Biomedical subjects

Marisa Carrasco

Publications and source records attributed to Marisa Carrasco.

4 recordsLinked to original sources

PubMed2003

Speed of visual processing increases with eccentricity.

The visual system has a duplex design to meet conflicting environmental demands: the fovea has the resolution required to process fine spatial information, but the periphery is more sensitive to temporal properties. To investigate whether the periphery's sensitivity is partly due to the speed with which information is processed, we measured the full timecourse of visual information processing by deriving joint measures of discriminability and speed, and found that speed of information processing varies with eccentricity: processing was faster when same-size stimuli appeared at 9 degrees than 4 degrees eccentricity, and this difference was attenuated when the 9 degrees stimuli were magnified to equate cortical representation size. At the same eccentricity, larger stimuli are processed more slowly. These temporal differences are greater than expected from neurophysiological constraints.

Differential Threshold↗

PubMed2002

Covert attention affects the psychometric function of contrast sensitivity.

We examined the effect of transient covert attention on the psychometric function for contrast sensitivity in an orientation discrimination task when the target was presented alone in the absence of distracters and visual masks. Transient covert attention decreased both the threshold (consistent with a contrast gain mechanism) and, less consistently, the slope of the psychometric function. We assessed performance at 8 equidistant locations (4.5 degrees eccentricity) and found that threshold and slope depended on target location-both were higher on the vertical than the horizontal meridian, particularly directly above fixation. All effects were robust across a range of spatial frequencies, and the visual field asymmetries increased with spatial frequency. Notwithstanding the dependence of the psychometric function on target location, attention improved performance to a similar extent across the visual field.Given that, in this study, we excluded all sources of external noise, and that we showed experimentally that spatial uncertainty cannot explain the present results, we conclude that the observed attentional benefit is consistent with signal enhancement.

Adult↗

PubMed2002

Covert attention increases spatial resolution with or without masks: support for signal enhancement.

Visual attention can increase spatial resolution even when it leads to a decrease in performance. Whether this effect is mediated by reduction of external noise or by signal enhancement is an unsettled question. Although we previously demonstrated that attention can improve speed and accuracy in an acuity task, those experiments made use of a local postmask, which could be considered a source of external noise. In this work, a peripheral cue improved observers' abilities to indicate which side of a Landolt-square target had a gap whether or not a local postmask was used and with both central- and spread-neutral cues. In addition, we documented the presence of visual field inhomogeneities in a resolution task. Given that these experiments presented the target alone with no external noise added (i.e., without distracters or masks), our results indicate that transient attention enhanced the quality of the stimulus representation. Furthermore, because performance in the Landolt-square task indexes resolution, this attentional benefit indicates that transient attention can produce signal enhancement through finer spatial resolution.

Attention↗

PubMed2002

Vertical meridian asymmetry in spatial resolution: visual and attentional factors.

We investigated whether spatial resolution would be the same in the lower and upper halves of the vertical meridian (VM) of our visual field and whether attention would affect them differentially. It has been reported that (1) attending to the target's location improves performance in a texture segregation task when the observer's spatial resolution is too low (peripheral locations) but impairs it when resolution is already too high (central locations) for the task. This finding indicates an enhanced spatial resolution at the attended location (Yeshurun & Carrasco, 1998,2000), (2) observers' contrast sensitivity is higher in the lower than in the upper VM, a phenomenon known as vertical meridian asymmetry (VMA), an asymmetry determined by visual rather than by attentional factors (Carrasco, Talgar, & Cameron, 2001). In the present texture segregation task, performance was assessed under neutral- and peripheral-cue conditions. Transient covert attention was systematically manipulated by using a peripheral cue that indicated the target's location and its onset. Observers reported the interval containing a target patch appearing at one of a number of eccentricities in a large texture pattern along the VM. We found that (1) performance peaked at farther eccentricities in the lower than in the upper visual VM, indicating that resolution was higher in the lower half, and (2) the peripheral cue affected performance along the VM uniformly, indicating that the degree of enhanced resolution brought about by transient attention was constant along the VM. Thus, we conclude that the VMA for spatial resolution is determined by visual, not transient covert attentional, constraints.

Adult↗