Characterization of Area 18 modulation from stimulation outside the receptive field of Area 17 cells in the cat

Abstract

A stimulus placed outside the classic receptive field (CRF) of an Area 17 cell does not evoke an excitatory response. However if the CRF is stimulated to excite the cell, a second stimulus placed in the periphery modulates this baseline activity. Two questions address this phenomenon: 1) what CRF properties are modulated by these influences and 2) from where do they originate? In cats paralyzed and anesthetized with Propofol and N2O, we investigated which properties (i.e. orientation, spatial frequency, temporal frequency, contrast and diameter) of the Area 17 cells' RFs were modulated by a peripheral stimulus and whether the modulation originated in Area 18. We inserted a tungsten electrode in Area 17 and a multi-barrel injection and recording pipette in Area 18 and identified restricted sites with non-overlapping RFs. Optimal stimulus parameters for the Area 17 cell were determined and control tuning curves for each parameter were measured. We then optimized stimulation parameters for the Area 18 site and verified that iontophoretic administration of GABA (0.5 M) silenced the response to that stimulus. To determine the influence of the peripheral site on the response of the Area 17 cell, both sites were stimulated simultaneously and each curve was remeasured. The spatial frequency and contrast responses generally remained unchanged, but the orientation (5/10), temporal frequency (4/10) and diameter tuning (4/10) curves decreased and/or shifted according to the 95% confidence interval. To determine if these influences came from Area 18, GABA was applied to the Area 18 site and the curves were remeasured using both stimuli. In half the cases, the responses returned to control levels indicating that Area 18 activity caused the modulation. This result supports our hypothesis that top down influences from Area 18 make a significant contribution to the widespread integration of information by Area 17 cells.