Auditory laterality and attentional deficits after thalamic haemorrhage

Thalamic lesions have been shown to produce severe cognitive deficits invol ving language and memory. A majority of the studies have reported cognitive deficits after lesions in the anterior and dorsomedial thalamic nuclei. We report five case studies of effects on language processing after postero-d orsal thalamic haemorrhages. Four of the patients had lesions on the right side, and one patient had a lesion on the left side. Effects on language pr ocessing were investigated with the dichotic listening test with consonant- vowel syllables. This test, in which conflicting auditory stimuli are prese nted simultaneously to the two ears, has been used to probe differences in language processing in the left and right hemispheres. The four patients wi th right-sided lesions reported almost none of the syllables presented to t he left ear, and were unable to modify this massive right ear advantage by directing attention to the left or right ear. The patient with a left-sided lesion showed a weaker left ear advantage, and was able to modify his resp onses by shifting attention, to an extent similar to that of healthy refere nce individuals. When tested with monaural stimulus presentation, the score s of all patients rose to almost 100 % correct for each ear. The pattern of effects with dichotic stimuli under different instructional conditions can not be accounted for in purely structural terms, and indicates that lesions in the posterior part of the thalamus, including the pulvinar nucleus and medial geniculate body, produce deficits not only in processing of complex auditory stimuli but also in the allocation of attention to input from one ear or the other.