Biomedical subjects

Bruno G Breitmeyer

Publications and source records attributed to Bruno G Breitmeyer.

9 recordsLinked to original sources

PubMed2006

Target recovery in metacontrast: the effect of contrast.

The visibility of a target stimulus (T) can be reduced by an aftercoming and spatially non-overlapping mask stimulus (M1), a phenomenon known as metacontrast masking. Interestingly, the visibility of the masked target can be recovered when a secondary mask (M2) is added to the T-M1 sequence. We analyzed a computational model of retino-cortical dynamics (RECOD) and derived the prediction that contrast dependence of metacontrast and target recovery should parallel the contrast dependence of afferent magnocellular and parvocellular pathways, respectively. In a psychophysical experiment, we tested this prediction by systematically varying (a) M2's contrast and (b) the M1-M2 onset asynchrony (SOA). At the optimal M1-M2 SOA, target recovery effect increased with M2's contrast without saturating, but at the optimal M1-M2 metacontrast SOA, reduction of M1's visibility saturated very rapidly as M2's contrast increased. Quantitative comparisons of psychophysical results with model simulations provide support for our prediction. We conclude that metacontrast masking is driven by signals originating from the magnocellular pathway and target recovery in metacontrast is driven by signals originating from the parvocellular pathway.

Adult↗

PubMed2006

Meta- and paracontrast reveal differences between contour- and brightness-processing mechanisms.

We investigated meta- and paracontrast masking using tasks requiring observers to judge the surface brightness or else the contours of target stimuli. The contour task revealed strongest metacontrast at SOAs shorter than those obtained for the brightness task. Paracontrast revealed related temporal differences between the tasks. Additionally, the paracontrast results support the existence not only of prolonged inhibitory effects but also of facilitatory effects. The combined results comport with the existence of cortical mechanisms for: (i) fast contour processing, (ii) slow surface-brightness processing, (iii) prolonged inhibition, and (iv) facilitation.

Adult↗

PubMed2006

The path of visual attention.

Visual cuing is one paradigm often used to study object- and space-based visual selective attention. A primary finding is that shifts of attention within an object can be accomplished faster than equidistant shifts between objects. The present study used a visual cuing paradigm to examine how an object's size (i.e., internal distance) and shape, influences object- and space-based visual selective attention. The first two experiments manipulated object size and compared attentional shift performance with objects where the within-object distance between cued and uncued target locations was either equal to the between-object distance (1:1 ratio condition) or three times the between-object distance (3:1 ratio condition). Within-object shifts took longer for the larger objects, but an advantage over between-object shifts was still evident. Influences associated with the shapes of the larger objects suggested by the results of the first two experiments were tested and rejected in Experiment 3. Overall, the results indicate that within-object shifts of attention become slower as the within-object distance increases, but nevertheless are still accomplished faster than between-object shifts.

Analysis of Variance↗

PubMed2005

Visual processing in schizophrenia: Structural equation modeling of visual masking performance.

Schizophrenic patients consistently demonstrate performance deficits on visual masking procedures. In visual masking, the subject's ability to process a target stimulus is reduced by another stimulus (mask) presented either before (forward masking) or after (backward masking) the target. Masking procedures employed in schizophrenia research have used several experimental paradigms. Most early studies have used high-energy masks (i.e., the mask is stronger than the target) and spatially overlapping target and mask. More recently, studies have begun to employ relatively weak (i.e., low-energy) masks, as well as masks that surround, but do not spatially overlap, the target. Data for forward and backward masking components of four masking conditions (target location and identification with a high-energy mask, target identification with a low-energy mask, and target identification with equal energy paracontrast/metacontrast) were collected from 75 patients with schizophrenia. Based on theoretical distinctions among masking procedures, we compared four models of visual masking using structural equation modeling. Although high zero-order correlations were found among the masking parameters, a four-factor model, in which factors were separated on the type of response (target location and identification), the shape of the function (monotonic and non-monotonic), and the overlap of the stimuli (overlapping and non-overlapping), provided the best fit for the data. These findings suggest that the four masking procedures used in this study may tap unique aspects of visual processing and are not redundant. The results also support theories of the different mechanisms underlying performance on these measures.

