Abstract
Neurons in the prearcuate region of prefrontal cortex (PFC) receive inputs from the motion-processing area MT and during motion discrimination exhibit direction selective (DS) responses suggestive of their MT origins (Zaksas & Pasternak, 2006). This region also sends direct top-down projections to MT and the nature of activity in both areas recorded during the same motion task indicates strong functional links between them. We examined the contribution of top-down PFC influences to MT responses and to motion perception while a monkey with a unilateral ibotenic PFC lesion discriminated directions of two sequential stimuli, sample and test, separated by a delay.
Recordings from the ipsilateral MT revealed abnormalities during all phases of the task. During the sample, a significant decrease in responses and a drop in DS, was accompanied by a transient increase in response variability. During the delay, suppression in activity and a subsequent anticipatory increase in firing rates, common in normal MT, were absent. This was accompanied by abnormally low variability of delay activity, suggesting that the top-down PFC signals may be among contributors to the variability in delay activity in normal MT. During the test, on trials when its direction matched the direction of sample, the normally transient response suppression was more sustained, suggesting PFC involvement in the sensory comparison phase of the task.
Behavioral testing revealed deficits in direction thresholds at longer delays, indicating a disruption in the maintenance and/or attentional components of the task. These deficits were most dramatic when the task required rapid reallocation of spatial attention. The lesion effects were confined to the contralesional visual field, suggesting a link to retinotopic areas involved in motion processing (eg. MT). Our results demonstrate the importance of PFC influences for normal MT activ
Supported by EY11749 NHMRC (Aus) Post-Doc fellowship to L.L. and P30 EY01319 to CVS.