Biomedical subjects

Charles A Heywood

Publications and source records attributed to Charles A Heywood.

5 recordsLinked to original sources

PubMed2005

Object onset and parvocellular guidance of attentional allocation.

The parvocellular visual pathway in the primate brain is known to be involved with the processing of color. However, a subject of debate is whether an abrupt change in color, conveyed via this pathway, is capable of automatically attracting attention. It has been shown that the appearance of new objects defined solely by color is indeed capable of modulating attention. However, given evidence suggesting that the visual system is particularly sensitive to new onsets, it is unclear to what extent such results reflect effects of color change per se, rather than effects of object onset. We assessed attentional capture by color change that occurred as a result of either new objects appearing or already-present "old" objects changing color. Results showed that although new object onsets accrued attention, changing the color of old objects did not. We conclude that abrupt color change per se is not sufficient to capture attention.

Adult↗

PubMed2004

Visual salience in the change detection paradigm: the special role of object onset.

The relative efficacy with which appearance of a new object orients visual attention was investigated. At issue is whether the visual system treats onset as being of particular importance or only 1 of a number of stimulus events equally likely to summon attention. Using the 1-shot change detection paradigm, the authors compared detectability of new objects with changes occurring at already present objects--luminance change, color change, and object offset. Results showed that appearance of a new object was less susceptible to change blindness than changes that old objects could undergo. The authors also investigated whether it is onset per se that leads to enhanced detectability or onset of an object representation. Results showed that the onset advantage was eliminated for onsets that did not correspond with the appearance of a new object. These findings suggest that the visual system is particularly sensitive to the onset of a new object.

Attention↗

PubMed2003

Achromatopsia, color vision, and cortex.

Brain damage can entirely abolish color vision in cases of complete achromatopsia. Other processes that depend on wavelength differences, however, can be retained. Form and motion defined by pure color differences can be perceived readily even when the colors themselves cannot be told apart. The loss of color vision in cerebral achromatopsia has been equated with the loss of a "color center" presumed indispensable for the phenomenal experience of hue. The "color center" has been assigned a role in the cortical construction of color, specifically in implementing the computations that underlie color constancy. Many features of the condition are consistent with this account. Other neurologic patients, however, retain conscious experience of hue, yet fail to disentangle the illuminant and the reflectance properties of surfaces. For them, color experience is determined by the wavelength composition of light reflected from a surface. If their wavelength-dependent vision is mediated by activity in early visual areas, then it is difficult to understand why these areas are unable to perform a similar role when they remain intact in achromatopsic observers. The prevalence of cells in the ventral visual areas of the monkey brain that code color and the further fractionation of color-related areas in human observers revealed by functional imaging suggest multiple color areas. Their different contributions are only just beginning to become apparent.

Brain Diseases↗

PubMed2003

Visual sensitivity in search tasks depends on the response requirement.

This paper reports a comparison between two tasks of visual search. Two observers carried out, in separate blocks, a saccade-to-target task and a manual-target-detection task. The displays, which were identical for the two tasks, consisted of a ring of eight equally spaced Gabor patches. The target could be defined by a difference from the distractors along four possible dimensions: orientation, spatial frequency, contrast or size. These four dimensions were used as variables in separate experiments. In each experiment, performance was measured over an extensive range of values of the particular dimension. Thresholds were thus obtained for the saccade and the manual response tasks. The nature of the response was found to modify the relative visual sensitivity. For orientation differences, manual response performance was better than saccade-to-target performance. The reverse was true for spatial frequency and contrast differences, where saccade-to-target performance was better than manual response performance. We conclude that saccade-selection in a search task draws on different visual information from that used for manual responding in the equivalent task. The two tasks thus differ in more than the different response systems used: the results suggest the action of different underlying neural visual mechanisms as well as different neural motor mechanisms.

Contrast Sensitivity↗

PubMed2003

Detectability of onsets versus offsets in the change detection paradigm.

The human visual system is particularly sensitive to abrupt onset of new objects that appear in the visual field. Onsets have been shown to capture attention even when other transients simultaneously occur. This has led some authors to argue for the special role that object onset plays in attentional capture. However, evidence from the change detection paradigm appears contradictory to such findings. Studies of change blindness demonstrate that the onset of new objects can often go unnoticed. Assessing the relative detectability of onsets compared with other visual transients in a change detection procedure may help resolve this contradiction. We report the results of four experiments investigating the efficacy with which onsets capture attention compared with offsets. In Experiment 1, we employed a standard flicker procedure and assessed whether participants were more likely to detect the change following a frame containing an onset or following a frame containing an offset. In Experiment 2, we employed the one-shot method and investigated whether participants detected more onsets or offsets. Experiment 3 used the same method but assessed whether onsets would be detected more rapidly than offsets. In Experiment 4, we investigated whether the effect obtained in Experiments 1-3 using simple shapes would replicate when images of real-world objects were used. Results showed that onsets were less susceptible to change blindness than were offsets. We argue that the preservation of information is greater in onsets than in offsets.

Attention↗