Orientation tuning and receptive field structure in cat striate neurons during local blockade of intracortical inhibition.

The contribution of intracortical inhibition to orientation tuning in the cat striate cortex (area 17) was studied by responses to flashing light bars of different orientations and lengths in 68 single-units before and during microiontophoretical application of bicuculline, a GABAA antagonist, A three-fold increase in the background activity (13.3 +/- 1.3 vs 4.4 +/- 0.5 imp/s) and 4.4-fold increase in the maximal discharge frequency (264.4 +/- 22.3 vs 60.6 +/- 5.3 imp/s) was found in 96.0% of the cells studied during microiontophoresis. In most units all characteristics of orientation tuning significantly changed during application of bicuculline: i) tuning width increased in 76.3% of cells from 52.7 +/- 2.8 degrees in control to 85.2 +/- 4.6 degrees, ii) tuning selectivity diminished in 63.6% of cells by a factor of 1.5, and iii) tuning quality dropped in 68.5% of cases by a factor of 2.5. The threshold ejection current of bicuculline for widening of tuning was in 2/3 of the cells in the range from +10 to +40 nA (+31.0 +/- 4.5 nA) and the maximum effect was obtained in 3/4 of units with +30(-) + 100 nA (+67.1 +/- 6.0 nA). Unmasking of additional excitatory inputs to the studied cells due to blockade of the inputs from inhibitory interneurons in considered as the main mechanism of the described bicuculline effects.