Biomedical subjects

Adam Reeves

Publications and source records attributed to Adam Reeves.

10 recordsLinked to original sources

PubMed2006

Isolation and characterization of dominant mutations in the Bacillus subtilis stressosome components RsbR and RsbS.

The general stress response of Bacillus subtilis is controlled by the activity state of the sigma(B) transcription factor. Physical stress is communicated to sigma(B) via a large-molecular-mass (>10(6)-Da) structure (the stressosome) formed by one or more members of a family of homologous proteins (RsbR, YkoB, YojH, YqhA). The positive regulator (RsbT) of the sigma(B) stress induction pathway is incorporated into the complex bound to an inhibitor protein (RsbS). Exposure to stress empowers an RsbT-dependent phosphorylation of RsbR and RsbS, with the subsequent release of RsbT to activate downstream processes. The mechanism by which stress initiates these reactions is unknown. In an attempt to identify changes in stressosome components that could lead to sigma(B) activation, a DNA segment encoding these proteins was mutagenized and placed into B. subtilis to create a merodiploid strain for these genes. Eight mutations that allowed heightened sigma(B) activity in the presence of their wild-type counterparts were isolated. Two of the mutations are missense changes in rsbR, and six are amino acid changes in rsbS. Additional experiments suggested that both of the rsbR mutations and three of the rsbS mutations likely enhance sigma(B) activity by elevating the level of RsbS phosphorylation. All of the mutations were found to be dominant over wild-type alleles only when they are cotranscribed within an rsbR rsbS rsbT operon. The data suggest that changes in RsbR can initiate the downstream events that lead to sigma(B) activation and that RsbR, RsbS, and RsbT likely interact with each other concomitantly with their synthesis.

Alleles↗

PubMed2006

Lighting, backlighting and watercolor illusions and the laws of figurality.

We report some novel 'lighting' and 'backlighting' effects in plane figures similar to those which induce the 'watercolor illusion', that is, figures made with outlines composed of juxtaposed parallel lines varying in brightness and chromatic color. These new effects show 'illumination' as an emergent percept, and show how arrangements of 'dark and light' along the boundaries of various plane figures model the volume and strengthen the illusion of depth. To account for these various effects we propose several phenomenological 'laws of figurality' to add to the Gestalt laws of organization and figure-ground segregation. We offer a set of meta-laws which are speculative but which serve to integrate and organize the phenomenological laws. These laws indicate how luminance gradient profiles across boundary contours define both the 3D appearance of figures and the properties of the light reflected from their volumetric shapes.

Adaptation, Ocular↗

PubMed2005

The role of attention in binding shape to color.

Pictures of easily-identifiable objects with novel colors (e.g. a blue frog) or of forms with arbitrary colors (e.g. a green triangle) were presented briefly at 10.6 degrees eccentricity. Stimuli had strong outlines and vivid fill colors (red, green, yellow, blue, or purple). The same pictures were repeated once in each block of 30 trials for 6, 9, or 12 blocks, and recognition was probed after each block. Shapes were acquired quickly, within 3-4 blocks, whether attention was focused on the pictures or split to a demanding foveal task. Color-shape acquisition was also fast with focused attention, but stabilized at a low level with split attention. Delaying the foveal task restored color-shape acquisition. We suggest that attention facilitates the creation and maintenance of novel color-shape bindings in the visual periphery; without attention, binding is less effective.

Adult↗

PubMed2005

Coexpression patterns of sigma(B) regulators in Bacillus subtilis affect sigma(B) inducibility.

RsbT is an essential component of the pathway that activates the Bacillus subtilis sigma(B) transcription factor in response to physical stress. rsbT is located within an operon that includes the genes for its principal negative regulator (RsbS) and the stress pathway component that it activates (RsbU), as immediate upstream and downstream neighbors. In the current work we demonstrate that RsbT's ability to function is strongly influenced by coexpression with these adjoining genes. When rsbT is expressed at a site displaced from rsbS and rsbU, RsbT accumulates but it is unable to activate sigma(B) following stress. RsbT activity is restored if rsbT is cotranscribed at the alternative site with the genes that normally abut it. Additionally, an rsbS allele whose product allows constitutively high RsbT-dependent sigma(B) activity displays this activity in rsbS merodiploid strains only when cotranscribed with rsbT and is recessive to a wild-type rsbS allele only if the wild-type rsbS gene is not cotranscribed with an rsbT gene of its own. The data suggest that RsbS and RsbT are synthesized in equivalent amounts and interact coincidently with their synthesis to form stable regulatory complexes that maintain RsbT in a state from which it can be stress activated.

Bacillus subtilis↗

PubMed2005

A random-ray model for speed and accuracy in perceptual experiments.

