C.A. Marzi

Distribution in the visual field of the costs of voluntarily allocated attention and of the inhibitory after-effects of covert orienting

By using a simple reaction time (RT) paradigm we have investigated the spatial distribution of the benefits and costs of voluntarily directed attention and of the inhibitory after-effects of covert orienting. In the first experiment subjects deliberately allocated attention to each one of five stimulus positions disposed along the horizontal meridian, while at the same time fixing their eyes on the central position. The separation in visual angle between the central position and the two nearest positions, one on the left and the other on the right, was 10 degrees; that between the central position and the two most eccentric positions was 30 degrees. By comparing RT to brief flashes of light presented at each position during directed attention with RT to identical flashes at the same position during diffuse attention (i.e. in a condition in which subjects paid equal attention to all five positions), it was possible to determine that benefits, that is RT decreases relative to the diffuse-attention condition, were strictly limited to the attended position. Costs, i.e. RT increases relative to the diffuse-attention condition, showed a more diffuse and complex spatial pattern. When attention was directed to one of the noncentral positions, costs were apparent at the two contralateral positions and at the central position, but not at the ipsilateral position. When attention was directed to the central position, costs occurred at all other positions. This suggests a special role for the vertical meridian in delimiting the area of costs when one covertly orients towards the opposite right or left visual half field. Work of others and our preliminary evidence indicate that the area of costs is similarly limited by the horizontal meridian when one orients toward the opposite upper or lower visual field. In the second experiment we studied the inhibitory after-effect of covert orienting. Orienting to a light stimulus without moving the eyes to it may induce a short-lived facilitation of the speed of response to a second stimulus presented at the same position, but this facilitation is followed by a profound and prolonged RT retardation. By using a two-flashes paradigm we observed this RT retardation not only when the two stimuli appeared at the same position, but also when they occurred at different locations in the same altitudinal or lateral visual hemifield. There were no inhibitory after-effects when the two stimuli appeared on opposite sides of the vertical or horizontal meridian.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial summation across the vertical meridian in hemianopics: A test of blindsight

Twenty hemianopic patients with retrochiasmatic lesions have been tested for spatial summation of pairs of flashes simultaneously presented either to the same hemifield or to opposite hemifields across the vertical meridian. In such a task normal subjects show summation, i.e. a faster reaction time in response to a pair of stimuli than in response to a single stimulus. Such an effect is present both for pairs of stimuli presented within the same hemifield and for pairs of stimuli in which the two flashes are presented one in the right and the other in the left hemifield. In contrast to normals, hemianopics as a group did not show interfield summation although, like normals, showed summation within one hemifield. A single-case analysis, however, revealed that in one patient there was a reliable overall interfield summation and that in three others there was evidence of summation in at least one testing session. The presence of interfield spatial summation between the normal and the affected hemifield of hemianopics thus provides further evidence of blindsight in a task paradigm in which guessing strategies and stimulus artefacts can be eliminated. The very small proportion of patients showing blindsight can be in part related to the relatively low stimulus intensity and the very brief stimulus exposure duration used.

Spatial distribution of the inhibitory effect of peripheral non-informative cues on simple reaction time to non-fixated visual targets

It is known that reaction time (RT) for the detection of a light target at extrafoveal locations is lengthened by a previous non-informative light cue at the same location. We describe an additional inhibitory effect from cues remote from the target but occurring within the same lateral or altitudinal visual hemifield. Subjects made a speeded key-press response to the second of two successive light flashes in a pair while maintaining fixation. Each of the two flashes could appear at random in one of four positions, two in the right and two in the left visual fields, or two in the upper and two in the lower visual fields. We found an RT prolongation not only for cued over uncued positions, but also for within-field non-coincident cue-target pairs over between-fields cue-target pairs. The within-field inhibitory effect, though smaller than the same-location effect, was fully apparent even when the target occurred at 1 degree of visual angle from the midline and at 29 degrees from the cue. Both effects were seen with cue-target asynchronies ranging from 0.2 to 1.5 sec. The results are relevant to the understanding of the neural mechanisms for covert shifts of attention across the main meridians of the visual field.

Why are left-right spatial codes easier to form than above-below ones?

Previous studies have shown that spatial compatibility is stronger in the left-right than in the above-below dimension. This left-right dominance cannot be attributed to a better representation of the effectors in left-right than in above-below locational codes or to the fact that incompatible left-right stimulus-response pairings cross the body midline, whereas incompatible above-below ones do not. Nicoletti and Umiltà (1985) proposed that the left-right dominance should be attributed to the allocation of attention to the more difficult discrimination, which, in vision, is that concerning the left-right dimension. This attentional hypothesis was tested in the present study, in which we used the auditory modality. We reasoned that because in the auditory modality the above-below discriminations are more difficult than left-right ones, attention should be preferentially allocated to the former. Therefore, in audition an above-below dominance should replace the left-right dominance observed in vision. Experiments 1 and 2 showed a clear-cut compatibility effect in the auditory modality for both the left-right and above-below dimensions. Experiment 3 showed that spatial compatibility was still stronger for the left-right than for the above-below dimension. Since the left-right one proved to be the more discriminable dimension, this finding rules out the attentional hypothesis, at least in the version originally proposed.

Pathways of Interocular Transfer in Siamese Cats

Siamese cats share with other albino mutants of numerous mammalian species a profound abnormality of their visual systems (see Guillery et al., 1974 for review). In addition to the lack of pigment in the eye and the frequent but by no means constant presence of strabismus (Rengstorff, 1976), their visual pathways are almost totally crossed. Exaggerating a trend already present in ordinary cats (Kirk et al., 1976) fibres from the centremost 20–25 degrees of temporal retina, with the exclusion of a small medial normal segment (Guillery et al., 1974; Shatz, 1977a), instead of projecting ipsilaterally, cross at the optic chiasm and terminate in the contralateral visual centres. The presence of such an extra portion of contralateral visual pathways can be precisely documented both anatomically and electrophysiologically (Guillery and Kaas, 1971; Kalil et al., 1971; Shatz, 1977a) at the level of the lateral genicu-late nucleus (LGN). Also, in the optic tectum there is consistent anatomical and electrophysiological evidence for an almost totally contralateral visual input (Berman and Cynader, 1972; Kalil et al., 1971; Lane et al., 1974; Weber and Hartig, 1976).

Neurophysiologic and neuropsychological aspects of cutaneous perception

Abstract Just a century ago, the discovery of sensory spots by Blix (1884), 1 Goldscheider (1884), 2 and Donaldson (1885) 3 gave impetus to differentiating sensory submodalities in touch, which Weber, 4 in 1846, considered the only cutaneous sense.