Alterations in visual receptive fields in the cat superior colliculus induced by bicuculline methiodid

The precise pharmacological and physiological mechanisms involved in establishing receptive field (RF) properties in the superior colliculus (SC) are not clearly understood. This study has investigated the effects of iontophoretic application of bicuculline methiodide (BMI) on the RF properties of visual cells in the superficial layers of the cat SC. Two methods were employed in determining changes in RF size when plotted using a moving stimulus: (1) a subjective manual assessment, and (2) an objective computer-controlled assessment. Manual assessment showed the average RF expansion to be over three times the control RF sizes in 18 of 23 cells. Computer-controlled assessment using a moving stimulus confirmed RF expansion in 15 of these 18 cells. The RF area measured using a computer-controlled flashing stimulus also increased substantially in response to the application of BMI. When direction selectivity was exhibited by a cell, the magnitude of selectivity decreased dramatically after BMI application. These results suggest the importance of a GABA-ergic mechanism in the organization of RF properties in the SC. The results of this study are compared to previous research investigating pharmacologically-induced changes in RF properties, and possible models of the pharmacology and physiology of RFs in the superficial layers of the SC are discussed.