We present a 'random ray' model to describe Yes/No reaction times (RTs) and errors in perceptual experiments. The ray model is analogous to a random walk, but it is computationally simpler, requiring only elementary geometry. Ray parameters control the drift rates to the Yes and No decision boundaries, bias, and a termination or 'time-out' rule. Rays are normally distributed, but predicted RT distributions are skewed by projection onto the boundaries. Model parameters can be estimated directly from the 16th, 50th, and 84th percentiles of the RT distributions on hit, correct rejection, false alarm, and miss trials, if the data satisfy three easily testable constraints. Examples are given from visual search and object recognition.

Attention↗

PubMed2004

The roles of distractor noise and target certainty in search: a signal detection model.

Observers searched arrays of briefly presented near-isoluminant colored disks for a single disk of known color (feature search) or unknown color (oddity search). Speed and accuracy were converted to a single, model-based measure of performance (Perf), in units of (d')2 per second of latency. Perf decreased with set size in feature search and increased in oddity. In both types of search, grouping the distractors, making them homogeneous in color, and reducing their saturation, all increased Perf. These commonalities suggested an SDT-based model in which distractors increase noise in the same way in both types of search. However, in oddity, though not in feature search, distractors must be attended and so adding distractors also boosts the effective target contrast, overcoming the added noise. A model with two free parameters for noise and one for attention accounted for every combination except for oddity searches among heterogeneous grouped distractors.

Adolescent↗

PubMed2004

Transient lumanopia at high intensities.

Transient lumanopia is the loss of sensitivity to a flicker burst presented in early dark adaptation. We earlier reported that lumanopia with 18 Hz flicker was greatest after turning off 400 td adapting fields and progressively disappeared as the adapting field was intensified. We now report that lumanopia can occur with intense adapting fields, but only with faster flickers (e.g. 40 Hz, 5000 td fields). Dimming the field (but not brightening it) can also produce lumanopia. The results illustrate frequency-dependent attenuation by a temporal filter whose parameters are set by light adaptation and which change abruptly when the field is dimmed or extinguished.

Dark Adaptation↗

PubMed2004

Rapid iconic erasure without masking.

We report on the erasure of the iconic memory of an array of 12 black letters flashed on a continuously- present white field. Erasure is accomplished by replacing the 16 ms letter array (frame 1) with a blank white frame for 16 ms (frame 2). The letter array returns in frame 3, with from one to six letters missing. Report of the missing letters is accurate without the blank white frame but is impoverished with it, as if interposing the blank erases the icon. Erasure occurs without any obvious luminance masking, 'mud splashes', pattern masking (backward, forward, or metacontrast), lateral masking, or masking by object substitution. Erasure is greatly decreased if the blank is presented one frame earlier or later. We speculate that erasure is due to a rapid reset of the icon produced by an informational mis-match.

Humans↗

PubMed2002

The use of word-picture verification to study entry-level object recognition: further support for view-invariant mechanisms.

There are substantial logical and empirical reasons for rejecting the popular view that plane-misoriented objects are identified after normalization of global orientation. Our subjects determined the entry-level identity of common objects (line drawings) before determining basic orientation (upright vs. rotated)--as they must if they are to begin to know how to restore the image to the canonical upright. We used a description-picture matching procedure in which 7596 of the trials involved a mismatch in identity, orientation, or both, so that comparisons could be based on one response (no). Times to verify identity were faster than times to verify orientation and did not increase with rotations in the picture plane. That mismatch objects were positively identified at the entry level was shown both in a surprise recognition test for object names and through the transfer of priming from the matching task to a subsequent object-naming task. We conclude that classic mental rotation-like effects on naming times do not reflect early object encoding and recognition processes, which are view invariant, but may stem from double-checking at a postrecognition verification stage.

Adult↗

PubMed2002

Generating visual mental images: latency and vividness are inversely related.

In three experiments, participants pressed a reaction time key and rated vividness after reading descriptions of common objects and imagining them as accurately as possible within a visual display subtending 1.2 degrees or 10.8 degrees. Display size had small effects on vividness and on image latency. Latency was much faster (approximately 2.5 sec) for vivid images than for nonvivid ones (approximately 7.5 sec), regardless of display size and initial size at which the images were generated (natural image size). Our findings characterize images as (1) detailed, as indicated by ratings of detail and by lack of category membership effects; (2) as occurring at many natural image sizes, so that time-consuming size adjustments are typically not needed; and (3) generated not concurrently with but after lexical access, as shown by latencies for the same image evoked by synonyms like pail and bucket. We conclude that image vividness and latency reflect the activation level of the visual imagery system, implying that image generation in everyday life mostly involves retrieving already-computed representations episodically stored in memory, which seldom require extensive re-elaboration such as size adjustment.

Humans↗