Proceedings of The Physiological Society
University of Bristol (2005) J Physiol 567P, C128
Interocular cross-orientation suppression in cat striate cortex involves intracortical inhibition
Sengpiel, Frank; Vorobyov, Vasily;
1. School of Biosciences, Cardiff University, Cardiff, United Kingdom.
Cross-orientation suppression (COS) in V1 is commonly thought to result from intracortical inhibition. This hypothesis was rejected by Freeman et al. (2002), who suggested an origin in the LGN. We examined the nature of COS elicited by dichoptically presented gratings in V1 of anaesthetized neuromuscularly blocked cats, employing single-cell recordings. Details of animal preparation have been described elsewhere (Sengpiel et al. 1995). Briefly, anaesthesia was induced with an i.m. injection of ketamine (20-40 mg/kg) and xylazine (2-4 mg/kg). Following tracheal cannulation, animals were artificially ventilated and anaesthetized with a mixture of N2O (55-65%), O2 (35-45%) and isoflurane (2-2.5% during surgery, 1-1.5% during recording). During recording the animal was neuromuscularly blocked with a continuous i.v. infusion of gallamine triethiodide (10 mg/kg/h) in glucose-saline. E.C.G. and E.E.G. were constantly recorded to monitor the state of anaesthesia. Animals were killed with an overdose of barbiturate at the end of the experiment. First, we studied the dependence of COS on the drift rate of the suppressing grating. Temporal frequency tuning of monocular cross-orientation suppression was recorded in 73 neurons, interocular suppression in 74 cells. Of those, 46 neurons were tested under both suppression paradigms. Overall, the tuning of interocular suppression closely matched the tuning of the cells' excitatory responses, with cut-off rates of 8-16 Hz. In contrast, monocular COS was observed for drift rates of up to 32 Hz, similar to LGN responses. Second, we tested whether interocular COS was affected by adaptation to the suppressor. If COS were of geniculate origin, adaptation to the suppressor should not affect its strength, since firing rates in the LGN decrease only slightly. We measured contrast-response functions in the presence and absence of a suppressing grating and found that mean interocular COS was reduced from 33.4% ± 2.9% (mean ± SEM) without prior adaptation to 5.6% ± 8.7% after adaptation to the masking grating. Suppression under these two conditions differed significantly (p < 0.001, paired t-test). Third, we locally antagonized intracortical inhibition by bicuculline micro-iontophoresis and found that interocular COS was reduced from 46.1% ± 4.0% (mean ± SEM) prior to application of bicuculline to 18.1% ± 4.9% during application of bicuculline. Suppression under these two conditions differed significantly (p < 0.001, paired t test). Taken together, our data demonstrate that interocular COS is substantially different from monocular COS and that it is mediated by inhibitory circuitry within the visual cortex.
Where applicable, experiments conform with Society ethical requirements