Unilateral and bilateral electrotactile stimuli were delivered to both hands of 11 right-brain-damaged (RBD) patients with left tactile extinction and 20 healthy subjects. Bimanual stimuli could be presented simultaneously or with varying stimulus onset asynchronies (SOAs). Subjects indicated their detection of unilateral or bilateral stimuli, their judgements of whether stimuli were simultaneous or successive, and, in the latter case, which side came first. In RBD patients, extinction was maximal with simultaneous presentations and decreased as SOA increased. With short SOAs, omissions of left-sided stimuli occurred with both right-side and left-side stimulus precedence, suggesting a forward and backward interference of the right stimulus on the processing of the left stimulus within a time window of at least 100 msec. In contrast, there was no interference of the left stimulus on the detection of the right stimulus. Unlike controls, extinction patients rarely expressed simultaneity judgements, but those that were produced tended to be veridical or nearly so, like in normal controls. Whereas controls expressed generally accurate judgements of right or left precedence, patients showed a bias toward a right precedence and a maximal uncertainty between left-first and right-first choices when the left stimulus had a lead between 100 and 200 msec. The results are consistent with the hypothesis that, as in other sensory modalities, tactile extinction is associated with an abnormal persistent bias of attention toward the ipsilesional side that delays the processing of contralesional stimuli. However, the finding that both extinction and explicit judgements of simultaneity tended to occur with simultaneous bilateral stimuli suggest the presence of some residual neural capacity to detect precise temporal coincidence.

Detection reaction time (RT) at an extrafoveal location can be increased by noninformative precues presented at that location or ipsilaterally to it. This cue-induced inhibition is called inhibition of return or ipsilateral inhibition. We measured detection RT to simple light targets at extrafoveal locations that could be designated for covert orienting by local or distant cues. We found that cue-induced inhibition co-occurred in an additive fashion with the direct effects of covert orienting, i.e., it detracted from facilitation at attended locations and increased the disadvantage for unattended locations. Thus, cue-induced inhibition cannot be suppressed by a volitional covert orienting to the cued location; the cooccurrence of different facilitatory and inhibitory effects confirms the simultaneous operation of multiple independent, attentional mechanisms during covert orienting.

The lateral organization of the gustatory pathway in man is incompletely understood. Majority of the studies support an uncrossed projection from each side of the tongue to the cortex, but reports of an opposite crossed organization continue to appear in the neurological literature. We studied the lateral organization of the gustatory pathway in normal controls, a man with a complete callosal agenesis, and a man with a complete section of the corpus callosum, a right anterior-frontal lesion, and language in the left hemisphere. Sapid solutions were applied to one or the other side of the tongue, and subjects reported the taste of the stimulus either verbally or by manually pointing to the name of the taste. There were no differences in accuracy and reaction time between the right and left hemitongues of the controls and the genetically acallosal observer. By contrast, the callosotomy subject showed a constant marked advantage of the left hemitongue over the right for both accuracy and speed of response, though performance with right stimuli was clearly above chance. The left advantage can be attributed to the left hemisphere being favored by the essentially verbal nature of the task, or to the presence of a lesion in cortical gustatory areas in the right hemisphere, or to both factors. Whichever of these hypotheses turns out to be correct, the results unequivocally reject the notion of an exclusively crossed organization of the gustatory pathway from the tongue to the cortex, and favor the notion of a bilaterally distributed organization of this pathway with a marked predominance of the uncrossed over the crossed component.