Differential behavior of simple and complex cells in visual cortex during a brief IOP elevation.

PURPOSE

To study and compare responses of different types of cortical neurons in the primary visual cortex in cats to grating stimuli before and during brief elevation of intraocular pressure (IOP).

METHODS

Single-unit electrophysiological recordings were performed in anesthetized and paralyzed cats. The IOP was elevated by injecting saline into the anterior chamber of the cat's eyes through a syringe needle. The IOP was elevated to a level at which the retinal perfusion pressure (arterial pressure minus IOP) was maintained at approximately 30 mmHg for a period of 4 minutes. The responses of simple and complex cells in the primary visual cortex to visually drifting sinusoidal gratings were measured before and during the elevation of IOP.

RESULTS

The response amplitude of all the cortical cells in the primary visual cortex declined during a brief elevation of IOP. The decrease in the response of simple cells was always more significant than that of complex cells. The differential decrease between the two major types of cells was independent of the cell's receptive field location and cortical depth. There was a mild tendency for cells with higher preferred spatial frequencies to be more sensitive than those with lower frequencies. The preferred orientation and direction of most cortical cells remained roughly unchanged though their orientation and direction biases decreased. An increase in the animal's blood pressure, which returned the retinal perfusion pressure to a normal level, compensated for the decreased response induced by the elevation of IOP.

CONCLUSIONS

The differential effects of a brief elevation of IOP on the response of simple and complex cells in the visual cortex are general and may originate from the retina through the lateral geniculate nucleus (LGN), where different effects of elevation of IOP are exerted on X- and Y-type retinal ganglion cells. The results may suggest differential behavior of neurons tin the parvo and magno pathways of the primate.