Catherine Craver-Lemley

Visual Imagery Selectively Reduces Vernier Acuity

Mental imagery interferes with perception. This, an example of the ‘Perky effect’, was studied for vernier acuity. Mean accuracy for reporting the offset of vertical line targets declined from 80% to 65% when subjects were requested to imagine vertical lines near fixation. Images of horizontal lines or of a grey mist in the fixation region lowered accuracy to a similar extent. However, accuracy was barely affected when the image was requested 1.5 deg or more from the target. The Perky effect remained strong for at least 4 s after an instruction to ‘clear’ the image away. The results were not due to imagery-induced changes in fixation, pupil diameter, or accommodation, or (at least primarily) to central attentional or decisional factors. Rather, imagery produces a local, pattern-insensitive, and relatively long-lasting reduction in visual sensitivity. The sensitivity loss may be mimicked by a 0.24 log unit reduction in target energy.

Is it what you see, or how you say it? Spatial bias in young and aged subjects

Healthy subjects demonstrate leftward bias on visual-spatial tasks. However, young controls may also be left-biased when drawing communicatively, depicting the subject of a sentence leftward on a page relative to the sentence object, that is, a spatial-syntactic, implicit task. A leftward visual-spatial bias may decrease with aging, as right-hemisphere, dorsal, visual-spatial activation may be reduced in elderly subjects performing these tasks. We compared horizontal and radial (near-far) visual spatial bias, and spatial-syntactic bias, in healthy young and aged participants. Both horizontal and radial visual-spatial bias were smaller in aged participants when explicitly, but not implicitly assessed. Mean implicit far bias was greater in aged subjects, although this varied by task. We observed less implicit, spatial-syntactic left bias in aged than young participants. These results may be consistent with relatively less dominance of right hemisphere, dorsal spatial systems with aging.

Asymmetrical visual-spatial attention in college students diagnosed with ADD/ADHD.

ObjectiveResearch indicates that individuals with attention deficit disorder (ADD)/attention deficit hyperactivity disorder (ADHD) may exhibit left-right asymmetric spatial attention, with deficient processing of stimuli in the left visual hemispace. However, there is controversy as to when this phenomenon can be observed. BackgroundPeople with ADD/ADHD do not have obvious spatial bias when performing everyday tasks. Visual cancellation tasks have demonstrated behavioral asymmetry in ADD/ADHD, but results have not been consistent across studies. Children and older adults with ADD or ADHD have been assessed, but previous studies of college students with ADD/ADHD are not available. MethodWe tested 24 students with ADD or ADHD and 24 control students on a verbal and nonverbal cancellation task. ResultsThe ADD/ADHD group made significantly more left-sided omission errors than controls on a letter cancellation task. This group difference was not observed for a shape cancellation task, however. ConclusionsThese results support possible left visual inattention in college students with ADD/ADHD. Studies of functional correlates of these attentional phenomena are needed.

Illusory" Illusory Conjunctions: The Conjoining of Features of Visual and Imagined Stimuli

The question of whether illusory conjunctions would occur with visual mental imagery wasinvestigated. In 4 experiments, participants were tachistoscopically presented displays ofgeometrical figures (varying in shape, color, and solidity) flanked by 2 digits. For half of thetrials, participants imagined one of the figures in the display. Illusory conjunctions occurredbetween the features of the physical (cued) and imagined figures, which suggests that imageryinfluences perception at the level of visual processing at which features are combined.Moreover, the conjunction errors induced by an imagined figure were similar to those inducedby a physical figure with the same features. The pattern of errors could not be accounted for byguessing. Together, these findings support the view that there can be correspondence betweenvisual imagery and visual perception.

Unmasking the Perky Effect: Spatial Extent of Image Interference on Visual Acuity

We have previously argued that visual mental images are not substitutable for visual percepts, because the interfering effects of visual stimuli such as line maskers on visual targets differ markedly in their properties from the interfering effects of visual images (the “Perky effect”). Imagery interference occurs over a much wider temporal and spatial extent than masking, and unlike masking, image interference is insensitive to relative orientation. The lack of substitutability is theoretically interesting because the Perky effect can be compared meaningfully to real line masking in that both types of interference are visual, not due to optical factors (accommodative blur or poor fixation) or to high-level factors (attentional distraction, demand characteristics, or effects of uncertainty). In this report, however, we question our earlier position that spatial extents of interference are markedly different: when images and real lines are matched in contrast, which was not done previously, their interference effects have very similar spatial extents. These data add weight to the view that spatial properties of images and percepts are similar in respect to extent. Along with the wider temporal extent and the insensitivity to orientation, the new results remain compatible with our older hypothesis that to create a clear mental image in a region of visual space, incoming signals from the eye must be suppressed (Craver-Lemley and Reeves, 1992). We have pursued this idea in this report using “unmasking,” in which adding elements to the visual image in the region beyond the zone of suppression reduces the Perky effect.

Imagery Interference Diminishes in Older Adults: Age-Related Differences in the Magnitude of the Perky Effect

Studies have documented the negative effects of mental imagery on perception (also known as the Perky effect) in younger adults, but imagery-interference effects in older adults have never been assessed. Two experiments examined this issue directly. Experiment 1 demonstrated that visual mental images diminish visual acuity in younger adults (mean age = 19.0) but not older adults (mean age = 73.6). Experiment 2 obtained parallel results, showing that visual imagery interfered with performance on a visual detection task in younger (mean age = 18.7) but not older adults (mean age = 66.7). Processes underlying age-related differences in imagery-interference effects are discussed and implications of these results for changes in cognitive performance in older adults are considered.