Principal Investigator
Contribution of top-down projections from the PFC to motion processing in area MT
Reduced direction selectivity in the ipsilesional MT after PFC lesion.
Neurons in the prearcuate region of prefrontal cortex (PFC) receive inputs from the motion-processing area MT and during motion discrimination exhibit direction selectivity (DS) suggestive of their MT origins.1 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. In this still ongoing project, we have been studying 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 delay2. 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 (A), was accompanied by a transient increase in response variability.
Ipsilateral prefrontal lesion leads to more sustained match suppression of test responses in MT.
During the delay, suppression in activity and a subsequent anticipatory increase in firing rates, common in normal MT, was absent. During the test, the normally transient response suppression on trials with matched directions during the test and the sample was more sustained (see B), 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 task components. These deficits were most dramatic when the task required rapid reallocation of spatial attention. The lesion effects were most pronounced in 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 activity during motion discrimination and for normal motion perception.
Footnotes
- 1
Zaksas D, Pasternak T (2006 Nov 09). Directional signals in the prefrontal cortex and in area MT during a working memory for visual motion task. J Neurosci. 26, 11726-42.
- 2
Lui LL, Mancarella MA, and Pasternak T (2009). A unilateral PFC lesion affects neuronal activity in area MT during motion discrimination tasks. Journal of Vision Abtracts.
