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Biomedical subjects

D G Purcell

Publications and source records attributed to D G Purcell.

13 recordsLinked to original sources
PubMed1998

A word-superiority effect in the presence of foveal load.

Foveal stimuli have been shown to disrupt visual information processing in the parafovea and periphery by their mere presence. In the present study, 6 subjects were presented letter triads 3.58 degrees to the right or left of the point of fixation. At the same time, a single letter was presented at the point of fixation that was either the same as the middle letter in the triad or different from any of the triad letters. On other trials, no letter was presented at the point of fixation. Analysis indicated a word superiority effect when a foveal letter was presented that was the same as the letter in the triad. Performance between words and nonwords did not differ significantly when the foveal letter was different or absent. It was concluded that the mere presence of foveal load alone is not disruptive to performance. Depending on the visual context of the target to be reported, the presence of a foveal stimulus may improve performance.

Discrimination, Psychological↗
PubMed1996

The effects of foveal load and visual context on peripheral letter recognition.

Peripheral letter recognition was examined by presenting subjects with a letter triad centered 7.16 degrees to the right of fixation. At the same time, a single letter was presented at the point of fixation that was either the same as one of the letters in the triad or different from any of the triad letters. On other trials, no letter was presented at the point of fixation. Results supported and extended previous foveal load research. The disruptive effect of foveal load does not depend solely on the presence or absence of a foveal stimulus. Recognition of the first letter in non-words was disrupted when a foveal letter was presented that was different from the peripheral letter. Recognition of the middle letter for both words and non-words was disrupted when a foveal letter was presented that was the same as the middle letter. A significant interaction between foveal letter and triad type was also found. No evidence was found to suggest that recognition of a peripheral letter is significantly better when that same letter is also present at the point of fixation.

Adult↗
PubMed1996

It takes a confounded face to pop out of a crowd.

It is widely reported that a picture of an angry face seems to figuratively pop out of an array of happy faces, although all of these reports are based on a single experiment by Hansen and Hansen. Pop out, when it occurs, indicates that an observer has located the target by means of a preattentive, parallel search. Hansen and Hansen concluded that it was the affect displayed by the face which caused it to pop out from its surrounding distracters. However, Hansen and Hansen's angry faces contained extraneous dark areas which were introduced when they transformed Ekman and Friesen's photographs of angry and happy faces into black-on-white sketches. When the original artifact-free gray-scaled versions of angry and happy faces were used no evidence for pop out was found. All target faces were found during a serial, self-terminating search regardless of their expression. The angry face in Hansen and Hansen's experiments may have popped out from a crowd of happy faces because of a contrast artifact inadvertently introduced when they created their stimuli.

Artifacts↗
PubMed1994

The responses of neurons in the temporal cortex of primates, and face identification and detection.

The ability of a human observer to detect the presence of a briefly flashed picture of a face can depend on the picture's spatial configuration, that is on whether its features are rearranged (jumbled) or are in their normal configuration. The face-detection effect (FDE) is found under conditions of backward masking, when the presence of a face can be detected with shorter masking intervals when it is in the normal than when in the rear-ranged configuration. A similar effect is found when the subject is asked to classify the face as rearranged or not - the face-classification effect (FCE). Part of the interest of the FDE and the FCE is that they show how the configuration of a stimulus can be an important factor in the perceptual processing which leads to detection and classification of the stimulus. To analyse these effects we recorded from single neurons in the cortex in the superior temporal sulcus of macaques when they were shown (in a visual fixation task) normal and rearranged faces under backward masking conditions shown in experiments 2 and 3 to produce, with the same apparatus, the FCE, and also to produce comparable effects on the identification of which face was present (called hereafter the face-identification effect), and also of the clarity of the face. We found in experiment 1 that there are some face-selective neurons which respond to faces only, or better, when the features in the faces are in their normal configuration rather than rearranged. We also showed in this experiment that the difference in the response to the normal as compared to the rearranged faces became greater when the masking stimulus was delayed more. Thus, at intermediate delays, there are more neurons active for the normal than for the rearranged face. We therefore propose that the FDE, the FCE, and the face-identification effect arise because the total number of neurons activated by faces in their normal configuration is greater than that activated by rearranged faces, because of the sensitivity of some face-selective neurons to the spatial arrangement of the features. The experiments also show that backward visual masking does produce abrupt termination of the firing of neurons in the temporal cortical visual system, so that the duration of a neuronal response is very short when visual stimuli can just be perceived.

Analysis of Variance↗
PubMed1991

The object-detection effect: configuration enhances perception.

Line drawings used by Weisstein and Harris (1974) are seen as box-like three-dimensional figures if the lines are arranged properly. A flat two-dimensional pattern is seen when these same lines are disarranged. A target line contained within the three-dimensional figure is identified more readily than is the same line contained within a two-dimensional figure. This finding was extended in the present experiments: The three-dimensional stimulus was detected more quickly than the two-dimensional stimulus, under conditions of visual backward masking. Three-dimensional stimuli were also classified more quickly than two-dimensional stimuli. Just as with the face-detection effect and the word-detection effect, object detection can be affected by the form of the visual stimulus.

Adult↗
PubMed1982

Recovery and nonmonotone masking effects.

Recovery was produced by a homogeneous flash of light (M2). With M2 absent, correct letter report was a U-shaped curve when plotted against the interval separating the onset of a letter target and the onset of a patterned masking stimulus (M1). With the homogeneous flash of light ed to the stimulus sequence (target + M1 + M2), recovery occurred for all the shortest delays between the onsets of target and M1. Recovery peaked at a constant separation between the onsets of the target and M2, regardless, of the separation between the onsets of the target and M1. Current explanations of recovery cannot account for this result.

Adult↗
PubMed1967

Recovery of masked visual targets by inhibition of the masking stimulus.

Theories of visual backward masking all assume that a masked target is eliminated from the visual system. Experiments on reaction time to masked signals suggest otherwise, as does a recent demonstration that a masked target can be restored to phenomenal awareness by backward masking of the target's mask. Two experiments are reported here that substantiate the possibility of recovering a masked target, by using different stimulus materials and a more elaborate design than was employed in the first demonstration of this effect.

Humans↗