Dependence by any other name smells just as sweet: reply to van der Velde and van der Heijden (1997)

The hypothesis that people selectively attend to entire objects predicts that all attributes of an object will be reported either very accurately (if the object was attended) or very inaccurately (if it was unattended). Hence, reports of object attributes should show positive dependence. M. Monheit and J. Johnston (1994) have confirmed this prediction. F. van der Velde and A. H. C. van der Heijden (1997), however, have argued that dependence in the overall data is spurious. They advocate a model that partitions the data into 2 subsets, 1 for perception trials and 1 for guessing trials, each of which separately exhibits independence. Here, the authors argue that this treatment of guessing is misguided because, in effect, guesses are discarded rather than treated as failures of perception. The Monheit and Johnston analysis, on the other hand, is fundamentally sound and demonstrates precisely the kind of dependence predicted by the spatial attention hypothesis.

Spatial attention to arrays of multidimensional objects.

Encoding briefly displayed arrays of multidimensional objects appears to require selective attention, but this hypothesis is challenged by M. J. Nissen's (1985) finding that properties of an object are reported independently. Selective attention to some objects but not others should produce positive dependence. Theoretical analysis shows that deviations from independence would have been difficult to observe in Nissen's data because of high guessing rates and small sample sizes. Four new experiments showing strong positive dependence in property reports are described. Deviations from independence were highly significant for most Ss. Quantitative modeling shows that selective attention to subsets of objects would produce about the amount of dependence obtained. Rather than challenging attention theories, the amount of dependence in encoding multidimensional objects is consistent with selective attention to either objects or locations.

Unconscious perception of "extinguished" visual stimuli: reassessing the evidence.

When parietal-damaged patients fail to report a contralesional stimulus because of extinction, is this because the stimulus is not perceived, or because it is perceived but cannot reach conscious awareness? VOLPE et al. [10] reported an intriguing study that seemed to locate the problem at least partly in the transfer of information to conscious awareness. They showed patients with extinction pairs of stimuli, one in each hemifield. Although patients were predictably poor at reporting the identity of the contralesional stimulus, they were able to make accurate same/different judgements comparing the two stimuli. This was interpreted as evidence that both stimuli were perceived. In the present paper, we point out that the dissociation between identification and same/different matching could also be due to the possibility that less visual information about the contralesional stimulus is necessary to make a same/different judgement than to identify the stimulus, and that chance performance is considerably higher in the first than in the second type of task. In Experiment 1, we verified this by degrading one side of a stimulus display and "replicating" the dissociation with normal subjects. We also equated the amount of visual information needed for the two tasks by yoking the stimulus pairs on "different" trials of the same/different matching task with the choice pairs on a forced choice identification task. Under these conditions, the dissociation vanished. In Experiment 2, we administered these tasks to three parietal-damaged patients with extinction. When the original method was used, same/different matching was better than identification of the contralesional stimulus. With the forced choice identification method, the dissociation again vanished.

Optimal geometry for oval sprint tracks.

Track aspect ratio is defined as the percentage of lap length devoted to turns on an oval running track. Equations based on experiments are developed to model a composite runner with a specified top speed, during an acceleration phase in the straightaways and a centripetal phase in the turns. We calculate velocity deficits for several common track sizes over the range of aspect ratios and predict that, under our assumptions, a perfect circle is the optimal track shape.

Parietal lobe mechanisms of spatial attention: modality-specific or supramodal?

Is the spatial attention system divided into separate, modality-specific subsystems, or is there a supramodal spatial attention system? More specifically, does the role of the parietal lobe in spatial attention involve modality-specific or supramodal mechanisms? We addressed this question using a variant of Posner's spatial cuing task. Parietal-lesioned patients performed a simple reaction time task to lateralized visual target stimuli, preceded on each trial by either non-predictive lateralized visual cue stimuli or non-predictive lateralized auditory cue stimuli. With both types of cues, we found disproportionate slowness in responding to invalidly cued contralesional targets, indicative of an impairment in disengaging attention from the ipsilesional to the contralesional side of space. The finding of an attentional disengagement impairment for visual targets with auditory cues implies that the parietal lobe's attentional mechanism operates on a representation of space in which both visual and auditory stimuli are represented, in other words, a supramodal representation of space.

Brain Activity Underlying Mental Imagery: Event-related Potentials During Mental Image Generation.

This article addresses two issues about the neural bases of mental imagery. The first issue concerns the modality-specificity of mental images, that is, whether or not they involve activity in visual areas of the brain. The second issue concerns hemispheric specialization for the generation of mental images. We compared event-related potentials recorded under two conditions: one in which subjects were shown words and asked to read them and one in which subjects were shown words and asked to read them and generate visual mental images of the words' referents. Imagery caused a slow, late positivity, maximal at the occipital and posterior temporal regions of the scalp, relative to the comparison condition, and consistent with the involvement of modality-specific visual cortex in mental imagery. Also noted was an asymmetry in the imagery-related ERP, consistent with left-hemisphere specialization for mental image generation. Similar results were obtained when subjects listened to auditorily presented words with and without instructions to generate mental images. To assess the specificity of the relation between these ERP effects and mental imagery, we compared the ERP changes brought about by imaging with those brought about by another effortful task using the same stimulus words: proofreading the words for occasional misspellings. This produced changes that differed in polarity, time course, and scalp distribution from the imagery-related changes.