processing. These findings are not consistent with the assumption that RT advantage and interference reflect order of processing in a simple way.

Aged↗
PubMed1988

The role of perceptual reference frames in visual field asymmetries.

The direct access model of hemispheric asymmetry was tested in a letter reflection (normal of reflected) judgement task. In the baseline condition letters were presented in their upright orientation, and reaction times were faster to letters presented in the right visual field than to those presented in the left visual field. In two other conditions the slides, and thus the letters, were rotated +90 degrees clockwise (Rotated +90) from upright or -90 degrees counterclockwise (Rotated -90). This resulted in the letters in both rotated conditions being shown in the upper or lower visual field through the sagittal plane but rotated 90 degrees. Despite this fact a "right field" advantage was again found when the right field was defined relative to the orientation of the tops of the rotated letters (lower visual field in the Rotated +90 condition and upper visual field in the Rotated -90 condition). These results demonstrate that the internal representation of locations in space is more important in predicting visual field asymmetries, at least in the present task, than the field of stimulus presentation relative to the fovea.

Adult↗