Adult↗

PubMed2004

Unconscious priming by color and form: different processes and levels.

Using a metacontrast masking paradigm, prior studies have shown (a) that a target's color information and form information, can be processed without awareness and (b) that unconscious color processing occurs at early, wavelength-dependent levels in the cortical information processing hierarchy. Here we used a combination of paracontrast and metacontrast masking techniques to explore unconscious color and form priming effects produced by blue, green, and neutral stimuli. We found that color priming in normal observers is significantly reduced when an additional paracontrast mask precedes the target at optimal masking SOAs. However, no reduction of form-priming effects was obtained at similar optimal paracontrast SOAs. We conclude that unconscious color priming depends on an early, wavelength- or stimulus-dependent response of color neurons located at early cortical levels whereas unconscious form priming occurs at later levels.

Adult↗

PubMed2004

A comparison of masking by visual and transcranial magnetic stimulation: implications for the study of conscious and unconscious visual processing.

Visual stimuli as well as transcranial magnetic stimulation (TMS) can be used: (1) to suppress the visibility of a target and (2) to recover the visibility of a target that has been suppressed by another mask. Both types of stimulation thus provide useful methods for studying the microgenesis of object perception. We first review evidence of similarities between the processes by which a TMS mask and a visual mask can either suppress the visibility of targets or recover such suppressed visibility. However, we then also point out a significant difference that has important implications for the study of the time course of unconscious and conscious visual information processing and for theoretical accounts of the processes involved. We present evidence and arguments showing: (a) that visual masking techniques, by revealing more detailed aspects of target masking and target recovery, support a theoretical approach to visual masking and visual perception that must take into account activities in two separate neural channels or processing streams and, as a corollary, (b) that at the current stage of methodological sophistication visual masks, by acting in more highly specifiable ways on these pathways, provide information about the microgenesis of form perception not available with TMS masks.

Consciousness↗

PubMed2004

Unconscious color priming occurs at stimulus- not percept-dependent levels of processing.

Although past studies have shown that visual information can be processed without awareness, the types and levels of this processing have yet to be determined. We used metacontrast masking to explore unconscious priming effects of white, blue, and green stimuli generated on a color video display. We found that a white prime tends to act more like a green than a blue one. Color confusions among unmasked and masked primes and calibrations of the display phosphors show that physical rather than perceptual properties of the stimuli best explain the white prime's effects. We conclude that unconscious color priming in normal observers occurs at early wavelength-dependent levels of processing prior to later color-percept-dependent levels.

Color Perception↗

PubMed2003

The what and where in visual masking.

A metacontrast mask suppresses the visibility of, without influencing the reaction time (RT) to, the target. We investigated whether this dissociation results from a sensori-motor pathway immune to masking effects or from the characteristics of stimulus timing in mutually inhibitory sustained and transient channels. For target visibility, para- and metacontrast yielded the usual U-shaped functions. Peak paracontrast occurred at stimulus onset asynchronies (SOAs) of -150 to -100 ms. RTs were relatively low for metacontrast and did not show a systematic change as a function of SOA. The RT contribution from contour-masking was greatest at an SOA of -150 ms (paracontrast) and declined to near zero in the metacontrast regime. The dissociation between visibility and RT seen in metacontrast did not occur in paracontrast, rejecting the theory that RTs are elicited by a single sensori-motor pathway immune to masking. The dependence of the dissociation on stimulus timing can be explained by RECOD, a dual-pathway model wherein fast and slow activities interact.

Contrast Sensitivity↗

PubMed2002

In support of Pockett's critique of Libet's studies of the time course of consciousness.

Susan Pockett presents sound arguments supporting her reinterpretations of data that Libet and co-workers used to support a number of intriguing and influential conclusions regarding the microgenesis and timing of (a) conscious sensory experience and (b) volitionally controlled motor responses. The following analysis, extending and elaborating some of her main arguments, proposes that Libet's experimental methodologies and rationales, and thus also his interpretation of data, are flawed and that neglect or ignorance of methodological and empirical constraints well known to sensory psychologists risks drawing premature or faulty conclusions about the timing of conscious experience.

Chronobiology Phenomena↗