Functional asymmetry in sheep temporal cortex.

Sheep, like humans, show a bias in favour of the left visual field when discriminating familiar faces. This, in humans, is thought to be caused by a right hemisphere dominance for processing faces involving the temporal cortex. We have directly investigated inter-hemispheric differences in face-processing in sheep by recording the frequencies and response profiles of single cells in the temporal cortex which respond selectively to faces. While there was no evidence for increased frequencies of face-sensitive neurones within the right temporal cortex, or their relative selectivity for individual faces, there was a pronounced response latency difference between the two hemispheres. The cells in the left hemisphere responding selectively to particular faces did so up to 400 ms later than those in the right and with a greater degree of temporal variability between cells. No hemispheric latency differences were found, however, in other cells responding to general visual stimuli or to many faces. The data suggest that, while specialised face-sensitive neural circuits in the right hemisphere may play a key role in the rapid (< 200 ms) identification and discrimination of facial identity, those on the left may be involved more with slower processes associated with integrating the emotional or mnemonic consequences of recognition.