Attention involves selecting a subset of the environment to undergo more elaborate processing in the brain. To respond appropriately to events in the world one must not only orient attention in space, but also in time. As it turns out, the brain regions most clearly implicated in spatial attention – the parietal lobes – are also thought to be involved in temporal attention, particularly ventral parietal regions such as the temporo-parietal junction (TPJ).
A recent study reported in the Journal of Cognitive Neuroscience provides further causal evidence in support of the view that the right TPJ in particular is dominant for temporal processing. The study utilized a novel simultaneity judgement task in two experiments that involved patients with lesions to the TPJ and healthy participants that had inhibitory TMS delivered to the TPJ. Participants were presented 4 flashing discs for 3 seconds (alternating uniform black and white) that were presented in the corners of an invisible square. On each trial, one disc was randomly selected to flash in counter phase to the other 3 discs (i.e., oddball disk was white when other disks were black). Prior to target onset, either the left or the right pair of disks was cued and participants were asked to judge whether cued pair flashed synchronously.
A staircase procedure showed that healthy controls and patients with damage to left TPJ could perform the simultaneity judgement at 80% accuracy when the flash rate for the array of items was approximately 9 Hz. In contrast, average flash thresholds for right TPJ patients were markedly worse, with 80% threshold observed when the flash rate was approximately 4 Hz.
The follow up experiment involving Transcranial Magnetic Stimulation (TMS) with healthy controls showed a similar pattern of results. In this experiment, inhibitory 1Hz TMS was applied for 20 minutes either to the left TPJ, the right TPJ or over early visual cortex. Simultaneity thresholds after TMS were worse (compared to pre-stimulation thresholds) only when the right TPJ was inhibited. Thresholds did not vary from baseline after inhibition of left TPJ, and inhibition of early visual cortex showed a slight improvement in flash thresholds.
By combining lesion and TMS methods, the results of the study provide convincing causal evidence that the TPJ is involved in temporal attention. Brain-imaging studies have previously reported TPJ activation during simultaneity tasks, however imaging studies are correlational and cannot say anything about the causal role of the TPJ in these processes. Indeed, the inclusion of TMS is important since many of the patients that participated in the study had very large lesions, whereas the effect of TMS is comparatively more focal.
However, the evidence that the right TPJ is dominant for temporal attention is somewhat ambiguous. It is difficult to tell from the data presented in the paper whether the extent of the brain damage observed in the left and right TPJ patients was the same. Moreover, the results of the TMS experiment did not provide strong evidence for the dominance of right TPJ for temporal processing. Although the right TPJ impaired temporal processing (compared to baseline), the magnitude of the impairment was not significantly different from the impairment observed in the left TPJ (however there was a trend toward significance). Strictly speaking then, the results of the TMS study do not provide firm support for the claim of selectivity. Nevertheless, the present paper adds to a growing number of studies examining the neural correlates of time perception using techniques that infer causality (TMS, TDCS etc) in a field that is largely dominated by brain imaging techniques.
Bronson Harry
The MARCS Institute, Western Sydney University
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